TEPHROCHRONOLOGY AND GEOCHEMICAL FINGERPRINTING OF EOCENE AND OLIGOCENE VOLCANIC ASHES OF EAST AND CENTRAL TEXAS

Sedimentary formations of east-central Texas contain many Eocene to Oligocene volcanic ash beds, with many of the younger ash layers containing hydrated but otherwise unaltered glass shards. This study analyzed samples of 16 volcanic ash beds using instrumental neutron activation analysis of bulk ash and glass shards, inductively coupled plasma mass spectrometry of bulk ash, and electron microprobe analysis of both apatite phenocrysts and glass shards to characterize their geochemistry. Rare-earth element data indicates light-rare-earth enrichment with a moderate Europium anomaly. New 40Ar/39Ar dating of tiny single crystal sanidine gives a precise age of 34.130±0.007 Ma for the youngest (South Somerville) sample at the top of the Jackson Group and 41.78±.01 Ma for the oldest (Hurricane Bayou) sample. Geochemical fingerprinting, particularly apatite phenocryst chemistry, supports the correlation of the Little Brazos volcanic ash from Brazos to Houston County, Texas, and provides supporting evidence for equivalence to the St. Johns bentonite in Louisiana. Geochemical fingerprinting also suggests equivalence of the Caddell (Koppe Bridge) ash to deposits within the Gonzales Co. bentonite mines of south-central Texas. Major-element electron microprobe data from 6 samples of volcanic glass shards indicate alkaline rhyolitic volcanism and the trace-element characteristics of all 16 ashes are consistent with subduction-related sources. The Sierra Madre Occidental of Mexico or the Trans-Pecos of Texas are possible eruptive source regions. 40Ar/39Ar dates are ±1 sigma analytical error only and relative to Fish Canyon Tuff sanidine at 28.201 Ma and a 40K decay constant of 5.463e-10 /a.