Geology of the Dallas-Fort Worth Metroplex
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The greater Dallas-Fort Worth Metroplex 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 Mesoproterozoic and are about 1600 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 region of central Texas. 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 Pangea 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 Pangea broke up, forming the Atlantic Ocean-Gulf of Mexico basin. Because the Pennsylvanian collision was followed by uplift associated with Pangea break-up, the DFW region was probably a mountainous region (rift-flank uplift) that was eroded for about 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 Pangea broke up, eventually reaching DFW about 100 million years ago, in the middle of Cretaceous time.
The DFW metroplex sprawls across a 100 km (60 mi) wide N-S trending belt of outcropping Cretaceous sediments. Fort Worth in the west is neatly built on Early Cretaceous (Comanche Series) and 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 kept rising for another 30-50 million years, so that by the time the coccolithophorid Austin chalk was deposited, the "Octopus Garden" that became DFW lay 100 m (330 ft) or more below the sea surface. The inexorable rise in sea level was only interrupted by tectonic rumblings in southern Arkansas and Oklahoma, shedding copious amounts of Woodbine Sandstone to the south. These sandstones underlie the cities of Denton, Grapevine, and Arlington, Texas. The Cretaceous sediments dip a degree or so to the east, so the Cretaceous sediments get younger towards the east. Sediments deposited during the Cretaceous-Tertiary extinction event, when the dinosaurs became extinct, lie near the town of Terrell, at the eastern edge of the DFW metroplex.
People enjoy searching for fossils in the rocks around Dallas. Remnants of dinosaurs and Late Cretaceous marine reptiles such as Mosasaur are found. One species of Mosasaur was named after the city: Dallasaurus turneri.
The Trinity River has been important in shaping the DFW metroplex. Dallas was situated at the best ford, downstream from where the Elm Fork joins the main stream, where the river flows southeast over the chalk. This provided a place where travelers need only cross the river once, at a place with relatively firm landings and bottoms. This was the best place to cross the Trinity from the earliest days, best for fordings, ferries, and bridges. During the days of the Republic of Texas, the DFW metroplex was mostly uninhabited by Europeans, but settlers began to find their way north in the 1840s. The route north naturally followed the low hills and gentle ridges of Austin chalk hills to the river ford that soon became Dallas. The future site of Dallas was selected by Bryan as the place for his trading post to overlook the ferry that he operated at the crossing.
Dallas was also affected subtly by much younger geologic formations deposited by an older, more vigorous Trinity River. The northern hemisphere Ice age occurred in Pleistocene time, when a continental ice sheet reached as far south as Kansas during the Wisconsin glaciation. The ice age climte had two effects on the Trinity River: It caused downcutting (few people know that there is a 100 m (330 ft) deep buried canyon beneath the Trinity in Dallas) [citation needed], and a wetter climate caused much more water to flow in the river. The greater river flow generated great sedimentary terraces. From time to time these terrace deposits reveal bones of extinct giant mammals, such as Mastodons and Mammoths [citation needed]. The Pleistocene terraces affected the development of Dallas, providing a rich alluvial soil and a perched aquifer, very useful indeed during the early years. Downtown Dallas is built on a series of these terraces, rising subtly eastward from the Trinity river.
The DFW metroplex had an additional, if subtle, geologic advantage. The Trinity is not good for navigation by boats butis great for drinking. Trinity River water is better than either of the larger rivers to the north and south, the Red River and the Brazos River. The larger rivers are longer and flow over salt-bearing Permian sediments, well west of the Trinity headwaters. The Trinity is consequently sweeter water than either the larger Brazos or Red rivers. Life is better and easier near sweet water, and this simple fact helped DFW prosper relative to settlements on the larger rivers to the north and south. Because the Trinity is not suitable for navigation, the metroplex could not have grown to be a large city until the railroad arrived, which happened early in the Metroplex's history, in the early 1870s. (See: 19th Century) The Dallas-Fort Worth Metroplex is thus truly a modern metropolitan area, because it could not have grown so large until mechanical transportation systems made the Trinity disadvantage in river navigation insignificant.
[edit] External links
- A Natural History of N. Central Texas
- [1] Dallas Paleontological Society
- Barnett Shale
- [2] Dallasaurus story
- [3] USGS realtime data for Texas rivers. Compare conductance data for Brazos, Trinity, and Red rivers
- [4] U Texas at Dallas Geoscience department
