The Age and Geochemistry of Volcanic Ash in the Catahoula Formation of Louisiana, Mississippi, and Texas, USA

New radiometric dating of volcanic ash in Oligocene-Miocene nonmarine deposits in Louisiana, Mississippi, and Texas reveals regional distribution of early Oligocene volcanic ash across the northern part of the Gulf of Mexico from Texas to Mississippi. Fallout volcanic ash in the Chalk Hills Member of the lower Catahoula Formation type section is dated with cooccurring sanidine and zircon, yielding a sanidine Ar-40/Ar-39 date of 30.149 +/- 0.011 Ma and a zircon U/Pb date of 29.98 +/- 0.05 Ma; bentonite in quarry exposure of the Glendon Formation at Brandon, Mississippi, yields a zircon U/Pb date of 32.59 +/- 0.13 Ma. These dates are close to the age of 30.65 +/- 0.06 Ma volcanic ash in the lower Catahoula Formation of east Texas. The geochemistry of the Louisiana Chalk Hills Member volcanic ash matches that of the lower part of the Catahoula Formation in Texas. Trace-element and rare earth element (REE) compositions of single glass shards of east Texas volcanic ash reveal a dual-source composition. Trace-element and REE compositions indicate a normal mid-ocean ridge basalt composition and a subduction-melt source. This correlates with the west North American ignimbrite flare-up of Mexico. The problematic palynological late Miocene (similar to 9 Ma) dating of upper Catahoula Formation strata in Catahoula Parish, Louisiana, is reconciled by proposing the presence of a fault in the Catahoula Formation type section. Better dating of the Catahoula Formation is needed to improve reconstructions of Oligocene-Miocene continental climates, drainage patterns, and paleohydrology in the region.