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Sanidine 40Ar/39Ar dates and zircon U-Pb dates of middle Eocene to late Oligocene volcanic ash beds provide high-resolution geochronology for the northern Gulf of Mexico. The dates coincide with silicic volcanism generated as North America moved closer to the Pacific spreading axis. Ten new dates are reported, five for upper Eocene Jackson Group strata and five for Oligocene Catahoula Group strata. Dating is extended to south Texas and the Rio Grande Valley. Clusters of radiometric dates at ca. 34.1–34.5 Ma and at ca. 35.7–35.8 Ma indicate times of greater volcanic eruptive activity during the late Eocene. The ca. 34.1–34.5 Ma cluster occurs across the northern Gulf of Mexico from south Texas to Mississippi. Airborne volcanic ash plumes carried sanidine grains as coarse as 500 μm as much as 650 km away from the closest eruptive center, and grains to 150–200 μm occur in ash beds 600–800 km from the closest source. Volcanic ash bed dates do not correspond with any dated calderas in the southwestern United States. Long-distance transport of relatively coarse crystals has important implications for use of detrital mineral geochronology in paleodrainage studies. Early Rupelian dates of lower Gueydan Formation strata in the Catahoula Group are coeval to lower Vicksburg Group in the subsurface. The previous interpretation of a long-duration early Oligocene depositional hiatus in Texas is not supported and is replaced with the interpretation of early Oligocene nonmarine fan deposition in south Texas.

Eocene–Oligocene chronostratigraphy of ignimbrite flareup volcanic ash beds on the Gulf of Mexico coastal plains