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.
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 an 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.
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.
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 -long 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.
