Llano Uplift

located in Central Texas

About Llano Uplift

The Llano Uplift forms the Hill country around Llano. The uplift also exposes a rim of lower Paleozoic rocks that are typically hidden under the broad expanse of Cretaceous limestone in Texas. Since building stone, talc, serpentine, iron ore, copper, silver, and gold have been produced in the past, the area has become known as the "Central Mineral Region." It turns out that there is some disagreement and uncertainty about just how long the Uplift has been an uplift. You could say the "why" is because of the granites. The Llano Uplift is an uplift because the earth's crust is thicker there than in the areas around the Uplift. Specifically the crust is both thick and composed of relatively light (probably granitic) rocks at depth. This thick light crust "floats" high on the dense rocks of the Earth's mantle, bringing very old rocks up to the surface. The reason or reasons why the crust is thicker in the Llano Uplift is more difficult to explain: The crust can be thickened when tectonic plates collide and mountain ranges are formed (as in the Himalayas today). This may have played a part, but it is not clear why this would be localized where it is, instead of being spread out along the continental margin. The Llano Uplift is also the location of a bend in the ancient continental margin, and areas like that have increased intrusion of granitic rocks.

Crustal thickening did occur during the Proterozoic (a.k.a. late Precambrian), as evidenced by a positive Bouger Gravity anomaly centered on the current Llano Uplift. However, examination of isopach (unit thickness) maps of sedimentary rocks deposited on the eroded igneous and metamorphic rocks during the Paleozoic (Barnes and Bell, 1977) show a variety of patterns that do not provide clear evidence of the uplift having always been a positive topographic feature, even relative to surrounding areas. However the units deposited in the uplift region throughout time have been shallow water faces. At no time since the Precambrian have the rocks of the Llano Uplift ever been at more than a kilometer or two of water depth. One consistent feature of the rocks deposited throughout the lower Paleozoic is that, where it is possible to interpret a depositional setting, the rocks are shallow marine (ocean) to terrestrial (land) in origin.In the Cambrian, the Hickory Sandstone was deposited on the eroded surface of the Proterozoic rocks. The lowest portions of the unit are coarse-grained fluvial and alluvial deposits. Local topographic variations of the surface of the Proterozoic of up to 243 m. [800 ft.] have been documented, and account for much of the local thickness variation of the Hickory Sandstone. Deposition of the Hickory Sandstone was uneven, and some areas of Proterozoic rock were high enough not to be covered by the Hickory Sandstone. However, the variations seem local in nature, and overall the thickness of the Hickory SS. thins to the northwest with no evidence for a specific "uplift".When the thicknesses of other sediments deposited in the various Cambrian units are examined the thickness do not always bear a relation to the uplift area as delineated by the gravity anomaly. The Lion Mountain. Sandstone of the Cambrian Riley Fm. is the first unit that has an Iopach map which suggests a high spot corresponding to the current uplift shape. In younger units, the Cambrian Wilberns Fm. as a whole does seem thinner over the Llano Uplift, but the Ordovician Ellenburger Fm. has a more complex pattern. Rocks of the Llano Uplift seem to have been re-uplifted, as it were, several times after the Cambrian.