CHARACTERIZATION OF MASSIVE GRAY CHERT OCCURRANCES ALONG STRIKE OF THE LATE OLIGOCENE CATAHOULA VOLCANIC ASH FROM STARR COUNTY TO DUVAL COUNTY, SOUTH TEXAS

More than 50 siliceous knobs are the most conspicuous element of the landscape in southern McMullen and northern Duval counties. These knobs, 10-35 m high, occur along the trend of the Catahoula Ash. First reported by Bayley (1926); later, McBride et al. (1968) presented the first basic descriptions of these rocks. Examination of these knobs, including field and thin section observations, and NAA and pXRF minor/trace element data, suggests that similar geochemical processes locally altered the ash over a distance of >200 km producing resistant light gray, fine-grained chert which is not visually distinguishable among occurrences in Nuevo Leon, Mexico, Starr County, and western Duval County. The chert is massive, dense and lacks stratification. Noticeable is the variable amount of vugs ranging from mm to 10 cm, some partially or completely filled by chalcedony (González et al., 2014), often feathery. In thin sections the dense chert is composed of quartz with fragmental or microbreccia texture. Small fractures are filled with brecciated silica, often in a matrix of Fe-rich, dark phases. Opal-A fills white veins up to 1 cm wide. PXRF analysis of more than 100 samples of this chert from Starr County, with a range of colors and textures, yield uniquely high values of Zr and Ti in every sample, 4-20 times greater than any sample of a representative suite of cherts from other sources. Analyses of samples from western Duval County match those from Starr County. The elevated levels of Zr and Ti, and other high field-strength elements, compared to bulk Catahoula Ash and other cherts, suggest mobilization of silica was part of this chert-forming process. The chert from these knobs was a valuable lithic resource used by Native Americans for the manufacture of tools ranging in age from Early Archaic (3500-6000 BC) to Late Prehistoric (AD 700) times. Further geochemical characterization of these rocks will be valuable for archaeologist working in northern Mexico and south Texas to understand sourcing and dispersal patterns of lithic artifacts.