PETROLOGY OF THE EOCENE SABINETOWN-CARRIZO

The lower Eocene Sablnetown Formation of Bastrop County, Texas, is a moderately sorted, silty, very fine-grained subgraywacke sand which is overlain with marked disconformity by the middle Eocene Carrizo, a medium-grained subgraywacke sand. The deltaic Sabinetown, which attains a thickness of 73 to 100 feet in Bastrop County and strikes northeastward with a southeast dip, is characterized by low-rank, chloritic, metasedimentary rock fragments, basic volcanic rock fragments, plagioclase, glauconite, and a heavy mineral suite which is rich in garnet, zircon, and sphene. The fluvial Carrizo is characterized by high-rank metaquartzite, acid volcanic quartz pheuocrysts, and a very strong kyanite-staurolite suite. Petrographic studies and general field relationships indicate that the Sabinetown is mainly com-posed of a complex, heterogeneous association of detritus probably derived from the Ouachita Moun-tains of Oklahoma nd Arkansas and intimately mixed with abundant material eroded from the sands and silts exposed in more local outcrops. Volcanic material, possibly from south Texas, was also entering the basin of deposition. The marked disconformity which separates the Sabinetown and Carrizo formations i the result of a tectonic pulse which caused a regression of the lower Eocene sea and permitted erosion to attack the newly deposited Sabinetown sediments. The Carrlzo accumulated in the erosional lows with its continental, crossbedded sand resting directly on the glauconitic, fine-grained marine Sabinetown. This tectonic activity further caused a significant readjustment in drainage patterns and the dominance of the Ouachita source during the deposition of the Wilcox Sabinetown was weakened by the beginning of the Clalborne when the southern Appalachians began providing high-rank metaquartzite and a strong kyanite-staurolite suite to the overlying Carrizo Formation