|District||Texas Hill Country, Llano County, Texas|
|Age of rock||Mesoproterozoic|
|Type of rock||Precambrian and Paleozoic inlier surrounded by Cretaceous uplands.|
The Llano uplift is a low geologic dome that is about 90 miles (140 km) in diameter. It consists of an island-like exposure of Precambrian igneous and metamorphic rocks surrounded by outcrops of Paleozoic and Cretaceous sedimentary strata. At their widest, the exposed Precambrian rocks extend about 65 miles (105 km) westward from the valley of the Colorado River and beneath a broad, gentle topographic basin drained by the Llano River. The subdued topographic basin is underlain by Precambrian rocks and bordered by a discontinuous rim of flat-topped hills. These hills are the dissected edge of the Edwards Plateau, which consist of overlying Cretaceous sedimentary strata. Within this basin and along its margin are down-faulted blocks and erosional remnants of Paleozoic strata which form prominent hills.
The Llano Uplift is well known for its large, granite domes, such as Enchanted Rock. The area includes several major rock quarries like Granite Mountain that mine the distinctive pink granite. Further, 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. What is not in doubt is 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.
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 orogeny.
Central Mineral region
The Llano Uplift region is also called the Central Mineral region of Texas because of the occurrence of the great variety of minerals found in and the numerous ore propsecting pits dug into exposed Precambrian rocks and Lower Paleozoic strata. Over the decades, a few small mines have yielded yttrium and other rare-earth minerals, magnetite, feldspar, vermiculite, serpentine, and gem quality topaz. Briefly, galena as lead ore was mined from limestone lying uncomfortably upon granite knobs that were once hills before being submerged by rising relative sea level in the Cambrian. Minor showings of gold, silver, copper, tin, bismuth, molybdenum, tungsten, and uranium minerals have been found and explored in prospecting pits. Before it closed in 1980, the Southwestern Graphite mine northwest of Burnet, Texas, was the only major producer of high-purity graphite in North America for several decades. Also, in the past, large quantities of soapstone were excavated from outcrops south of Llano, Texas, and ground for use as insecticide carrier and inert filler in various products. The principal mineral resources currently produced from Central Mineral region consist of fracturing sand (“Frac sand”), crushed stone, and building stone. Granite has been quarried from almost innumerable localities and the active production of dimension stone continues today from a dome of coarse pink Town Mountain Granite near Marble Falls, Texas.
- Barnes, V.E., Bell, W.C., Clabaugh, S.E., Cloud, P.E., Jr., Young, K., and McGehee, R.V., 1962. Field Excursion No. 1, November 10-11, 1962: Geology of Llano Region and Austin Area, in Rainwater, E.H. and Zingula, R.P., eds., Pp 58-61. Geology of the Gulf Coast and Central Texas, and Guidebook of Excursions. Houston Geological Society, Houston, Texas. 391 pp.
- Clabaugh, S.E., and McGehee, R.V. 1972, Precambrian rocks of Llano region, in Barnes, V.E., Bell, W.C., Clabaugh, S.E., Cloud, P.E., Jr., McGehee, R.V., Rodda, P.U., and Young, K., eds., Pp. 9-23. Geology of the Llano region and Austin area. Texas Bureau of Economic Geology Guidebook 13, University of Texas, Austin, Texas. 77 pp.
- Seddon, G., 1970. Pre-Chappel conodonts of the Llano region, Texas. Texas Bureau of Economic Geology Report of Investigation no. 68, University of Texas, Austin, Texas. 130 pp.
- Petrossian, R., Michael Jacobs, P.G., Meinshausen, M., Guide, F., Mine, V.S. and Maymi, N., 2016. Economic Geology Resources of the Llano Uplift Region and the Historical Impacts to the Region’s Growth. Guidebook to the Texas Section- American Institute of Professional Geologists Spring Field Trip, Llano Uplift Region, Central Texas: May 14-15, 2016. American Institute of Professional Geologists, Houston, Texas. 71 pp.
- Reed, Robert M. and Helper, Mark A. (1994) Evidence for solid-state deformation of ~1.1 Ga "anorogenic" granites in the Llano Uplift, Texas. Geological Society of America Abstracts with Programs, 26, 1, 25
- Town Mountain Granites map
- Central Mineral Region field trip. September 19, 1998
- Reed, Robert M., et. al., SEDIMENTARY STRUCTURES, PALEO-WEATHERING, AND PROTOLITHS OF METAMORPHIC ROCKS, GRENVILLIAN LLANO UPLIFT, CENTRAL TEXAS
- Rainwater, E.H. and Zingula, R.P., 1962. Geologic History of Central Texas: Precambrian Rocks of Llano Region. in Rainwater, E.H. and Zingula, R.P., eds., pp. 62-106,1962. Geology of the Gulf Coast and Central Texas and Guidebook of Excursions. Houston Geological Society, Houston, Texas. 391 pp.