Paper No. 1
Presentation Time: 1:15 PM


ALLISON, David T., Earth Sciences, University of South Alabama, LSCB 136, Mobile, AL 36688,

The Llano Uplift of central Texas is an excellent destination for geological field trips or field courses focused on poly-deformed terranes and basement-cover relationships. For trips originating in the southeastern U.S., the region is more geographically proximal than other regions of equivalent exposure quality and complexity. The Llano Uplift region consists of a regional topographic basin surrounded by the higher Edwards plateau, itself underlain by Gulf Coastal Plain units. The elevations in the Llano Uplift range from approximately 900 to 1400 feet above sea level with a typical local relief of 200-300 feet. The local terrain is a grassland savanna with dispersed copses of mesquite, cedar scrub, and Texas live oak. The open terrain makes the region very accessible to geologists because of the lack of thick vegetation. In addition the modest topographic relief and low absolute elevation negate the need for a period of acclimation for those residing near sea-level. Although high temperature can be an issue in mid-summer, the generally warm climate makes field trips to the region possible at any time of the year.

The Grenville basement rocks of the Llano Uplift are composed of a poly-deformed eugeoclinal tectonostratigraphy intruded by mainly post-orogenic granitic intrusions. Metamorphic grade ranges from middle-amphibolite to granulite facies. The Coal Creek ophiolite sequence marks the suture between Rodinia and an accreted terrane.

The Grenville basement rocks are non-conformably overlain by a series of transgressive Cambro-Ordovician sedimentary cover sequences dominated by carbonate sedimentary rocks. Several units such as the Lion Mt. sandstone member of the Riley formation are fossiliferous, containing trilobite bioherms. Structurally the Precambrian basement and Paleozoic cover are cut by normal faults related to the Marathon Uplift (Ouachita orogeny). Examples of drag-folding along fault contacts can be observed at the Lake LBJ scenic overlook. The topographic expression of the faulting produces map-scale northeast-trending ridges that mark down-faulted grabens. The topographic inversion is a result of carbonate cover rocks being more erosionally resistant than basement rocks.