PRECISE 27 MA U/PB ZIRCON AGES OF THE MASSIVE LATE OLIGOCENE CATAHOULA ASH EXPOSED AT RIO GRANDE CITY, TX

The 20 m thick exposures of the Catahoula Ash adjacent to the Rio Grande floodplain in Starr County, Texas have been interpreted to represent a single, massive, rhyodacitic volcanic ash depositional event. The age of the Catahoula ash has previously been estimated to be ~25-30 million years based on its stratigraphic position; it overlies dark unconsolidated clay of the Frio Formation and is unconformably capped by a resistant, well-cemented fluvial conglomerate of the Pliocene Goliad Formation. Here we present the first radiometric ages from this unique ash from a sample collected 4-5 m above the contact with the underlying Frio Formation at the “Tampico” location. Sixteen euhedral to subhedral zircons averaging ~30x100 microns were analyzed at the UCLA ion microprobe dating lab. The main cluster of 12 concordant grains have a weighted U/Pb age of 27.2 Ma, ± 0.4 Ma (2 sigma). Three older, inherited(?) grains have U/Pb ages of 36, 38 and 336 Ma; these older grains have significantly greater U content than the 12 concordant grains. Depth profile analyses on 9 zircons by laser ablation mass spectrometry at University of Houston yielded a U/Pb zircon rim age of 26.7 Ma, ± 0.5 Ma (2 sigma). These independent ages overlap and are consistent with several felsic volcanic sources in the Sierra Madre of Mexico, the closest being ~400 km distant. Determining the age of this rapidly deposited volcanic unit is important as it represents an excellent stratigraphic marker in southeast Texas.