This will mark the fourth in a series of writings on the Geological History of Texas. Part IV will cover the Triassic, Jurassic and the Cretaceous periods within Texas.
Triassic period (208-245 million years ago)
By Triassic time, colorful shales and sandstones were still being deposited, although in more restricted areas of the Panhandle. The Dockum group is well exposed at Palo Duro Canyon and Caprock Canyons State parks and in the breaks along the Canadian River west of Amarillo. A wide patch or Triassic rocks also occurs east of Big Springs. However, rumbles of change in the configuration of continents are seen in Texas as the supercontinent of Pangea begins to split apart during the Triassic. The Gulf of Mexico begins to shudder open, and red shale, siltstone and sandstone are first the deposits to be shed into the down-warping south-east and east side of the Ouachita Mountain line. On land, ferns being anew, and there is a reawakening of the bryozoans.
Jurassic period (144 to 208 million years ago)
During Jurassic time the breakup of Pangea began in earnest. The Rocky Mountains were rising and the Gulf of Mexico occupied the new gap between North and South America. At the beginning the Gulf was a shallow sea not well connected to the other oceans. It dried up often leaving vast salt plains. The Louann Salt, the motherlode of salt domes in the Gulf Coast was thus born. Limestones of the subsurface Smackover formation were deposited when deeper marine water conditions prevailed. The dinosaurs were in full force by the Jurassic period and the first kinds of flowering plants and early rodent-like mammals also appeared. Jurassic rocks are virtually absent at the surface in Texas though Jurassic limestone, sandstones and shale beds can be seen along I-10 west of Sierra Blanca in westernmost Texas.
Cretaceous period (66 to 144 million years ago)
Texas Cretaceous rocks are fascinating! Lower Cretaceous rocks virtually blanket the center half of the state. Limestone cliffs, caverns, canyons, springs, abundant fossils and dinosaur tracks are all part of the scene. Upper Cretaceous rocks are found in a band from the Red River southward through Dallas/Fort Worth, to Austin, San Antonio and westward to Del Rio. The I-10 Interstate from San Antonio to Fort Bend runs entirely on Cretaceous rocks and wonderful canyon outcrops and roadcuts are seen along the road. The continents continues to pull apart in Cretaceous time. The Rocky Mountains underwent their major push and shallow seas on the continent's margins advanced and retreated repeatedly. Some sea advances filled the trough in front of the Rocky Mountains, creating a connecting seaway all the way from the Arctic Ocean to the Gulf of Mexico. The shallow Cretaceous seas over Texas were filled with calcareous shelled organisms, and thick deposits of limestone were laid down. On the sandy shorelines and mudflats of these seas dinosaurs roamed freely, leaving evidence of their passing in fantastic fossilized footprints and trackways all across Texas. Many formation names are applied to descrive the complex suite of Texas Cretaceous rocks. The Lower Cretaceous portion is divided into Washita, Fredricksburg and Trinity groups from top to bottom, while Upper Cretaceous rocks are assembled into Navarro, Taylor, Austin, Eagle Ford and Woodbine groups from top to bottom. Cretaceous rocks form impressive cliffs at Santa Elena and Boquillas Canyons in Big Bend Park. The Hill Country around San Antonio and Kerrville is carved in Cretaceous rocks, as are the Colorado River Canyon north of Austin and the Devils River-Rio Grande Canyon west of Del Rio. While marsupials and bats arose in Cretaceous, and though dinosaurs and flowering plants proliferated, the winds of major change blew for life on Earth. At the close of the Cretaceous, the dinosaurs and many of their relatives disappeared forever. But strangley plant life marched across the Cretaceous boundary and into the Tertiary period virtually unchanged, as did the birds. Though the picture of dinosaurs choking to death from the cataclysmic dust of a meteor impact is a commonly touted idea, it does not explain why flowering plants or birds survived the holocaust. More work needs to be done on the geologic causes of the so-called mass extinction at the end of the Cretaceous.
The next series of The Geology of Texas will cover the Tertiary and Quaternary periods to present day Texas.
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