THE LLANO UPLIFT: AN OUTSTANDING DESTINATION FOR GEOLOGICAL FIELD TRIPS AND FIELD GEOLOGY COURSE MAPPING PROJECTS
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.