The Llano Uplift is a geologically ancient, low geologic dome that is about in diameter and located mostly in Llano, Mason, San Saba, Gillespie, and Blanco counties, Texas. 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 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.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.
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.
In 1992, the Texas Department of Health identified the area as one of four regions with high potential for the presence of hazardous levels of radon gas.
Geology
The Llano Uplift can be considered an uplift by either its pattern on a geological or structural map of the top of the Precambrian rocks. It qualifies as an uplift because it consists of an extensive Precambrian basement high that is exposed by virtue of its surface lying significantly above in elevation the surface of surrounding Precambrian basement. However, the Llano Uplift may not have been ever uplifted as a distinct entity and at a single time as a basement high. Rather, it formed by the areas surrounding it having subsided around it and the Precambrian rocks underlying it having been elevated by the formation and interaction of multiple geologic structures at multiple times during the Carboniferous and Cretaceous periods.Ewing, T.E., 2016. Texas Through Time: Lone Start Geology, Landscapes, and Resources. Texas Bureau of Economic Geology, Austin TX; 431 p.
Precambrian rocks Precambrian rocks directly underlie the surface of the central and topographically lowest part of the Llano Uplift within a low-relief basin drained of the Llano River and eastward to the valley of the Colorado River. These rocks consist of about of Middle Proterozoic crystalline basement exposed in an erosional window eroded through overlying Phanerozoic sedimentary strata. The Precambrian basement is cut by numerous normal and oblique-slip faults, the result of the Ouachita Orogeny, that juxtapose Paleozoic strata with the Precambrian rocks.Barnes, V.E., Bell, W.C., Clabaugh, S.E., Cloud, Jr., P.E., McGehee, R.V., Rodda, P.U., and Young, K., 1972, Geology of the Llano region and Austin area, field excursion The University of Texas at Austin, Bureau of Economic Geology, Guidebook no. 13, 77 p.Mosher, S., 1998. Tectonic evolution of the southern Laurentian Grenville orogenic belt. Geological Society of America Bulletin, 110(11), pp. 1357-1375.Mosher, S., Helper, M., and Levine, J., 2008, The Texas Grenville Orogen, Llano Uplift, Texas, Field trip guide to the Precambrian geology of the llano uplift. Trip 405 for the Geological Society of America (GSA) Annual Meeting, Houston, Texas, GSA Structural Geology and Tectonics Division, Boulder, Colorado.
The Precambrian rocks consist of multiply deformed, metasedimentary, metavolcanic and metaplutonic rocks that range in age from 1.37 to 1.23 Ga. These metamorphic rocks have been intruded by 1.13 to 1.07 Ga, syntectonic to post-tectonic granites. These rocks can be divided into three fault-bounded blocks of strata called domains. They are called the Valley Spring, Packsaddle, and Coal Creek domains. Each of these domains contain distinctive rock types and ages and were either erupted, intruded, or deposited in three separate areas and later tectonically juxtaposed during the Grenville Orogeny. The Valley spring domain consists mainly of gneiss that is composed of quartz and microcline feldspar with minor biotite and hornblende. Likely, this gneiss consists of highly metamorphosed sedimentary, volcanic, and intrusive rocks that include rhyolite lava flows and ash-flow tuffs; igneous intrusions; and arkose interbedded with minor limestone, and shale. The age of these metamorphic rocks range from range from 1.29 to 1.23 Ga. The Packsaddle domain consists mainly of schists composed of hornblende, biotite, muscovite, and actinolite; marbles and calc-silicate rocks; quartzites; and quart-feldspar gneiss. These rocks were likely originally marine limestone, shale, and sandstone interbedded with mafic and felsic volcanic rocks and intrusive sills. They date from 1.27 to 1.25 Ga. Granitic sills that intrude these rocks have been dated from 1.255 to 1.250 Ga. The Coal Creek domain consists of a mass of serpentinite (The Coal Creek Serpentinite) that is surrounded by meta-igneous quartz-plagioclase gneiss of the Big Branch Gneiss. The gneiss has been dated at 1.33 to 1.30 Ga and was metamorphosed about 1.29 Ga, earlier than any other Llano metamorphic rocks. Coal Creek domain also contains diorite plutons, gabbro, amphibolite, mafic schist, minor talc, and smaller serpentinite bodies, all of which were metamorphosed about 1.26 Ga. The Coal Creek domain appears to represent fragments of an island arc with a slice of oceanic mantle faulted into it.
After 1.2 Ga, a global cycle of continental collision and the resulting mountain formation, globally called the Grenville orogeny, and locally called the Llano Orogeny, tectonically shoved and interleaved together these strata. They were also further altered by metamorphism into the rocks that outcrop today in the Llano Uplift. Large granitic plutons that locally form a large percentage of the outcrop in some areas and a llanite dike also intruded them. During the 400-million year interval between the emplacement of llanite and the start of Middle Cambrian sedimentation, erosion removed several kilometers of Precambrian rock.
Lower Paleozoic (Cambrian and Ordovician) Within and around the Llano uplift are erosional remnants and down-faulted blocks of Lower Paleozoic sedimentary strata. Within the Llano Uplift, these remnants and fault blocks often form prominent hills. The Lower Paleozoic strata are composed of over of Cambrian sandstones, limestones, and dolomites of the Moore Hollow Group and Lower Ordovician limestone and dolomite of the Ellenburger Group. The Moore Hollow Group which consists of the Hickory sandstone, Cap Mountain limestone, and Lion Mountain sandstone, and the Wilberns Formation, which consists of sandstone, limestone, shale, and an upper mixture of limestone and dolomite.Barnes, V.E., and W.C. Bell, 1977, The Moore Hollow Group of Central Texas. Report of Investigations 88, 169 p. University of Texas, Austin, Bureau of Economic Geology. The Ellenburger Group is an incomplete sequence of Lower Ordovician strata, which are divided, from the base up, into the Tanyard, Gorman, and Honeycut formations. These formations contain limestones and dolomites, which are typically nonglauconitic and sparingly fossiliferous. An erosional, pre-Devonian paleosurface with well-developed paleokarst truncates the Ellenberger Group such that it thins from a thickness of in the southeastern corner of the Llano region to only in the northwestern corner of the region.Cloud, P. E., and Barnes, V. E., 1948, The Ellenburger Group of Central Texas. Publication 4621, 473 p. University of Texas, Austin, Bureau of Economic Geology.
The Moore Hollow Group records the advancing of a sea from the southeast across eroded Precambrian rocks during Middle to Late Cambrian times and subsequent burial beneath coastal and nearshore marine sediments. The Cambrian sea spread northward across the eroded surface an area of Precambrian rocks that had a local relief as great as . As a result, sediments composed of locally derived residuum, often wind-abraded accumulated as a thin, discontinuous cobble conglomerate overlying Precambrian strata at the base of the Moore Hollow Group. Following the deposition of the uppermost Cambrian limestones and dolomites, the Lower Ordovician Ellenburger Group (composed of the Tanyard, Gorman, and Honeycut Formations) accumulated within shallow-water carbonate platforms. At the end of the Lower Ordovician, the Central Texas region was tilted eastwards and exposed to subaerial erosion and karstification.Cloud, P.E. and Barnes, V.E., 1948. Paleoecology of the early Ordovician sea in central Texas. In National Research Council, Division Geology and Geography, Report of The Committee on a Treatise on marine ecology and paleo-ecology. 8, pp. 29-83.Cloud, P.E. and Barnes, V.E., 1957. In H.S. Ladd, ed. Treatise on marine ecology and paleoecology. Geological Society of America Memoir 67:163– 214.
Possible reworked Middle Ordovician conodonts (Chirognathus) have been found in younger strata and a pocket of Upper Ordovician limestone, the Burnam Limestone, is preserved in a collapse structure in Burnet County. The reworked conodonts and the Burnam Limestone indicate that the region of the Llano Uplift was likely either partially and briefly submerged during the Middle and Upper Ordovician only to have the sediments deposited during these inundations removed by later erosion.Barnes, V.E., Cloud Jr, P.E. and Duncan, H., 1953. Upper Ordovician of central Texas. American Association of Petroleum Geologists Bulletin, 37(5), pp. 1030-1043.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.
Middle Paleozoic (Silurian and Devonian)