EL SAUZ CHERT, A LIMITED-SOURCE, CHEMICALLY UNIQUE LITHIC ARCHAEOLOGICAL RESOURCE MATERIAL FROM THE LOWER RIO GRANDE VALLEY OF TEXAS

Over 50 years ago archaeologists studying projectile points in the Lower Rio Grande Valley (LRGV) noted a distinctive light gray, but sometimes colorful, variety of high-quality chert not found in the coarse local sedimentary units, the Goliad and Uvalde Formations. The sources of this unique lithic resource dubbed of El Sauz Chert (ESC) are two small, (20m x 20m) isolated mounds in Starr County, near the village of El Sauz, that appear to be genetically related to the volcanic ash of the Catahoula Formation that surrounds them. Evidence of extensive prehistoric quarrying is suggested by the occurrence of large quantities of debitage and worn hammer stones surrounding the mounds. Artifacts made of ESC range in age from Early Archaic (3500-6000 BC), to Late Prehistoric (AD 1200-700), suggesting that native populations used the chert to manufacture tools for about 8,000 years. A recent survey sourcing lithic resources of the LRGV found that ESC represents approximately 6% of all tools analyzed (n = 986), surprising given how remote and geographically restricted the two quarried mounds are. The bulk of the ESC is light to medium gray, but approximately 10% of the samples exhibit a wide range of colorations, including pink, red, yellow, orange, purple and caramel, which occur as mottles and smeared patterns rather than solid colors. The gray shades are the most common and possess the best flaking properties but identifying them as ESC can be challenging as they can be mistaken for other abundant gray cherts. For positive discrimination of El Sauz-type chert from other LRGV and Texas chert sources, we present the first geochemical fingerprint of ESC, consisting of unusually high concentrations of zirconium (up to 180 ppm) and titanium (up to 2000 ppm), measured with portable X-ray fluorescence. The restricted occurrence of ESC, and the fact it was quarried for 8,000 years, provides a unique opportunity to study prehistoric exchange and dispersal patterns along the LRGV.