ABSTRACT

Detrital volcanic glass has been identified in fluvial deposits of the middle Frio Formation of South Texas. Petrographic analysis of core samples from Seeligson and Stratton fields reveals abundant unabraded to slightly abraded, very fine sand- to silt-sized glass shards. Gamma-ray log response correlates with the presence of volcanic glass. Volcanic glass and API unit counts markedly increase through the medial third of the middle Frio and diminish toward the upper third of the middle Frio.

The stratigraphic and aerial distribution of volcanic glass detritus in middle Frio sandstones and mudstones suggests that volcanic ash was deposited rapidly on the Oligocene coastal plain of South Texas. Ash was apparently also deposited in the catchment basin of fluvial systems delivering sand to the Stratton area. Given the regional biostratigraphic constraints of middle Frio fluvial deposits, the interval containing volcanic glass is regarded as representing the 30- to 34-mya eruptive event in the Trans-Pecos region.