Lab 8: Geologic Maps - Geology of Texas


Geologic Regions


Texas is a large state with varied geology, ranging from the simple dipping-strata structure of the Gulf Coastal Plain, North-central Texas, and the Panhandle, to the complicated geological structure of the Llano Uplift (L.U. in the center of the map below), the Marathon Uplift, and the Big Bend Region. The colors show the age of rock at the surface -- this is a geologic map, so the colors represent standard color definitions for the geologic periods. The Cretaceous Period is represented by green colors, for example. Study the two maps below, then look at the region-by-region notes at the bottom of the page.




The "T" symbols on this map are strike-and-dip symbols. Look back at that part of the lab if you need a refresher on how to visualize the orientation of the strata.


Gulf Coastal Plain


Texas is on the western end of this broad province, which wraps around the Gulf of Mexico from Mexico, through the Gulf states, and over to Florida and Georgia where the same strata continue as the Atlantic Coastal Plain. The age on outcrop is as old as Cretaceous, but Jurassic, Triassic, and slightly older rocks lie beneath. From the Cretaceous outcrop belt along the inner margin, the strata dip gently (less than 5 degrees in most places) toward the coast. That means the Cretaceous beds dip under the younger beds closer to the coastline. The Cretaceous - Tertiary boundary is shown by the dashed line (K stands for Cretaceous, as the C was already taken up by the Cambrian). From the K/T boundary, the outcropping rock layers get younger, and finally along the coast we have modern day deposition -- you might say modern sediments have an age of zero. The age of outcropping beds is already getting to one million years old just landward of Corpus Christi, Houston, and Beaumont, and East Texas and South Texas generally range in age from 0 at the coast to 65 million years old at the K/T boundary. The Edwards Plateau and Lampasas Cut Plain are the landward edges of the western Gulf Coastal Plain, the so-called Hill Country, dominated by Cretaceous limestone strata across the outcrop.


North-central Texas


As shown by the blue colors on the map in this area, the rock layers are Paleozoic in age, ranging from the middle Paleozoic near the Llano Uplift to a broad area of Pennsylvanian and Permian age strata west of Fort Worth, over toward Abilene. Note that these strata dip generally toward the west, representing a completely different geological area than the Gulf Coastal Plain.


Llano Uplift


The Llano Uplift has the oldest rocks in Texas on outcrop, getting as old as about 1.1 billion years. The Precambrian granite and metamorphic rock is flanked by Paleozoic rocks. The geological structure is more complicated than in adjacent areas, with many faults and folded strata.


Big Bend


The Big Bend area, where we find Big Bend National Park and Big Bend State Park, is the area shown in the circle, where the Rio Grande River makes a big bend into Mexico. Big Bend geology is the most complicated of the state, with many faults, localized uplifts and folding, and volcanism. The age of outcropping rocks ranges from Paleozoic to Recent, but most of the area is covered by Cretaceous and Tertiary age strata.


Marathon Uplift


The Marathon Uplift, named after the little town of Marathon there, is small, but loaded with geological structure and interesting Paleozoic rock units. This is an area where highly folded Paleozoic strata can be seen along the road. The folding happened when South America, Africa, and North America collided to form the supercontinent Pangaea around 300 million years ago.


Guadalupe Mountains


The highest point in Texas, over 8000 feet in elevation, is found at Guadalupe Peak, just on the Texas - New Mexico border. Permian age limestone strata, including ancient reef deposits, crop out along a high escarpment here, and continue into southern New Mexico. Carlsbad Caverns formed in these limestone strata, and is just across the border in New Mexico.




Finally, the Panhandle is an area of gently undulating strata, flat-lying across many areas. Rocks at the surface range in age from early Mesozoic to Recent. Quite a bit of the Panhandle area is covered by a veneer of young sedimentary materials, on which much irrigated farm land has been developed. Palo Duro Canyon, near Amarillo and Lubbock, is an area of eroded canyons formed by headward erosion of the Red River, which is just a small stream here (The Red River goes on to form the southern border of Oklahoma with Texas).