Abstract

The lower Eocene Sabinetown Formation of Bastrop County 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 75 to 100 ft in Bastrop County and strikes northeastward with a SE. 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 phenocrysts, and a very strong kyanite-staurolite suite. Petrographic studies and general field relationships indicate that the Sabinetown is mainly composed of a complex, heterogeneous association of detritus probably derived from the Ouachita Mountains of Oklahoma and Arkansas and intimately mixed with abundant material eroded from the sands and silts exposed in more local outcrops. Volcanic material, possibly from S. Texas, was also entering the basin of deposition. The marked disconformity which separates the Sabinetown and Carrizo formations is 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 Carrizo 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 Claiborne when the southern Appalachians began providing high-rank metaquartzite and a strong kyanite-staurolite suite to the overlying Carrizo Formation.

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