- Geology of the Dallas-Fort Worth Metroplex
The geology of the
Dallas-Fort Worth Metroplex is relatively stable, due to the metropolitan area's position on the western edge of slantingCretaceous sediment plains and on the eastern edge of theFort Worth Basin , both resting atop the TexasCraton .Summary
The area sits on a surface of gently tilted
sediments of mostlyCretaceous age, but this sedimentary cover obscures a much longer geologic record.North Texas sits near the edge of theNorth American craton ofPrecambrian age. The oldest rocks in Texas date from theMesoproterozoic and are about 1,600 million years old. This represents thecontinental crust of Texas and is overlain by sediments ofPaleozoic age; these mostly cannot be seen in the DFW area but are exposed in southernOklahoma and around theLlano Uplift region ofCentral Texas . Some of these sediments contain important deposits ofnatural gas , especially theBarnett Shale ofMississippian age. The region west ofWeatherford, Texas consists ofPennsylvanian sediments that tilt a few degrees west. These sediments were deformed whenGondwana collided withLaurasia to formPangaea about 300 million years ago (Ma). A great mountain range formed, the Marathon-Ouachita-Appalachian-Variscan cordillera, which stretched from west Texas into easternEurope . This mountain belt collapsed during theTriassic andJurassic time as Pangaea broke up, forming theAtlantic 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 about 190 million years. For this reason, sediments of thePermian ,Triassic ,Jurassic , and most of the earlyCretaceous are not found in or beneath the DFW region, although thick continental deposits of these ages are found to the west, inWest Texas ,New Mexico , andArizona . Sea level rose as thesupercontinent Pangaea broke up, eventually reaching DFW about 100 million years ago, in the middle ofCretaceous time.The DFW Metroplex sprawls across a convert|100|km|mi|0|sp=us|lk=on wide N-S trending belt of outcropping
Cretaceous sediments.Fort Worth in the west is neatly built onEarly Cretaceous (Comanche Series) and Dallas in the east is built onLate Cretaceous (Gulf Series) sediments. The location of modern 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 Austinchalk was deposited, the "Octopus Garden" that became DFW lay convert|100|m|ft|0|sp=us|lk=on 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. The Cretaceous sediments dip a degree or so to the east, so the Cretaceous sediments get younger towards the east. Sediments deposited during theCretaceous-Tertiary extinction event , when thedinosaur s became extinct, lie near the town of Terrell, at the eastern edge of the DFW metroplex.Fossils
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 ". [" [http://www.smu.edu Southern Methodist University] - [http://www.smu.edu/smunews/dallasaurus/news-release.asp SMU and Dallas Museum of Natural History Announce Missing Fossil Link Dallasaurus] ." Originally published15 November 2006 . Retrieved28 February 2007 .]Trinity Rver
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 byJohn Neely Bryan as the place for his trading post to overlook theferry 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 inPleistocene time, when a continental ice sheet reached as far south asKansas during theWisconsin glaciation . The ice age climate had two effects on the Trinity River: It caused downcutting (few people know that there is a convert|100|m|ft|0|sp=us deep buried canyon beneath the Trinity in Dallas) Fact|date=February 2007, 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 asMastodons andMammoths Fact|date=February 2007. The Pleistocene terraces affected the development of Dallas, providing a rich alluvial soil and a perchedaquifer , very useful indeed during the early years.Downtown Dallas is built on a series of these terraces, rising subtly eastward from the Trinity river.Water quality
The DFW Metroplex had an additional, if subtle, geologic advantage. The Trinity is not good for navigation by boats but is 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-bearingPermian 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:History of Dallas, Texas (1874-1929) ") 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.References
External links
* [http://www.nhnct.org/geology/geo2.html A Natural History of North Central Texas]
* [http://www.dallaspaleo.org/ Dallas Paleontological Society]
* [http://www.aapg.org/explorer/2005/05may/barnett_shale.cfm Barnett Shale]
* [http://blumtexas.blogspot.com Barnett Shale Maps]
* [http://barnettshaleoil.com Barnett Shale Oil]
* [http://waterdata.usgs.gov/tx/nwis/current?type=quality&group_key=basin_cd USGS realtime data for Texas rivers. Compare conductance data for Brazos, Trinity, and Red rivers ]
* [http://utdallas.edu The University of Texas at Dallas] - [http://utdallas.edu/geosciences/ Geosciences Department]
* [http://www.dallasgemandmineral.org/ Dallas Gem and Mineral Society]
* [http://www.natureandscience.org/exhibits/default.asp Museum of Nature & Science exhibits]
* [http://www.dgs.org/index.php Dallas Geological Society]
* [http://geosociety.org/ Geological Society of America]
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