Tephrochronology and Stratigraphy of Eocene and Oligocene Volcanic Ashes of East and Central Texas



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Sedimentary formations of east and central Texas contain many Eocene to Oligocene volcanic ash beds, with some of the younger ash layers containing hydrated but otherwise unaltered glass shards. This study analyzed samples of 15 volcanic ash beds using neutron activation analysis (NAA) of bulk ash and glass shards, inductively coupled plasma mass spectrometry (ICPMS) of bulk ash, and electron microprobe analysis of both apatite phenocrysts and glass shards to characterize their geochemistry.

^(40)Ar/^(39)Ar dating of single sanidine phenocrysts gives an age of 30.64 ?0.03 Ma for the youngest (Sam Rayburn) sample to 41.79 ?0.02 Ma for the oldest (Hurricane Bayou) sample. The nine radiometric dates obtained by this study serve to better constrain the ages of the Claiborne and Jackson Groups and the Catahoula Formation of Texas with the Conquista and Hurricane Bayou ash beds being possible calibration points for the Eocene/Oligocene and Lutetian/Bartonian boundaries, respectively.

Geochemical fingerprinting, particularly apatite phenocryst chemistry, supports the correlation of the Little Brazos volcanic ash in Brazos County to volcanic ash deposits in 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) volcanic ash to deposits within the Gonzales County bentonite mines of south-central Texas.

Major element electron microprobe data from seven samples of volcanic glass shards indicate the ash was produced from sub-alkaline rhyolitic volcanism and the trace-element characteristics of all 15 ashes are consistent with subduction-related sources. Rare earth element (REE) data indicates light rare earth element (LREE) enrichment with a moderate Europium anomaly. The Sierra Madre Occidental of Mexico is the likely source area, but the Trans-Pecos of Texas and Mogollon-Datil of New Mexico cannot be definitively ruled out as possible eruptive source regions.