Wednesday, February 6, 2008

Geology of Texas Part III - Pennsylvanian to Permian periods

This will mark the third in a series of writings on the Geological History of Texas. Part III will cover the Pennsylvanian period and the Permian periods within Texas.

Pennsylvanian period (320 to 286 million years ago)
About 300 million years ago the Ouachita Mountain range rose to form a distinct feature across Texas, as the then North America continent collided with another continent to become part of the Pangea supercontinent . To the west of the uplifted Ouachita Mountains, the crust sagged in response and several basins formed. Seas and sediments found their way into the basins over time. Thick sections of Pennsylvanian aged marine limestones, along with shales and sandstones, underlie most of West Texas. The Cisco, Canyon, Strawn, and Bend are Pennsylvanian groups of rocks arranged in descending order of age.

Pennsylvanian rocks are found in roadcuts north and east of Marathon. From the Llano uplift northward to Jacksboro and Bowie lies a wide inclined belt of Pennsylvanian rocks arranged in bands, oldest to youngest, east to west. Marine fossil snails, clams, trilobites, bryozoans and ammonites are found in Strawn group rocks east of Mineral Wells. Canyon group snails, clams and crinoids are found along US 377 southwest of Brownwood. Cisco group brachiopods and clams occur northeast of Cisco. Fossils of fusulinids, one celled organisms that look like wheat, are very common and characterize rocks of Pennsylvanian and Permian age. Marble Falls limestone can be seen along the river on the east side of the Llano uplift.

Extensive forests of conifers, ferns, seedferns and horsetail trees in the Pennsylvanian period gave rise to coal deposits and reptiles first roamed these forests towards the end of the period. The Mississippian and Pennsylvanian are together called the "carboniferous" over much of the world because of the abundant coal deposits laid down during this time.

Permian period (245 to 286 million years ago)
Permian rocks are a geologic delight in Texas. Colorful red beds at the edge of the high plainsin the Panhandle are Permian, as are magnificent reef limestones on El Capitan in Guadalupe National Park, as well as oil-rich limestones in West Texas. Great climatic changes occured in the Permian as major grops of corals, bryozoans, brachiopods, fusulinids and crinoids became extinct along with the entire trilobite tribe. But the vertebrates such as mammal-like reptiles flourished. Land plants also changed, and the ferns, seedferns, and horsetail trees began to declineduring the Permian. But Plesiosaurus, lizards, thecodonts (precursors of dinosaurs and crocodiles) had their beginnings.

While marine reefs, and banks prevailed in shallow marine waters of West Texas, near-shore evaporation flatsin the Panhandle area produced deposits of bright red shales as well as salt and gypsum deposits.

Permian rocks at the surface in vast expanses of terrain in north Texas from the edge of the high plains eastward to Mineral Wells and Wichita Falls, southward past Abilene to San Angelo. Tracts of Permian rocks extend from northward from I-10 to Guadalupe National Park and westward to El Paso. There are dozens of formation names to describe various Permian-age rocks in Texas. some worth mentioning here are the upper Permian Quartermaster formation (red sandstones, shales, evaporite minerals) in Palo Duro Canyon. Others are the Capitan, Goat Seep, Brushy Canyon, and Bone Springs formations in Guadalupe National Park.

The next series of The Geology of Texas will cover the Triassic, Jurassic and Cretaceous periods.

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