Historical Geology - Chapter 14

 

The Breakup of Pangea

 

The breakup of Pangea took place in four stages:

·        Rifting between Laurasia and Gondwana (Rifting of N. America from S. America).  This expanded ocean circulation & evap. dep.

·        Rifting and movement of various Gondwana continents during late Triassic and Jurassic.

·        Rifting between S. America and Africa in L. Jurassic.  Clockwise rotation of Laurasia begins to close E. end of Tethys Sea.  Evap. deposited in narrow basin between S. America and Africa.

·        Australia and Antarctica separate by the end of the Cretaceous. Greenland rifts from N. America in the Cenozoic.

 

A global rise in sea level during the Cretaceous, due to higher heat flow and increased rates of spreading led to worldwide transgressions (Zuni Sequence Early Jurassic to Paleocene), otherwise most of the N. American craton during the Mesozoic was above sea level.

 

East Coast

During the E. to M. Triassic the newly formed Appalachians were being actively eroded and reduced to a low-lying plain.  Block faulting developed with the breakup of Pangea in a zone stretching from Nova Scotia to N. Carolina.  Red bed development (Newark Group in N. Jersey contains numerous fossil footprints of reptiles).  Intrusions of dikes, sills and extensive lavas filled the basin floors.  Famous intrusion - Palisades sill along the Hudson River in the N. York-N. Jersey area.  Rifting ceases as Atlantic Ocean grows.  Eastern margin of N. America becomes a passive continental margin.  During the Cretaceous the Appalachians were re-elevated perhaps as a result of hot-spot activity.

 

Gulf Coast

As N. Am separated from S. am during the L. Triassic, the Gulf of Mex. Began to form a shallow restricted basin - ideal for evaporite formation. Evaporite formation ceases in the Jurassic and the area is covered by northward transgressing seas during the Cretaceous.  Extensive reef development during the Cretaceous formed excellent petroleum reservoirs.

 

West Coast

The west coast of N. Am. during the Mesozoic is dominated a convergent plate boundary.  The results sequence of orogenic events have collectively produced what is known as the Cordilleran mobile belt (Jurassic-Cretaceous).  Activity is dominated by east-dipping subduction zone under N. Am.

·        Sonoma Orogeny:  Suturing of a Permian Island Arc to N. Am. at the Permian-Triassic boundary.

·        Nevadan Orogeny:  Late Jurassic and Cretaceous event that formed the Sierra Nevada, S. California, Coast Range and Idaho(?) granite batholiths.  Also formed mtn ranges in Nevada, Utah and Idaho. Franciscan Complex v. Great valley Group.  Development of 2 subduction zones dipping in opposite directions produces a 2nd island arc which collides w/ continent in L. Jurassic. Melones and Bear Mtn. Faults represent old subduction zones.

·        Sevier Orogeny: Cretaceous event caused by continued subduction of Pacific plate beneath N. Am.  Shallowing of the angle of subduction creates numerous low-angle thrust faulting to develop in Nevada.

·        Laramide Orogeny:  Late Cret. and early Tertiary event which begins the development of the Rocky Mtns. (discussed in Ch. 16).

 

Triassic sedimentation in the western U.S. dominated by regression of Absaroka seas during the Triassic leaves much of the seafloor exposed to erosion.  Deposits of colorful redbeds abound.  Moenkopi Fm of Colorado Plateau (mudcracks, reptile tracks, fossil amphibians and reptiles), overlain by Shinarump Conglomerate and the U. Triassic Chinle Fm. (Petrified Forest Natl. Park. Preserved logs of gymnosperms (conifers and cyads).due to silicification. The Chinle is overlain by the Wingate Sandstone (desert dune deposits) and the Kayenta Fm. (stream and lake deposits).

 

Jurassic sedimentation in the west is dominated by Navajo Sandstone (coastal dune deposits with some crossbeds more than 25 m high).

Marine conditions return in middle Jurassic to form Sundance Sea.  Detrital sediments from west (Nevadan Orogeny) form the Sundance Fm.  Multi-colored sandstones began filling the Sundance Sea.  Morrison Fm. (richest assemblage of Jurassic dinosaurs skeletons) forms in Colorado and Utah as semi-tropical coastal swamp, lake and stream deposits coalesce.

 

In L. Cretaceous arctic waters enter the craton from the north to create the Cretaceous Interior Seaway. Part of the Cretaceous transgression due to the fragmentation of Pangea.  At the end of the Cretaceous the Sea regresses to the north and south and marginal marine and continental deposition forms widespread coal deposits on the coastal plain.

 

Microplate Tectonics

Much of the material accreted to continents along convergent plate boundaries is eroded older continental crust along with significant amounts of igneous rock formed as a result of subduction.  However, many mtn. Systems are the result of small accreted lithospheric blocks of foreign origin called microplates. These microplates can be identified by their:

·        Different fossil content

·        Stratigraphy and structure

·        Paleomagnetic properties

 

Many microplates are so different from the surrounding mountain systems that they are thought to have been carried great distances as parts of other plates before they collided with the continent.  25% of the entire Pacific coast from Alaska to Baja Calif. Consist of microplates.

·        Volcanic arcs

·        Ocean ridges and Seamounts

·        Fragments of continents scraped off a subducting plate

 

Microplates probably played a role in the formation of the Appalachians and other mountain systems but are now difficult to recognize because of greater deformation and erosion.

 

Mesozoic Mineral Resources

Important coal deposits (lignite and bituminous) in Rocky Mtn. States.  More than 50% of all petroleum deposits come from the Middle East (formerly Tethys Sea).  Same is true of Gulf Coast deposits of the U.S. Rich uranium deposits (carnotite) form in Mesozoic rocks in the Colorado Plateau region. Fe deposits in w. Europe; Cret. diamonds from Kimberlites in S. Afr.

Gold deposits of the "gold rush" in Calif.  Porphyry Cu deposits in SW U.S.

 

 

 

 

 

 

By the end of the Permian Period, Pangea extended from pole to pole, covered about 1/4 of the earth's surface and was surrounded by the Panthalassa, a global ocean that encompassed about 300 degrees of longitude.  Such a configuration exerted enormous influence on the climate and generally resulted in arid conditions over large parts of Pangea's interior.

 

The world's climates result from the complex interactions between wind and ocean currents and the location and topography of the continents.