GEOARCHAEOLOGICAL EXAMINATION OF CULTIVATED SOILS AT LA PLAYA, AN EARLY AGRICULTURAL PERIOD SITE IN SONORA, MEXICO

The La Playa archaeological site is located on the southern floodplain of the Boquillas River in Sonora, Mexico. This site has evidence of a Cienega phase Early Agricultural period earthen irrigation canal system (800 B.C.- A.D. 200). Due to rapid modern floodplain erosion, most of the irrigation canals have been destroyed. Low-energy sheet wash erosion has destroyed many of the agricultural fields, removing cultivated soils and leaving behind the rock-lined field borders.

Magnetic gradiometry surveys at La Playa identified 8700 m² of intact agricultural field soils buried beneath the floodplain alluvium. These irrigated field soils were excavated, examined, and samples were collected for phytolith, textural, and micromorphology analyses. Micromorphology is the petrographic study of intact soils. Results from these geoarchaeological analyses indicate that field soils were fine-textured, organic-rich sediments that were saturated with water.

These micromorphology and textural samples were also compared to samples from canals and other areas on the floodplain that were not cultivated in order to understand how humans modified the floodplain sediments to create productive agricultural soils. The documentation of these anthropogenically modified floodplain soils that are at risk of erosion is important in the study of sustainable agricultural practices in arid lands.