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Geologic History

Overview
The 600+ million year geologic history of North Central Texas and all of Texas involved several periods of mountain-building, tectonic activity. The “pre-Texas” landmass, attached to the North American continental core, was repeatedly whacked into other landmasses, forcefully pulled apart again, and moved about the globe on mobile crustal plates. (An excellent virtual-reality visual representation of this process can be seen at the Institute for Geophysics, UT Austin website.)

Over millions of years, sediments were pushed up, some collapsed, and others were folded and warped. Volcanoes spewed molten rock and ash. Deeply buried sediments were heated forming magmas that were injected into other sediments. These processes created mountains and inland seas. Continuous erosion flattened mountains to plains and filled in the seas. Vast amounts of sediment washed into the Gulf creating thick, steeply-dipping layers of rock. These events set the stage to form the Texas we know today.

The following is a summary of events by geological era. (Click for a geologic time scale with links to period paleomaps.)

Pre-Cambrian
Thick sequences of coarse grading to fine sediments were washed from the continental core into an ancient sea - into the location of what would become Texas. Tectonic collisions and plate movement built mountains, created metamorphic structures, and formed massive granitic bodies. Most of these features were eventually eroded into a flat, rocky tableland. The ancient sea coast stretched from Dallas to Del Rio.

Paleozoic
Shallow seas invaded flat, rocky “Pre-Texas” providing a repository for eroded continental sediments arriving from the north and west. During the 250 million years from the Cambrian to the Mississippian, coarse sands and gravels accumulated in the near shore environment while limestones, dolomites, and cherts were deposited in the deeper waters to the south and east. Flora and fauna proliferated. The fossil record shows an abundance of life.

During Pennsylvanian time the North American plate collided with the newly forming Pangea landmass, squeezing and buckling Texas rocks once again. The sweeping, S-shaped feature seen on the Texas geologic map, the Ouachitas, is a result of this collision. Believe it or not, a huge, sweeping mountain range runs right through North Central Texas. You can’t see it since what remains is buried deep within the earth. As mountains rose in Dallas, the earth’s crust to the west warped downward forming large basins which eventually filled with water and fine sediment creating brackish seas. Texas rested, tectonically speaking, during the Permian. Erosion continued unabated wearing down the mountains, washing sediment into the western basins from the east and the west. Organically-rich sediments, reef structures, and coastal deposits were covered, buried, and compressed. Abundant plant and animal remains buried within porous, sedimentary rocks, laid the ground work for the future wealth of many a Texas oilman.

 

Mesozoic
During the Triassic and Jurassic periods, restless plates reactivated. Texas and the parent continental plate pulled away from Pangea opening the chasm for the Gulf. The remains of the old Ouachita mountain range and Dallas were on the Texas gulf coast. The rocks farther to the east and south warped downward and began to fill with sediment.

The Cretaceous was a particularly tectonically active period for the continent. The Rockies had a major upward thrust and seas rose and fell continuously across the continent and Texas. 200 million years of geologic history wore down the North Texas highlands nearly flat, warped them downward, and buried them beneath thousands of feet of Cretaceous sediments. A significant event for this time period was when the dinosaurs met their demise at the close of the Cretaceous (beginning of the Paleocene) 66 million years ago.

Cretaceous North America Texas In The Cretaceous
Fig.1 - Texas Under Water During the Cretaceous.


Cenozoic
The Tertiary was a time of massive sediment influx to the east into the widening Gulf. Volcanism and block faulting were the major events to the west. Texas was beginning to look like Texas. In the Miocene, the Rockies were thrust upward again providing vast amounts of clastic sediment for rivers to wash southeastward into Texas. In the late Miocene, the buried lower-Cretaceous Texas coastline beneath Dallas provided sufficient crustal weakness for the uplifting of the Edwards Plateau. The Brazos and the Trinity watersheds eroded the north/south trending escarpment over time from its original location in Dallas to the present location nearly 200 miles to the west. The Tertiary was the age of mammals and grasses. Texas life proliferated.

The "Ice Age" periods of the Quaternary provided substantial climatic changes to make several adjustments to our topography. Wind-blown soils built the foundations for prairies. Massive pluvial runoff from increasing available moisture from melting glaciers helped build the major watersheds of the Trinity and the Brazos. Wildlife in our region was thriving and abundant. Fossil remains of mammoths, camels and bison have been found in Quaternary deposits in North Central Texas. In the latter part of the Holocene, Aggies appeared. Six hundred million years of Texas geology would never be the same.

  - Doug G. Pierson
Credits:
Darwin Spearing, "Roadside Geology of Texas", Mountain Press Publishing Co., 1991

Donald E. Trimble, The Geologic Story of the Great Plains, Geological Survey Bulletin 1493, 1980
http://library.ndsu.edu/exhibits/text/
greatplains/text.html
 
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