Download Earth Science Chapter 23: The Paleozoic, Mesozoic, and Cenozoic

Earth Science
Chapter 23: The Paleozoic, Mesozoic, and Cenozoic Eras
Chapter Overview
Section 1: The Paleozoic Era
1. Paleozoic Paleogeography
Geologists refer to the ancient geographic setting of an area as its paleogeography. The
supercontinent Rodinia was present at the end of the Proterozoic. By the Cambrian, the ancient
North American continent of Laurentia has split off from Rodinia, was located near the equator,
was surrounded by ocean, and was almost completely covered by a shallow, tropical sea.
Throughout the Cambrian Period, there was no plate tectonic activity on Laurentia. The edge of
the continent, called a margin, was a passive margin.
• Characteristic Sediments
Because of this paleogeography, Laurentia developed a characteristic pattern of sandstone,
shale, limestone deposition. Large sandy beaches developed along the shores of the tropical,
which later developed into sandstone formations. Very fine grain sediments were deposited in
slightly deeper water and resulted in the formation of shale layers. Carbonate sediments
accumulated in even deeper water as the calcium carbonate skeletons of organisms fell to
the ocean floor; these skeletons developed into limestone.
2. Changes in Sea Level
• A transgression occurs when sea level rises and the shoreline moves further inland. The
result of a transgression is deeper water deposits overlying shallower water deposits
• A regression occurs when sea level falls and causes the shoreline to move seaward. This
results in shallow water deposits overlying deeper water deposits
• Sea level rose during the Early Ordovician, and a beach environment covered much of
Laurentia’s margins. Overlying a sandstone formation is thin layer of shale and extensive
limestone deposits. These limestone deposits contain the fossils of the first corals that built
organic reefs. The reefs affect the environments in which they grow. Behind them the water is
warm and calm, and many fragile organisms can thrive. Also, the reefs restrict water flow so
the water between them become oversaturated with calcium and sodium resulting in the
formation of slat deposits as the water evaporates.
• The Late Paleozoic began with Laurasia still covered by a shallow tropical sea; so
Mississippian rocks throughout North America are predominately limestone. A major
regression of the sea marked the end of the Mississippian. The Pennsylvanian Period began
with a slow transgression, and the sediments that were deposited have different
characteristics than the earlier Mississippian rocks
⇒ Cyclothems
Pennsylvanian rocks in North America display a repeating cyclic pattern of sediments
stacked one on top of another, a pattern called a cyclothem. Cyclothems record two
changes in sea level. A regression, represented by layers of marine limestone and
sandstone, and a transgression represented by sandstone, clay, coal, shale, and marine
limestone
⇒ Reefs and Evaporites
During the Permian much of eastern North America was dry land and a shallow sea
covered much of what is now western North America. A large barrier reef complex was
formed, and because of the restricted water flow caused by the reefs, large evaporite
deposits formed behind the reef during the Middle and Late Permian. Two important
mineral deposits formed in this environment. One is the salt deposits that are
commercially exploited and can be used as a storage facility for wastes. The second, the
Permian Reef itself, is very porous and formed a very large oil reserve
3. Mountain Building
During the Middle Ordovician, an ocean-continent collisional boundary developed in what is now
eastern North America that resulted in a mountain building event called the Taconic Orogeny
•
Orogenies and Deformation
From the Late Silurian through the Early Mississippian, Laurentia was involved in several
mountain building episodes including the Caledonian Orogeny in Late Silurian and the
Devonian, the Acadian Orogeny, and, during the Late Devonian and Early Mississippian, the
Antler Orogeny. The Caledonian and Acadian Orogenies impacted what was to become the
Eastern North American continent, while the Antler Orogeny occurred along the Laurentia’s
western margin
• During the Late Paleozoic, the supercontinent Pangaea formed. In the Southern Hemisphere
a series of collisions between the continents of South America, Africa, Antarctica, and India
resulted in the formation of the supercontinent Gondwana
• Continental Collisions and Mountain Building
The Late Paleozoic was a time of active mountain building. Gondwana collided with Laurasia
during the Ouachita Orogeny. Because this collision was so intense it caused the crust to
uplift many miles inland from the continental boundaries, forming a mountain range called the
Ancestral Rockies. In the Late Pennsylvanian, the part of Gondwana that is now Africa
collided with Laurasia in the Alleghanian Orogeny
4. Paleozoic Life
• Cambrian Explosion
There was an increase in the diversity and abundance of life forms near the beginning of the
Cambrian Period. Organisms representing all but one of major marine groups appeared at
this time. Some of the best fossilized Cambrian organisms come from the Burges Shale in
the Canadian Rocky Mountains
• Mass Extinctions
When an unusually large number of organisms become extinct in a relatively short period of
geologic time, it is called a mass extinction. Two of the greatest extinction events in Earth’s
history occurred during the Middle Paleozoic. The first extinction occurred at the end of the
Ordovician Period when approximately 57% of all marine genera became extinct. The second
mass extinction occurred during the late Devonian when approximately 50% of the marine
genera were wiped out. In both cases, the extinctions were the result of global cooling. In the
first cooling, widespread glaciation caused the lowering of sea level in addition to world wide
cooling. In the second case, the cooling caused a general lowering of the amount of oxygen
in the oceans through a process called overturning, which stressed the marine life forms.
• Terrestrial plants
Simple land plants began to appear on Earth in the Ordovician. During the Carboniferous, the
first land plants with seeds appeared and diversified. The development of seeds enabled
terrestrial plants to survive in a variety of environments
⇒ The Permian Mass Extinction
The largest mass extinction in the history of life on Earth marks the end of the Paleozoic
Era. Nearly 95% of all marine species, 65 % of amphibians and reptiles, and one-third of
all insects did not survive.
Section 2: The Mesozoic Era
1. Mesozoic Paleogeography
The Mesozoic Era consisted of the Triassic, Jurassic, and Cretaceous periods
• The Breakup of Pangaea
⇒ By the Late Triassic, the brittle lithosphere of Pangaea had cracked and broken apart. As
some of the large rifts widened and as the landmasses spread apart, the ocean flooded
the rift valleys and formed new oceans that divided the nearly separated continents. The
breakup of Pangaea resulted in the formation of the Atlantic Ocean
• Seaways and Sand Dunes
⇒ As Pangaea broke up in the Early and Middle Triassic, deep valleys formed and heavy
sedimentation occurred. By the early Jurassic, western North America was much more
arid and was covered with a thick blanket of sand, which string winds shaped into dunes.
2. Mountain Building
• Active subduction occurred along the western coast of North America through the Middle
Triassic. The mountain ranges that formed in this time period are called the Cordillera
• Three major episodes of orogenies occurred along western North America during the
Mesozoic. The oldest orogeny was characterized by a tremendous number of igneous
intrusions. The next orogeny was characterized by low-angle thrust faulting and folding. The
third orogeny was characterized by vertical uplifts.
3. Mesozoic Life
The base of the food chain that supported all the large animals consisted of tiny, ocean-dwelling
organisms called phytoplankton. Phytoplankton float near the surface of oceans and lakes and
make their own food through the process of photosynthesis
• Plant life
As the planet warmed, the plant life underwent a major change. The swamps dried, and tall
cycad trees, along with ginkgo, pine tree, and other conifers, appeared. Flowering plants
appeared during the Cretaceous period
• Terrestrial animals
mammals appeared during the late Triassic around the same time as the dinosaurs. The
dominant Mesozoic animals were the reptiles.
⇒ Unlike amphibians, the reptiles laid their eggs on dry land. The dinosaur egg was the first
amniotic egg, an egg that contains the food and water required by developing embryos
inside
• Mass Extinctions
A major mass extinction event ended the Mesozoic. Most major groups of organisms were
affected and all known species of dinosaurs, pterosaurs, ammonites, and plesiosaurs
became extinct. The case of the extinction could have been the impact of a large meteorite
near the Yucatan Peninsula that resulted in a change in the global climate. The evidence to
support this idea includes a thin layer of iridium has been found in cretaceous-Paleogene
boundary sites around the world. Iridium is an element that is extremely rare on Earth but
common in meteorites and asteroids.
Section 3: The Cenozoic Era
1. Cenozoic Paleogeography
the Cenozoic Era encompasses about 1.5 percent of Earth’s total history
• Global cooling
As Australia separated from Antarctica, the world-wide climate began to cool. This cooling
trend is thought have caused, in part, by a change in ocean currents.
⇒ At this time Antarctica formed a permanent ice cap
• Throughout the Pleistocene, ice covered much of Earth’s northern hemisphere. Glaciers
advanced and retreated in at least four stage over North America and the northern latitudes
• Cenozoic Mountain Building
⇒ Subduction in the West
Volcanism returned to the western coast of North America at the end of the Eocene. The
Cascade Mountains in the Pacific Northwest are the result of the subduction of an
oceanic plate beneath the western coast of North America. During the Miocene, the North
American Plate was forced over the East Pacific Rise resulting in the creation of the San
Andreas Fault. The subduction of the East Pacific Rise coincides with extensional
tectonism in the southwestern United States. A series of that trend north to slightly
northeast are separated by long, linear valleys. This area is called the Basin and Range
Province
⇒ While the final breakup of Pangaea occurred during the Cenozoic, plate tectonics also
brought continents together during this time. The result one such collision is the
Himalayan Mountains. India traveled north and collided with the southern margin of Asia.
That Alps were formed by a collision between Africa and the connected landmass of
Europe and Asia. Before the collision with Eurasia, a narrow sea called the Tethys Sea
separated the two continents
2. Cenozoic Life
• Life on Land
Most of the currently living groups of mammals had evolved by the Eocene. Forests
dominated North America during the Paleocene and Eocene. As the climate cooled during
the late Eocene, however, the forests gave way to open land. The grasslands supported a
large diversity of mammals, most of which are members of groups living today
• Pleistocene Mammals
As the Pliocene ice age began; the great grasslands were replaced by more arid land. A new
group of mammals evolved to populate the land as the Late Pliocene-Pleistocene ice age
cooled North America
• Primates and Humans
⇒ Primates
Primates are distinguished from other mammals by a grasping hand with an opposable
thumb and two eyes directed forward that result in stereoscopic vision. Our species,
Homo sapiens, belongs to a group of primates called hominids, which are part of a larger
group called hominoids. Hominids also include the great apes. The most recognizable
feature that separates a hominids from other hominoids is that hominids are bipedal, they
walk upright on two legs
⇒ The Rise of Homo Sapiens
Tracing the ancestry of Homo sapiens began in 1850 with the discovery of a fossilized
skull in Neander Tal in Germany. There is relatively little fossil record of the development
of Homo sapiens, but earliest known hominid was alive 3.5 million years ago