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Dallas–Fort Worth sits above Cretaceous-aged strata, dates ranging from ~145-66 Ma (million years ago). These Cretaceous-aged sediments lie above the eroded Ouachita Mountains and the Fort Worth Basin, which was formed by the Ouachita Orogeny. Going from west to east in the DFW Metroplex and down towards the Gulf of Mexico, the strata gets progressively younger. The Cretaceous sediments dip very gently (about 1°) to the east. == Structural and tectonic history of the DFW Metroplex == The geology of the DFW Metroplex consists of gently tilted sediments of mostly Cretaceous age, which also obscures a much older geologic record. Sediments older than Cretaceous can only be found at the surface in the extreme western part of the DFW Metroplex, in the area around Weatherford, Texas. Ancient folded mountains formed by the Ouachita orogeny existed in the eastern part of the Metroplex 300 million years ago. These ancient mountains were reduced by erosion and rifting associated with the opening of the Gulf of Mexico in Jurassic time and were buried beneath younger Cretaceous sediments. Although the Ouachita Mountain roots are not visible in the DFW Metroplex since they are buried, they can still be recognized by boreholes and other data. In west Texas near Marathon, the mountain range makes an appearance to the surface, and is known as the Marathon Uplift. To the north of the DFW Metroplex, we can see the roots of these mountains in SE Oklahoma. We know of these today as the Arbuckle mountains, despite the fact that they are far from what the untrained eye would consider a former vast mountain range. The Marathon-Ouachita-Appalachian-Variscan cordillera, which stretched through central Texas, around Arkansas, up through the Appalachian Mountains and eventually into eastern Europe, occurred when the supercontinents Pangea and Laurussia collided to form Pangea in the late Paleozoic ~300 Ma. The zone of deformation known as the Ouachitas marks a zone of weakness that was exploited when the Gulf of Mexico opened about 165 Ma, in Jurassic time. The oldest rocks in Texas date from the Precambrian, specifically the Mesoproterozoic and are about 1,600 million years old, and mark the southern limit of the North American craton. These rocks are mostly buried beneath Phanerozoic sediments, but are exposed in the Llano area, where previous Precambrian igneous and metamorphic rocks where uplifted and exposed at the surface. These billion year old rocks can only be seen several thousand feet in the subsurface by boreholes and other data in the DFW Metroplex. The Fort Worth Basin which lies beneath Cretaceous sediments west of Dallas formed as a foreland basin during the Ouachita orogeny. Horizontal shortening caused flexual isostasy to bend the lithosphere. The bent lithosphere to the west of the Ouachita mountains caused a bowl shaped depression to form, known as a foreland basin, preserving the Mississippian sediments of the Barnett Shale and other Paleozoic sediments; these sediments mostly formed before the Pangeic collision. Significant deposits of hydrocarbons such as natural gas have economic importance as is seen in formations like the Barnett Shale. Pangea started to break up during the Triassic ~225Ma. Rifting affected regions which became the central Atlantic (between North America and Africa) and the Gulf of Mexico at about the same time. This rifting created a divergent plate margin that would play an integral role of the future geologic processes to follow. Rifting which involves the stretching of pre-existing crust and mantle lithosphere was initiated by the existence of sufficient horizontal deviatoric tensional stress that broke the lithosphere. Eventually rifting gave way to sea floor spreading in the Atlantic and Gulf of Mexico in the mid Jurassic, around ~165 Ma. Sea floor spreading is where new oceanic lithosphere is being created by upwelling of material, unlike rifting where it only involved the stretching of the crust. Convection currents in the sub-lithospheric mantle are the driving mechanisms that caused sea floor spreading to occur. New lithosphere is made when hot material beneath ocean ridges is brought to the surface by these cells. As the new lithosphere moves horizontally away from the ridges, the new crust added to the Gulf of Mexico and the Atlantic caused the continents of North America and South America to be moved apart. Seafloor spreading in the Gulf of Mexico ceased by the beginning of the Cretaceous and spreading shifted to the proto-Caribbean. Around 110-85 Ma, there was worldwide oscillatory increases in ocean floor spreading rates. The increase in the amount of basalt being injected into the ocean caused a displacement of water from the ocean basins, which resulted in sea level rise, flooding the coasts of the Texas margin and other bordering continents around the world. The major sea level rise that took place due to an occurrence of an oscillation is known as the Cenomanian transgression, which is the most well known and last major transgression in the Cretaceous. The dispersal of extra magma warmed the water in the ocean, and was a conducive environment for calcareous-shelled organisms, which eventually died and sunk to the bottom of the ocean floor creating thick deposits of limestone. In addition to the displacement of water, an increase in injected magma raised CO₂ levels to around 2-6 times the current level. The increase in CO₂ levels along with the extra production of crust caused global temperatures to rise, which would also play an integral role in the future development of different Cretaceous formations. When the sea floor spreading rates slowed around ~85 Ma, so did the amount of basaltic material being thrown into the ocean which caused the initial water displacement. As seen around the DFW Metroplex, the Cretaceous rocks deposited during this time were directly influenced by increased sea floor spreading rates. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geology of the Dallas–Fort Worth Metroplex」の詳細全文を読む スポンサード リンク
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