GEOCHEMICAL CHARACTERIZATION OF EOCENE AND OLIGOCENE VOLCANIC ASHES OF EAST AND CENTRAL TEXAS

Sedimentary formations of east and central Texas contain many volcanic ash beds in strata of Late Middle Eocene to Oligocene age with many of the younger volcanic ashes containing abundant volcanic glass shards. This study analyzed 16 samples of volcanic ash beds from Brazos County and adjacent areas using electron microprobe, individual neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). Major-element data from 6 samples bearing volcanic glass were analyzed by electron microprobe and indicate that the volcanism is alkaline rhyolitic and the trace-element characteristics of all 16 ashes are consistent with subduction-related sources. The timing and geochemical characteristics of the volcanism suggests that the Sierra Madre Occidental of Mexico or the Trans-Pecos of Texas as the likely eruptive source. Rare-earth element data indicates light-rare-earth enrichment with a moderate Europium anomaly. Geochemical fingerprinting confirms correlation of the Little Brazos volcanic ash from Brazos to Houston County, Texas, and shows equivalence to the St. Johns bentonite in Louisiana. Major- and trace-element groupings on variation diagrams suggest the possibility of further refining regional correlations using these geochemical fingerprinting methods, particularly when combined with our ongoing efforts to add apatite phenocryst chemistry.