ABSTRACT
A clearer picture of when and how the Llano uplift attained its present segmented domal form is sought by interpretation of data from various outcrop and subsurface studies.
A series of thickness and structural maps have been prepared. These indicate that broad initial warping of the Concho arch, the uptilted southeast part of which forms the present Llano uplift, occurred during the interval between deposition of Ellenburger and Mississippian rocks. Pronounced development of the northeast, southeast and southwest flanks of the Llano uplift took place during Medial Pennsylvanian (Lampasas-Strawn) time when the deep Strawn and Kerr basins were formed as foredeeps paralleling the Ouachita orogenic system. Much rifting accompanied or followed these subsidences, particularly where the connecting trough crossed the central and eastern parts of the Llano region.
Considerable folding and elevation occurred along northeast-southwest trends at the close of Lampasas and probably at intervals during Strawn time. Maximum uplift and erosion evidently centered in northwest Mason County, Canyon sediments having been deposited on Cambrian rocks in this area where the extensive Bend flexure-Richland Springs and Concho axes intersect. Development of a pronounced west flank and general erosion of Paleozoic rocks of the Llano uplift awaited regional elevation to the east and subsidence to the west, mainly during Permian and Triassic time.
Periodic accentuation of the Concho-Llano-San Marcos structural axis may be attributed to the presence of a relatively buoyant or stable region in the earth’s crust. However, the absence of positive tendencies along this trend during the Cambrian and Early Ordovician seems to contradict this theory. Deltaic loading, particularly by Lower Pennsylvanian sediments in the Ouachita and Marathon regions, may have brought compensating positive tendencies to the broad intervening region, thereby influencing subsequent diastrophic and depositional developments.
