The area sits on a surface of gently tilted sediments of mostly Cretaceous age, but this sedimentary cover obscures a much longer geologic record. North Texas sits near the edge of the North American craton of Precambrian age. The oldest rocks in Texas date from the Meso-proterozoic and are about 1,600 million years old. This represents the continental crust of Texas and is overlain by sediments of Paleozoic age; these cannot be seen in the DFW area but are exposed in southern Oklahoma and around the Llano Uplift region of Central Texas (See the Enchanted Rock link above). Some of these sediments contain important deposits of natural gas, especially the Barnett Shale of Mississippian age. The region west of Weatherford, Texas consists of Pennsylvanian sediments that tilt a few degrees west. These sediments were deformed when Gondwana collided with Laurasia to form Pangaea about 300 million years ago (Ma). A great mountain range formed, the Marathon-Ouachita-Appalachian-Variscan cordillera. This mountain belt collapsed during the Triassic and Jurassic time as Pangaea broke up, forming the Atlantic Ocean-Gulf of Mexico basin. Because the Pennsylvanian collision was followed by uplift associated with Pangaea break-up, the DFW region was probably a mountainous region (rift-flank uplift) that was eroded for around 190 million years. For this reason, sediments of the Permian, Triassic, Jurassic, and most of the early Cretaceous are not found in or beneath the DFW region, although thick continental deposits of these ages are found to the west, in West Texas, New Mexico, and Arizona. Sea level rose as the supercontinent Pangaea broke up, eventually reaching DFW about 100 million years ago, in the middle of Cretaceous time. (Beach front property anyone?)
Fig 1; Metroplex Underlying Geology
The DFW Metroplex sprawls across a 100 kilometers (62 mi) wide N-S trending belt of outcropping Cretaceous sediments. Fort Worth in the west is built on Early Cretaceous Comanche Series. Dallas in the east is built on Late Cretaceous Gulf Series sediments. DFW lay on the beach about 110 Ma, during early Cretaceous time. The water systematically rose for another 30-50 million years, so by the time the coccolithophorid Austin chalk was deposited, the benthic environment that became DFW lay 100 meters (328 ft) or more below the sea surface.