CONSTRAINING THE THICKNESS, ENVIRONMENT OF DEPOSITION AND BULK CHEMISTRY OF THE CATAHOULA VOLCANIC ASH AT RIO GRANDE CITY, TEXAS

The estimated age, stratigraphic position and nature of the erosive upper contact of the completely devitrified Catahoula Ash at Rio Grande City (RGC) Texas, were previously summarized by González and Hinthorne (2014, GSA SC section meeting). New field recognizance on the lower reach of Arroyo Los Olmos, RGC, coupled with ground penetrating radar (GPR) imaging, have allow quantifying the total thickness of the Catahoula ash as 20 m. This Oligocene-Miocene age volcanic ash, interpreted to represent a single air-fall event, appears to have been deposited in a low-energy marine environment, close to the shoreline. This interpretation is supported by the presence of a ~3 m thick, ash-clay mixed zone, observed on the stream banks. Below the mixed zone is pure, dark clay. A sharp topographic break marks the mixed zone to clay transition. A 30 m long GPR profile near the base of a 16 m thick ash outcrop, shows a persistent horizontal reflector at a depth of 3.5 m below the surface, this reflector is interpreted to be the mixed zone to clay transition.

Nine samples of the ash unit from different stratigraphic positions were analyzed by neutron activation yielding data on 4 major and 25 minor and trace elements. Na and K both average 2.0% (element by weight), Fe 2.1%, and Ca varies from <2% to 12% due to variable calcite/caliche cementation; Ba has a strong correlation with Ca. The main systematic trend observable is a decrease in alkalis (Na, K and Cs) upward in the ash unit. U varies from <3 ppm to 5 ppm; Walton, et al. (1981) report 2-9 ppm in altered Catahoula sediments 200 km north of RGC.