| Llano Uplift | |
|---|---|
  location of Llano Uplift inTexas | |
| Geography | |
| Country | United States |
| State | Texas |
| Region | Texas Hill Country |
| County | Llano County |
| Municipality | Llano, Texas |
| Range coordinates | 30°10′27″N99°03′55″W / 30.17417°N 99.06528°W /30.17417; -99.06528 |
| Geology | |
| Orogeny | Grenville orogeny |
| Rock age | Mesoproterozoic |
| Rock type(s) | Precambrian and Paleozoicinlier surrounded by Cretaceous uplands. |
TheLlano Uplift is a geologically ancient, lowgeologic dome that is about 90 miles (140 km) in diameter and located mostly inLlano,Mason,San Saba,Gillespie, andBlanco counties, Texas. It consists of anisland-like exposure ofPrecambrianigneous andmetamorphic rocks surrounded by outcrops ofPaleozoic andCretaceoussedimentarystrata. At their widest, the exposed Precambrian rocks extend about 65 miles (105 km) westward from the valley of theColorado River and beneath a broad, gentle topographic basin drained by theLlano 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 theEdwards Plateau, which consist of overlying Cretaceous sedimentary strata. Within this basin and along its margin aredown-faulted blocks and erosional remnants of Paleozoic strata which form prominent hills.[1][2][3]
The Llano Uplift is well known for its large,granite domes, such asEnchanted Rock. The area includes several major rock quarries likeGranite Mountain that mine the distinctive pink granite. Further, the area contains the only known deposits ofllanite.[4]
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.[citation needed]
The Llano Uplift can be considered an uplift by either its pattern on ageological orstructural map of the top of the Precambrian rocks. It qualifies as an uplift because it consists of an extensive Precambrianbasement high that is exposed by virtue of its surface lying significantly above in elevation the surface of surrounding Precambrianbasement. 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 multiplegeologic structures at multiple times during theCarboniferous and Cretaceous periods.[5]
Precambrian rocks directly underlie the surface of the central and topographically lowest part of the Llano Uplift within a low-reliefbasin drained of theLlano River and eastward to the valley of theColorado River. These rocks consist of about 9,000 km2 (3,500 sq mi) of MiddleProterozoiccrystalline basement exposed in anerosional window eroded through overlyingPhanerozoicsedimentarystrata. The Precambrian basement is cut by numerous normal and oblique-slip faults, the result of theOuachita Orogeny, that juxtaposePaleozoic strata with the Precambrian rocks.[2][6][7]
The Precambrian rocks consist of multiply deformed,metasedimentary,metavolcanic and metaplutonic rocks that range in age from 1.37 to 1.23 Ga. Thesemetamorphic rocks have beenintruded by 1.13 to 1.07 Ga, syntectonic to post-tectonicgranites.[2][5][6] These rocks can be divided into threefault-bounded blocks of strata calleddomains. They are called theValley Spring,Packsaddle, andCoal Creek domains. Each of these domains contain distinctive rock types and ages and were eithererupted, intruded, ordeposited in three separate areas and later tectonically juxtaposed during theGrenville Orogeny. TheValley spring domain consists mainly ofgneiss that is composed ofquartz andmicroclinefeldspar with minorbiotite andhornblende. Likely, this gneiss consists of highly metamorphosed sedimentary,volcanic, andintrusive rocks that includerhyolitelava flows andash-flowtuffs; igneous intrusions; andarkose interbedded with minorlimestone, andshale. The age of these metamorphic rocks range from range from 1.29 to 1.23 Ga. ThePacksaddle domain consists mainly ofschists composed ofhornblende,biotite,muscovite, andactinolite;marbles and calc-silicate rocks;quartzites; and quart-feldspar gneiss. These rocks were likely originally marine limestone, shale, andsandstone interbedded withmafic andfelsic volcanic rocks and intrusivesills. 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. TheCoal Creek domain consists of a 6.4-kilometer-long (4.0 mi) mass ofserpentinite (TheCoal Creek Serpentinite) that is surrounded by meta-igneous quartz-plagioclase gneiss of theBig 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 containsdioriteplutons,gabbro,amphibolite, mafic schist, minortalc, and smaller serpentinite bodies, all of which were metamorphosed about 1.26 Ga. The Coal Creek domain appears to represent fragments of anisland arc with a slice of oceanicmantle faulted into it.[2][5][6][7]
After 1.2 Ga, a global cycle ofcontinental collision and the resultingmountain formation, globally called theGrenville orogeny, and locally called the Llano Orogeny, tectonically shoved and interleaved together these strata. They were also further altered bymetamorphism into the rocks thatoutcrop 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 MiddleCambriansedimentation,erosion removed several kilometers of Precambrian rock.[2][5][6]
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 600 meters (2,000 ft) of Cambrian sandstones, limestones, anddolomites of theMoore Hollow Group and Lower Ordovician limestone and dolomite of theEllenburger Group.[2][5] The Moore Hollow Group which consists of theHickory sandstone,Cap Mountain limestone, andLion Mountain sandstone, and theWilberns Formation, which consists of sandstone, limestone, shale, and an upper mixture of limestone and dolomite.[2][8] The Ellenburger Group is an incomplete sequence of LowerOrdovician strata, which are divided, from the base up, into theTanyard,Gorman, andHoneycut formations. These formations contain limestones and dolomites, which are typicallynonglauconitic and sparinglyfossiliferous. An erosional, pre-Devonianpaleosurface with well-developed paleokarst truncates the Ellenberger Group such that it thins from a thickness of 570 meters (1,870 ft) in the southeastern corner of the Llano region to only 250 meters (820 ft) in the northwestern corner of the region.[2][9]
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 marinesediments. The Cambrian sea spread northward across the eroded surface an area of Precambrian rocks that had a local relief as great as 240 meters (790 ft). As a result, sediments composed of locally derivedresiduum, often wind-abraded accumulated as a thin, discontinuouscobbleconglomerate 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-watercarbonate platforms. At the end of the Lower Ordovician, the Central Texas region was tilted eastwards and exposed to subaerial erosion andkarstification.[8][10][11]
Possible reworked Middle Ordovicianconodonts (Chirognathus) have been found in younger strata and a pocket of Upper Ordovician limestone, the Burnam Limestone, is preserved in acollapse structure in Burnet County. Thereworked 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.[12][13]
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 theSilurian 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 ofresiduum developed by the subaerial, in situ dissolution of underlying limestones and dolomites and residuum eroded and redeposited by an advancing marine shoreline.[12][13][14][15]
Like the Devonian strata found in the Llano Uplift, earlyCarboniferous (Mississippian) strata, the youngest black shale of the Houy Formation, thecrinoidal 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 shallowepicontinental seas and marine sedimentation alternating with long periods of terrestrial exposure during which these marine sediments were almost completely removed by erosion.[13][14][15][16][17]
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 nearMarble 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.[2][16][17]
The succession of Carboniferous strata within and adjacent to the Llano Uplift records the pronounced subsidence and filling of the adjacentFort Worth Basin by the westward progradation ofdelta and associatedfluvial systems from the uplifted Ouachita Mountains to the east. The Chappell Limestone and Barnett Shale represent episodic early Carboniferous inundations of the Llano region followed its submergence and formation of a carbonate platform within which the Marble Falls Limestone accumulated as a lateral equivalent to deeper water Smithwick Shale. As the Fort Worth Basin deepened and sank in responses to the Ouachita Orogeny, Smithwick Shale was deposited on former sites of Marble Falls sedimentation within the Llano Uplift region. In the deeper part of the Llano region, basin-fill shale andsubmarine fan deposits that form the lower Strawn Group accumulated. As the basin filled during the remainder of the Carboniferous and Permian, fluvial-deltaic sediments and associated shallow marine continental shelf deposits of the upper Strawn, Canyon, and Cisco group accumulated within the Fort Worth Basin.[16][18][19]
The only Mesozoic rocks that are known in the Llano region are those of the Cretaceous system. ThroughoutTriassic andJurassic 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.[20]
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 Wichitapaleoplain.[21] What little research has been conducted on the Wichita paleoplain estimates that as much as 33 meters (108 ft) 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.[5][21][22]
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 orescarpment is common.[23][24]
The Llano Uplift region is also called the Central Mineral region of Texas because of the occurrence of the great variety ofminerals found in and the numerousore prospecting pits dug into exposed Precambrian rocks and Lower Paleozoic strata. Over the decades, a few small mines have yieldedyttrium and otherrare-earth minerals,magnetite,feldspar,vermiculite,serpentine, and gem qualitytopaz. Briefly,galena aslead ore was mined from limestone lying unconformably upon granite knobs that were once hills before being submerged byrising relative sea level in the Cambrian. Minor showings ofgold,silver,copper,tin,bismuth,molybdenum,tungsten, anduranium minerals have been found and explored in prospecting pits. Before it closed in 1980, the Southwestern Graphite mine northwest ofBurnet, Texas, was the only major producer of high-puritygraphite in North America for several decades. In the past large quantities ofsoapstone were excavated from outcrops south ofLlano, 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 offracturing sand (“Frac sand”),crushed stone, and building stone. Granite has been quarried from almost innumerable localities and the active production ofdimension stone continues today from a dome of coarse pink Town Mountain Granite nearMarble Falls, Texas.[4][25]
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