The Geology of Texas - Part 1 Precambrian and Cambrian Periods

This will mark the first in a series of writings on the Geological History of Texas. Part 1 covers the Precambrian period of 570-4500 millions years ago and the Cambrian period 505-570 million years ago.

Before we start picture Texas from Northwest flowing southeastward to the Gulf of Mexico . As you move from the panhandle to the coast rocks get progressively younger.

Precambrian Texas -
The geologic record begins in Texas over a billion years ago when thick sequences of course, fine sediments were dumped into an ancient sea as North and South America pulled apart forming the Gulf of Mexico. Eventually the continent collided with another continent or an ocean margin in a plate tectonic event that buried, squeezed, and heated the borderlands, including the sediment piles. The collision built mountains and created metamorphic schist and gneiss out of the deeply buried sedimentsand generated molten magmas which colled to form granite bodies. Erosion then flattened this range to a table top by Cambrian times. Precambrian rocks are seen in the Franklin Mountains near El Paso , in Llano County of central Texas, and in the west Texas ranges near Van Horn. Precambrian rocks have also been reached by oil drilling over much of central and west Texas, though little is known about Precambrian rocks beneath the Gulf Coast Plain because they are buried so deeply.

Cambrian Texas-
Shallow marine seas that transgressed across Texas in Cambrian time bordered the low, erosion worn central core of the North American continent. Sandy sediments were deposited at the margin of these seas from streams carrying their loads eroded from the low continental terrain to the north and west. Cambrian sandstones around the Llano uplift in central Texas and in the Marathon region of West Texas are examples of this sedimentation.

Farther from shore in the clear marine water, dolomite and limestone accumulated from the shells of Cambrian organisms. Outcrops of Cambrian limestones are also found in the Llano uplift area of central Texas. Cambrian time is noteworthy because it represents the appearance , rather suddenly in the geologic record, of abundant fossils. Trilobites, brachiopods, sponges, snails, clams, and bryozoans were all present by Cambrian time, whereas late Precambrian rocks display only rare fossils of algae and a few soft bodied marine animals.

Part II of the series will cover the Ordovician, Silurian, Devonian and Mississippian time periods in Texas ranging from 505 million years ago to 360 million years ago.