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Llano Uplift
From Wikipedia, the free encyclopedia
The Llano Uplift is a roughly circular geologic dome of Precambrian rock, primarily granite, in Central Texas in the United States. It is located in the eastern region of the Edwards Plateau, west of the Texas Hill Country. The region is characterized by a central exposure of Precambrian granite (Town Mountain Granite), surrounded by a ring of gneiss and schist (Valley Spring Gneiss and Packsaddle Schist respectively, also Precambrian), which is in turn surrounded by uplifted lower Paleozoic strata. This area is rimmed by limestone ridges.
The Llano Uplift is best known for its large, grantic protruding rock formations such as Enchanted Rock. The area includes several major quarries that mine the distinctive pink granite. Furthermore, the area contains the only known deposits of Llanite.
The area is termed an uplift due to the raised status of the Precambrian rocks in comparison to adjacent, completely buried Precambrian strata, as well as Paleozoic rocks that are buried elsewhere in central Texas. However, the exposures of Precambrian rock are generally located at the lowest surface elevations of the region. This is best pictured by imagining the igneous roots of an ancient mountain range buried entirely beneath newer sedimentary strata, followed by the erosion of the newer strata to the extent that some of the "highest" igneous rocks are exposed.
The origin of the Town Mountain Granite--tightly-dated to late Mesoproterozoic times--is a minor mystery, stemming from a debate over whether or not it is related to the Grenville orogeny.[1], [2] What is not in doubt in that the Valley Spring and Packsaddle formations are heavily metamorphosed sedimentary rocks, originally deposited south of the North American craton, and altered by subduction or continental collision.[3], [4]
The ancient faulting that produced the Llano Uplift is believed to be related to that which produced the Marathon Uplift in west Texas. Both may have been formed around the time of the Ouachita Mountains orogeny.
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