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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