Recognition and Implications of Volcanic Glass Detritus in the Fluvial Deposits of the Middle Frio Formation, South Texas

KERR, DENNIS R., and JEFFRY D. GRIGSBY, University of Texas at Austin, Austin, TX

Detrital volcanic glass has been identified in fluvial deposits of the middle Frio Formation (Oligocene) of south Texas. Petrographic analysis of core samples from Seeligson and Stratton fields reveals abundant unabraded to slightly abraded, very fine sand- to silt-size glass shards. Gamma-ray log response faithfully records the presence of volcanic glass with a marked increase in API units through the middle Frio interval containing volcanic glass detritus.

Occurrence of volcanic glass is limited to the medial third of the 2000- to 2500-ft-thick middle Frio Formation. Detrital frame-work composition coupled with a sharp increase in API unit counts on gamma-ray logs suggest that volcanic ash was deposited rapidly on the coastal plain and was, at least in the Stratton area, conducted to and deposited in channel systems. The amount of ash/glass diminishes with time. Given the regional biostratigraphic constraints of the age of middle Frio fluvial deposits, the interval containing volcanic glass is regarded as representing the 30-34 Ma eruptive event in the Big Bend region.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)