12.3+Phanerozoic+Earth+History

Lesson Objectives

 * The Phanerozoic is divided into the Paleozoic, Mesozoic, and Cenozoic.
 * Marine transgressions and regressions were common during the Paleozoic and Mesozoic.

Introduction
Compared with the long expanse of the Precambrian, the Phanerozoic is recent history. Much more geological evidence is available for scientists to study so the Phanerozoic is much better known.

Paleozoic
The Paleozoic is the furthest back era of the Phanerozoic and it lasted the longest. But the Paleozoic was relatively recent, beginning only 570 million years ago. The paleogeography of the Paleozoic begins and ends with a supercontinent.

Marine Transgressions and Regressions
Some of the most important events of the Paleozoic were the rising and falling of sea level over the continents. Sea level rises over the land during a **marine transgression**. During a **marine regression**, sea level retreats. During the Paleozoic there were four complete cycles of marine transgressions and regressions (**Figure** [|below]). Six marine transgressions and regressions have occurred during the Phanerozoic. Geologists know about marine transgressions and regressions from the sedimentary rock record. These events leave characteristic rock layers known as sedimentary **facies**. On a shoreline, sand and other coarse grained rock fragments are commonly found on the beach where the wave energy is high. Away from the shore in lower energy environments, fine-grained silt that later creates shale is deposited. In deeper, low-energy waters, carbonate mud that later hardens into limestone is deposited. The Paleozoic sedimentary rocks of the Grand Canyon (**Figure** [|below]) contain evidence of marine transgressions and regressions, but even there the rock record is not complete. Look at the sequence in the figure below and see if you can determine whether the sea was transgressing or regressing. At the bottom, the Tonto Group represents a marine transgression: sandstone (11), shale (10), and limestone (9) laid down during 30 million years of the Cambrian Period. The Ordovician and Silurian are unknown because of an unconformity. Above that is freshwater limestone (8), which is overlain by limestone (7) and then shale (6), indicating that the sea was regressing. After another unconformity, the rocks of the Supai Group (5) include limestone, siltstone, and sandstone indicative of a regressing sea. Above those rocks are shale (4), sandstone (3), a limestone and sandstone mix (2) showing that the sea regressed and transgressed and finally limestone (1) indicating that the sea had come back in. The Paleozoic sedimentary rocks of the Grand Canyon were deposited during marine transgressions and regressions. One of two things must happen for sea level to change in a marine transgression: either the land must sink or the water level must rise. What could cause sea level to rise? When little or no fresh water is tied up in glaciers and ice caps, sea level is high. Sea level also appears to rise if land is down dropped. Sea level rises if an increase in seafloor spreading rate buoys up the ocean crust, causing the ocean basin to become smaller. What could cause sea level to fall in a marine regression? Geologists think that the Paleozoic marine transgressions and regressions were the result of the decrease and increase in the size of glaciers covering the lands.

Plate Tectonics
A mountain-building event is called an **orogeny**. Orogenies take place over tens or hundreds of millions of years. At the beginning of the Paleozoic, the supercontinent Rodinia began to split up. At the end, Pangaea came together. As continents smash into microcontinents and island arcs collided, mountains rise. Geologists find evidence for these collisions in many locations. For example, Laurentia collided with the Taconic Island Arc during the Taconic Orogeny (**Figure** [|below]). The remnants of this mountain range make up the Taconic Mountains in New York. The Taconic Orogeny is an example of a collision between a continent and a volcanic island arc. Laurentia experienced other orogenies as it merged with the northern continents. The southern continents came together to form Gondwana. When Laurentia and Gondwana collided to create Pangaea, the Appalachians rose. Geologists think they may once have been higher than the Himalayas are now. Pangaea was the last supercontinent on Earth. Evidence for the existence of Pangaea was what Alfred Wegener used to create his continental drift hypothesis, which was described in the Plate Tectonics chapter. As the continents move and the land masses change shape, the shape of the oceans changes too. During the time of Pangaea, about 250 million years ago, most of Earth’s water was collected in a huge ocean called Panthalassa (**Figure** [|below]). Pangea was the sole landform 250 million years ago, leaving a huge ocean called Panthalassa, along with a few smaller seas.

Mesozoic
The Mesozoic is known as the age of the dinosaurs, but things were happening geologically as well. The Mesozoic was dominantly warm and tropical.

The Breakup of Pangaea
At the end of the Paleozoic there was one continent and one ocean. When Pangaea began to break apart about 180 million years ago, the Panthalassa Ocean separated into the individual but interconnected oceans that we see today on Earth. Why would a supercontinent break up after being together for tens of millions of years? A continent is a giant insulating blanket that does not allow mantle heat to escape very effectively. As heat builds up beneath a supercontinent, continental rifting begins. Basaltic lavas fill in the rift and eventually lead to seafloor spreading and the formation of a new ocean basin. In the Afar region of northern Ethiopia, Africa is splitting apart, causing active volcanism throughout the province. The Atlantic Ocean basin formed as Pangaea split apart. The seafloor spreading that pushed Africa and South America apart is continuing to enlarge the Atlantic Ocean (**Figure** [|above]).

Plate Tectonics
As the continents moved apart there was an intense period of plate tectonic activity. Seafloor spreading was so vigorous that the mid-ocean ridge buoyed upwards and displaced so much water that there was a marine transgression. Later in the Mesozoic those seas regressed and then transgressed again. The moving continents collided with island arcs and microcontinents so that mountain ranges accreted onto the continents’ edges. The subduction of the oceanic Farallon plate beneath western North America during the late Jurassic and early Cretaceous produced igneous intrusions and other structures. The intrusions have since been uplifted so that they are exposed in the Sierra Nevada Mountains (**Figure** [|below]). The snow-covered Sierra Nevada is seen striking SE to NW across the eastern third of the image. The mountain range is a line of uplifted batholiths from Mesozoic subduction. A marine transgression during the Cretaceous covered much of the North American continent with water (**Figure** [|below]). Lands that had been uplifted during tectonic activity remained above water during this marine transgression that took place during the Cretaceous.

Cenozoic
The Cenozoic began around 65.5 million years ago and continues today. Although it accounts for only about 1.5% of the Earth’s total history, as the most recent era it is the one scientists know the most about. Much of what has been discussed in the first chapters of this book describes the geological situation of the Cenozoic.

Plate Tectonics
The paleogeography of the era was very much like it is today. Early in the Cenozoic, blocks of crust uplifted to form the Rocky Mountains, which were later eroded away and then uplifted again. Subduction off of the Pacific Northwest formed the Cascades volcanic arc. The Basin and Range province that centers on Nevada is where crust is being pulled apart. The San Andreas Fault has grown where the Pacific and North American plates meet. The plate tectonic evolution of that plate boundary is complex and interesting (**Figure** [|below]). This figure shows the evolution of the San Andreas Fault zone from 30 million years ago (bottom) to present (top). The Farallon Plate was subducting beneath the North American Plate 30 Ma. By 20 Ma the Pacific Plate and East Pacific Rise spreading center had started to subduct, splitting the Farallon Plate into two smaller plates. Transform motion where the Pacific and North American plates meet formed the San Andreas Fault. The fault moved inland and at present small sea floor spreading basins along with the transform motion of the San Andreas are splitting Baja California from mainland Mexico. Although most plate tectonic activity involves continents moving apart, smaller regions are coming together. Africa collided with Eurasia to create the Alps. India crashed into Asia to form the Himalayas.

Ice Ages
As the continents moved apart, climate began to cool. When Australia and Antarctica separated, the Circumpolar Current could then move the frigid water around Antarctica and spread it more widely around the planet. Antarctica drifted over the south polar region and the continent began to grow a permanent ice cap in the Oligocene. The climate warmed in the early Miocene but then began to cool again in the late Miocene and Pliocene when glaciers began to form. During the Pleistocene ice ages, which began 2.6 million years ago, glaciers advanced and retreated four times (**Figure** [|below]). During the retreats, the climate was often warmer than it is today. Glacial ice at its maximum during the Pleistocene. These continental ice sheets were extremely thick, like the Antarctic ice cap is today (**Figure** [|below]). This continental glacier over Antarctica is up to 4,000 meters (12,000 feet) thick.

Lesson Summary

 * The Phanerozoic began 570 million years ago and continues today.
 * The Paleozoic was a time of four marine transgressions and regressions, which left characteristic sedimentary facies.
 * An orogeny is a mountain building event that takes place when a continent runs into another continent, a microcontinent, or a volcanic island arc.
 * The general climate trend in the Cenozoic was cooling, leading to the Pleistocene ice ages from 2.6 million to about 10,000 years ago.

Review Questions
1. What are the possible causes of a marine transgression? Of a marine regression? 2. What rock sequence indicates a marine transgression? 3. What rock sequence indicates a marine regression? 4. How do the rocks of the Grand Canyon indicate marine transgressions or regressions? 5. What was the configuration of oceans during the time of the supercontinent Pangaea? 6. What geologic evidence is left after a continent breaks apart? 7. What was the Pleistocene climate like? 8. Using the map (**Figure** above), describe the geologic history of North America. In what order did events occur? What is the cause of the orogenies lining the western and eastern continental margins? 9. Look at Africa. Why is there no orogenic province on the western or eastern margins of the continent? What is the cause of the purple province in the northeast? 10. Where are the mountains of South America located? Why are there no mountains on the opposite coast? Why are there mountains on both coasts of North America?

Vocabulary
orogeny A mountain building event, usually taking place over tens or hundreds of millions of years. marine transgression The rising of sea level over the continents. marine regression The falling of sea level so that seas no longer cover the continents. facies Characteristic sedimentary rock layers that indicate the processes and environments in which they were formed.

Points to Consider

 * How did the paleogeography of the planet affect the evolution of life?
 * How did climate affect the evolution of life?
 * How was human evolution related to major climatic events?