Physical Geology (EES 1000, Dynamic Earth) is a prerequisite to Historical Geology (EES 1004). To pass historical geology with a C or better, without having a background in physical geology, is difficult. You will have to do extra work on your own time! Don't attempt this unless you are willing to buy a physical geology text and learn it on your own. The review of physical geology done in historical geology is brief.
These notes on the web contain the topics that are covered by the instructor in the class lectures. The lecture notes are not intended to be a book, and students are expected to use their class notes together with the class book, or an other historical geology test, or web resources to understand the topics. A list of review questions and specific topics for each chapter are given at the end of each chapter in the lecture notes. These are questions and concepts that the student should understand for the test. After a chapter is covered in class, this review list may be updated by the instructor. Students are responsible for checking the web site to get the update prior to a test.
Tests: Three 100 point multiple-choice tests. The first test will cover Chapters 1-5, reviewing Physical Geology concepts, Fossils, and Evolution. The second test will cover Chapters 6-10, reviewing the origin of the solar system, the Hadean, Archean, Proterozoic Eons and the Paleozoic Era. The last test covers Chapters 11-15, reviewing the Mesozoic and Cenozoic Eras and the origin of humans.
The numerical grades of the 3 tests will be averaged and the final grade format is 87 or above, A; 74 up to 87, B; 61 up to 74, C; 48 up to 61, D; below 48, F. Students caught cheating will receive a zero on the test.
Students who miss one of the first two exams can take a makeup in the class period prior to the last (or third test). The makeup covers all of the material covered in the first two tests. A student who took both of the first two tests can also use the makeup to substitute for the lowest grade on one of those two tests. A student missing both of the first two tests will need excused absences for both tests in order to get two makeups, one of which will be given during final week. A student missing the third test with an excused absence can get a makeup during final week. Makeups during final week will be given at the time scheduled for finals for this class.
Class role will be taken.
You can contact me by telephone, email, and before and after class to set up an appointment. Otherwise, look for me in the Geochemistry Lab (1059) or in my office (1034) on class days in the afternoon. Note that the general class notes for this course (Earth and Environment Throught Time or Historical Geology) on my web page (accessed from www.ronstoessell.org) are often updated following the lectures.
The lecture schedule may vary but the three listed test dates are fixed. A review is scheduled prior to each test. The date of the review may be shifted up if the test material is covered faster than scheduled, freeing up a class period to be used for study for that test.
Chapters Chapters Chapters
8/21/06 1 10/03/06 6 K. Der. 11/16/06 15
8/23/06 1 10/10/06 6 & 7 K. Der. 11/21/06 review
8/29/06 2 10/12/06 7 11/28/06 makeup on Chapters 1-10
(second to last class date)
8/31/06 2 10/17/06 8 11/30/06 test 3 on Chapters 11-15
(last class date)
9/05/06 4 K. Der. 10/19/06 9
9/07/06 4 K. Der. 10/24/06 10
9/12/06 3 10/26/06 review
9/14/06 3 10/31/06 test (6-10)
9/19/06 5 11/02/06 11
9/21/06 review 11/07/06 12
9/26/06 study day 11/09/06 13
9/28/06 1st test 11/14/06 14
chapters 1-5
| Chapter 1 | Chapter 2 | Chapter 3 |
| Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 |
| Chapter 10 | Chapter 11 | Chapter 12 |
| Chapter 13 | Chapter 14 | Chapter 15 |
The chapter is mostly a review of general concepts from EES 1000, Physical Geology. Read the chapter and understand the following concepts and terms listed below. In addition, memorize the Geologic Time Table given in the outline. You will have to know the time table for all three tests.
Hutton proposed "Uniformitarianism" or "The present is the key to the past."
Principle of Uniformitarianism - You need to be able to explain how this principle is applied to explain how ancient rocks formed. The idea is to study the processes forming rocks today to be able to understand how they formed in the past. When we see diagnostic structures in rocks forming today than we can often assume that ancient rocks with these structures formed in the same way, i.e., the same processes occurred in the past as occur today because the same laws govern these processes.
Scientific Method - The scientific method is used in the study of the Earth and consists of four steps: (1) gathering data, (2) forming a hypothesis to explain the data, (3) making predictions based on the hypothesis, and (4) testing these predictions to see if they are true - if so, the hypothesis becomes a theory. A theory which makes many correct predictions is called a law; however, as new data becomes available, theories and laws always have to be modified, i.e., there is no absolute law or truth in science. An example is conservation of energy in reactions and normal chemical processes, the first law of thermodynamics. It doesn't hold in nuclear reactions in which mass is converted to energy.
superposition, original horizontality, lateral
continuity
Older sedimentary beds lie under younger sedimentary
beds unless the beds have been overturned (superposition).
Sedimentary beds
are initially deposited as horizontal layers (original horizontality).
Beds
that are laterally continuous represent a single formation event which can cover
a lot of time (lateral continuity).
intrusive relationships and cross-cutting
relationships
Only a younger rock layer (intrusion, e.g., dike)
can cut an existing rock unit.
principle of components
The components making up a
sedimentary rock are older than the rock.
principle of biota (fauna and flora)
succession
Evolution occurs along a pathway without repeating
itself.
Absolute time measurements from radioactive decay involve measuring the
amount left of an unstable isotope which decreases by one half during each half
life. The unstable isotope decays to a daughter isotope which may also be
unstable and undergoing decay.
For example, 14C radioactive
carbon decays to 14N with a half life of 5700 years. After four half
lives (22,800 years), the amount of radioactive carbon has decreased to 1/16th
the amount initially present. After 10 half lives, the amount of any unstable
isotope is so small that it is difficult to measure accurately. The necessity
for a significant change in the unstable isotope and having enough of the
isotope to measure limits the use of 14C to ages between 1,000 and
100,000 years. Other unstable isotopes (of generally different elements) with
smaller and larger half lives are used to age-date rocks within smaller and
larger time intervals. In the case of many isotopes, the amount of unstable
isotope when the rock formed is not known; however, the unstable isotope is
incorporated in different minerals within the rock but in different
concentrations in each mineral. In this case, the dating is applied to the
different minerals to compensate for the lack of knowledge of the initial
concentration of the unstable isotope in the rock. For example, in
87Rb => 87Sr dating, each mineral starts out with the
same ratio of 87Sr to the stable isotope 86Sr but with
different concentrations of the unstable parent isotope 87Rb and of
the stable daughter isotope 87Sr. The increase in the ratio of
87Sr/86 in each of the minerals can be used to solve for
the age without knowing how much of the unstable parent isotope was initially
present.
The three types are angular unconformity (erosion rruface separating two "non-parallel sedimentary beds), disconformity (erosion surface separating two parallel sedimentary beds) and "nonconformity" (erosion surface on top of a crystalline rock). Frequently, soils form when unconformities occur.
The crust is floating on the mantle (like a log on water) and moves up if material is removed, e.g., from erosion of mountains or melting of ice, and moves down if material is added, e.g., in a depositional basin such as the Mississippi delta. The continental crust is much thicker than oceanic crust. The thickest continental crust occurs with the tallest mountains - just as the log floating on a pond extends further below the water if it sticks up higher above the water.
The temperature and structure of the earth passing from the center to the surface goes from hot to cold and from dense to less dense: inner core (solid iron with minor nickel and sulfur); outer core (liquid iron with minor nickel and sulfur - responsible for the earth's magnetic field); mantle (solid silicate except for plastic asthenosphere where convection cells occur); crust (solid silicate).
Lithosphere plates are rigid and composed of oceanic and continental crust and the underlying uppermost mantle. These plates ride on top of heat-driven convection cells which are located in the asthenosphere and probably extend deeper in the mantle. The plates converge at subduction zones where one plate carrying oceanic crust is subducted. This produces shallow to deep (large) earthquakes and a trench with composite volcanoes (andesitic magma) overlying the subduction zone, forming from magma released by partial melting of oceanic crust (mafic rocks, basalt and gabbro) of the subducting plate. Plates diverge at spreading ridges (diverging boundaries), characterized by shallow earthquakes, diverging from a rift valley with shield volcanoes (basaltic magam) forming from partial melting of peridotite, an ultra-mafic rock in the underlying asthenosphere. Plates slide past each other along faults called transform boundaries, producing large, shallow earthquakes, e.g., the San Andreas fault. In addition, when two plates with continental crust on their leading edges converge, they fuse, forming high mountains of folded sediment from the sediment on the continental margins, and large earthquakes. Continental crust (felsic rock, granite) is not subducted because it is too light (low density). Two converging plates can only fuse after all the oceanic crust (mafic rock, basalt and gabbro) between the converging continental crusts is destroyed in subduction zones.
One additional interesting plate tectonic process takes place on the interior of plates. Hot spots or mantle plumes are mafic (basaltic and gabbroic) magma plumes rising up from the mantle and melting through the overlying plate, producing shield volcanoes on the earth's surface, e.g., the Hawaiian Islands. Because the hot spot is stationary in the mantle and the overlying plate is moving, a linear chain of shield volcanoes and/or fissures forms on the surface of the overlying plate.
oceanic crust (mafic rock: basalt overlying gabbro) only about 7 km thick - less than 4 miles. Oceanic crust is created by partial melting of peridotite under diverging zones and destroyed in subduction zones.
continental crust (felsic rock: granite) - thicker than oceanic crust. Continental crust is created at subduction zones by partial melting of oceanic crust. Continental crust is never destroyed.
Please note that universal agreement does not exist on the Time Scale. For example, the Quarternary Period sometimes includes both the Holocene and the Pleistocene Epochs. And some authors use Precambrian as an Eon with Proterozoic and Archean as Sub Eons or even as Eras. The Hadean Eon is sometimes added as the earliest Eon to cover the time lacking a rock record on the Earth, taking time from the Archean Eon. Many authors use Tertiary as a Period and not Paleogene and Neogene Periods, and some authors use Carboniferous as a Period and not Mississippian and Pennsylvanian. The Time Table below is that consistent with your text and also with the concept that the Periods within an Era should roughly correspond to same length of time and that the Eons should correspond to the larger amounts of time. This splits the Carboniferous Period into the Mississippian and Pennsylvanian Periods and keeps the Archean and Proterozoic as Eons, not Eras. Use of Paleogene and Neogene Periods, rather than Tertiary Period, allows a split of a very long Period into three Epochs in each Period.
Eon Era Period Epoch
Quaternary Holocene (Recent) Since the end of the last Ice Age
----------------- 10,000
Neogene Pleistocene Ice Ages (sometimes included in Quaternary)
(included in Pliocene Ice Ages begin, Isthmus of Panama forms
Cenozoic the Tertiary) Miocene
(modern life)
-----------------
age of mammals
and birds Paleogene Oligocene Himalayas Form
(included in Eocene Alps form
the Tertiary) Paleocene mammals and birds battle for supremacy on land
65 my -----------------------------
Phanerozoic Cretaceous (Latin for chalk) age of dinosaurs & first angiosperms, Rockies form
(well-displayed Mesozoic Jurassic (Jura Mountains) age of dinosaurs, first birds, Sierra Nevada form
Pangaea breaks up
life) (middle life) Triassic (3 fold division) age of thecodonts, first mammals, dinosaurs,
and pterosaurs , hexacorals evolve
250 my ----------------------------- Greatest Extinction
Permian (Russian Province)
age of reptiles, Urals form, major glaciation
Pennsylvanian (US state)
age of amphibians and reptiles, Appalachians form
Mississippian (US river)
age of amphibians and crinoids
Paleozoic Devonian (Devonshire County)
Pangaea forms age of fish, first gymnosperms and amphibians,
major extinction and glaciation at end of period
(old life) Silurian (Celtic tribe)
abundant spore-bearing land plants, first fish with jaws
Ordovician (Celtic tribe)
abundant stromatoporids and tabulate corals, first spore-bearing land plants
major extinction and glaciation at end of period
Cambrian (Roman for Wales)
age of trilobites and nautiloids, first fish
540 my ------------------------------
Proterozoic (age of protoctists), part of Precambrian, Rhodina forms and breaks up, active plate tectonics, two
major periods of glaciation, multicelled plants and animals evolve
2,500 my --------------------------------------------
Archean (age of bacteria), part of Precambrian, the earliest 600 m.y. lacks a rock record and is sometimes called
the "Hadean Eon." Oceanic crust forms during Archean and then first continental crust froms from oceanic crust.
4,600 my --------------------------------------------
Beginning of the earth, mantle on surface
Rocks are composed of minerals. The bulk chemical composition of a rock is the average composition of the minerals. The most common minerals are the aluminum silicates that make up most of the igneous rocks, e.g., the micas (muscovite and biotite), the feldspars (orthoclase, albite, anorthite), pyroxene, ampohibole, and olivine. Other common minerals that occur in sedimentary rocks are composed of calcium carbonate (calcite and aragonite), iron oxides (hematite and magnetite), calcium sulfates (gypsum and anhydrite) and sodium chloride (halite). Minerals have different properties, i.e., melting temperatures, cleavage, hardness, etc. For this course, learning different minerals is not as important as understanding the different types of igneous, sedimentary, metamorphic, and hydrothermal rocks.
extrusive (has small crystals) - examples, basalt (oceanic crust) & andesite
intrusive (has large crystals) - example, granite (continental crust), gabbro (oceanic crust), & peridotite (mantle)
Igneous rocks form from crystallizing magma and have a crystalline
texture. They are composed of aluminum silicate minerals. These
minerals range in composition from ultramafic to felsic in which iron, calcium,
and magnesium-rich minerals are mafic minerals and silicon, potassium, and
sodium-rich minerals are felsic minerals.
Intrusive rocks
(plutonic) form under the earth's surface and have large crystals due
to slower rate of crystallization. Extrusive rocks (often volcanic) form
on the earth's surface on land or under water and have small
crystals due to rapid crystallization from the faster loss of heat. If
the magma cools very fast, a volcanic glass forms. Intrusive and extrusive
igneous rocks of the same composition (same minerals) have different names. The
chemical composition of the rocks varies from ultramafic to felsic as it changes
from being rich in iron, magnesium, and calcium to being richer in silicon,
potassium, and sodium. The ultramafic intrusive rock is
peridotite; while its extrusive equivalent (komatite) is very
rare. Within the earth's crust are three important estrusive and intrusive rock
pairs: basalt and gabbro forming mafic rocks; andesite
and diorite forming intermediate rocks; and rhyolite and
granite forming felsic rocks. The viscosity of the magma increases
going from mafic to felsic while its temperature of crystallization (rock
melting temperature) decreases. Because more felsic minerals melt at
lower temperatures than mafic minerals, more felsic rocks can be derived from
more mafic rocks by partial melting the more mafic rock and then crystallizing
the magma. mineerals that form in the crystallized rock will cover a
more felsic range (than the parent rock) due to the more felsic composition. The
process can be repeated several times to make an even more felsic rock.
This is how ultramafic rocks making up the mantle peridotites are
partially melted to generate magmas forming basalt and gabbro (oceanic crust) at
diverging boundaries (spreading ridges) and hot spots, and how basalts and
gabbros are partially melted to generate magmas forming andesite, diorite, and
granite, making new continental crust overlying subduction zones.
Oceanic crust forms at diverging ridges and over hot spots from magma
produced by partial melting of ultramafic rock in the mantle and is composed of
gabbro underlying basalt. Continental crust is composed mostly of granite.
Diorites and andesites form at subduction zones by partial melting of oceanic
crust. The earth's mantle is composed of peridotite, an ultramafic intrusive
igneous rocks.
An intrusive rock forming a tabular layer parallel to the
surrounding rock structure is called a sill. If the tabular
layer cuts the structure of the surrounding rock, it is a dike.
A massive intrusion is a pluton. If the pluton has a large surface exposure
(> 100 square kms), it is a batholith. A
stock is a pluton with a smaller surface exposure.
Lava is magma on the earth's surface, and it can be
emitted from a fissure or a vent on a volcano. Flat sheets of basaltic lava can
occur, forming flood basalts which have columnar jointing due
to crystallization on land and pillow structures due to
crystallization under water. Volcanic eruptions of magma more felsic than
basalt, often contain partly consolidated rock called
clastic or detrital - sediment lithified through diagenesis - exp., sandstone
chemical - exp., evaporites, some limestones, coal
Sedimentay rocks form from earth-surface processes and are usually
bedded in layers. Sedimentary rocks are commonly composed of sediment
grains of aluminum silicate minerals formed at higher temperatures and other
minerals formed at lower, earth-surface temperatures, e.g., calcium carbonate
minerals (calcite and aragonite), clays, iron oxides (hematite, limonite
goethite), chert (microcrystalline quartz), evaporites (halite and gypsum), and
recrystallization products such as dolomite (from calcite and aragonite)
anhydrite (from gypsum), and quartz (from chert).
Detrital or
clastic sedimentary rocks form from the weathered fragments (sediment)
of preexisting rocks. The rock names are often based on sediment size:
breccia and conglomerate [gravel size], sandstone [sand size],
siltstones [silt size], and shale [clay size] and shales and siltstones
are often called mudstones. Other rock names are based on composition, e.g.,
limestones are composed of broken fragments of calcium
carbonate fossils and cherts are composed of
microsiliceous (opaline) fossils; however, both limestones and
cherts can be chemical rocks composed of calcium carbonate and opal
precipitates, respectively. Breccias have angular grains while conglomerates
have rounded grains. Note that the weathering process and stream transportation
of sediment tends to round sediment, sort
sediments by size (due to weight), and destroy mafic minerals
relative to felsic minerals. So rounded felsic minerals are common in
sedimentary rocks. Detrital rocks become lithified upon deposition,
burial, and cementation of the grains together.
Chemical sedimentary rocks usually form as precipitates
from water. Evaporites are generally composed of gypsum and
halite and form as precipitates from evaporating sea water.
Limestones are composed of calcium carbonate minerals (calcite
and aragonite) which can be precipitated from sea water or fresh water when the
water degasses carbon dioxide to the atmosphere. Limestones also form as
cemented shell fragments of calcium carbonate minerals and hence can also be
considered a detrital sedimentary rocks. Chert is composed of
microcrystalline quartz and forms from lithication of hydrated silica (opal)
either as a direct precipitate or as cemented opaline shell fragments. Petrified
wood is chert precipitated from groundwater as a replacement for organic tissue.
As with limestone, chert composed of shell fragments can be considered a
detrital sedimentary rock.
Early in the earth's history, banded iron
deposits formed as a direct precipitate of iron minerals, a process that is no
longer possible because the high oxygen content prevents the build-up of iron in
seawater. The calcium carbonate in limestones often recrystallizes through the
addition of magnesium to form calcium magnesium carbonate rocks that are called
dolomite. Organic rocks such as lignite and oil shale are also chemical
sedimentary rocks, formed from the preservation and modification of organic
plant tissue.
Sedimentary rocks typically have structures which
can be used to identify the environment of formation. These include
presence of diagnostic fossils of species living in particular environments,
absence of bedding, and special types of bedding such as crossbedding and graded
bedding, ripple marks that are symmetrical or nonsymmetrical, and the presence
of mudcracks and varves (alternating layers of a thin, dark layer with thick,
light-colored layer. The degree of sorting of the size of grains, the shape of
the grains, the composition of the minerals, and the geometry of the sedimentary
rock deposit can also indicate the environment of formation.
foliated (has parallel crystal growth) - examples: slate, phyllite, schist, gneiss
nonfoliated (has nonparallel crystal growth)- examples: quartzite, marble, hornfels
Metamorphic rocks form by recrystallization of pre-existing rocks as
the result of temperature and pressure. The rocks recrystallize because
they are exposed to a different environment from that in which they formed. The
recrystallization requires a fluid, and components in the fluid or associated
gas phase may change the bulk composiiton of the rock, a process called
metasomatism. A rock that has been heated so that portions have
melted and then recrystallized is a migmatite.
Directed
pressure on the rock will cause a preferential growth of crystals during
recrystallization called foliation, giving the rock a layered
appearance (like a sedimentary rock). Foliation typicaly develops during
regional metamorphism associated with converging plate
boundaries. The maximum temperature of metamorphism is the melting temperature
of the rock. Metamorphism of shale under directed pressure will produce a slate
(perfect foliation layers of microcrystalline micas), which then converts to a
phyllite (not-so-perfect foliation of layers of megacrystalline mica), which
then converts to a schist (less than perfect foliation with non-segregated
megascopic minerals within the layers), which then converts to a gneiss
(segregated layers of minerals of different colors), which then converts to a
migmatite (contains portions of crystalline rock from partial melting and
crystallization of the magma).
Typically, around a cooling pluton the
rocks are baked (contact metamorphism) causing rocks without
foliation, e.g., marbles from limestone, quartzites from quartz sandstones,
skarns from shales and limestones, hornfels from shales or basalts. Cooling
plutions are common in subduction zones and associated with hot spots in
continental areas. Directed pressure metamorphism (with temperature) is also
characteristic of converging plate boundaries such as subduction zones. These
broad areas of convergence produce regional metamorphism, causing foliated
metamorphic rocks to form (slates, phyllites, schists, and gneisses).
For a rock of a particular bulk composition, the minerals that form can
often be correlated with the temperature and pressure of metamorphism. This is
known as the metamorphic grade and is used to tell the
temperature and pressure that the rock was metamorphosed at. The mineral
assemblage of a particular metamorphic grade is called the metamorphic facies.
Other types of metamorphism include burial metamorphism
of sedimentary rocks in which they are lithified and recrystallized as they are
gradually buried. This is a low-grade metamorphism that occurs with deltaic
sediments and they are usually nonfoliated. For example, this produces the
transformation of lignite to coal, of smectites to illite to micas, of opal to
chert to quartz.
Common examples are veins in country rock containing quartz or calcite.
Tectonic settings include the craton which is the stable interior of a continent, e.g., Kansas; active continental margins which include volcanos and forearc and backarc basins associated with a subduction zone, e.g., Cascade region of western Oregon and Washington; passive continental margins which are usually the site of sediment deposition of river deltas on broad continental shelves, e.g., the Louisiana coast; and deep oceanic basins which include the abyssal plains and mid-oceanic ridges of the oceans and features along the ocean margins. Within each tectonic setting are distinctive environments of deposition which produce rocks with diagnostic features that can be used to identify those environments in ancient rocks.
Some of the environments of deposition are discussed below with the characteristics or "sedimentary structures" used to identify them in ancient rocks. These sedimentary structures include fossils, e.g. marine shells of organisms; crossbeds which are nonparallel sedimentary layers deposited from a current of water or from wind; graded bedding which is a decrease in grain size upwards within a layer of sediment deposited from a turbulent mixture of sediment and water as it slows down; salt deposits precipitated from evaporating saline water; varves consisting of alternating thin and wide layers of lake-deposited sediment that are dark colored and light colored, respectively; and symmetrical and asymmetrical ripple marks found in stream deposits, wind deposits, and on beaches; and mud cracks found along the edge of low energy environments, e.g., lagoons.
Most sediments are deposited in shallow marine environments with only a minor amount deposited in the deep sea. Terrestrial environments include stream channels, natural levees of rivers, backswamps, lake deposits, wind-deposited sand dunes in deserts and on barrier islands, wind-deposited dust, and glacier-deposited sediment. On land, river transport of sediments tends to round the sediment and sort sediment by size; whereas, glacier transport does neither. Erosion features can be useful in identifying an environment. Rivers cut a V-shaped valley; whereas, glaciers cut a U-shaped valley and produce hanging valleys where glaciers once intersected. Glaciers also produce striations in the underlying bedrock. Wind transport causes sand grains to be pitted. Wind cannot carry anything heavier than sand and cannot carry sand grains more than a few feet above the ground, limiting erosion to the lower parts of standing rocks and producing arches and other interesting ventifacts. The lack of typical sedimentary structures is also useful, e.g., a lack of bedding, rounding, and sorting is typical of a glacier deposit. The shape of a deposit is a clue to the depositional environment, e.g., a beach deposit is wide and thin; whereas, an alluvial fan deposit is cone shaped at the mouth of a canyon.
River deltas composed of deposits from the distributaries with shoe-string
shaped sand and silt channel deposits having small crossbeds
and asymmetrical ripple marks; the channel deposits are bounded by
natural levee silt deposits; which are in turn bounded by
backswamp deposits of clay with organic debris (forming peat
and coal) from the continents;
Barrier islands with a
linear geometry with marine shells and with sand dunes making sandstones that
have large crossbeds;
Beach deposits with a sheet
geometry with symmetrical ripple marks and marine shells;
Lagoon
deposits (separating barrier islands and the shoreline) with clay
deposits and mud cracks;
Marshes and swamps deposits
with clay deposits and associated plant fossils;
Coral reefs
deposits with coral fossils of calcium carbonate;
Calcium carbonate sediment on carbonate platforms from
shells and fecal pellets of marine organisms and from inorganic precipitation
from sea water of oolites (spherical layered particles.
Note that the shape of a river delta along a coast can be a birdfoot (similar to the Mississippi River) if the wave action is weak, allowing the distributaries from the main channel to extend out into the oceans, or it can be shaped like a upside down V due to strong waves smearing the sediment along the coast as it is deposited. The latter is the normal shape of a delta. The longshore current in the Gulf of Mexico goes from east to west, carrying sediment westward from the Mississippi River delta; however, the sediment doesn't reach shore to rebuild coastal Louisiana because the river has been forced by man-made levees to build the delta far from the coast in deep water. Without these levees, the river would have switched its course (down the Atchafalaya) and built a new delta in shallow water near Morgan City.
Sediment moves downslope in turbidiy currents (high density underwater debris flows) carving out submarine canyons with some turbidity current deposits.
This is a wedge of sediment at the bottom of the continental rise containing turbidity current deposits with their characteristic graded bedding. A group of turbidity current deposits form an abyssal fan at the mouth of a submarine canyon. Lithified turbidity current deposits are called turbidites.
Slow forming deposits form on the deep sea floor: deposits of calcium carbonate plankton shells (carbonate oozes on less deep portions which eventually become limestones; siliceous plankton shells (siliceous oozes which eventually become cherts); pelagic clays (from wind-blown clays from the continents) which become pelagic shales, and manganese nodules which are concretions (resembling cannon balls) forming at the seawater-sediment interface. The calcareous oozees do not form at the deepest depths because deep seawater is undersaturated with respect to calcium carbonate, causing the dissolution of these plankton shells as they descend.
Fluvial (river) Floodplain has the same features as the delta, i.e., rounded coarse-grained channel deposits of sand and silt (with a shoe-string geometry) with small crossbeds and asymmetrical ripple marks; associated width less-coarse grain natural levee deposits; and grading dark-colored fine-grained backswamp deposits containing coal and peat.
Lacustrine (lakes) deposits typically have varved deposits in which the sediment deposited in the summer forms a thick lighter-colored layer and the sediment deposited in the winter forms a thin dark-colored layer. These varves are best developed in areas where the lake freezes over in winter, e.g., near a glacier. Lakes also have beach deposits (symmetrical ripple marks) similar to marine deposits but with fewer fossils of organisms.
Glaciers deposits typically have moraine deposits (called till) that are unsorted, unlayered, and unrounded. Lithified till is called tillite. Glaciers carve striations in rock that they move across. Glaciers carry and deposit rocks (exotics) that are different from the native rocks in an area, and melting icebergs drop large boulders (drop stones) on the deep sea floor that are unrelated to the normal fine-grained deep sea deposits.
Desert deposits contain several different depositional environments. Playa deposits are dried-up lake beds with mud cracks and salt deposits in the center of valleys lacking external drainage, i.e., through-flowing rivers. Eolian sand dunes deposits become sandstones with large crossbeds, and ventifacts are faceted boulders, carved by sandstorms. Loess deposits are wind-blown dust deposits that are characterized by straight vertical faces in roadcuts. Alluvial fan deposits are flash flood deposits at the mouth of canyons that have a typical cone-shape geometry. Desert soils have a layer of caliche (calcite) near the surface from evaporating groundwater. Desert varnish is a dark layer of manganese oxide forming on the surface of desert boulders. Note that loess deposits are formed in non-desert areas in times of drought, and that eolian sand dunes also form on beaches near the coast.
soil zones are usually identified from their red color from oxidation of iron and the presence of preserved root structures and hard pan layers which could be a caliche (calcium carbonate) layer in deserts and a clay pan or iron pan layer in temperate soils such as in Louisiana.
Rock units are named on the basis of distinguishable physical differences and
the geographic location of the best outcrop exposure. A sedimentary rock unit
can be delineated by lithology, producing a lithostratigraphic
unit. A lithostratigraphic unit is not usually age constant, i.e., the
age varies laterally. The lithostratigraphic units can be
formations and their subdivisions are called
members. The first word of the name is the "type" geographic
location (where the best exposure is) and the second word is the lithology,
e.g., sandstone, if the rock unit is composed of only one type of lithology.
Otherwise, the second word will be formation or member. Formations are combined
into a larger unit called a group, and groups can be combined
into a system. A system is usually defined to correspond to the
units deposited or otherwise forming in a geologic "Period." For example rocks
in the Permian System formed during the Permian Period. This may appear to be a
contradiction in the definition of a lithostratigraphic unit not representing
constant time. However, Geologic Periods were defined based on worldwide
unconformites at their boundaries so a lithostratigraphic unit would not be
expected to cross the Period boundary.
A sedimentary rock unit can also be delineated by fossil assemblage,
producing a biostratigraphic unit. A biostratigraphic unit
corresponds to a time interval of the existence of the organisms represented by
the fossil assemblage. The usual biostratigraphic unit is called a
zone. The fossil assemblage consists of index
fossils which are fossils of species that were widely dispersed,
existed for a short time period before becoming extinct, and are easily
recognized.
If a lithostratigraphic unit grades laterally into a
different unit, e.g., a sandstone into a siltstone, this is called a
facies change. This is a normal occurrence in the shallow
marine environment in which coarse-grained sediments are deposited close to
shore and fine-grained deposits are deposited in deeper water with less wave
action. Facies changes move laterally as sea level rises and falls because the
position of the shore moves landward as sea level rises (transgressive sea) and
seaward as sea level falls (regressive sea).
As sediment is deposited,
it compacts and the crust sinks so that the water depth decreases at a slower
rate than expected. Eventually, sea level changes will change sediment
deposition at any location in shallow marine water because it causes a shift in
the position of the facies change. Of course other factors can cause a shift in
the facies position, e.g., decreases in sediment delivered to the sea will shift
the facies change landward and vice versa. If sea level is rising, we
would normally expect the facies change to shift landward and coarse-grained
sediments to be overlain by fine-grained sediments and the opposite to occur if
sea level falls. If sea level falls to the point that the surface is exposed, no
sediment deposition would probably occur and an erosion surface (disconformity)
and/or soil surface would develop. Geologist use the vertical change in
sedimentation and the location of erosion surfaces within vertical cores to
delineate world-wide (eustatic) changes in sea level in the
geologic past. The causes of sea level rise can be due to melting of the ice
caps on Greenland and on Antarctica (which is happening today due to global
warming) or it can be due to an increase in spreading at the mid-oceanic ridges.
The latter is due to a decrease in volume in the ocean basins due to the
seafloor rising and expanding as hot basaltic magma fills the oceanic ridge
system. The normal changes in sea level are about 35 ft/million years; whereas,
changes in sea level due to ice ages are about 330 ft/ several thousand
years.
Geologic maps are used in this course to help understand an area's geology. The most common map shows the location of surface outcrops of formations together with information on the dip of the formations below the surface and the location of fault planes where they intersect the surface. Typically, such a map will also have a stratigraphic section for the area showing a vertical sequence of described units from top to bottom with information on age, depth, thickness, lithology, fossil content, composition, etc. Geologic maps of a single formation are commonly used to contour the depth below the surface to its upper or lower boundary or to contour its thickness. Geologic maps can be used to show the location of facies changes between formations that are laterally continuous across the area.
How would you recognize the following erosional or depositional environments in ancient rocks? This could be a sedimentary structure such as large or small crossbeds, a lack of bedding, presence of coal beds, the geometry of the deposit, type of fossil present, etc.
Know the answers to the questions below.
Fossils are preserved remains or tangible signs of ancient life which can be found in sedimentary rocks, most commonly from marine environments, e.g., coral reefs and shales, but also from some terrestrrial environments, e.g., swamps. Hard parts such as bone, teeth, and shells are frequently preserved, and evidence of hard parts also includes permineralization (e.g., petrified wood), molds and casts. Soft parts can be preserved in amber and in anoxic (without oxygen) environmenst as petroleum, coal, and peat. Evidence of soft parts can also form from impressions and carbonization (e.g., outlines of fish in shales). Trace fossils represent indirect evidence of organisms such as tracks, trails, burrows. The index fossils used by paleontologists to define biostratographic units (zones) and for general correlation of widely separated rocks, and for dating rocks are the fossils of organisms that were widespread (generally marine), existed as a species only a short time, and are easily identified in rocks.
Life has the capacity for self-replication (reproduction) and self-regulation (use raw materials to sustain chemical reactions). A virus does not fulfil this definition because it cannot self-replicate without entering a cell or use raw materials to sustain chemical reactions.
Taxonomy is the classification of living organisms. Organisms are divided into kingdoms. Bacteria are single-celled organisms lacking a nucleus or chromosomes and other internal cell structures such as mitochrondia (for respiration) and chloroplasts (for photosynthesis). Their cells are called prokaryote cells and they are often lumped into one kingdom Monera or split into Archaeobacteria which include primitive bacteria and Eubacteria which include cyanobacteria (blue green algae). Nonbacteria cells are called eukaryotes and contain internal cell structures. Protoctista (also called Protista) contains algae which are simple single and multi-celled plant-like organisms, e.g., seaweed, and simple single-celled animal-like organisms called protozoans, e.g., amoebas. Phytoplankton (diatoms and nannobacteria) and zooplankton (foraminifera and radiolarian) are in this kingdom. The more complex kingdoms include the animal-like Fungi, the animals in Animalia, and the plants in Plantae.
Phylogeny is the evolutionary history of a species. Species in Fungi, Animalia, and Plantae represent an evolutionary process that begin with Monera, passed through Protista before evolving into more complex species.
The kingdoms are subdivided into phylum, class, order, family, genus, species
An example of taxonomy is the classification of humans: kingdom, Animalia; phylum, Chordata; class, Mammalia; order, Primates; family, Hominidae; genus, Homo; species, Homo sapiens
A cluster of species that share an evolutionary ancestry is called a CLADE. Within a group of related species, some traits are primitive and some are derived and hence are not shared by all species. The common traits imply a common evolutionary origin. A cladogram shows the progression of clades sharing fewer common traits as evolution proceeds.
The groups are listed below and you should learn how they are related by evolution.
Bacteria cell fossils first appear in rocks of Archean Eon age.
Archaeobacteria - includes anaerobic bacteria which formerly lived on the earth's surface when there was little or no oxygen in the atmosphere.
Eubacteria - plant-like cyanobacteria (blue green algae) and animal-like bacteria responsible for decay of dead organisms. Cyanobacteria are photosynthetic and began the buildup of the earth's oxygen content in the atmosphere. They produce stromatolites which are boulder-shaped rocks composed of layers of algae mats covered with sediment.
Protists first evolved in the Proterozoic Eon. The earliest non-bacterial fossils are of acritarchs, thought to be single-celled phytoplankton, possibly a type of dinoflagellate. However, paleontologists think that plant-like eukaryotes (such as phytoplankton) evolved from animal-like eukaryotes because animal-like eukaryotes only have mitochondria and plant-like eukaryotes have both mitochondria and chloroplasts.
Protozoans - single-celled animal-like. Protozoans are thought to have evolved from an animal-like bacteria engulfing another (smaller) animal-like bacteria which survived to become a mitochondria in the cell. This explaijns the presence of DNA in mitochondria that is unrelated to the DNA in the chromosomes of a cell. Need to know important zooplankton: radiolaria ( have silicious (opaline) shells which produce siliceous oozes) and foraminifera (have calcium carbonate shells which produce calcareous oozes on the seafloor).
Algae - Thought to have evolved from a protozoan engulfing a cyanoacteria which survived to become a chloroplast within the cell. They are single-celled and simple multi-cellular plant-like organisms that have fertilization external to the plants in water. Fertilization in higher plants occurs within the parent plant. Learn important phytoplankton: diatoms (have silica shells) and nannoplankton (have calcium carbonate shells) and dinoflagellates (have chitin shells).
Multicelled plants containing organs of tissue. They evolved from algae in the Late Proterozoic and are listed below in order of evolution:
Spore-bearing plants> require moisture to reproduce. These include moses (primitive nonvascular plants) and spore-bearing ferns (vascular plants). They evolved in the Ordovician Period and were the first land plants, living in moist environments.
Vascular plants evolved in the Devonian Period. Vascular means that fluids can move up and down the plant stem.
Gymnosperms - naked seed-bearing plants, e.g., conifers such as pines and cycads, which evolved from spore-bearing plants in the Late Devonian Period and colonized dry land.
Angiosperms - flowering plants, e.g., oaks, which evolved from gymnosperms in the Cretaceous Period. These are the dominant plants today with a shorter reproductive cycle than gymnosperms.
Multicelled animals are composed of organs of tissue and evolved from
Protozoans in the Late Proterozoic. Skeletons first appeared at the
beginning of the Cambrian Period (beginning of the Phanaerozoic Eon and
Paleozoic Era which accounts for the abundant fossil record since this
time.
Unless otherwise noted, the major invertebrate groups first appeared in primitive forms in the Cambrian Period.
Phylum Coelenterata (Cnidarians)
They are suspension feeders with radial symmetry. Examples include floating jellyfish and sedentary corals (tabulates, horn corals, hexacorals) which form calcium carbonate reefs in warm, shallow waters.
Phylum Porifera
Sponges are suspension feeders which often contribute calcium carbonate skeletons to coral reefs, e.g., stromatoporoids.
Phylum Annelida (segmented worms)
These evolved in the Proterozoic Eon and are burrowing animals (worms) with a well-developed digestive tract
Phylum Arthropoda
Arthropods evolved in Proterozoic Eon and are segmented, joint-legged animals with a hard exoskelton. (Although the exoskelton was lacking in the Proterozoic Eon.) Arthropods include the trilobites, insects, crustaceans, spiders and scorpions.
Phylum Brachiopoda
These are bivalves that lack mirror symmetry and include articulate and inarticulate brachiopods.
Phylum Bryozoans (moss animals)
Evolved in the Ordovician Period: moss-looking corals which form colonies which help form reefs of calcium carbonate, e.g.,lacy bryozoan.
Phylum Mollusca
This group includes bivalve molluscs (clams), gastropods (snails), cephalopods (nautiloids, ammonoids, belemnoids, squids, octupus)
Phylum Echinodermata
They have radial symmetry. Examples include starfish, crinoids, (sea lilies), sand dollars, and sea urchins.
Phylum Chordata
Have a vertebrate column with a spinal cord. Cambrian conodonts were the earliest vertebrates. The following classes appeared in order of evolution - jawless fish, jawed fish, sharks and bony fish, amphibians (from lobe-finned fish), reptiles (including therapsids and thecodonts), dinosaurs (from thecodonts), pterosaurs (from thecodonts), mammals (from therapsids) and birds (from lizard-hipped dinosaurs). Jawless fish evolved in the Cambrian Period, and sharks and bony fish evolved from jawed fish in the Devonian Period. Amphibians evolved from bony fish in the late Devonian Period, and reptiles evolved from amphibians in the Pennsylvanian Period. Therapsids and thecodonts were reptile groups that evolved in the Permian and Triassic Periods, respectively. Dinosaurs and pterosaurs evolved from thecodonts in the Triassic Period, and birds evolved from dinosaurs in the Jurassic Period. Mammals evolved from the therapsids in the Triassic Period.
Evolution means to change or "Descent with Modification" for a species and was first proposed by Charles Darwin
Organic evolution refers to changes in populations of the same species. A basic restraint on evolution is the body structure of what is already present in a population, i.e., because evolution remodels rather than starts with a new design.
Natural Selection and Speciation
During evolution, a species as a whole can evolve in the course of time or can rapidly give rise to additional species. Natural selection is used to describe the gradual transformation of a species through transferral of selected traits through breeding from survival of the fittest. Speciation is used to describe the rapid development of new species from another species. Often, the new species is geographically isolated from the remainder of the parent species, thereby lacking the full genetic variability in the general population. Also, often no competitors exist in the environment.
Adaptations refer to specialized features of plants and animals that allow them to perform functions, e.g., the development of seeds in gymnosperms and the development of eggs in reptiles enabled both groups to colonize the dry interiors of continents.
The process of evolution involves changes in the gene pairs which are responsible for hereditary factors. These changes can involve genetic mutations of sex cells when dividing or or exposed to chemicals and/or radiation which are subsequently preserved through breeding or by new gene combinations through breeding. Each gene is a chemical segment in a DNA (deoxyribonucleic acid) strand within a chromosome in the nucleus of a cell. The chromosomes exist in pairs, half supplied by each parent during breeding.
DNA has the structure of a twisted rope ladder (double helix) in which the sides of the ladder are composed of alternating sugars (S) and phosphates (P). The steps are composed of pairs of nucleotide bases: either adenine (A) and thymine (T) or guanine (G) and cytosine (C). Genetic point mutations of DNA can occur during cell division or by an external source such as radiation or chemical exposure. It these mutations occur in sex cells thay can be passed on to offspring through breeding in which each individual of a pair contributes half the chromosomes by way of a gamete (e.g., egg and sperm). If the mutation is favorable, the individual is more likely to survive and continue to pass on his or her genes. In this way species evolve or change with time. Genetic point mutations probably result in speciation, a quicker process than natural selection.
Species may also evolve, in the absence of genetic mutation, through a new gene combination in breeding. This is a long-term process which is called natural selection. If the combination is favorable, the individual is more likely to survive and continue to pass on his or her genes, making it more likely for the gene combination to reoccur in related offspring. However, to produce a completely new species, genetic mutation may be required.
homology - The development of different functions for organs in different species which shared this organ in a common ancestor, e.g., bat wings, human hands, dog paws, and whale flippers all have the same basic five-finger structure. This was part of the basic evidence for evolution cited by Darwin.
vestigial organs - organs developed in ancestral species and retained through evolution but not serving a purpose, e.g., our ear muscles, the pelvic bones of whales and some snakes. Again, this is part of Darwin's evidence for evolution.
natural selection - survival of the fittest.
artificial selection - domestic breeding
mass (rapid) extinction - large numbers of species become extinct every 26 m.y. to 30 m.y. and this appeasr to be associated with asteroid impacts on the earth. The asteroid events deposits trace metals (e.g., iridium) in sediments which have been identified in the geologic record. These trace metals were within the asteroid which was vaporized upon impact. The vaporization process ejected trace metals into the atmosphere to spread arourd the earth and then slowly settle out onto the earth's surface. An explanation for the cycle is that the sun has a companion star, a red dwarf, not yet identified which is circling the sum every 26 to 30 m.y. When this "death" star or "nemisis" star passes near the region where comets are located that pass through our solar system, the comet orbits are disrupted, increasing the probability of collision between the Earth and a bolide or large meteorite associated with the comets. The variation from 26 m.y. to 30 m.y. could be due to the passage of other stars that disrupts the orbit of the companion star.
pseudoextinction - evolution into a new species, suggesting that the ancestors have become extinct.
adaptive radiation - rapid expansion of a group into many new species as the result of a friendly environment, e.g., other competing groups have become extinct or introduction into a new environment lacking competitors, e.g., the adaptive radiation of mammals in the Cenozoic following the extinction of the dinosaurs at the end of the Mesozoic. This is part of speciation.
adaptive breakthroughs - development of key features that along with ecologic opportunities allow adaptive radiation to take place. In corals, the development of a porous skeleton allowed more rapid growth and the development of the symbiotic relationship with dinoflagellates allowed for removal of their produced carbon dioxide as well as a convenient food supply.
extinction rates vary between different groups. Mammals have a survival rate of 1 to 2 m.y. The average extinction rate for all groups is about 3 m.y. Human species have been here for at least 200,000 years.
evolutionary convergence - adaptive radiation evolution of two different taxonomic groups, e.g., marsupials and placentals to produce similar-looking species to occupy similar environments, e.g., Tasmanian wolf and the wolf.
iterative convergence - evolution of a taxonomic group producing similar-looking species at different time periods; however, they are not identical because evolution does not repeat itself.
The history of evolution of species is called phylogeny. The sequence of development of an individual from its origin by fertilization of an egg to its death is called ontogeny. Frequently, phylogeny is preserved in ontogeny in the different embryonic stages, leading to the saying "Ontogeny recapitulates phylogeny." The preservation shows up in common embryonic stages between species having a common ancestor group. This evidence for evolution was cited by Darwin.
Cope's rule - Organisms increase in size through evolution (now somewhat discredited)
Organisms become more complex.
One common evolutionary trend is to eliminate the adult form or the latest stages of development or by retaining some of the juvenile or embryonic features. This involves the transfer of sexual maturity to an earlier juvenile stage and the arrestment of development at that stage. Amphibians that have an aquatic juvenile stage may remain in that stage, never progressing to four-legged terrestrial adults. The axolotl evolved from the juvenile stage of a salamander, through the loss of thyroxine from the thyroid gland, which is needed for transformation to an adult salamander. The domestic dog has lost the adult form found in wolves. Dogs like wolf pups bark, but adult wolves do not. Humans have lost the adult ape form through evolution. We lack the body hair and pointed faces of adult apes, retaining the juvenile, lesser hair, and flat face of young apes.
Most evolution probably involves speciation, evolving rapidly from existing species, rather than by natural selection, the slow transformation of an existing species through breeding.
The evolutionary trend may be controlled by species selection in which the significance of a species in evolution is related to its length of duration and its rate of producing descendant species. If a species has a long length of duration and produces a lot of descendant species than more of its traits are likely to be passed on.
Dollo's Law - Evolution doesn't reverse itself to perfectly duplicate an earlier extinct species.
Mass extinctions of species have occurred throughout the earth's history. these extinctions have been postulated as due to temperature changes on the earth's surface, possibly resulting from meteorite hits that have periodically occurred. Mass extinctions have occurred throughout the earth's history but there does appear to be a 26 million year cycle that is marked by mass extinctions. This has given rise to the Death Star hypothesis, a dark sun rotating our sun every 26 m.y. and bringing comets and meteorites with it to bombard the solar system.
Plate tectonics is a theory that is consistent with the observations about the interior of the earth and its crust. Seismic wave modelling indicates the earth has a molten outer core, so heat is flowing to the surface. Seismic waves also indicate the asthenosphere, underlying the uppermost mantle is plastic and hence can move in convection cells set up by this flow of heat towards the surface. The rigid plates of crust and uppermost mantle in plate tectonic theory move on top of these convection cells in which the upward arms occur at diverging boundaries and the downward arms occur at subduction zones. The processes at the diverging boundaries and hot spots account for the formation of oceanic crust with shield volcanoes and the processes at the subduction zones in converging boundaries account for the destruction of oceanic crust and the formation of continental crust with composite volcanoes and the inclusion of remnants of oceanic crust within continental crust. The complex folding of layered sedimentary rock into mountains occurs at converging boundaries where continental crust is sutured to continental crust following the destruction of intervening oceanic crust on the leading edge of plates in subduction zones. This process has produced the super continents such as Pangaea and the highest mountain ranges on the earth's surface. Breakup of the continents by rifting explains the formation of oceans and the occurrence of salt deposits and red beds along passive continental margins. The young age of the ocean basins together with the trend of increasing age from the diverging boundary at the center of an ocean to subduction zones at their margins and the old to young trend from the center of the continents to their margins are consistent with the processes associated at diverging and converging boundaries, respectively. The large faults that occur between plates and along fracture offsets of spreading ridges with strike-slip motion are consistent with the horizontal movement as one plate slides past another. The theory has been able to link all aspects ot the structure and composition of the interior and surface of the earth.
The Frenchman Antonio-Snider-Pellegrini published a map in 1858 suggesting that a supercontinent had broken apart to form North America, South America, Europe, and Africa, using the puzzle-like fit of the continents. Even earlier, the Englishmen Bacon had proposed this possibility.
Fossil evidence pointed for a connection between the continents. The name Gondwanaland, proposed by the Austrian geologist Edward Suess in the late 19th century, originally referred to a hypothetical southern hemisphere super continent containing land bridges connecting India, Antarctica, South Africa, South America, and Australia. All of these areas contained flora of fossil plants called the Glossopteris flora which are best exposed in Gondwana, India. The most conspicuous Glossopteris genus is the Glossopteriss seed fern. Early geologists speculated that land bridges of felsic rock had connected the continents and then subsequently sank. However, lower-density felsic rock cannot sink in the higher-density mafic rock.
Alfred Wegener, a German meteorologist, published "On the Origin of Continents and Oceans" in 1915, a book proposing continental drift in which a supercontinent called Pangea split apart in the late Mesozoic. Wegener used different lines of evidence besides the puzzle-like fit of the continents. He pointed out the African rift valleys were early stages of a continental rifting process. He used geologic structures and fossil evidence to support the occurrence of Pangea as well as paleoclimate data which different from present-day climates.
The South African geologist Alexander du Toit became the main supporter of Wegener and showed that striations from Permian glaciers indicated the impossible movement from oceans onto present landmasses. Du Toit supported continental drift with the present-day common occurrences of earthworm genera, the common presence of the Early Permian fresh-water fossils of Mesosaurus genera, and the Early Triassic mammal-like herbivore reptiles Lystrosaurus genera, and the common Gondwana sequence which includes Permian tillites, Triassic dune deposits, and Jurassic lava flows. Du Toit also pointed out that a reconstruction of Gondwanaland allowed the mountain ranges along their margins to form a continuous chain between the continents. Du Toit proposed that Pangea did not form until the late Paleozoic. The northern supercontinent Laurasia and the souther supercontinent Gondwanaland existed prior to the formation of Pangea. Unfortunately du Toit and Wegener could not come up with an adequate force to allow the continents to push through the oceanic crust.
Wegener confused the issue by pushing for an early Cenozoic split of Pangea, rather than the early Mesozoic split. This dating error confused palaeontologists who realized that not all of the evolution occurring on separate continents could have occurred within the Cenozoic time frame
In the 1950s', apparent magnetic polar wandering was used to support the movement of plates. Remnant magnetism can give the apparent location of the magnetic poles at the time the rock formed. The remnant magnetism from dated (by radioactive means) igneous rocks of the same geologic age on different continents usually points to different locations of the poles as a function of geologic time, indicating separate movements of the plates rather than true polar wandering.
In 1962, the American Harry Hess published the "History of Oceanic Basins," proposing the creation of sea floor at the midoceanic ridges and its destruction at trenches or subduction zones. He was able to explain the young age of the sea floor. He also could explain the movement of the continents as being carried along with the sea floor, rather than being pushed through the oceanic crust. He supported his evidence with the small numbers of volcanic seamounts, the presence of guyots (submarine volcanoes with wave eroded flat tops that occurred as the plate carrying the inactive volcano moved it below sea level. He proposed mantle convection as the force moving the sea floor and continents. In 1963, Vine and Matthew explained the magnetic sea-floor anomalies symmetrical to the midoceanic ridges using Hess's hypothesis.
The earth's magnetic field shows two changes with geologic time. First, the declination (difference between the magnetic poles and the geographic poles) shifts slowly (on the order of a few thousand years to make a circuit) as the magnetic poles rotate around the geographic poles. Second, on the order of every half million years, the polarity of the magnetic poles appears to rapidly switch so that the north and south magnetic poles reverse. The reason for this switch is the subject of debate.
The polarity reversal of the magnetic poles can be determined from studying the residual magnetism of rocks, primarily igneous rocks containing iron. The igneous rocks have the earth's magnetic field orientation frozen in as residual magnetism when they cool below about 500oC (their Curie points). This residual magnetism can be determine from an igneous rock and the rock's age can be determined by radioactive (radiometric) dating. By doing this for rocks from stacked lava flows, the polarity of the earth's magnetic field as a function of geologic time has been determined.
The polarity of the residual magnetism is said to be "normal" if the orientation is in the same general direction as today earth's magnetic field and "reversed" if the orientation is in the opposite direction. Note that the movements of plates (through plate tectonics) around the earth's surface may have changed the apparent orientation of the earth's magnetic field from the time the rock was formed. Plate tectonics makes determining the polarity more difficult in rocks older than Mesozoic. We generally know the movement of the continents in the Mesozoic and Cenozoic, so the change in orientation from plate movement can be subtracted out.
The magnetic reversals are used to date the sea floor. The oldest sea floor is Mesozoic in age. If an area of the sea floor has a residual magnetism parallel to the present earth's magnetic field, a positive (stronger) magnetic field is measured for the earth directly over that sea floor and vice versa. A stronger magnetic field is called a positive anomaly; whereas, a weaker magnetic field is called a negative anomaly.
The symmetrical magnetic anomaly pattern of positive and negative anomalies, relative to the mid-oceanic ridges, can be related to the orientation of the earth's magnetic field at the time the sea floor was created. The basaltic sea floor originally formed at the ridge, picking up a residual magnetism corresponding to the orientation of the earth's magnetic field. The sea floor then moved away from the ridge as new sea floor was created. This movement away from the ridge has created the present symmetrical pattern of magnetic anomalies. Since the ages of changes in magnetic polarity are known from radiometric dating on land, the magnetic anomaly pattern can be used to date the sea floor.
We can determine, from the residual magnetism of igneous rocks, the location of the two poles of the earth's magnetic field at the time of formation of the rock. The movements of the plates have produced an apparent movement in the positions of the magnetic poles. We know the poles haven't actually moved because the apparent positions of the magnetic poles are different for rocks of the same age that are located on different plates. The only explanation is that the plates are moving independent of each other.
Sea floor spreading explains the rift valleys (grabens and normal faulting) associated with the midoceanic ridges, the presence of strike-slip faults (transform faults connecting other plate boundaries), and the occurrence of pillow basalts in oceanic regions. The spreading is of the order of 5 cm/year for the Pacific sea floor and less for the Atlantic sea floor. Presently there are 8 large plates and several smaller plates.
The occurrence of shallow and deep earthquakes in the Benioff zones was explained by subduction in plate tectonics. The associated andesite volcanism in island arcs and on continental margins was explained as due to partial melting of the sea floor in the subduction zones. The arcuate shape of the volcanos is related to the arcuate shape of the trenches which is due to the spherical shape of the earth, e.g., try pressing down on a ping pong ball and see the arcuate outline of the depression). The great age of the continents and the general age trend to increasing age in the center of the continents was due to the inability to subduct the lighter continental crust and the gradual accretion of additional material to the continents along their margins. Hot spots or mantle plumes or thermal plumes are used to explain nearly immobile points in the mantle which serve as the source of magma for volcanoes erupting on the surface of the earth and then carried away from the magma source These hot spots create aseismic ridges or chains of volcanoes in which the only active volcano is over the hot spot, e.g., the big island of Hawaii in the Hawaiian Island chain.
Rifting appears to begin with a mantle plume, producing a three armed rifting center, a triple junction. The three arms may consist of a spreading zone, a subduction zone or a transform fault. In general, one of the arms become inactive and the other two arms connect up with active arms from other mantle plumes. The result is a linear zone of rifting which can begin under a continent or under an ocean. The failed rift arms are frequently the sites for river valleys, e.g., the Mississippi River and the Amazon River.
The rifting of a continent probably involves a nearly stationary continent, rather than an actively moving one over the asthenosphere. Doming occurs along with rifting as a continent splits. East Africa is nearly stationary and appears to be about to begin splitting along the East African Rift connecting to the Red Sea rift and the Gulf of Aden rift (triple junction). In this case all three arms of the triple junction are remaining active.
Continental rifting in the early stages results in basaltic magma with normal faulting (plateau or flood basalts and shield volcanoes and a rift valley). The early seaway is shallow, producing nonmarine clastics (red beds), followed by evaporites as the seaway enters the rifting region, followed by the development of passive continental margins as the continent separates. The passive continental margins accumulate large deposits of marine sediments.
Evidence of ancient subduction zones occurs in the formation of melanges in the accretionary wedge which separates the trench from the forearc basin lying in front of the active volcanoes. On continents, behind the zone of volcanoes, deep-water marine flysch is deposited in the foredeep basin, followed by shallow, non-marine mollase deposits. Ophiolites, representing ancient sea floor, are frequently exposed in regions where two continents have sutured together following the destruction of an intervening ancient ocean (in a subduction zone). In the process of suturing, the crust is thickened by thrusting and uplifted.
The coexistence of adjacnet terranes, formed in different regions by different processes, can be due to movement into present position along transform faults, e.g., Alaska and British Columbia. Terraines may also coexist due to suturing of island arcs to continents at converging boundaries, e.g., Sonoma County in California.
The process of mountain building is called orogenesis and a particular episode of mountain building is called orogeny. Mountain building occurs near subduction zones and where two continents are sutured together, following the destruction of an intervening ocean by subduction. The suturing is the result of the continental crust being too light to be subducted when continental crust converges against continental crust. Mountain building results in a thickening of the continental crust. Because of crustal underthrusting, the crust is doubled in thickness when two continents are sutured together. Pieces of ancient oceanic crust are often preserved as ophiolites, emplaced in the acretionary wedge when the subduction zone was active. Mountains associated with suturing will consist of folded sediments that have been pushed upward along the suture line and have then slid by gravity away from the suture line, e.g., the Alps and Himalayas. These sheets of folded sediment are called nappes. Erosion of these mountains leads to block faulting (isostatic adjustment) as the crust rises due to the loss of overlying weight, exposing granite and metamorphic rock which is eroded into mountains, e.g., the Appalachians.
Along a continental or island arc margin behind a subduction zone, there will be an igneous arc of composite volcanoes associated with partial melting of the oceanic crust, e.g., the Andes. The crust is thickened as the result of emplacement of plutons in the igneous arc and the extrusion of volcanics. On either side of the igneous arc will be a zone of metamorphism due to heat and compression. The mountain building can be due to compression from the subducting plate combined with gravity spreading once the crust has been thickened to the point that it is unstable vertically. Eventual erosion of the volcanoes will lead to block faulting (isostatic adjustment) as the crust rises due to the loss of overlying weight, exposing granite and metamorphic rock which is then eroded into mountains, e.g., the Sierra Nevada and the Rockies.
Towards the continental interior will be a fold and thrust belt bordering the foredeep or foreland or backarc basin. The foredeep or foreland basin will first fill with deep-water marine flysch (black shale and turbidite) deposits and then with fresh-water (alluvial fans, flood plains, etc,) molasse deposits. Thick molasse deposits are sometimes referred to as a clastic wedge. The thrusting in the fold and thrust belt is along an inclined basal thrust surface called a decollement. The decollement slopes upward towards the interior of the continent. Much of the movement along the decollement may be due to gravity spreading in the igneous arc and metamorphic zone.
Towards the subduction zone, is the forearc basin, filled with debris from erosion of the igneous arc and metamorphic zone. The accretionary wedge separates the forearc basin from the subduction zone. The accretionary wedge contains a melange of material derived from the subduction zone. Ophiolites are preserved in the accretionary wedge.
Archean Eon - from the beginning of the earth, about 4.6 b.y. ago to 2.5 b.y. ago. The end of the Archean is thought to represent the time of the beginning of modern plate tectonics on the earth's surface.
Archean time is part of the Precambrian time. Precambrian time refers to everthing before the Cambrian Period which started about 540 m.y. ago. No rocks are present in the earliest age of the earth - about 4.6 to 4 b.y. ago and sometimes this is called tha Hadean Eon - however, in this course the Hadean is included in the Archean Eon (4.6 b.y. to 2.5 b.y.). The Archean rocks are exposed in parts of the Precambrian shield areas in the centers of each continent. The Precambrian shield consists of exposed Precambrian rocks within the craton of each continent. The craton includes both the Precambrian shield and those areas covering the Precambrian rocks that have been undeformed since the beginning of the Paleozoic era.
Most of the Archean rocks are crystalline rocks, either igneous or
metamorphic. They are generally divided into two groups:
even-grained,
high-grade metamorphic rocks known as granulites (consisting of
the metamorphic rock granulite and the igneous rock granite) and thought to be
the core of volcanic rocks and
volcanic and sedimentary rock (derived
from forearc and backarc basins associated with subduction zones) known as
greenstone belts.
Because of their great age, most
sedimentary Archean rocks will have been metamorphosed or otherwise
destroyed.
Fossils are generally limited to stromatolites, made by cyanobacteria, also called blue green algae. These photosynthesis bacteria began producing the oxygen for the atmosphere.
Our solar system is about 4.6 b.y. old as shown by the ages of most meteorites which date back to that time.
stony or chrondite (rocky) equivalent to the earth's mantle.
iron (metallic) equivalent to the earth's core.
stony-iron (mixture) equivalent to a mixture of the earth's mantle and core.
carbonaceous chondrites (carbon-rich stony meteorites in which the carbon molecules contain amino acids that are different isomers (different structure) from similar amino acids produced by Earth's organisms. They also have different 13C/12C and 18O/16O ratios.
Another indication of the age of the solar system is that lunar rocks have been dated as old as 4.6 b.y. Ages of lunar rocks indicate heacy meteorite bombardment in the solar system occurred during the first 600 m.y., i.e., 4.6 to 3.9 b.y. ago.
The oldest rocks dated on earth are about 3.96 b.y in age in Canada. These are younger than lunar rocks and meteorites because the earth's crust is continually being destroyed by erosion and weathering, and the radiometric clocks are reset by metamorphism and igneous activity. Note that zircon grains in Archean sedimentary rocks, derived by erosion of older rocks, have been dated as old as 4.4 b.y. in Western Australia.
The universe must be older than 4.6 b.y. as shown by the Doppler effect of light waves. This is an increase in wavelength of waves reaching an observer if the object sending the waves is moving away from the observer. The effect is proportional to the speed of the object and can be used to calculate a time of 15 to 20 b.y. ago when the matter in the universe was at the same location. A subsequent explosion, the big bang, may have begun the expansion of the universe to the present day.
All the planets rotate around the sun in the same direction and all, with the exception of Pluto, revolve around the sun in nearly the same plane and all, except for Pluto and Venus, rotate around their axis in the same direction. Theses observations suggest the planets formed at the same time from the same rotating dust cloud. Pluto may not be a true planet but an asteroid and Venus's change in rotation around its axis may be due to an early collision when Venus was first forming.
Composition Density Temp. Range Atm. Magnetic
C Field
Mercury rocky 5.7 -175=>425 trace weak
(has cratered surface)
Venus rocky 5.2 =>475 CO2, H2SO4
(has cratered surface) high
pressure
Earth rocky 5.5 N2,O2 strong
Mars rocky 3.9 CO2 none
ice caps of H2O CO2 low
(has cratered surface) pressure
asteroid belt
Jupiter 1.35 H2, He, CH4, NH3
Saturn 0.70 H2, He present
(has iron core)
Uranus 1.3 -185 H2, CH4, NH3
Neptune 1.6 H2, He
Pluto 2.1
solar nebular theory - One possible origin begins with a
collapsing sun which heats up to form heavy elements in the
process by fusion, followed by the sun exploding as a
supernova. The explosion produces a dense cloud of cosmic dust,
a nebular. Because the parent star was rotating as it exploded,
the resulting cloud of dust rotated. In the explosion, the lighter elements were
thrown further out from the center of the cloud.
Other theories simply
begin with the preseence of cosmic dust cloud which collapses due to its
proximity to a supernova event.
The cloud condensed as particles stuck
together and begins to rotate more rapidly as it contracts in order to preserve
angular momentum. Less dense materials, e.g., gaseous elements, were expelled
towards the edge of the rotating cloud and beyond into space. With increasing
rotation speed, the cloud flattened into a disc which segregated into rings. The
material in each ring coalesced into a protoplanet with the young sun forming in
the center. The coalescence process involves gravity as the protoplanets and sun
grows in size. The accumulation leads to an increase in temperature due to heat
released by friction as particles are incorporated into the planets and sun,
eventually leading to molten planets and a temperature high enough in the sun to
support nuclear fission and fusion.
An alternative theory has the sun
and planets forming from separate dust clouds in which the planetary dust cloud
was captured by the sun's gravitational pull.
The origin of the earth's
moon is thought to be related to the earth as the result of a planetary
collision of the earth with another object. Moon rocks show a close relationship
with earth rocks, having similar 18O/16O ratios, which are
not generally similar with or between meteorites. Ancient moon rocks have a weak
residual magnetic field, indicating a molten core was present to explain the
presence of a magnetic field. Presently, the moon lacks a magnetic field so the
core is no longer molten.
Excesses (relative to meteorites from outside the solar system) in amounts of Xenon 129 and Plutonium 244 in some meteorites indicate the formation of these meteorites occurred within a 100 m.y. of the formation of the sun. Excess xenon 129 formed from iodine 129 which was created during the formation of heavy elements by the collapsing sun and preferentially incorporated into meteorites. The half life of iodine 129 is only 17 m.y., so the meteorites formed prior to the decay of all the iodine 129. Thus, the entire solar system may have formed in this time interval of about 100 m.y.
Concentric Layering of the Earth and the Development of the Oceans, Atmosphere, and Crust
Homogeneous accretion versus heterogeneous accretion
Homogeneous - condensed material of both heavy and light elements which were segregated gravitationally once the heat released by condensing, melted the material. The dense elements sank to the form the core, and less-dense elements floated nearer the surface to form the mantle. Radioactive elements followed the light elements because they formed compounds with these elements.
Heterogeneous - a dense core condensed first followed by condensations of less dense silicates - alternative theory to homogeneous condensation.
atmosphere - degassing of the earth, primarily from molten material during early formation of earth. Degassing continues with volcano emission. Present atmosphere is 78% N2 and 21% O2 with minor amounts of H2O and CO2. The O2 is the result of photosynthesis of plants and the early atmosphere was deficient in O2.
oceans - degassing of H2O from molten material during early stage of earth formation. Upon cooling of the planet's surface, the water condensed to form oceans. Dissolved salts are now in steady state with inputs balancing outputs in the oceans which acocunts for their constant composition.
oceanic crust - first crust to form. Basaltic magma formed from partial melting of ultramafic rock (peridotite) making up the mantle. Initially, there probably was no crust on the surface and no convection in the mantle. Partial melting of mantle rock below the surface formed less mafic magmas which rose by buoyancy to the surface to crystalize as basalt overlying gabbro (oceanic crust). Later, when mantle convection began, the oceanic crust formed from magma moving upward in spreading ridges.
continental crust - second crust to form. Felsic rock formed from partial melting of the underlying oceanic crust where it was thick. This magma rose by buoyancy to the surface to form continental crust. Later, when mantle convection began, partial melting of oceanic crust in subduction zones formed magma which rose by buoyancy to form andesitic volcanoes with roots of granite (continental crust).
mantle - ultramafic silicates, forming during gravitational segregation of lighter elements from heavy elements - has subsequently solidified by cooling from convection currents.
outer core - liquid iron with minor sulfur and nickel- formed during gravitational segregation of heavy elements from lighter elements - is still liquid - perhaps it is solidifying by increasing the size of the inner core.
inner core - solid iron with minor sulfur and nickel - formed during gravitational segregation of heavy elements from lighter elements - has subsequently solidified by cooling from convection currents.
The lunar rocks from the maria craters date at between 3.9 and 4.6 b.y., indicating great meteorite showers occurred during this time. Note that an asteroid impact melts the surface rock and resets the radiometric clock, causing the age dates on the rock to give the age of the impact. The composition of the earth's crust may have been altered by these meteorite showers. The actual impacts have been removed by erosion and other earth-surface processes.
Greenstone belts provide more information than Archean granulites which are rocks subjected to intense metamorphism. The greenstone belts typically occur as pods or synclines within granulites in which ultramafic volcanics (komatites) grade upward into mafic volcanics (basalt with pillow structures) into felsic volcanics (andesite to rhyolite) into sediments. The sediments are not commonly those of continental-shelf sequences, e.g., carbonates (limestones) or delta sequences with cross-bedded sandstones (quartz sandstones and feldspar-rich sandstones or arkoses). Instead, they are typically marine, e.g., mudstones (marine shales) and turbidites (greywackes and conglomerates), cherts, and banded-iron deposits with few indications of terrestrial or fresh water deposition. The presence of mostly deep-water sediments are explained by the presence of small protocontinents of felsic crust and the absence of large continents and their associated shelves during Archean time.
The first large continent apparently formed in South Africa, about 3 b.y. ago. Other large continents were formed between 2.1 to 2.7 b.y. ago as indicated by dating of metamorphism related to intrusion of magma in cratons. The end of the Archean time at 2.5 b.y. marks the beginning of the widespread occurrence of continental crust together with modern plate tectonics.
Banded iron formations (iron-rich layers alternating with cherts) are common during the Precambrian, particularly during the Proterozoic. They also occur during the Archean in greenstone belts and make up the oldest dated rocks. The lack of oxygen (no higher plants to make it by photosynthesis) in the Precambrian atmosphere may help explain these iron formations which are not forming today during Phanerozoic time. Oxygen tends to prevent accumulations of iron in aqueous solution, by causing precipitation of iron-rich minerals.
Graphite in banded-iron formations has 13C/12C ratios similar to those in organic tissue. Bacteria are the only life forms represented in Archean fossils. These include both animal-like and plant-like bacteria. Stromatolites, thought to have been formed by cyanobacteria (of which modern species undergo photosynthesis), occur in rocks dated as old as 3.5 b.y. They occur as layered structures of organic-rich calcium-carbonate sediment alternating with layers of calcium-carbonate sediment. Stromatolites grow in shallow seas on continental shelves and are thus rare in Archean rocks because of the scarcity of continental shelves around the small protocontinents existing during this time. They were much more abundant during the Proterozoic and become less abundant during the Phanerozoic when competitors evolved to limit their growth.
Bacteria fall in the kingdom Monera and are "prokaryotes", meaning that they lack a cell nucleus, DNA in chromosomes, and other cell organelles, that are present in the advanced cells called "eukaryotes" which did not exist in the Archean.
Amino acids are the building blocks of proteins needed by living organisms. There are 22 naturally-occurring amino acids. An amino acid contains the group:
H
|
-C-COOH
|
NH2
Proteins are formed by linking amino acids between the NH2 of one amino acid with the COOH groups of another amino acid. Each linkage is accompanied by the release of one molecule of water H2O. Amino acids are found in carbonaceous chondrites (carbon-rich stony meteorites) as well as on earth. Electrical sparks or ultraviolet light in a gas phase of ammonia (NH3), methane (CH4), hydrogen (H2), and steam (H2O) can produce amino acids. Other gas phases can be used such as carbon monoxide (CO), nitrogen (N2) and hydrogen (H2). Could this type of reaction have anything to do with the beginning of life on earth? ATP (adenosine triphosphate) is used by modern cells as the source of energy to build organic compounds. Instead of manufacturing ATP, it can be produced from simple gases. Could the early cells have obtained ATP from their environment rather than manufacturing it.
Note that bacteria can obtain energy to store as ATP in two ways:
Chemosynthesis - the breakdown of simple chemical compounds within a cell
fermenting bacteria - fermentation - breakdown of sugars to form alcohol and carbon dioxide and wate. These bacteria cannot tolerate oxygen and presently live out of contact with the earth's atmosphere.
methane-producing bacteria - Archaebacteria use organic compounds to form methane and sugars. These bacteria cannot tolerate oxygen and presently live out of contact with the earth's atmosphere.
sulfate-reducing bacteria - reduce sulfate to breakdown sugars to form sulfide and carbon dioxide and water. These bacteria cannot tolerate oxygen and presently live out of contact with the earth's atmosphere.
aerobic respiration - aerobic animals, animal-like cells, and animal-like bacteria reduce oxygen to breakdown sugars to form carbon dioxide and water - however, little oxygen was available in Archean time and there may not have been bacteria using aerobic respiration.
photosynthesis - use of light energy to form chemical compounds within a cell
photosynthesis - The common earth-surface process utilized by plants, plant-like cells, and plant-like bacteria in which light interacts with the green pigment chlorophyll. An Archean example is cyanobacteria (blue-green algae) which use carbon dioxide and water with chlorophyll and light energy to form oxygen and sugars. The buildup of oxygen began with photosynthesis of cyanobacteria.
sulfide-oxidizing bacteria - anaerobic photosynthetic purple and green bacteria which use carbon dioxide and hydrogen sulfide and light energy to form sulfur and sugars. These bacteria cannot tolerate oxygen and live out of contact with the earth's atrmosphere.
Proterozoic Eon, 2.5 b.y. to 540 m.y.
During the Proterozoic Eon, the modern style of orogeny (mountain building), which is related to plate tectonics, began. The first modern-style orogeny was the Wopmay orogeny of Canada which occurred between 2.1 and 1.8 b.y. ago along the western margin of the present-day Archean age microplates forming the core of the Canadian Shield. In this area, continental crust had rifted two continental masses apart and subsequently, a subduction zone developed along the boundary. The igneous belt, metamorphic belt, and a fold and thrust belt (containing the backarc (also known as the foredeep or foreland) basin sediments) are preserved in the rock record. Stromatolites are preserved in the preorogenic shallow marine sediments of the foredeep basin.
During the Proterozoic Eon deposition of sediment shifted from being primarily deep-water marine (in the Archean Eon) to shallow-water as the shelf areas along continents developed.
As described briefly below, mountain building occurs along plate margins where plates are converging.
Andesitic volcanoes form along the edge of an un subducted plate near a subduction zone in which oceanic crust is being destroyed, e.g., Cascades, Sierras, Rockies (difficult example), Aleutians, Japanese Islands, Philippine Islands, and Andes. Once all oceanic crust is destroyed in a subduction zone, continental crust will collide with continental crust, between the two plates. Because continental crust cannot be subducted (due too its low density), the two plates then fuse together and compress any sediment along the margins into mountains of folded sediment, e.g., Alps, Urals, Appalachians, and Himalayas. In both types of mountain building, subsequent erosion will result in the continental crust (granite) being uplifted by block faulting due to isostasy.
What type of rocks would we expect from mountain building?
Described below is a general vertical upward sequence of shallow-water to deep-water marine to shallow-water marine into fresh-water sediments - all in the backarc (foredeep or foreland) basin - and a lateral sequence of rocks going (away from the craton) from folded sediments (fold and thrust belt) to metamorphosed sediments, to igneous rocks. These sequences are associated with Phanerozoic mountain building such as the Wopmay orogeny of Canada.
A depositional basin exists between the volcanoes forming along the edge of a continent and the stable craton. The vertical trend in sediments in this basin grade upward from the (oldest) shallow-shelf marine shelf deposits which are preorogenic (sandstones and carbonates) to deep-water marine deposits (flysch: mudstones and turbidites) and then back to the (youngest) shallow-water marine and fresh-water deposits (molasse). The trend reflects the downwarping of a shallow basin by the stresses associated with mountain building (occurring further offshore), followed by filling the deep-water basin with sediments from the newly-formed mountains and from the craton. These sediments have been folded because of the compression associated with mountain building. In addition, the sediments closest to the mountain-building belt are metamorphosed because of the generally higher compression and the higher temperatures due to the presence of magmas associated with the volcanoes.
The lateral rock sequence going from the craton to the area of mountain building is a belt of folded and thrusted sediments, a belt of metamorphosed and folded sediments, to a belt of igneous rock. The first two belts are formed in the depositional basin described above and the third belt consists of rock making-up the volcanoes.
Early Proterozoic glaciation occurred about 2.3 b.y. ago, as shown by glacially-derived features (tillites, dropstones, laminated mudstones with varves due to deposition in glacial lakes in the Gowganda Formation in southern Canada. Similar age tillites are also found in Wyoming, Finland, southern Africa, and India. Thus the glaciation was world-wide.
Late Proterozoic glaciation occurred in several pulses between 850 and 600 m.y. ago, as shown by tillites on all major continents except Antarctica. A mass extinction occurred with the glaciation at 600 m.y. which affected the acritarchs and other species and may have been due to a cooling of the surface waters of the oceans.
Between 2 and 3 b.y. ago atmospheric oxygen was thought to have been only about 2% of present-day concentrations, reaching moderate (15%) concentrations at around 2 b.y. ago. Cyanobacteria and other organisms undergoing photosynthesis and producing oxygen, did not become abundant until about 2.3 b.y. ago, although cyanobacteria actually evolved about 3.5 b.y. ago. Sulfate-reducing bacteria, sulfide-oxidizing or purple-green bacteria, methane-producing bacteria, and fermenting bacteria cannot survive in oxygenated environments. These bacteria have become less common since Archean time, being restricted to anaerobic (without oxygen) environments, out-of-contact with the atmosphere.
The generally low levels of atmospheric oxygen in the Precambrian time are needed to explain the banded iron formations which commonly formed between 2.5 and 1.8 b.y. ago, as well as the occurrence of uranium minerals with uranium in a reduced form U4+. The common occurrence of pyrite (having both reduced iron and reduced sulfur) in Precambrian sediments, is another indicator of low levels of atmospheric oxygen. Pyrite is not stable in an oxygenated environment and decomposes. The banded iron formations consist of alternating bands of chert with layers that are usually composed of iron-rich minerals, with iron in a reduced state (Fe2+). Iron and uranium are sinks for oxygen (combine with it) and would not normally be deposited in reduced forms under earth-surface conditions, unless the oxygen in the atmosphere was at low concentrations. In addition, uranium is actually very soluble in its oxidized form and would have been dissolved quickly if deposited in an oxidizing environment.
Red beds, containing oxidized iron as Fe3+ in hematite, Fe2O3, appear in rocks younger than 2.3 b.y. ago, as the result of a gradual increase in oxygen content of the atmosphere. In addition, not all of the iron in banded iron formations is in the reduced state. Some of the minerals contain iron in both reduced and oxidized forms (e.g., magnetite, Fe3O4)) indicating some oxygen in the atmosphere.
Before the concentration of oxygen in the atmosphere could be built up to present-day levels by photosynthesis, various oxygen sinks had to be filled. Carbon monoxide (emitted by volcanoes) had to be oxidized to carbon dioxide in the atmosphere, sulfide minerals exposed at the earth's surface had to be oxidized to sulfates, and reduced iron, uranium, and other metals exposed to the atmosphere had to be oxidized to metal oxides, e.g., rust. A lot of the initial oxygen produced by photosynthesis was used up in these reactions. Once levels of oxygen were built-up in the atmosphere, they stopped increasing and reached a steady state concentration due to the evolution of organisms, using oxygen in respiration. In essence, photosynthesis became balanced by respiration, producing a balance between atmospheric oxygen and carbon dioxide.
At the beginning of the Proterozoic Eon, life consisted of single-celled prokaryotic producers of the kingdom Monera, such as cyanobacteria. By the middle of the Proterozoic Eon, life had evolved to produce single-celled eukaryotes in the kingdom Protoctista: first into protozoans, animal-like protoctists (more than 1.8 b.y. ago) and then into plant-like protoctists, such as acritarchs (about 1.4 b.y. ago). The acritarchs are thought to have been dinoflagellates. They underwent adaptive radiation between 900 and 700 m.y. ago before undergoing a mass extinction 600 m.y. ago at the time of the Late Proterozoic glaciation. Only a few spherical forms survived. Present-day representatives of acritarchs are dinoflagellates.
By the end of the Proterozoic Eon, multi-celled plant-like protoctists had evolved together with multi-celled plants of the kingdom Plantae and multi-celled animals of the kingdom Animalia, such as jellyfish and sea pens of the phylum Coelentera, segmented worms of phylum Annelida, and organisms with external skeletons and jointed appendages of phylum Arthropoda. The evolution of multicellular animals required high oxygen concentrations in the atmosphere and advanced nerve cells and occurred less than 1 b.y. ago (as evidenced from trace fossil evidence of burrows).
As noted earlier, stromatolites originated about 3.5 b.y. ago, became very abundant, as shallow water environments became more abundant, about 2.3 b.y. ago and exist today. They are not useful as index fossils because they tend to have different shapes in different environments. They are no longer abundant because multi-cellular animals evolved that feed on the cyanobacteria. They only survive today in shallow-water environments too harsh for these predators, e.g., tidal channels.
Eukaryotes, (cells containing nuclei, chromosomes, and other organelles) evolved from the prokaryotes in the Proterozoic about 1.8 b.y. ago, based on the first appearance of large cells with thick walls (cannot identify as to being plant-like or animal-like). The first appearance of large cells in the fossil record date back to 1.4 b.y. ago in the acritarchs. In general, prokaryotes have much smaller cell sizes than acritarchs.
Eukaryotes are thought to have evolved through the union of two or more prokaryotic cells. Protozoans, animal-like cells in the kingdom Protoctista, are thought to have evolved first. The evolutionary process consisted of one prokaryote devouring a smaller bacterium, in which the smaller cell survived, although altered, and became a mitochondrion or an organelle capable of using oxygen to break down organic compounds, i.e., respiration in the first animal-like cell. This scenario is supported by the observation that bacteria are known to inhabit single-cell organisms and perform respiration for their host.
The first plant-like cell in the kingdom Protoctista is thought to be the result of a protozoan consuming a cyanobacteria which became an intracellular body known as a chloroplast (where photosynthesis occurs). This possibility is supported by structural similarities between cyanobacteria and chloroplasts. The oldest plant-like protoctist fossils date back 1.4 b.y. (algae plankton called "acritarchs" and thought to be dinoflagellates); whereas, the oldest positively-identified fossils of protozoans date back only 0.8 b.y.. However, since the plant-like protoctists moat likely evolved from protozoans, the protozoans must have originated more than 1.8 b.y. ago.
Algae include plant-like organisms, ranging from cyanobacteria to single-cell and multi-cellular photosynthesis protoctists to multi-cellular plants that (unlike advanced land plants) lack multicellular reproductive structures to protect their eggs and embryos). Multicellular algae are present in the fossil record beginning about 0.9 b.y. ago.
Multicellular animals evolved from protozoans by developing multicellular body forms and advanced nerve cells. Between 900 to 600 m.y. ago, a series of accumulations of 13C-rich carbonate rock deposits, provide evidence that not much oxidation of organic material was occurring. (This follows because organic matter is rich in 12C, which upon oxidation would also be incorporated into carbonate rock, preventing carbonate rock from being rich in 13C.) The lack of oxidation of organic matter allowed rapid build-up of oxygen in the earth's atmosphere, which then promoted the evolution of multicellular animals, depended upon oxidation for respiration.
Multicellular animals may have not developed rapidly until near the end of the Proterozoic for other reasons besides a low atmospheric oxygen content. Evolution took time to produce the nerve cells which coordinate muscle movement. Perhaps, these nerve cells only developed near the end of the Proterozoic.
The oldest multi-celled animal fossil records are trace fossils (burrows, trails, tracks) of soft-bodied animals in rocks less than 1 b.y. old. Imprints of jellyfish and sea pens have been found. Fossil skeletons were not preserved until near the end of the Proterozoic Eon, about 600 m.y. ago, because organisms with skeletons evolved after the non-skeleton forms. The multicellular animals present at the end of the Proterozoic Eon included coelenterates (jellyfish and sea pens in phylum Coelenterata), segmented worms (annelids in phylum Annelida), and arthropods in phylum Arthropoda. The late Proterozoic Ediacara fauna in Australia contains fossils of many soft-bodied animals.
Continents can grow by
(1) suturing two continents together, following the destruction, in a subduction zone, of oceanic crust between them.
(2a) accretion by attachment of microplate (e.g., an island arc) to a large craton, following the destruction, in a subduction zone, of oceanic crust between the craton and the microplate. The microplates are called exotic terrains.
(2b) accretion by metamorphism of sediments, near a subduction zone, that are on the continental margin. These can also be oceanic sediments that were smeared onto the edge of the continental margin, as they were pushed into the subduction zone.
(2c) accretion through the introduction of magma in volcanoes overlying a subduction zone on the continental crust. The magma represents partially melted oceanic crust and melted oceanic sediments in the subduction zone.
(3) introduction of basaltic magma in failed rifts within a continent. Although, continents become smaller by rifting, if the rifting proceeds to split the continents.
During the Proterozoic Eon, a large supercontinent developed called Rodina. The continent eventually included North America, Greenland, Scotland, Ireland, eastern Russia, Baltica (Scandinavia), and possibly Siberia along its northeastern margin. Also attached to the continent were Australia (along its northern margin), Antarctica (along its western margin, and Africa (along its southwestern margin). The final stage of the supercontinent creation was the Grenville Orogeny in which South America sutured to Laurentia in Late Proterozoic. This supercontinent subsequently broke apart at the end of the Proterozoic: separating into Laurentia (North America and Greenland), Baltica, other northern hemisphere continents, the southern continents (that later joined in the early Paleozoic to become Gondwanaland), and several microcontinents. Eurasia did not exist during the Proterozoic Eon but formed later from fragments rifted from Laurentia and Gondwanaland in the Phanerozoic Eon. Note that the super-continent which existed at the end of the Proterozoic Eon was reassembled at the end of the Paleozoic Era to form the supercontinent named Pangaea. When Pangaea split into two supercontinents during the Mesozoic, these were Laurasia and Gondwanaland.
Laurentia began to form 1.95 to 1.85 b.y. ago through the assembly together of at least six microplates of Archean age to form the Canadian Shield. Note that Laurentia included Greenland which contains Archean age rocks. South of the shield area, a series of Proterozoic island arcs developed (in the region of the present United States) that were accreted onto the craton between 1.8 and 1.6 b.y. ago.
About 1.5 b.y. ago, Laurentia may have become smaller in size through the rifting of one of the microplates, Siberia, away from the western margin of the craton.
A rift began to develop in central Laurentia between 1.2 and 1.0 b.y. ago. The rift extended south from the Great Lakes region to Kansas, forming the Mid-continental rift (also called the Keweenawan Rift) which failed but can be recognized by basaltic rocks south of the Canadian Shield. The copper-rich Keweenawan basalts were extruded at this time.
Between 1.2 and 1.0 b.y. ago, the western margin of South America and Laurentia sutured together along the present eastern margin of the United States, during the Grenville orogeny to complete the formation of the supercontinent Rodina. During this time, Ireland, Scotland, Scandinavia, and eastern Russia were part of Baltica already joined to Laurentia. Siberia was already joined to Laurentia. The rest of what was to become Gondwanaland formed a crescent wrapping around Laurentia. Some of the suturing of these southern continents to Laurentia may also have occurred during the time of the Grenville Orogeny. The reassembly of the southern hemisphere continents in the vicinity of Laurentia follows the Samfrau belt of mountains, between South America and Africa, between South Africa and Antarctica and between Australia and Antarctica. The Samfrau mountains were active during the time of the Grenville Orogeny indicating suturing together of the southern continents at the same time they were possibly being sutured to Laurentia. The North America east coast suturing with South America produced a range of mountains in the Grenville Province. Remnants of those mountains are exposed in the Adirondack Mountains in New York. Rocks formed during the Grenville orogeny are also exposed in the Blue Ridge Mountains in Virginia, and in the Llano Uplift in Texas.
The supercontinent Rodina rifted apart between 0.8 and 0.6 b.y. ago. Laurentia and Baltica split apart from the continents which became Gondwanaland. There were several microcontinents such as northern Ireland, Scotland, southern Ireland, and England. The Iapetus Ocean separated the other continents from Laurentia. The rifting separating Gondwanaland from the western margin of Laurentia produced a clastic wedge of eroded sediments in basins preserved in the Belt Supergroup.
The sequence of accretion of continents to form Gondwanaland at the end of the Proterozoic is unknown. Metamorphic belts in the Proterozoic follow the present-day outlines of the different continents which formed Gondwanaland. However, these metamorphic belts appeared to form in the interior of Gondwanaland and the different continents later broke apart along these belts. The origin of the metamorphic belts is unknown. They are not the result of the normal mountain-building processes of plate tectonics.
Phanerozoic Eon and Paleozoic Era
Cambrian Period 543-510 m.y. ago
Ordovician Period 510-439 m.y.
ago
Silurian Period: 408 to 438 m.y. ago
Paleogeography and Plate Tectonics
Near the end of the Proterozoic Eon, the continents had merged briefly into Rodina, a supercontinent (through collisions) which included North America. Subsequently, the supercontinent continued splitting apart at the Laurentia (North America and Greenland), Baltica (Scandinavia), and Gondwanaland (South America, Africa, Antarctica, Australia, India, Arabian Peninsula). The Iapetus Ocean formed as Laurentia (North America) and Gondwanaland separated by rifting of Rodina.
Gondwanaland remained intact throughout the Paleozoic Era, reconnecting to other continents in Pangea, and not breaking apart until the Mesozoic Era. Eurasia did not exist as a continent at the beginning of the Paleozoic Era. Eurasia began forming during the Paleozoic Era by accretion of continental fragments onto Laurentia to form Laurasia (N. Am., Greenland, and Eurasia). These fragments included Baltica, and the suturing of the Russian and Siberian platforms at the Ural Mountains at the end of the Paleozoic.
During the Cambrian Period the continents are thought to have been near the equator and not near the poles. A near-equatorial location explains the common presence of reef limestones, requiring both shallow waters and warm waters. Sea level was low at the end of the Proterozoic Eon so most of the continental landmasses were exposed. A low sea level explains the general absence of marine sediments marking the transition between the end of the Proterozoic Eon and the beginning of the Phanerozoic Eon (Paleozoic Era and Cambrian Period). However, during the Cambrian Period, sea level rose and flooded most of the continents: Sauk Sea {Cambrian} and Tippecanoe Sea {Ordovician and Silurian} in Laurentia.
Sea level gradually rose throughout the early Paleozoic, reaching a maximum level in Late Ordovician and in the Silurian, followed by a gradual drop until the end of the Paleozoic. In each period however, there were smaller rises and falls of sea level, e.g., the sea level fall at the end of the Ordovician and Devonian Periods that corresponded to major glaciation.
Note that the center of the craton of Laurentia was generally unflooded during the time of high sea levels in the early Paleozoic. This region called the Transcontinental Arch served as a source of sediment for deposition in the interior epicontinental seas. Between Late Proterozoic and Middle Cambrian time, an active rift existed in Oklahoma, the Southern Oklahoma Rift which contain basalts and was covered with shallow limestones from Late Cambrian to Early Devonian time after the rift became inactive.
During the Ordovician Period, Gondwanaland begin moving south towards the south pole, eventually covering part of the pole by what is now North Africa during the end of the Ordovician Period. A major period of glaciation occurred on Gondwanaland at this time; however, Laurentia and Baltica did not experience glaciation because they were too close to the equator (but Baltica was further south than Laurentia). Sea level fell during this time of glaciation and a major mass extinction of marine organisms occurred at the end of the Ordovician Period. The mass extinction was greatest in tropical waters which presumably underwent global cooling due to glaciation occurring in Gondwanaland.
As Gondwanaland moved south during the Ordovician Period, Baltica and two small micro-continents (England and the south half of Ireland) were moving north and approaching Laurentia, closing the Iapetus Ocean. During late Ordovician time, mountain building, associated with island arcs near subduction zones, occurred along the eastern margin of Laurentia and along the margin of Scotland. This first period of Appalachian mountain building is called the Taconian orogeny. Eventually, in Silurian and Devonian time, Baltica and the two small microcontinents fused onto the eastern margin of Laurentia, producing the second phase of Appalachian mountain building called the Acadian or Caledonian orogeny. The former is the American name and the latter is the European name.
Typically, during the early Paleozoic Era, around each continent were concentric belts of sediment changing laterally in a seaward direction from shallow deposits of siliciclastic debris (due to weathering of silicates on the craton) to limestones (carbonate platform deposits, reefs of coral and stromatoporoids or sponges, stromatolites) to deep water deposits of muds and breccias (turbidites or flysch deposits). Added to the deep water deposits were weathered debris from volcanics if a subduction zone existed along the continental margin. The carbonate platform bordering the eastern margin of Laurentia was destroyed by subduction associated with the closing of the Iapetus Ocean during late Ordovician time. However, the carbonate platform along the western margin remained intact into middle Paleozoic time. It was not until the end of the Devonian Period that orogeny affected the western margin. The Burgess Shale of British Colombia, which contained many soft-bodied fossils, formed at the base of the Middle Cambrian Period. The shale formed at the seaward foot of the carbonate platform.
Permian: 290-245 m.y. ago, characterized by drier climates and evaporites
Pennsylvanian: 323-290 m.y. ago, called Late Carboniferous, characterized
by coal-bearing deposits and limestones of fusalind
foraminifera.
Mississippian: 363-323 m.y. ago, called Early Carboniferous,
characterized by limestone composed of crinoids.
Devonian Period: 360 to
408 m.y. ago, characterized by reefs of tabulate and rugosa corals.
High sea level occurred throughout most of the Devonian Period. Laurentia was flooded by the Kaskaskia Sea (Devonian). Lower sea level occurred at the transition between the Silurian and Devonian periods and at the transition between the Devonian and Mississippian Periods which was also a period of glaciation in Gondwanaland (late Devonian). From the end of the Devonian to the end of the Paleozoic, sea level gradually falls. Note that because the periods were distinguished on the basis of unconformities by the early geologists, it is not surprising that sea level was low at the end of each period (which accounts for the unconformities).
Late Paleozoic glaciation was initiated at the end of the Devonian Period on Gondwanaland and persisted in non-tropical regions throughout the end of the Paleozoic. Sea level had dropped at the beginning of the Mississippian Period (due to Late Devonian glaciation), at the beginning of the Pennsylvanian Period (due to expansion of Gondwanaland glaciation in late Mississippian) and at the end of the Permian (due to glaciation developing on parts of Pangaea existing near both the North and South Poles.
The epicontinental seas that flooded the continental interior of North America were the Kaskaskia Sea during the Devonian and Mississippian Period and the Absaroka Sea during the Pennsylvanian and Permian Periods.
Abundant warm-water deposits (reefs and evaporites) during the Silurian and Devonian Periods imply that Laurentia and Baltica stayed close to the equator. Gondwanaland contains glacial deposits at the end of the Devonian but not at the end of the Silurian. The period of glaciation at the end of the Ordovician on Gondwanaland ended in early Silurian as Gondwanaland moved away from the South Pole. Gondwanaland moved back over the South Pole in the Devonian. The Late Devonian glacial deposits correspond to the Devonian mass extinction. Australia had a Great Barrier Reef in Late Devonian time so not all of Gondwanaland was near the South Pole.
During the Mississippian Period, Gondwanaland stretched from the South Pole to near the equator. The Old Red Sandstone continent reached from the equator to the middle latitudes in the northern hemisphere, separated from Gondwanaland by the Tethys Sea. Siberia lay further north, near the North Pole. Kazakhstania and China lay east of the Old Red Sandstone continent and north of the equator. A continental ice sheet covered the area near the South Pole. During the Pennsylvanian Period, glaciers reached within 30o of the equator, while tropical coal swamps existed near the equator in the Old Red Sandstone continent. The tropical regions can be distinguished from the nontropical regions by the absence of tree rings in the fossil tree record. Colder climates produce seasonal tree rings (due to different growth rates in different seasons) that are lacking or poorly developed in tropical climates. During the Permian Period, the climate was much drier in much of North America and Europe. The formation of Pangaea created a supercontinent, stretching from the South Pole to the North Pole, that included land areas far from a source of moisture, e.g., the oceans. Most of the mountain ranges lay along the eastern coasts in the equatorial regions, producing large rain-shadow areas to the west (because of the prevailing easterly winds). Evaporite and dune deposits are characteristic of the Permian Period. The transition of the Permian to the Triassic is marked with disconformities due to low sea level, resulting from glaciation on Pangaea near both the north and south poles.
During the Silurian Period, Baltica was approaching Laurentia and the Iapetus Ocean was being destroyed in a subduction zone (near Laurentia's eastern margin). Eventually, in the Devonian time, Baltica was sutured onto Laurentia, forming another part of the Appalachian Mountain trend in the Acadian orogeny of North America (called the Caledonian orogeny in Great Britain and Scandinavia). This new continent (the so-called Old Red Sandstone Continent) was close to Gondwanaland during the Devonian Period, as suggested by the common Late Devonian genera of marine invertebrates. They may have even been sutured together in late Devonian and then separated by rifting? We don't know. However, Pangaea, the supercontinent at the end of the Paleozoic formed later in the Pennsylvanian and Permian Periods by the suturing of the Old Red Sandstone Continent to Gondwanaland. This later suturing event completed forming the Appalachian Mountain trend with the Alleghanian orogeny. The Alleghanian orogeny is called the Variscan or Hercynian orogeny in Europe.
During late Devonian, an island arc lay offshore of the western margin of the Old Red Sandstone continent (North America). The intervening seaway was destroyed by a subduction zone dipping under the island arc (to the west). The remnants of this arc are recorded in ophiolite deposits in the Klamath Mountains of California. (An ophiolite is a cross-section of oceanic crust which was not destroyed in a subduction zone but smeared and later uplifted onto the overriding edge of a subduction zone.) During the late Devonian and Mississippian, this island arc was accreted onto the western edge of the Old Red Sandstone continent in the Antler orogeny, creating the Klamath Mountains. This accretion formed the Roberts Mountains Thrust in Nevada in which shelf sediments were thrust eastward onto the craton.
Gondwanaland and the Old Red Sandstone continent were eventually sutured together to form Pangaea during the Pennsylvanian Period in the Alleghanian orogeny (American term), also called the Variscan or Hercynian orogeny (European terms). This orogeny extended the Appalachians to the southwest, through the Ouachita mountains, and also formed the Hercynian mountains in Europe that extended into northwest Africa. Kazakhstania sutured onto Siberia during the Pennsylvanian Period. In North America, weathering and erosion of the young Appalachian Mountains produced molasse deposits to the west in central North America. The ancestral Mississippi River was one of the rivers (the Michigan river) draining sediment from the Appalachian mountains.
The Ancestral Rocky mountains developed during the Pennsylvanian Period, as the Uncompahgre Uplift in central Utah and the Ancestral Front Range trending across southwest Colorado, both associated (somehow?) with the Ouachita-Appalachian mountains created during the suturing of Gondwanaland to the Old Red Sandstone continent to form Pangaea. The ancestral Rockies lay to the west of the modern Rockies which formed later in the Cretaceous Period. The Pennsylvanian Fountain Arkose, containing hogbacks and cuestas (and forming the Garden of the Gods in Colorado Springs), was formed from sediment eroded from the Ancestral front Range.
Some of the coal swamps of North America (and Europe) in the Pennsylvanian Period formed in the floodplains of rivers draining the Appalachians and Hercynians. These were floodplain forests growing in the backswamps behind the natural levees of meandering rivers. Other widespread, thin coal beds in central North America formed in swamps associated with shallow epicontinental seas. Such a coastal swamp would exist during times when sea level fell (due to an increase in glaciation near the polar regions) and would be covered with marine deposits as sea level rose (due to melting ice). Both types of coal deposits produce cyclic coal deposits as described below.
The backswamp deposits produce a cyclic deposit of coal as the result of the river meandering, depositing channel deposits over old backswamps, as the river raises the surface of its floodplain. The swamp associated with an epicontinental sea will produce a coal deposit at low sea level, which is then covered with marine deposits from a transgressing sea as sea level rises, followed by deposits from a regressing sea as sea level falls, before another coal seam is formed. The complete cycle is called a cyclothem. The Everglades is an example of a coastal swamp which would be affected by small changes in sea level, resulting in cyclic deposits of coal over time spans of tens or hundreds of thousands of years. Cyclothems did not form during the recent glacial times in the Pleistocene because of the general absence of these shallow epicontinental seas that would expand and contract over large areas of land surface with minor changes in sea level.
Because, the central western area of North America was near the equator in Pennsylvanian time, rain shadows lay west of mountain ranges. A rain shadow was created to the west of the Ancestral Rocky Mountains in the Paradox Basin of southern Utah, northern Arizona, and western New Mexico, where Pennsylvanian evaporites accumulated. Similarly, in western Texas in Permian time, the rain-shadow effect of the Ouachita mountains to the east produced evaporite deposits in the Delaware Basin. This same basin has the beautifully preserved El Capitan carbonate reef, composed of lacy bryozoan, calcareous algae, and calcareous sponges, along its margins. The development of evaporites took place in subsiding basins flooded by shallow epicontinental seas.
Further west, off the margin of the North American craton, a volcanic arc, the Golconda Arc, attached to a microcontinent (Sonomia) was shedding sediment during the late Paleozoic. The subduction zone dipped westward under the Golconda arc, producing an accretionary wedge. In late Permian time, the accretionary wedge, the Golconda terrain, was thrust eastward onto the continental margin in Nevada (similar to what was done in the Antler orogeny in late Devonian and early Mississippian time). This is called the Sonoma orogeny and the additional sediment resulted in the westward growth of the North American continental margin. The orogeny continued into the Triassic in which the Sonomia microcontinent was fused to North America, producing present-day southwestern Oregon and northwestern California.
Pangaea continued to enlarge during the Permian Period. Siberia (including Kazakhstania) was sutured to eastern Europe forming the Ural Mountains. Siberia remained near the North Pole. Pangaea stretched from the South Pole to the North Pole with the zone of attachment between Gondwanaland and Laurasia near the equator (Fig. 14-35). Only China was unattached to Pangaea at the end of the Paleozoic Era and it was attached during the Mesozoic Era. The water body adjacent and east of Pangaea along the Equator was called the Tethys Sea, the same name previously given to the ocean destroyed by the collision of the Old Red Sandstone with Gondwanaland in the Alleghanian orogeny and the same name subsequently used to describe the Mesozoic Sea formed in the breakup of Pangaea by the rifting between Laurasia and Gondwanaland.
In eastern Europe, lower Permian evaporite deposits accumulated along the western margin of the Ural mountains. The deposits formed in a subsiding basin (foredeep) which was collecting sediment from the Ural mountains. Evaporites precipitated as the result of evaporation and restricted circulation of sea water in the foredeep. During the late Permian, evaporites were deposited across northern Europe which was also acting as a subsiding basin. The region was flooded with sea water from the north during four successive periods. Each time evaporites precipitated because of evaporation and the restricted circulation of sea water back with the open ocean. These four periods of salt deposition produced the Zechstein salt deposits of northern Germany and the North Sea area. These salt deposits underlie the oil and gas production in the North Sea just as the Louann Salt underlies the oil and gas production in the Gulf Coast region.
During the Permian Period, the climate was much drier than during the Carboniferous. Evaporites were deposited and plants flourished that could survive under drier conditions.
Permian: 290-245 m.y. ago, characterized by drier climates and limestones of fusalind foraminifera, and glaciation on Pangaea and ending with the greatest mass extinction in earth's history.
Pennsylvanian: 323-290 m.y. ago, called Late Carboniferous, characterized by coal-bearing deposits and limestones of fusalind foraminifera and the evolution of reptiles.
Mississippian: 363-323 m.y. ago, called Early Carboniferous, characterized by limestones composed of crinoids and a mass extinction at the end of the period.
Devonian Period: 408 to 363 m.y. ago, characterized by evolution of differnt fish groups, followed by evolution of amphibians. Ammonoids evolved and abundant reefs occurred of tabulate corals and stromatoporoids. A mass extinction occurred at the end of the period during global glaciation.
Silurian Period: 439 to 408 m.y. ago, characterized by reefs of tabulate corals and stromatoporoids. The first land animals appeared.
Ordovician Period: 510-439 m.y. ago, characterized by reefs of tabulate and rugosa corals and stromatoporoids and a mass extinction at the end of the period during global glaciation. The first land plants appeared.
Cambrian Period: 543-510 m.y. ago, characterized by reefs of archaeocyathids and later by reefs of rugose corals and stromatoporoids, and the evolution of trilobites and nautiloids.
Near the end of the Proterozoic Eon, there was a mass extinction followed by adaptive radiation which produced many soft-bodied animals such as jellyfish (phylum Coelenterata) and worms, e.g., polychaete worms (phylum Annelida). By the beginning of the Paleozoic, the seas had also become inhabited by small, shelled animals, e.g., primitive mollusks in phylum Mollusca, with external skeletons. This occurred during the Tommotain Stage at the base of the Cambrian. The evolution of skeletons may be due in part to the development of carnivores and possibly a change in sea-water chemistry. The skeletons were made of either calcium carbonate or calcium phosphate. Note that calcium phosphate is what our teeth are made of.
The Cambrian life that has been preserved in the fossil record is of marine life, without any terrestrial life or fresh-water life. The first reefs, formed during the Tommotain Stage, were composed mainly of the cone-shaped archaeocyathids (phylum Archaeocyatha), together with calcareous algae. Archaeocyathids became extinct during the Cambrian and later Cambrian reefs were composed of corals, e.g., rugose (horn) corals (phylum Coelenterata) and stromatoporoids (sponge-like in phylum Porifera); however, reefs did not become abundant again until the Ordovician Period. Other marine organisms which evolved during the Cambrian include crinoids (sea lilies in phylum Echinodermata); brachiopods (unsymmetric bivalves in phylum Brachiopoda); conodonts (tooth structures of ell-like creatures in phylum Chordata); trilobites, ostracods (phylum Arthropoda); bivalves, snails, and nautiloids (phylum Mollusca); jawless fishes (phylum Chordata); and graptolites (phylum Hemichordata).
The dominant skeletonized fossils during the Middle and Late Cambrian Period are of trilobites which evolved in the Early Cambrian Period (following the appearance and extinction of the Tommotian fauna). The trilobites which lived in warm, shallow seas underwent several extinctions during the late Cambrian Period, followed by adaptive radiations (speciation). Trilobites living in cool, deep waters did not undergo mass extinctions, suggesting that the periodic cooling of the surface waters may have caused the extinctions. Nautiloids, which evolved in the Late Cambrian, underwent adaptive radiation in the Late Cambrian Period, followed by a mass extinction at the end of the Cambrian Period. Nautiloids have chambers in their shell which contain gas used for buoyancy control as they swim. Stromatolites (that first appeared during the Archean) remained abundant throughout the Cambrian Period. The phytoplankton Acritarchs (the earliest preserved eukaryotes that first appeared in the Proterozoic and suffer a mass extinction at the end of the Proterozoic Eon) were also abundant during the Cambrian Period. The zooplankton radiolarian and foraminifera plankton evolved and produced siliceous and calcium carbonate shells in deep sea sediments.
During the Ordovician Period, stromatolites began to decrease in abundance due to the evolution of animals that graze on algae mats. The mass extinction of trilobites at the end of the Cambrian Period began the decline in overall abundance of trilobites in the Ordovician Period. Adaptive radiation of other groups of organisms evolved many new species of phyla that first appeared during the Cambrian Period. Reefs were composed of bryozoans, rugose and tabulate corals, and stromatoporoids. Brachiopods, graptolites, and conodonts evolved rapidly and provide important index fossils for the Ordovician Period. The bryozoans or moss animals (phylum Bryozoa) which live on the sea floor, first appeared during the Ordovician Period as did sea urchins and starfishes) (phylum Echinodermata). Tabulate corals (phylum Coelenterata) first appeared in the Ordovician.
There were about 400 known families of marine invertebrate families at the end of the Ordovician Period as compared to 150 known fauna at the end of the Cambrian Period. A great adaptive radiation occurred in the Ordovician. Since that time the number of marine invertebrate families of marine animals have remained about the same through the end of the Paleozoic Era. This may be due to (1) filling environments to the point that it is difficult for new life to evolve, (2) evolution of effective predators limited the development of new life, and (3) animals became too specialized to easily evolve into totally new different species.
The mass extinction which ended the Ordovician Period wiped out about 100 of the families of marine animals; however, these were replaced in the Silurian Period.
Non-seed bearing plants (spore-releasing mosses) probably first appeared on land in moist environments during the late Ordovician Period. However, land plants did not become abundant until the Silurian Period.
The large mass extinction at the end of the Ordovician was probably due to cooling from glaciation on Gondwanaland.
Trilobites failed to recover fully after the mass extinction at the end of the Ordovician, becoming less abundant. Stromatolites continued their decrease in abundance. In general, the other groups of animals recovered by undergoing adaptive radiation. Acritarchs were the dominant phytoplankton. Mollusks and brachiopods were abundant as were bryozoans and graptolites. Reefs were abundant, composed primarily of rugose and tabulate corals and stromatoporoids together with bryozoans. The dominant reef-building organisms were the tabulate corals and the stromatoporoids, resulting in the name "tabulate-strome" reefs.
Ammonoids evolved from nautiloids in Devonian time. Relative to nautiloids, the septa separating the internal chambers of ammonoids are crenulated which makes the chamber walls stronger. Hence the shells can be made thinner and lighter. The ammonoids generally had a curved shape which produced better balance as the organism reached out to grab prey.
Fish moved from marine environments into fresh-water environments during the Silurian. The jawless ostracoderms evolved in Silurian time but became extinct by the end of the Devonian. These ostracoderms still had bony plates rather than scales and lacked paired fins. Fishes with jaws (acanthodians, jaws evolved from gill supports and teeth evolved from the scales on the gill supports) evolved in Late Silurian. The acanthodians were the first fish to have paired fins and scales covering their bodies. They underwent adaptive radiation in the Devonian which is known as the "Age of the Fishes". Placoderms were armored fish (with bone over much of their bodies) that evolved in the Late Silurian and were the dominant predators during the Devonian. They were too slow and eventually became extinct during the Early Mississippian.
Eurypterid arthropods (scorpion-like relatives of spiders) were common and lived in fresh and brackish waters. Eurypterids appeared on land in Late Silurian time.
Cartilage fish such as sharks evolved in the Early Devonian. Bony fish such as the ray-finned fish and the lobe-finned fish also evolved in the Devonian. The ray-finned fish have radiating bones from the body supporting the fins; whereas, the lobed-finned fish have a single bone attaching the fins to their body. Ray-finned fish are the first modern-type fish. The primitive ray-finned fish lacked a symmetrical tail and their scales did not overlap. Lobe-finned fish declined after the Devonian but are still present today as a few living fossil species. The lobe-finned fish have secondary lungs, allowing them to live in shallow-water environments as they dry up. The lungs evolved from air sacks which fish used to maintain buoyancy in water. The lobe-finned fish appear to have been the ancestors of amphibians which evolved soon after. The two pairs of legs of amphibians apparently evolved from the single bone supports of ventral fins of which there are two pairs on the lobe-finned fish. Perhaps, amphibians, reptiles, dinosaurs, birds, and mammals would have had more than two pairs of appendages if lobe-finned fish had possessed more pairs of ventral fins.
The ammonoids, eurypterids, and armored fish were predators which helped in the decline of trilobites and other defenseless organisms (including the ostracoderms).
Plant Invasion of Land
Spore-bearing (nonvascular plants) probably first appeared on land in the late Ordovician Period and became common in the Silurian Period. The development of vascular tissue, two sets of tissue (one to carry water and nutrients and another to carry manufactured food) was needed to evolve plants with roots and leaves. Vascular spore-bearing plants evolved in late Silurian, and into male and female plants in mid-Devonian (e.g., the lycopods). Spore-bearing plants require a moist environment for the fertilization process. The first forests were of spore-bearing plants in late Devonian. The great swamps of the Pennsylvanian that produced the coal beds were of lycopods. Seed-bearing plants (gymnosperms) evolved from the male and female spore-bearing plants in the late Devonian and do not require a moist environment because fertilization is internal. Dry land was invaded successfully by gymnosperms to form forests in the Late Devonian. Note that advanced seed plants with flowers (angiosperms) did not evolve until the Cretaceous Period.
More on Animal Invasion of Land
Arthropods (euryptids, relatives of spiders) had invaded land in the late Silurian Period; however, vertebrate animals (Phylum Chordata), such as amphibians (frogs, toads, salamanders), did not invade land until late Devonian. The amphibians are thought to have evolved from lobe-finned fishes (have paired fleshy fins and the ability to breath air) due to adaptive radiation on land to fill an environment without competition. Note that the amphibians evolved after vascular plants. Like the spore-bearing plants, they required a moist environment, because amphibians hatch from eggs in water and have an aquatic juvenile stage.
More on Reefs
Remember that reefs composed of archaeocyathids and calcareous algae were common in the early Cambrian. Although stromatoporoids and rugose corals evolved in the Cambrian, reefs were uncommon until the middle Ordovician when reefs of bryozoan, rugose and tabulate corals, calcareous algae, and stromatoporoids were common until the end of the Devonian. In the Devonian Period, reefs were particularly abundant along the western continental shelf of Laurasia (western Canada) and along the margin of Gondwanaland corresponding to the northwestern margin of Australia. These must have been warm-water areas. The Australian reefs are unusual in that they are built not only of corals and stromatoporoids, but also stromatolites. Predators in this region were not effective in destroying the cyanobacteria that built the stromatolites.
A mass extinction occurred during late Devonian time that affected marine organisms living in warm, shallow waters or surface waters, e.g., trilobites, gastropods, brachiopods, corals, stromatoporoids, acritarchs, ammonites, rugose corals, tabulate corals, stromatoporoids, and Devonian fish. All colonial species of rugose and tabulate corals became extinct. The acritarchs were hard hit in the Late Devonian extinction and never recovered after the end of the Devonian to become as abundant. Organisms living in colder water were unaffected. The extinction must be related to glaciation and global cooling of the surface ocean that occurred during the late Devonian. The decline in reef-building organisms meant that reefs almost ceased to be built at the end of the Devonian. Reefs were not important for the rest of the Paleozoic.
Vascular plants on land were not affected by the late Devonian mass extinction.
Fish and other families of marine life became more mobile (faster swimmers) after the Devionian Period during the late Paleozoic, resulting in the decline of heavily-armored animals such as the armored fishes and heavy-shelled nautiloids. These heavier predators couldn't catch their faster prey and gradually became extinct. The armored placoderms became totally extinct in early Mississippian time. Similarly, the heavier prey were slower, falling victim to faster predators. There was a major extinction of ammonoids at the end of the Mississippian Period.
Trilobites finally became extinct at the end of the Paleozoic Era. The acritarchs (phytoplankton), tabulates (corals), and stromatoporoids (sponges) persisted but did not re-expand after the Devonian extinction. Both tabulate and rugose corals became extinct at the end of the Paleozoic Era. While the bryozoans were reduced at the mass extinction at the end of the Paleozoic, they persisted into the Mesozoic and Cenozoic. The general absence of reef-building structures such as the tabulates and the stromatoporoids is reflected in the general absence of reefs in the late Paleozoic. Reefs did not become abundant again until the Triassic Period when the modern Hexacorals evolved, although low reef mounds were built out of bryozoans (lacy bryozoan), calcareous sponges, and calcareous algae (red algae) in the late Paleozoic, e.g., El Capitan reef in southwest Texas.
Brachiopods, gastropods, and crinoids flourished during the late Paleozoic. In particular, the crinoids underwent adaptive radiation in Mississippian Period, before being diminished by mass extinctions at the end of Mississippian time. Benthic fusulinds (foraminifera with calcium carbonate shells), living on the shallow sea floor, expanded in Pennsylvanian (late Carboniferous) and Permian time before becoming extinct at the end of the Paleozoic Era. Limestones in the Mississippian were frequently composed of crinoids; whereas those in Pennsylvanian and Permian time were frequently composed of fusulinds.
The belemnoids probably evolved from the ammonoids in the Pennsylvanian Period (?). They lack an external skeleton and had an internal weight attached to an air sac which was used to control buoyancy.
Terrestrial Plants
Lycopods, spore-bearing trees, were the predominant trees in the Mississippian and Pennsylvanian Periods (Carboniferous); however, cordaites (primitive seed-bearing gymnosperms) trees were abundant in non-moist habitats in the Pennsylvanian as were seed ferns, spore ferns, and other spore plants called sphenopsids. A modern sphenopsid is the horsetail. Fossils of Glossopteris, the seed fern, were abundant in Gondwanaland. Remember, these were used by Wegener to argue for continental movement). By the end of the late Paleozoic, the remaining lycopods and sphenopsids were much smaller and less abundant. The cordaites were totally extinct. Other gymnosperms such as the conifers evolved and predominated in the Permian Period. The gymnosperms subsequently prevailed throughout much of the Mesozoic: the Triassic Period, Jurassic Period, and early Cretaceous Period, until the appearance of the (flower-bearing) angiosperms.
Terrestrial Fish
Ray-finned fishes, sharks and mollusks expanded into fresh-water habitats.
Terrestrial Insects
During the late Paleozoic, insects (phylum Arthropoda) became abundant, having first appeared in early Devonian. Winged insects had evolved wings which in general were unfoldable (like dragonflies) by Early Pennsylvanian time. They had probably evolved in the Mississippian but the fossil record is poor. Insects with foldable wings evolved in late Pennsylvanian and became common in the Permian Period.
Terrestrial Amphibians and Reptiles
Amphibians dominated terrestrial habitats during the Carboniferous time before being replaced by terrestrial reptile groups in the Permian Period. Amphibians were the only vertebrates in the Mississippian (early Carboniferous) Period. The amphibians required a moist environment and lacked teeth capable of tearing up flesh, necessitating swallowing an entire organism. Reptiles evolved from amphibians in the Pennsylvanian Period and became common in the Permian Period. They had the advantage of reproduction with eggs, allowing them to live in non-moist environments. The "amniote" egg of reptiles (and also birds which evolved in the Mesozoic, probably from the dinosaurs) contains two sacs in a nutritious yoke; one for the embryo and one for body wastes, and is covered with a shell. Later reptiles also developed another advantage over amphibians, they evolved a jaw which could apply pressure on food, capable of tearing food into smaller chunks. Eventually, the mammal-like reptiles developed different teeth for chopping as well as tearing food.
By Permian time, mammal-like reptiles became the dominant large animals. The primitive mammal-like (cold-blooded or ectothermic) pelycosaurs, or finback reptiles, dominated in the early Permian Period and the partly (?) warm-blooded or endothermic therapsids dominated during the late Permian Period. The therapsids had their feet positioned more vertically beneath the body than other reptiles (better for balance and increased mobility). Warm-blooded animals have greater endurance than cold-blooded animals, an inherent advantage for both predator and prey. Mammals are thought to have evolved from the therapsids in the early Mesozoic. It is somewhat surprising that dinosaurs (classed as reptiles), rather than mammals became the dominant large animals of the Mesozoic Era. Perhaps the dinosaurs were also warm-blooded, explaining their effective competition with mammals. The dinosaurs did not evolve until the Triassic Period and they evolved from the thecodonts, a reptile group that also didn't evolve until the Triassic.
Limestones were commonly composed of crinoids during the Mississippian Period and of foraminifera fusulinds during the Pennsylvanian and Permian.
At the separation between the Mississippian and Pennsylvanian Periods (early and late Carboniferous), sea level fell and a mass extinction occurred. Presumably, an expansion in glaciation lowered sea level and cooled the surface water of the oceans, causing the extinction. Crinoid and ammonoid species were particularly reduced in this mass extinction. There does not seem to have been a major mass extinction between the Pennsylvanian and the Permian.
During the Permian Period, the climate was much drier than during the Carboniferous. Evaporites were deposited and plants flourished that could survive under drier conditions.
The mass extinction at the end of the Permian Period was the largest mass extinction in the history of the earth. It coincided with a drop in sea level and probably global cooling of the surface sea water. Although the period of glaciation near the South Pole on Gondwanaland was on the decline, the presence of the large land mass of Siberia near the North Pole, added to the overall global glaciation. On land, most of the therapsids, the mammal-like reptiles, failed to survive into the Triassic Period. In the marine world, all species of the shallow-water foraminifera fusulinds, the remaining genera of rugose and tabulate corals, and the trilobites became extinct. Only a few species of ammonoids survived, and there were major losses of species of brachiopods, bryozoans, and stalked echinoderms such as crinoids. Of the bryozoans that were reef builders, the lacy bryozoans became extinct. Bivalve mollusks and gastropods were eliminated in lesser amounts.
Cretaceous, 65 to 135 m.y. ago, named after chalk clifts in England
Jurassic 135 to 208 m.y. ago, named after the Jura mountains in France
Triassic 203 to 245 m.y. ago, named after a depositional basin in Germany
The similar fossil record of terrestrial animals confirms that Pangea existed as a single supercontinent throughout early and middle Triassic. Pangaea began to break apart at the end of the Triassic Period into Laurasia and Gondwanaland; however, the breakup was minimal at the end of the Triassic Period. The breakup began between Laurasia (in Eurasia) and Gondwanaland (in Africa), forming the Tethys Sea in the region of the present Mediterranean Sea in the late Triassic Period. Note that Eurasia did not include the Indian subcontinent which was still attached to Gondwanaland. This rifting produced a narrow seaway which contains Triassic-age evaporites (off the present northwestern coast of Africa). The evaporites formed from the limited sea-water circulation during early rifting.
During Jurassic time, the rifting zone extended between North America and Africa and then westward through the Gulf Coast region between North and South America. Note that the rifting did not continue to the point of separation of South America and Africa until the Cretaceous Period. The rifting between North America and Africa followed closely the old suturing region of the Allegheny (also called in Europe the Hercynian or Variscan) orogeny. An extension of the rifting to the west in the Gulf Coast region connected with the Pacific Ocean. Jurassic-age evaporites accumulated along the rift and were prominent in two regions: the area between Morocco and Nova Scotia where northwestern Africa and southeastern Canada were splitting and in the Gulf Coast Region. Jurassic-age evaporites in the Gulf Coast formed as the result of restricted sea-water circulation coming out of the west from the Pacific Ocean and out of the northeast from the Tethys Sea. These Jurassic-age salt deposits form the Louann Salt which underlies Louisiana and is exposed in the salt domes which have pierced the overlying sediments, as at Avery and Weeks Islands. Much later, Cretaceous age evaporites also formed between South America and Africa when the zone of rifting extended southward in early Cretaceous time.
Associated with the evaporite deposits forming with the rifting of a continent are red-bed deposits of continental sediments. These red beds are sediments which have been rapidly eroded and deposited in depositional basins (formed by normal faulting) along the edge of the rift. The thick red-bed sediments have not been weathered enough to remove unstable minerals and can be easily recognized in the geologic record. They are very characteristic of the early stages of rifting of a continent. The Gulf Coast red beds that coincide with the Louann Salt form the Eagle Mills Formation. The Newark Supergroup along the east coast of the United States is composed of red beds associated with the late Triassic and Early Jurassic rifting between North America and Africa, for which the associated salt deposits occur off the northwestern African coast.
The west coast of North America was built up by the accretion of exotic terranes such as that which occurred during the Sonoma orogeny in Permian-Triassic time. The Sonoma orogeny sutured an exotic terrane (Sonomia with the attached Golconda Arc) and the accretionary wedge of sediment separating the Golconda Arc from the continent (the Golconda terrain) to the west coast of North America. The suturing occurred due to the destruction of ocean floor in a subduction zone between Sonomia and North America in which the subduction zone dipped westward under the island arc. As the continent approached and reached the subduction zone (as the intervening oceanic crust was destroyed), subduction stopped and the island arc and wedge of sediment were sutured onto the west coast of North America.
Subduction zones later developed in the Triassic that dipped eastward under the west coast of North America, producing mountains equivalent to the Andes today along the western margin of the continent. The Sierra Nevada Mountains formed as the result of an easterly-dipping subduction zone in Late Jurassic in the Nevadan Orogeny. The Franciscan Formation, underlying San Francisco, was an accretionary wedge of deep-water sediment overlying this subduction zone near the North American margin.
The Great Valley sediments east of the Franciscan Formation are marine sediments from the Forearc Basin. The folded mountains in Nevada represent the fold and thrust belt of this same orogeny, and the Morrison Formation in Utah and western Colorado represents the Foreland Basin or Backyarc Basin molasse deposits forming associated with the Sundance Sea.
In Europe during the Triassic, the deposition (mostly non-marine) consisted of three divisions for which the name "Triassic" originated. The sequences were the Bunter (lowest, non-marine silic-clastics); the Muschelkalk (middle, marine mussel limestone); and the Keuper (upper, non-marine). During the Jurassic, the deposition was again in three divisions: the Liassic (lower, black shales); Dogger (middle, brown iron-rich sandstones and limestones); and the Malm (upper, white limestones). The Solnhofen Limestone in which the first bird fossils were found is part of the Malm.
In the Triassic, Pangaea spanned the equator, ranging from the South Pole (Australia in Gondwanaland) to the North Pole (Siberia in Laurasia) with the Gulf Coast region and North Africa on the equator. The interior of the continent was dry because of great distances from water. Little of the interior of the continent was flooded due to low sea level. The ocean to the east of Pangaea was called the Tethys Sea and the ocean to the west was the Pacific Ocean. The fauna could be split into a tropical flora called the Euroamerican flora and two cold floras, the Siberian and Gondwanaland floras. Evaporite deposition in Early Triassic was unrelated to rifting but due to arid climate in the center of Pangaea in the equatorial regions. The continent was just so large, that its interior climate was arid. The mass extinction at the end of the Triassic appears to be unrelated to glaciation.
Sea level was low at the beginning of the Triassic Period (remember there was a time of major glaciation at the end of the Permian Period) and gradually rose throughout the Triassic, Jurassic, and Cretaceous Periods. Sea level was high in Late Jurassic, flooding most of the interior of the continents, e.g., the Sundance Sea in the western United States was one of these epicontinental seas. The marine organisms living near the northern continents lived in cooler waters (the Boreal realm) than those living near the southern continents (the Tethy realm). In particular, coral reefs characterize the Tethy realm in the late Jurassic Period, suggesting a tropical climate. The high sea level corresponded to a lack of glaciation and gentle surface temperature gradients with latitude, i.e., wasn't as cold near the poles, as today.
The fragmented continents of Pangea were still clustered together at the start of the Cretaceous Period. North America had separated from Eurasia along its southern margin by the ancestral opening of the Atlantic Ocean; however, the two continents were still attached to the north through Greenland. North America and South America were separated by a narrow extension of the Tethys Sea which also narrowly separated Eurasia from Africa. The Tethys Sea was oriented as an east-west warm-water passageway between the continents which had originally formed Laurasia and those that had originally formed Gondwanaland. Evaporites formed along the margin of the Tethys Sea during the Early Cretaceous. South America was still connected to Africa. Antarctica, Australia, and India were still attached together and possibly to Africa.
The rifting continued during Cretaceous time, connecting the Arctic and North Atlantic Oceans through rifts on both sides of Greenland, splitting South America from Africa to form the South Atlantic Ocean, and splitting Australia, India, and Antarctica together away from Africa. At the end of the Cretaceous, India separated from the Antarctica plate and moved northward towards Eurasia. Not until the Eocene Epoch in the Paleogene Period did Australia (on the same plate as India) separate from Antarctica.
Sea level rose to its maximum level during the Cretaceous period. It had been rising since the end of the last previous major Ice Age at the end of Permian time. Warm temperatures during the Cretaceous may have been due to excess volcanic activity, releasing carbon dioxide to promote a greenhouse effect. High sea level flooded the interiors of continents with warm-water seas. These included the Mowry Sea in Early Cretaceous time and the Cretaceous Interior Seaway in Late Cretaceous time in western North America. In Europe, the Chalk Sea covered much of North Sea area during Late Cretaceous time. The marine deposits from these interior seas are exposed today, providing an excellent fossil record for Cretaceous time. Since the end of the Cretaceous time, sea level has gradually fallen.
The warm seas during the Cretaceous Period helped produce anoxic (low oxygen) waters in the oceans. Cold surface waters have a high density and will sink to ocean depths. This occurs today in the polar regions of the oceans. The cold sinking waters are oxygenated and bring oxygen to the deep ocean. These waters flow along the bottoms of the oceans and are eventually returned to the surface waters by upwelling. The absence of this cycle during the Cretaceous helped produce anoxic ocean waters during the Cretaceous. Even the shallow epicontinental seas were sometimes anoxic, resulting in the deposition of organic-rich black shales, e.g., the black shales deposited in the Mowry Sea during Early Cretaceous time in western North America. The sinking of cold surface waters in the oceans did not reoccur until the Eocene Epoch in the Paleogene Period.
During Jurassic time, orogenic activity (mountain building) was occurring along the western edge of North America in the Nevada Orogeny, forming the Sierra Nevada Mountains. During the Cretaceous time, the orogenic activity shifted eastward, inland from the western margin of the coast. The Cordilleran Mountain Belt, east of California and Washington and Oregon, was forming in the Sevier Orogeny as a result of a shallow-dipping subduction zone, dipping to the east under the western edge of the continental margin. The shallow dip pushed the mountain building eastward, because magma could only be generated where the subduction zone had reached sufficient depth within the mantle. The elevation of the Rocky Mountains began in Late Cretaceous time in the Laramide Orogeny but reached its climax in the Cenozoic Era.
The concept of explaining mountain building in the interior of a continent by a shallow-dipping subduction zone extending from the continental margin, is not (to your instructor) a satisfactory explanation; however, it is the only explanation consistent with plate tectonics. The explanation for the Laramide Orogeny, forming the Rocky Mountains in Colorado, is particularly weak. Colorado is so far eastward from the western continental margin, that the subduction zone would have to dip eastward to the base of the continental crust, run horizontal for a thousand kilometers, and then dip downward into the mantle.
The most important marine invertebrates during the Triassic Period were the bivalve and gastropod mollusks which had been the groups least affected by the mass extinction at the end of the Permian Period. Sea urchins expanded in the Triassic and the Jurassic Periods. Hexacorals (modern reef corals) evolved in the Triassic but were much more abundant in the Jurassic Period. The symbiotic relationship of hexacorals with dinoflagellates did not develop until the Jurassic Period. Some of the Triassic coral reefs formed in deeper waters than later coral reefs did, because later reefs have dinoflagellates which require photosynthesis, i.e., must be in the photic zone or shallow-water zone.
During the Triassic Period, the belemnoids (which evolved in the late Paleozoic) became common (more abundant in the Jurassic Period). The belemnoids were squidlike cousins of the ammonoids and nautiloids and lacked an external shell. Instead, they had an internal gas sac for buoyancy with a stone weight which acted as a counterbalance. The stone weight had a bullet shape and often looks (as a fossil) like a large caliber shell, e.g., a 50 caliber shell.
The primitive ray-finned (bony) fish had scales; however, they didn't overlap completely as they do on modern fish. They developed a swim bladder (gas-filled bladder) from a residual lung (the same lung earlier passed on to amphibians), to help regulate buoyancy. Unlike modern bony fish, their bones were partly made of cartilage and their tails were asymmetrical (characteristics inherited from Paleozoic bony fishes). Some of the fish had rounded pegs for crushing rather than sharp teeth.
The primitive shark group, the hybodonts, were prominent in the Triassic seas and had rounded pegs for teeth for crushing.
One reptile branch returned to the sea: the marine swimming reptiles (our sea monsters) evolved and included those with armored turtle-like bodies (placodonts), not to be confused with the placoderms or armored Devonian fishes; with paddlelike limbs (nothosaurs); with winglike or whalelike limbs (plesiosaurs); and the fishlike reptiles (ichthyosaurs). The placodonts and nothosaurs became extinct by the end of the Triassic Period.
The plesiosaurs evolved from the nothosaurs in Triassic time and survived until the end of the Cretaceous. The ichthyosaurs also evolved in the Triassic and survived until the end of the Cretaceous. Terrestrial crocodiles evolved in the Triassic and developed marine forms in the Jurassic.
Other (land-water) reptile groups evolving in the Triassic were turtles and crocodiles. The amphibian group of frogs dates back to Early Triassic.
During the Late Triassic, small rodent-like mammals evolved from the therapsids (mammal-like reptiles) and the dinosaurs and flying reptiles (pterosaurs which lacked feathers) evolved from the thecodonts (a reptile group evolved in the Early Triassic which was commonly bipedal). The thecodonts became extinct at the end of the Triassic Period. The thecodonts were the dominant vertebrates in the Triassic.
Active flying (i.e., wing flapping) pterosaurs probably had to be warm-blooded in order to maintain the energy requirements of flying - otherwise, cold weather would ground them. Species that did mostly gliding, rather than flying were probably cold-blooded. The early dinosaurs were bipeds and more agile than the mammals which apparently led to their dominance over mammals. They may not have been cold-blooded, but warm-blooded? There were two types of dinosaurs: (1) bird-hipped herbivore dinosaurs and (2) lizard-hipped herbivore and carnivore dinosaurs. Dinosaurs became much more abundant in the Jurassic.
The mass extinction at the end of the Permian Period did not affect the plants to the extent of the animals. The gradual decline of the spore-bearing plants had already begun in the Permian Period. Ferns and seed ferns were abundant in the Triassic Period. Gymnosperms continued their dominance of plants, dating back to the Permian Period. The major groups were the cycads, the cycadeoids (close extinct relatives of the cycads), the conifers, and the ginkgos.
A major extinction occurred at the end of the Triassic Period, eliminating all species of the conodonts, placodonts, nothosaurs, thecodonts and most species of bivalves, ammonoids, plesiosaurs, ichthyosaurs, large amphibians, and therapsids and other mammal-like reptiles. The resulting void in large terrestrial species allowed the dinosaurs to undergo adaptive radiation.
During the Jurassic Period, the dinoflagellates (phytoplankton) underwent adaptive radiation. They replaced the acritarchs which were last important during the Devonian. Calcareous nannoplankton (phytoplankton) first evolved in early Jurassic and began to form chalks. They later became much more abundant in the Cretaceous. Globigerina foraminifera (zooplankton) also evolved in the Jurassic. Radiolarians (zooplankton) had evolved earlier in the Cambrian and formed siliceous-rich radiolarites. Note that radiolarians are not diatoms which are phytoplankton having siliceous shells that evolved in the Cretaceous.
The bivalves, ammonoids, plesiosaurs, and ichthyosaurs recovered from the end of the Triassic mass extinction and become abundant. Gastropods, belemnoids, ray-finned fish, hybodont sharks were abundant in the marine environment. Modern mackerel and tiger shark families evolved in the Jurassic Period. Some crocodiles became marine in the Jurassic Period.
Hexacoral reefs were abundant and mark the Jurassic Period as a time of major reef building. Remember that the symbiotic relationship with dinoflagellates, which began in the Jurassic, involves the dinoflagellates serving as a source of food and removing carbon dioxide from the coral polyps.
The dinosaurs expanded to fill up the void due to the elimination of thecodonts, and most large species of therapsids and other reptiles, and amphibians. Pterosaurs were abundant.
The bird-hipped dinosaurs developed into two herbivore groups: bipedal varieties such as ornithopods and quadrupedal varieties such as stegosaurs, e.g., the Jurassic-age Stegosaurus and the ceratopsians, e.g., the Cretaceous-age Triceratops. The lizard-hipped dinosaurs included both herbivores (sauropods such as Jurassic-age Brontosaurus) and carnivores (theropods such as the Cretaceous-age Tyrannosaurus). The large sauropods were generally quadrupeds because of their weight; whereas, the more nimble theropods were generally bipeds. The Morrison Formation in the western United States is famous for containing dinosaur bones. The Morrison Formation formed from the previously mentioned epicontinental Sundance Sea.
The earliest known fossils of birds are from the Late Jurassic (Archaeopteryx) Solnhofen limestone in Germany; however, some fossils from the Late Triassic may actually be of birds? Birds probably evolved from the lizard-hipped dinosaurs or possibly from thecodonts but not from the flying reptiles (pterosaurs).
Mammals were small and characterized by rodent-like creatures.
The plants continued to be dominated by ferns and the gymnosperm groups: the cycad, cycadeoids, conifers, and ginkgos; however, the cycads were the major plant group in the Jurassic Period.
There was not a major extinction at the end of the Jurassic Period, however, the Stegasauria dinosaurs and the larger sauropods did not pass through to the Cretaceous Period. Also the therapsids died out during the Jurassic Period.
Several groups of phytoplankton became very abundant: the dinoflagellates (without shells), the calcareous nannoplankton and globigerina foraminifera with their calcareous shells and the diatoms with their siliceous shells.
The abundant Cretaceous chalks were formed from calcareous nannoplankton (phytoplankton) shells and globigerina foraminifera (zooplankton) shells, often preserved in marine deposits of shallow epicontinental seas on the underlying continental rocks. They are thus exposed today as chalk deposits in the interior of continents. Remember that calcareous nannoplankton (phytoplankton) and globigerina foraminifera (zooplankton) had previously evolved in the early Jurassic. Diatoms (phytoplankton), which have shells of silica, evolved in the Cretaceous. Radiolarians (zooplankton), which also have shells of silica, evolved back into the Cambrian but did not become abundant until the Mesozoic.
Ammonoids and belemnoids were common during the Cretaceous; however, they would become extinct at the end of the Cretaceous Period.
Sea floor predators became more efficient. Crabs developed with crushing claws, e.g., the brachyuran crabs, and some gastropods developed the ability to drill through shells. The result was that benthic organisms were forced to evolve into forms that could swim or burrow actively, e.g., burrowing bivalve mollusks, or to develop heavy protective shells, e.g., gastropods. The efficient predation resulted in the decline of brachiopods and crinoid species, which have not become abundant again. However, bryozoans and benthic foraminifera species expanded during the Cretaceous Period.
Sea grass (a marine plant, not a true grass) evolved in the Cretaceous.
Bivalved mollusks evolved species of enormous sizes. Large coiled oysters existed. Rudists evolved in the Cretaceous and grew to enormous sizes, a meter in length. They looked like garbage cans with a huge lower lid and a small upper lid and were stacked on top of each other to form reefs. Rudists are thought to have had the same symbiotic relationship with dinoflagellates as did the hexacorals. Rudists became extinct at the end of the Cretaceous.
Hexacorals and calcareous algae (coralline algae) were present but were not the predominant reef-building organisms in Middle and Late Cretaceous. That position, which they had occupied in Jurassic and Early Cretaceous time, became filled by the rudists. However, when the rudists became extinct at the end of the Cretaceous, hexacorals became again the dominant reef-building organisms.
Modern ray-finned fish (bony) fish, the teleost fish, evolved with symmetrical tails, rounded scales, specialized fins, and short jaws. Modern sharks were present.
The top marine carnivores were still the marine reptiles: the fish-like reptiles called ichthyosaurs and the wing-limbed reptiles called plesiosaurs (both of which had evolved in the Triassic), and mosasaurs (marine lizards which evolved in the Cretaceous). These three groups of marine reptiles became extinct at the end of the Cretaceous.
Giant marine turtles were present, e.g., Archelon. Flightless diving birds had evolved, e.g., Hesperornis.
The cycads (gymnosperms), which had dominated land plants in Jurassic time, had been replaced by the conifers (gymnosperms) in Early and Middle Cretaceous. However, the flowering plants (angiosperms) evolved in Middle Cretaceous time and became the dominant plants by Late Cretaceous time. Today there are about 200,000 species of angiosperms and only about 550 species of conifer gymnosperms. The angiosperms have a shorter reproductive cycle than the gymnosperms, giving them an advantage in colonizing bare ground. In addition, the flowers of angiosperms attract insects, which provides an advantage in pollination and spreading seeds. Because insects often feed on flowering plants, the rapid adaptive radiation of angiosperms led to an adaptive radiation of insects. Grasses are angiosperms; however, they did not evolve until the Paleocene Epoch in the Cenozoic Eon.
The largest herbivores, the lizard-hipped sauropods, had died out in the Jurassic Period, as had the bird-hipped stegosaurs. During the Cretaceous Period, the bird-hipped, duck-billed dinosaurs were the abundant herbivores, forming great herds. The bird-hipped, rhinoceros-like triceratops were perhaps the last dinosaurs to become extinct at the end of the Cretaceous. The herbivores were eaten by the lizard-hipped carnivore dinosaurs called theropods, e.g., Tyrannosaurus. There was a general evolutionary trend in all dinosaurs towards larger body size (Cope's rule), although small dinosaurs existed. Pterosaurs were abundant of which the largest was Quetzalcoatus with a wingspread of 35 feet. The Cretaceous birds were probably shorebirds, equivalent to herons and cranes. The crocodiles approached the dinosaurs in size. Snakes evolved in the Cretaceous Period. The present-day constrictors, e.g., pythons, are descendants of the primitive snake groups.
Mammals remained small in size during the Cretaceous Period. They can be distinguished from reptiles in the fossil record on the basis of a single bone making up the lower jaw, greater complexity of cheek teeth, and a relatively larger brain case. Some important differences between mammal and reptiles include the following. Mammals stop growing as adults and have only two sets of teeth; however, reptiles continue to grow in size through life and their teeth are continuously replaced. Mammals are warm-blooded; whereas, reptiles are cold-blooded. Mammals are usually covered with hair for insulation, while reptiles have exposed skin or are covered with scales. Mammal young are usually born alive and nursed; whereas, reptiles usually lay eggs and do not nurse their young. The two major mammal groups, placentals and marsupials, evolved in the Late Cretaceous. The marsupials are especially diverse in Australia and the placentals are diverse everywhere else. Placentals nurture their young in the uterus through the placenta; whereas, marsupials bear their young at an early stage of development and transfer them to a pouch which contains the mother's teats.
Ammonoids, belemnoids (a few species may have survived into the Cenozoic?), dinosaurs, plesiosaurs, rudists, ichthyosaurs, mosasaurs underwent total extinction; however, the extinction was gradual through the end of the last 4 million years of Cretaceous time, called Maastrichian time. Calcareous nannoplankton suffered severe losses. A sharp pulse of extinction at the very end of the Cretaceous decreased planktonic foraminifera and seed-bearing plants, all of which later recovered in the Cenozoic Era. Plant species with smooth leaf margins (indicating tropical environments) underwent greater extinctions than those with jagged leaf margins (indicating cooler environments). This suggests gradual extinction due to a stressed environment followed by a major environmental shock. The rock record at the very end of the Cretaceous shows evidence of a bolide impact. Excess iridium with microspherules and shocked grains of quartz occur at the extinction boundary on all continents. A bolide impact would liquify rock creating microspherules as the liquid cooled and would have shocked quartz by the force of the impact. The high iridium could come from vaporization of the bolide which is rich in iridium. The impact would have thrown up all these components as dust in the atmosphere, allowing the earth's surface to cool and affecting planktonic and shallow marine organisms in tropical waters. There is a large crater near the Yucatan Peninsula which is thought to represent a large meteorite hit at the end of the Cretaceous Period.
Cenozoic Period Epoch Quaternary Holocene (Recent) 0.01 m.y. - present Pleistocene 1.75 - 0.01 m.y. Neogene Pliocene 5.3 - 1.75 m.y. Miocene 23.5 - 5.3 m.y. Paleogene Oligocene 33.7 to 23.5 m.y. Eocene 57 to 33.7 m.y. Paleocene 65 to 57 m.y.
Note that in the alternate classification of the Cenozoic Era, using the Tertiary and Quaternary Periods, the Tertiary Period includes all of the Paleogene and Neogene Epochs.
At the end of the Cretaceous, the Mid-Atlantic Rift separated Greenland from both North America and Europe in two rifts forking away from the single rift in the central Atlantic Ocean. In the Paleocene Epoch, the rift separating North America from Greenland became inactive. Greenland began to move with North America away from Europe. The rifting continued between Greenland and Scandinavia, opening a deep-water connection between the Arctic Ocean and the North Atlantic during Eocene time.
North America has remained connected to Eurasia through continental crust connecting Alaska with Siberia. At present, the Bering Sea, a shallow sea, overlies this continental crust. The Bering land bridge was generally open throughout the Paleogene Period. Only in the Neogene Period, has seawater frequently flooded this corridor.
Australia separated away from Antarctica at the end of the Eocene Epoch and began to move northward with India, as part of a single plate separated by oceanic crust, to its present position. Antarctica was already over the South Pole.
The average sea level, temperature, and rainfall dropped throughout the Paleogene Periods. Superimposed upon this trend were cycles of changes in temperature and sea level. The modern psychrosphere, cold deep ocean waters, originated at the end of the Eocene Epoch when Antarctica separated from Australia. The separation disrupted warm water currents flowing off the coast of Antarctica, resulting in forming cold, dense waters near coastal Antarctica (between Antarctica and Australia). The water sank and spread northward, as recorded by the extinction of benthic foraminifera. Another factor in the origin of the psychrosphere was the further opening of the Arctic Ocean to the North Atlantic by rifting between Greenland and Scandinavia. The frigid Arctic waters entered the North Atlantic and sank, spreading southward.
The climate trend was to more arid throughout the Paleogene Period. The Oligocene Epoch was marked by savannah (grasslands) rather than the semi-tropical wet conditions characteristic of the Eocene Epoch. Part of the increase in aridity was due to the general drop in sea level which increases the distance of the continental interiors from the oceans,
The Cannonball Sea occupied the central North American continent during the Paleocene Epoch. This sea did not completely withdraw from the present Gulf Coast region until the Oligocene Epoch. The thick sequence of Cenozoic sediments deposited in the Gulf Coast region are the source rocks (shales) and the reservoir rocks (sandstones) for much of the hydrocarbon production in this region.
The Laramide Orogeny, which had begun elevating the Rocky Mountains in Colorado at the end of the Cretaceous, persisted into Eocene time. This orogeny can only be explained in terms of plate tectonics by postulating a very shallow dipping subduction zone, extending eastward from the western margin of the continent. The orogeny was marked by uplifts of blocks of continental crust, e.g., Pike's Peak is an uplifted granite block near Colorado Springs as are the Black Hills of South Dakota. Volcanism was not common along the eastern margin of the Rocky Mountains. By the end of the Eocene, the Rockies had been largely eroded away. They later rose again as uplifted blocks of granite and crustal sediments in the Neogene Period.
The Laramide Orogeny ended when the Farallon Plate bordering California was totally destroyed in the eastward dipping subduction zone. In this area, the subduction zone was replaced with the San Andreas fault. However, remanents of the Farallon Plate still exist to the north along the coast of Oregon and Washington as the Juan de Fuca plate and to the south along the Mexican coast as the Cocos plate, and the Nazca Plate west of South America. These plate remanents are being destroyed in subductions zones while creating the magma forming the Cascades in the Pacific northwest, the Cordilleran mountains in Mexico and Central America, and the Andes in South America.
Along the continental margin in Washington, a subduction zone began to create the Olympic Range of volcanoes (near Seattle) during the Paleogene Period.
The oil shales in western United States in the Green River formation were laid down around the margins of lakes during Eocene time as the result of the trend towards a drier climate in the Paleogene.
The Turgai Strait, a seaway paralleling the eastern margin of the Ural Mountains served to isolate Asia from Europe during much of the Eocene and Oligocene.
The North Sea flooded central Europe during the Eocene and Oligocene Epochs. The major marine depositional basin centered on the Rhine Graben (a failed rift extending to the North Sea and underlying the present Rhine River valley).
The Alps were forming in Europe in the Paleogene Period (Eocene and Oligocene Epochs) as the result of a collision between the Italian Adriatic subcontinent and the Eurasian continent. The two land masses had been separated by the Penninitic Ocean which had formed as the result of Mesozoic rifting between them. Eurasia was moving south as the Penninic Ocean was destroyed in a subduction zone which dipped to the south near the northern margin of the Adriatic subcontinent. When the two land masses fused together, the sediment along the margins of the continents was pushed up and slid to the north to form the Western Alps of Switzerland and the Eastern Alps of Austria and slid to the south to form the Dolomites or Southern Alps of northern Italy. At about the same time, the Carpathian Mountains were forming by similar processes to the east as the result of fusion of continental fragments to southeastern Europe.
The Alps are formed by three large nappes, sediment layers that slide under the influence of gravity: the Pennides, the Austrides (other names are also used), and the Helvetides. The beds dip to the north in which the Pennides underlie the Austrides which underlie the Helvetides. The Pennides are exposed along the southern margin of the Alps (particularly in Switzerland), the Austrides are exposed further north (particularly in Austria), and the Helvetides are exposed along the northern margin of the Alps (particularly in Switzerland). The Alpine orogenic belt actually extends all the way to the Himalayas which were produced by the collision of the Indian subcontinent with Eurasia in the Neogene Period.
The Atlas mountains of North Africa and the Pyrenees Mountains also formed in the Eocene due to the closure of the two land masses of North Africa with Spain. This occurred earlier than the fusion of the Adriatic subcontinent with Eurasia. However, the formation of the Atlas Mountains, Pyrenees, Alps, Carpathians (Mountains in Eastern Europe), Caucasus Mountains (Between Black and Caspian Sea), and Himalayas form a line of mountain building due to closures of land masses against Eurasia.
The Atlantic Ocean continued to widen in the Neogene Period.
India's northward movement resulted in the subcontinent being sutured on to Asia in Middle Miocene time, creating the Himalayan Mountains which are still forming today. The movement of India was as part of the northward movement of the Australian plate which also contained both Australia and New Guinea and part of New Zealand. While India was suturing on to Asia, Australia and New Guinea were colliding with the Asian plate in the vicinity of Indonesia. The collision brought the distinctly different Australian biota into contact with the Eurasian biota by late Miocene time. The zone of contact of biota, where intermingling has occurred, is south of the Indonesian Island of Sulawesi (Celebes), and is called Wallace's line.
The present Andes have been forming since the Pliocene Epoch as the result of subduction along the western margin of South America.
The Tethys Sea gradually closed in the Miocene with the suturing of Africa with Arabia onto Eurasia (created Caucasus Mountains). This caused the Tethy Sea to separate into two branches without an outlet into the present Indian Ocean. The western branch became the Mediterranean Sea and the eastern branch, the Paratethy Sea, is represented today by the present Black Sea, Caspian Sea, and the Aral Sea.
During the Pliocene Epoch, isostatic uplift produced block faulting which elevated the Sierra Nevada Mountains in eastern California (originally formed in the Jurassic Period and subsequently eroded down) and during the Miocene Epoch, isostatic uplift raised the Rocky Mountains and the Colorado Plateau (both had originally formed in the Eocene Epoch and had been eroded down). A Miocene extension of rifting from the East Pacific Rise, running up the Gulf of California under the western United States, may have formed the Basin and Range Province of Nevada and the basaltic Columbia Plateau in eastern Washington and western Oregon. The present volcanoes of the coastal Cascades are Pliocene through Recent in age; however, volcanism, associated with the subduction zone along the western coast of northern California to northern Washington, has occurred throughout the Neogene Period. The San Andreas transform fault has been active since the Oligocene Epoch along the western boundary of southern California, connecting the East Pacific Rise in the Gulf of California with the subduction zone north of San Francisco that has produced the Cascades. The fact that the San Andreas has been active throughout the Neogene Period casts some doubt on a Miocene extension of the East Pacific Rise to form the Basin and Range Province and the Columbia Plateau.
The Appalachians were elevated in the Miocene Period by block faulting in response to isostatic uplift.
The initial forming of the present Andes corresponded to the forming of the Isthmus of Panama 3.5 m.y. ago, an event that isolated the Caribbean plate from the Pacific Plate by joining North America to South America. The isthmus of Panama was an island arc formed by subduction (dipping to the east). The occurrence of the Isthmus of Panama allowed plants and animals to migrate between the two continents. South America, like Australia, contained some interesting marsupials and also placentals which had developed since the continent had been isolated from Africa by the end of the Cretaceous Period. North America was not so isolated due to the Bering land bridge with Asia in the Paleogene Period. This same land bridge was sometimes present in the Neogene Period.
Africa was sutured to Eurasia by way of the Arabian Peninsula in early Miocene time, closing the eastern end of the Tethy Sea. At present, Africa is splitting away from the Arabian Peninsula with the opening (rifting) of the Red Sea and the Arabian Sea. This rifting began during the Miocene Epoch. A "dead" zone of rifting, the East African Rift Valley, connects with the active rifting at the intersection of the Red and Arabian Seas and extends southward downs the eastern margin of Africa.
The present Mediterranean Sea dates back to the Miocene Epoch with the closure of the eastern edge of the Tethys by the suturing of Africa and the Arabian Peninsula to Eurasia, creating the Caucasus Mountains. The sea then dried up between 6 and 5 m.y. ago, producing evaporite deposits, at the end of the Miocene Epoch as the result of closing of the narrow connection at Gibraltar with the Atlantic Ocean through a natural dam and the drop in sea level due to Antarctica glaciation known as the Messinian Event (discussed below).
The climate became drier and cooler throughout the Neogene Period.
Remember that sea level has gradually fallen since the end of the Cretaceous Period. At the end of the Miocene Epoch (about 5 m.y. ago), sea level apparently fell about 50 meters in the Messinian event as the result of major accumulation of glacial ice in Antarctica. Glaciation began about 3 m.y. ago, in the Pliocene Epoch and extended throughout the Pleistocene Epoch, causing sea level to fluctuate by more than 100 meters, i.e., dropping as ice accumulated and rising as ice melted in the interglacial periods. The Recent Epoch is an interglacial period that would probably lead to another ice age without the intervention of man through global warming produced by the greenhouse effect.
The Ice Ages began (as mentioned above) in the Pliocene Epoch about 3 m.y. ago, shortly after North America and South America were connected through the Isthmus of Panama (3.5 m.y. ago). This connection may have something to do with the beginning of the Ice Ages. The modern Gulf Stream (flowing clockwise between Europe and North America in the Atlantic) was strengthened (by the closure of the Panama seaway) and brought more warm water to the north Atlantic, resulting in more evaporation and consequently more snow to fall in the northern hemisphere. As snow accumulated and formed ice on land, the temperature was lowered through the reflection of solar heat (albedo effect) back to space.
Remember that there are several factors, besides ice, which have an effect on the earth's surface temperature. The cyclic nature of the earth's orbit affects the amount of solar heat reaching the earth's surface. An increase in atmospheric volcanic dust will lower the temperature by blocking solar radiation from the sun. An increase in atmospheric carbon dioxide will raise the temperature by preventing solar heat reflection. Carbon dioxide in the atmosphere should increase during an ice age as plants die back from the colder temperatures, helping to end an Ice Age. Remember that plants (in photosynthesis) use up the carbon dioxide produced by animals (in respiration).
The major centers of accumulation of glacial ice were on Antarctica, Northern North America, Greenland, and in Scandinavia. Continental ice sheets did not occur in South America, Australia, and Africa. Today we still have continental ice sheets on Antarctica and Greenland. The pulses of continental glaciation occurred about every 100,000 years with smaller cyclic variations every 20,000 to 25,000 years. These cycles often appear to conform to astronomical cycles corresponding to the 92,500 year cycle in the shape of the earth's orbit as it varies from elliptical to nearly circular and a 22,000 year cycle relative to the orientation of the earth's rotational axis as it varies where it points (the north end presently points at Polaris). There is also another cycle of about 41,000 years which reflects changes in the angle of the earth's rotational axis with the orbital plane of the earth, i.e., changing the position of true north in the sky. There does appear to be a relationship between the earth's astronomical cycles and the earth's surface temperature. However, something else began the sequence of Ice Ages because the Ice Ages have only occurred at infrequent times throughout the earth's history. The earth's surface temperature was only about 5oC (9oF) colder on the average, than today, during the periods of glaciation.
The time periods of glaciation have been recorded in the oxygen 18 to oxygen 16 ratio in the shells of marine life, preserved in deep sea sediment. The shells of globigerina foraminifera (zooplankton) have been used to date periods of glaciation as a function of age. The 18O/16O ratio increases in shells during times of glaciation on the continents. This is because the heavy oxygen 18 isotope remains preferentially behind in ocean water during the preferential removal of the lighter oxygen isotope in water vapor evaporated from the oceans. The glacial ice on land forms primarily from water vapor from the oceans, increasing the ratio of oxygen 18 to oxygen 16 in the ocean water.
Preserved pollen of terrestrial plants and different beetle species (found in fine-grained lacustrine sediments) have also been used to define the changes in climate on the continents during the Ice Ages.
The last major interval of glaciation in the Pleistocene Epoch is called the Wisconsin and it was separated by the second to last major interval of glaciation, the Illinoisan, by the Sangamon Interglacial Interval, 125,000 years ago. Both the Wisconsin and the Illinoisan Glacial Periods consisted of two pulses of glaciation.
Reefs did not become abundant until the Eocene Epoch. The extinction of the rudists together with depletion of species of hexacorals at the end of the Cretaceous resulted in few reefs forming during the Paleocene Epoch.
Calcareous nannoplankton recovered from the partial extinction at the end of the Cretaceous and together with diatoms and dinoflagellates were the dominate autotrophs in the oceans during the Paleogene Period. Note that diatoms and dinoflagellates were not affected by the mass extinction at the end of the Cretaceous.
Sand dollars (Echinoderms) evolved in the Eocene Epoch.
Whales evolved during the Eocene Epoch from carnivorous land mammals. The penguins or swimming birds also evolved in the Eocene Epoch. The top carnivores were the whales and enormous sharks. The pinnipeds which include walruses, sea lions, and seals probably evolved in Oligocene time, however, no Paleogene fossil record exist for these species.
Grasses evolved in the Paleocene and finally reached their present abundance at the end of the Oligocene. Grasses grow from the bottom of leaves rather than from the top. The early grasses did not have continuous growth of their leaves, thus they could not withstand heavy grazing by herbivores. Once they developed this ability, they spread rapidly to form grasslands. Subsequently, grasses evolved the addition of silica to their leaves to wear down the teeth of herbivores. Mammals have only one set of adult teeth. (The front teeth of rodents and rabbits are an exception.) Mammal herbivores' teeth have evolved by becoming taller (hight crown) with the addition of enamel and intricate dentine structure to become more resistant to abrasion. Grasses cannot depend upon insects for pollination because of the enormous number of individual plants. Instead, grasses depend upon wind pollination and asexual reproduction known as budding.
Adaptive radiation of mammals and birds occurred in the Paleocene to replace the void left by the extinction of the dinosaurs and pteropods (pterosaurs) at the end of the Cretaceous. The major groups were already present at the beginning of the Cenozoic. These included monotremes (egg-laying mammals) and marsupials and placentals. However, the dinosaurs had suppressed their diversification until htis point. The new mammals included ancestors of bats, primates, horses, cattle, rodents, and modern placental carnivores.
Horses and carnivores evolved in the Paleogene Epoch.
The modern hoofed herbivores began to appear in the Eocene Epoch, known as ungulates and are divided into odd-toed ungulates (perissodactyls e.g., horses, tapirs, and rhinos) and even-toed ungulates (artiodactyls, such as ruminants, e.g., such as cattle, sheep, goats, pigs, bisons, deer, camels). In the Eocene Epoch, the odd-toed ungulates outnumbered the even-toed ungulates; however, the situation was permanently reversed in the Oligocene Epoch. Odd-toed ungulates have only one toe or finger that has developed into a hoof; whereas, even-toed ungulates have two toes or fingers that have developed into a hoof. The evolution to hoofed animals was an adaption to gain speed.
Ancestors of elephants, dogs, cats, weasels evolved in the Eocene Epoch and underwent adaptive radiation in the Oligocene Epoch.
Huge flightless birds, the diatrymas, evolved in Eocene time but became extinct by the end of the Eocene. Most of the birds present were shore birds. Song birds did not evolve until the Neogene.
In Oligocene time, the largest land mammal to ever live on the surface of the earth evolved from the rhino family, the Indrichotherium. Also present in Oligocene time were the titanotheres, rhinolike animals with blunt horns rather than sharp ones. The Indrichotherium and titanotheres became extinct at the end of the Oligocene Epoch.
Monkeys and apelike primates evolved during the Oligocene Epoch. Earlier primate ancestors had evolved by the Eocene.
Reptiles and amphibians were relatively inconspicuous during the Paleogene Period. Modern frogs first appeared in the fossil record in the Eocene Epoch.
Mass Extinctions of the Late Paleogene Period
The decrease in smooth-margin leaf plants, relative to jagged-margin leaf plants implies that the temperature became colder during the Late Paleogene Period. Globigerina foraminifera suffered a major extinction by the end of the Eocene in which more species of the spiny (tropical environment) globigerina foraminifera became extinct than did species of the spineless (colder environment) globigerina foraminifera. Also many species of calcareous nannoplankton, which prefer tropical waters, became extinct during Late Paleogene time.
The colder temperatures occurred after early Eocene time in several pulses. The temperature remained cold in the Oligocene Epoch through the Neogene Period. This colder climate corresponded to the expansion of polar ice, particularly in Antarctica in the Oligocene Epoch and to the formation of the psychrosphere at the end of the Eocene. The mass extinctions during the Paleogene Period did not occur at the end of the Oligocene Epoch but occurred in about 5 pulses from Middle Eocene through Middle Oligocene. The formation of the psychrosphere (cold deep ocean water) resulted in the extinction of some benthic organisms, such as deep-water foraminifera.
Fresh-water diatoms (phytoplankton), which had evolved in the Paleogene, expanded for the first time in the Miocene Epoch, leaving their silica shells (which look like tiny pill boxes) in the non-marine rock record.
Remember that diatoms like colder waters. Thus the marine diatoms tended to expand while the calcareous nannoplankton (phytoplankton) retrenched when the marine surface waters became colder during the Ice Ages of the Pliocene and Pleistocene Epochs.
Globigerina foraminifera (zooplankton) expanded in the Miocene Epoch after their major extinction at the end of the Eocene. Interestingly, the new species resembled the extinct Eocene species, an example of iterative evolution, in which the basic body structure limits the direction of evolution, so that the organism may re-evolve along a similar (but not identical) path.
Calcareous algae or coralline algae evolved into forms that could cover the seaward edge of a reef with an algae reef, allowing coral reefs to grow along coasts pounded by heavy surf.
Modern whales underwent adaptive radiation in the Miocene Epoch, and dolphins (which are whales) first appeared in this epoch.
Herbs, which are non-woody plants that die back to the ground after releasing seeds, underwent adaptive radiation in the Neogene Period, in response to the continuing trend towards a drier climate that started in the late Paleogene. Note that grasses are non-woody plants with hollow jointed stems and narrow, sheathing leaves. Remember that grasses evolved in the Paleogene Period and first underwent adaptive radiation in Oligocene Epoch in response to the climatic trend towards drier conditions.
The trend in the Paleogene Period towards a drier climate continued in the Neogene Period through the end of the Pliocene Epoch, in which grasslands were favored at the expense of woodlands. The cooling trend in the Paleogene Period, leveled off in the Miocene and Pliocene Epochs with the climate only becoming slightly cooler until the Ice Ages in the Pleistocene Epoch.
The Neogene Period was characterized by adaptive radiation of frogs and toads (amphibians), snakes (reptiles), songbirds (birds), and rodents (mammals). The adaptive radiations can be related to the expansion of new species of grasses and herbs as the climate became drier. Rodents eat the plants and are in turn eaten by the snakes. Frogs and toads eat the insects which expanded to take advantage of new species of grasses and herbs. Songbirds evolved to eat insects feeding on the plants and the seeds of the plants.
The odd-toed ungulates have undergone a gradual decline in numbers of species during the Neogene Period. The even-toed ungulates continued to expand in the Miocene Epoch in response to the increase in grasslands but have since undergone a gradual decline in species. Carnivorous animals which fed on the ungulates underwent adaptive radiation in the Miocene Epoch as their food supply increased.
Elephant species were diverse during the Miocene and Pliocene Epochs, including the mastodons (with long jaws, tusks in both jaws, low-crowned teeth for browsing) and mammoths (with shorter jaws, tusks in only the upper jaw, and grinding teeth). Modern elephants evolved from the mammoths. Mammoths and mastodons only became extinct near the end of the Pleistocene Epoch (10,000 years ago), perhaps due to overhanging by early humans.
During the Pleistocene Epoch, the large size of the mammals may have evolved due to the cooler climate. The larger the animal, the smaller the heat loss because large animals have smaller surface area for their volume than do smaller animals.
The elevation of the Isthmus of Panama occurred about 3.5 m.y. ago during the Pliocene Epoch, allowing a great faunal exchange between the North America and South America. South American marsupials, e.g., opossum, and unusual placentals, e.g., anteaters and armadillos, moved northward. The North American species moving south included most modern placental animals.
Monkeys were present in the Oligocene Epoch. The Old World monkeys lived in Eurasia and Africa. The New World monkeys appeared in South America just before the end of the Oligocene Epoch, apparently crossing the Atlantic from Africa. Relative to Old World monkeys, New World monkeys possess prehensile (or grasping) tails, flatter faces, and wide, flat noses. An interesting and not well-known difference is that New World monkeys have wet noses and Old World monkeys have dry noses. Both Old World and New World monkeys underwent adaptive radiation in the Neogene Period. Apelike primates were also present in the Oligocene Epoch and flourished in the Miocene Epoch.
Apes separated from orangutans about 10-11 m.y. ago. Gorillas and chimpanzees separated from hominoids 5 to 7 m.y. ago,
Humans belong to the same superfamily, the Hominoidea, as do gibbons, orangutans, gorillas, and chimpanzees. The Hominidae family contains today only the single human species (Homo sapiens) and has a fossil record that extends back 7 m.y. (late Miocene). The members of this family are thought to represent evolution from living in trees to living in grasslands, requiring upright walking and higher intelligence to escape the fast ground-based predators. Most of the fossil record on Hominidae only goes back about 4 m.y. ago in the Pliocene Epoch and begins with the Australopithecus genus. Australopithecus anamensis fossils date from 4.2 to 3.9 m.y., Australopithecus afarensis dates from 3.8 to 2.9 m.y., and Australopithecus africanus dates from about 2.6 to 2.4 m.y. ago. Australopithecus species disappeared from the fossil record about 2.3 m.y. ago.
The Australopithecus species had a heavy brow, a low sloping facial region below the eyes (without a pronouced chin) and much smaller brains (about the size of a chimpanzee) than modern humans; however their pelvis was designed to support an upright body. Their teeth were intermediate between apes and humans. The fossil record of the Australopithecus species are confined to Africa as are all the early fossils in the Hominidae family..
The Homo genius was derived from one of the later Australopithecus species (probably Australopithecus africanus) and has fossil records extending back more than two million years. Most of the species of this genius also appear to have evolved in Africa. The oldest species was Homo habilis, also known as Homo ergaster, which had a relatively large brain, 650 to 800 cubic centimeters, as contrasted with the smaller brains of the Australopithecus and the much larger 1330 cubic centimeters average of modern humans. The evolution of a larger brain lies in continued growth of the brain cavity after birth. Other primates are born with a similar brain-size cavity at birth (relative to the total body size); however, their brain cavity growth is minimal after birth. Members of the Homo genius had the ability to make tools, a skill supposedly lacking in the earlier Australopithecus species; however, this may not have been the case.
Homo habilis had similar facial features as the Australopithecus species and eventually evolved into the larger Homo erectus about 1.6 m.y., about the time that it became extinct. Homo erectus had a larger brain size, ranging from 800 to 1300 cubic centimeters. They produced magnificent tools such as hand axes. Their tool culture is called Acheulian. The body structure of Homo erectus contains a narrow pelvis, suggesting endurance during locomotion. Their fossil record is extensive throughout Eurasia and Africa, extending to 300,000 years ago. Their fossils have been known by the terms: Java man, Peking man, and Pithecanthropus.
Modern humans (Homo sapiens) apparently evolved from Homo erectus. Our brain size ranges from 1200 to 1500 cubic centimeters and our pelvis is wider in order to allow for the birth of children with much larger heads than those of Homo erectus. We have sacrificed locomotion skills for brain power during evolution.
The modern human fossil record of Homo extends back about 100,000 years with Neandertal (Homo neandertalensis or Homo sapiens neandertalensis) and Cro-Magnon (Homo sapiens). Unfortunately, the poor fossil record between 400,000 and 100,000 years ago has obscured the evolutionary record of Homo sapiens from Homo erectus. Homo sapiens are thought to have originated about 300,000 years ago.
DNA evidence indicates Neandertals may have been a different species or a subspecies of modern humans. Neandertals partially overlapped with Cro-Magnon in Europe who arrived later by way of the Middle East. The extensive fossil record of Neanderthals in Eurasia suggests they evolved from Homo erectus in Europe; whereas, the early fossil record of Cro-Magnon suggests evolution from Homo erectus in Africa. Neandertals were physically more massive individuals than Cro-Magnon with long, low, sloping foreheads, having a prominent brow ridge, a projecting mouth, and a receding chin. They also had a slightly larger brain (on the average) than Cro-Magnon. Their burial practices indicated a belief in an afterlife. They did not appear to interbreed with Cro-Magnon and eventually died out during the end of the last inter-glacial time and the beginning of the last Ice Age, about 28,000 years ago. The Neanderthal stone culture is known as Mousterian. Neandertals never made it across the Bering land bridge into North and South America.
Cro-Magnon has a fossil record extending as far back as Neandertal. They were in Europe about 35,000 years ago so they co-existed wuth Neandertal for a few thousand years. Their culture is known as Late Neolithic and they produced magnificent cave paintings. Cro-Magnon are our direct ancestors. Humans were present in the New World at least 30,000 years ago, having crossed the Bering land bridge. The extinction of many of the large mammals existing at the end of the Pleistocene (about 10,000 years ago), such as mastodons, mammoths, elephant-size bison, giant beaver may be related to hunting pressure from humans using newly developed weapons.