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.
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. 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 paleo
karst 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.
Middle Paleozoic (Silurian and Devonian) 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.
Late Paleozoic (Carboniferous and Permian) 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.
Mesozoic 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.
Cenozoic 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. == Central Mineral region ==