Abstract

New single-crystal sanidine 40Ar/39Ar dates of 12 middle Eocene to Oligocene rhyolitic volcanic ash beds of the Texas coastal plains range from 30.64 ± 0.03 Ma (lower Catahoula Formation) to 41.841 ± 0.016 Ma (lower Crockett Formation). These dates are from Texas coastal plains strata in the Crockett marine transgression and the Yegua and Jackson depositional wedges. Radiometric dating of sanidine and electron microprobe analysis of volcanic apatite phenocrysts validate previous regional correlation of the Upper Alabama Ferry volcanic ash from Brazos County to Houston County, Texas, and provide support for correlation of this bed with the St. Johns bentonite deposit in Louisiana. Middle Eocene ash beds are distinguished by differences in apatite phenocryst composition. The wide distribution of the thick Alabama Ferry volcanic ashes points to an early age (41.841 ± 0.016 Ma) for the start of major explosive rhyolitic volcanism in western North America. Volcanic ash bed samples have subalkaline rhyolite composition with geochemical characteristics consistent with derivation from volcanic arc rocks. Trace-element data show a temporal change in volcanic ashes: Middle Eocene ashes are depleted in heavy rare earth elements compared to late Eocene and Oligocene units. This change in ash composition coincides with the time of transition from subduction compression to arc-rifting on the western margin of Mexico, and compositional data are consistent with the Sierra Madre Occidental being the source for the volcanic ash. Volcanic units of the eastern portion of the Trans-Pecos of Texas and Mogollon-Datil area of New Mexico overlap temporally, but geochemical characteristics and K/Ca ratios in sanidine indicate they are geochemically unlikely to be sources for the Texas coastal plains middle Eocene to Oligocene volcanic ash beds.

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