1/30/2024 0 Comments Xsection with alluvium![]() The dispersal of extra magma warmed the water in the ocean, and was a conducive environment for calcareous-shelled organisms, which after death sank to the bottom of the ocean floor creating thick deposits of limestone. 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 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. Seafloor spreading in the Gulf of Mexico ceased by the beginning of the Cretaceous and spreading shifted to the proto-Caribbean.Īround 110-85 Ma, there was worldwide oscillatory increases in ocean floor spreading rates. As the new lithosphere moved 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 move apart. New lithosphere is made when hot material beneath ocean ridges is brought to the surface by these cells. Convection currents in the sub-lithospheric mantle are the driving mechanisms that caused sea floor spreading to occur. 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. Eventually rifting gave way to sea floor spreading in the Atlantic and Gulf of Mexico in the mid Jurassic, around ≈165 Ma. 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. This rifting created a divergent plate margin that would play an integral role of the future geologic processes to follow. Rifting affected regions which became the central Atlantic (between North America and Africa) and the Gulf of Mexico at about the same time. Pangea started to break up during the Triassic ≈225Ma. Significant deposits of hydrocarbons such as natural gas have economic importance as is seen in formations like the Barnett Shale. 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. Horizontal shortening caused flexural isostasy to bend the lithosphere. ![]() The Fort Worth Basin which lies beneath Cretaceous sediments west of Dallas formed as a foreland basin during the Ouachita orogeny. These rocks are mostly buried beneath Phanerozoic sediments, but are exposed in the Llano area, where previous Precambrian igneous and metamorphic rocks were uplifted and exposed at the surface. 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. 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 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. To the north of the DFW Metroplex, the roots of the mountains can be identified in SE Oklahoma as the Arbuckle mountains, heavily eroded from the original vast mountain range. In west Texas near Marathon, the mountain range appears on the surface, and is known as the Marathon Uplift. Although the Ouachita Mountain roots are not visible in the DFW Metroplex since they are buried, they can still be recognized through boreholes and other data. These ancient mountains were reduced by erosion and rifting associated with the opening of the Gulf of Mexico in the Jurassic and then were buried beneath younger Cretaceous sediments. Ancient folded mountains formed by the Ouachita orogeny existed in the eastern part of the Metroplex 300 million years ago. 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. ![]() The geology of the DFW Metroplex consists of gently tilted sediments of mostly Cretaceous age, which also obscures a much older geologic record. Structural features located within East and Central Texas Structural and tectonic history of the DFW Metroplex
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