Stratigraphy and Geochronology of La Playa Archaeological Site, Sonora, Mexico

Abstract

The current study examines the stratigraphy, geochronology, and paleoecology of La Playa, an Early Agricultural period archaeological site (3600-1800 BP) located in northern Sonora, Mexico. We distinguished seven stratigraphic units ranging in age from >44,570 to 680 cal yr BP. All of the cultural remains are contained in Unit B, which spans from 4700-1580 BP. Deposits from Unit B represent overbank deposition from the nearby Rio Boquillas. The majority of cultural materials come from Units B4 and B5, which correspond to the Cienega phase (2800-1800 BP) of the Early Agricultural period. This period coincides with the first sedentary agricultural populations in the region and is marked by thousands of archaeological features including roasting pits, human burials, and extensive canal systems at La Playa. The presence of semi-aquatic and aquatic snails demonstrates that water was present year round in the canal system. The stable and radiometric isotopic evidence suggests that the early agriculturalists diverted ground water, likely from the nearby Rio Boquillas. Cultural remains from Unit C spanning the period <1580-680 BP are rare, suggesting major population decline during this time. There is little to no evidence of bioturbation in Unit C, suggesting that the landscape was thinly vegetated at this time. La Playa has experienced up to five meters of erosion during historic times, exposing a complex alluvial stratigraphy and numerous cultural features, which has greatly complicated archaeological interpretations at the site.