Техас штатындағы геологиялық дөңгелек (Texas shtatyndaghy geologiyalyq döńgelek)

Geologic dome in Texas, United States

The Llano Uplift is a geologically ancient, low geologic dome that is about 90 mile (145 km) 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 65 mile (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.

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. 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. 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. 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.

Within the Llano Uplift, fossiliferous Silurian and Devonian strata occur preserved as the fills of solution and collapse structures that vary in size from large structural sinks to crack fillings a few inches or less in width. Isolated deposits of fossiliferous Starcke Limestone preserved in ancient sinkholes developed in the Ellenberger Group provide definite evidence of the Llano region having been inundated by marine waters at least once during the Silurian Period. Fossiliferous Devonian limestones of various types that are preserved in cave fills, collapse depressions, and other paleokarst features develop in the Ellenberger Group also demonstrate that the Llano region was also episodically inundated by marine waters during the Devonian period. During periods of subaerial exposure, these deposits were largely stripped from the region of the Llano Uplift. The pockets and remnants of Devonian strata preserved in paleokarst included the Bear Spring Formation, Pillar Bluff Limestone, Stribling Formation, and, in part, the Houy Formation. Breccias found at the base of the base of the pockets of Devonian strata likely represent a mixture of residuum developed by the subaerial, in situ dissolution of underlying limestones and dolomites and residuum eroded and redeposited by an advancing marine shoreline.

Like the Devonian strata found in the Llano Uplift, early Carboniferous (Mississippian) strata, the youngest black shale of the Houy Formation, the crinoidal limestone of Chappel Limestone and the black shale Barnett Formation consist of at most a few meters of strata preserved within collapse structures and other paleokarst. As in case of the Devonian and Silurian strata found within the Llano uplift, these strata represent brief periods of inundation of the region by shallow epicontinental seas and marine sedimentation alternating with long periods of terrestrial exposure during which these marine sediments were almost completely removed by erosion. Late Carboniferous ( Lower Pennsylvanian ) strata are in large part exposed in three non contiguous areas. First, an isolated areal exposure of Smithwick Shale and underling Marble Falls Limestone occurs near Marble Falls, Texas, area in southwestern Burnet County. Second, in southwestern Mason County and northeastern Kimble County, late Carboniferous Marble Falls Limestone overlying relatively thin early Carboniferous strata is exposed within a half dozen, isolated fault blocks on the southwestern periphery of the Llano region. Finally, late Carboniferous are exposed as within a triangular shaped region that is bisected by the Colorado River along the northwest, north, and northeast periphery of the Llano region in McCulloch, San Saba, and Lampasas counties. In this area, Marble Falls Limestone, Smithwick Shale, and lower Strawn Group are well exposed. The strata of the lower Strawn Group are truncated by an erosional unconformity that is overlain by much younger Cretaceous strata.

The only Mesozoic rocks that are known in the Llano region are those of the Cretaceous system. Throughout Triassic and Jurassic periods, the Llano region was eroded. The accumulation of Triassic, terrestrial red beds of the Dockham Group may have reached to the western edge Llano region. However, they were eroded back to its present position and underlying strata eroded during the Triassic and Jurassic in response regional tilting and uplift. By the time that the regions of the Llano Uplift was slowly covered by Cretaceous sedimentary deposits, it had been reduced by erosion to a low relief erosion surface termed the Wichita paleoplain. What little research has been conducted on the Wichita paleoplain estimates that as much as of relief exists on this surface cut into the underlying strata. During the Cretaceous, this surface was progressively buried by the accumulation of fluvial and coastal sediments of the Trinity Group and later by the Walnut, Comanche Peak, and Edwards formations.

Erosion that has occurred since the withdrawal of Cretaceous seas has resulted in a topographic inversion. As a result, the oldest and structurally highest rocks tend to occur at the lowest topographic elevations. Where the Cretaceous rocks rim the Llano uplift, a sharp topographic rise or escarpment is common.

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 prospecting 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 unconformably 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. 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.