- Llano Uplift
The Llano Uplift is a roughly circular geologic dome of
Precambrian rock, primarilygranite , in CentralTexas in theUnited States . It is located in the eastern region of theEdwards Plateau , west of theTexas Hill Country . The region is characterized by a central exposure of Precambriangranite (Town Mountain Granite), surrounded by a ring ofgneiss andschist (Valley Spring Gneiss and Packsaddle Schist respectively, also Precambrian), which is in turn surrounded by uplifted lowerPaleozoic strata. This area is rimmed bylimestone 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 ofLlanite .Geology
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 theGrenville orogeny . [http://uts.cc.utexas.edu/~rmr/SCGSA94.html] , [http://uts.cc.utexas.edu/~rmr/tmg.html] What is not in doubt in that the Valley Spring and Packsaddle formations are heavily metamorphosedsedimentary rock s, originally deposited south of theNorth America ncraton , and altered bysubduction orcontinent al collision. [http://education.ollusa.edu/nasa/inserv/rocktrip9-19.html] , [http://uts.cc.utexas.edu/~rmr/protolith.html]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 theOuachita Mountains orogeny .ee also
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Geology of Texas
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