Tancy and three co-authors reported in 2018 that 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.
Outcrops of the Catahoula Formation across Texas and Louisiana (gray); gaps in
the belt are areas covered by younger deposits. Triangles represent sample
collection sites. Inset map shows locations of the Sierra Madre, the Yellowstone
eruptive center, and the Central American Volcanic Arc (CAVA) relative to the
Catahoula Formation outcrops.(Yancey, 2019)
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 clos-est 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 Guey-dan 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.
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