GEOARCHAEOLOGY AND SITE FORMATION PROCESSES OF A FINE-GRAINED ALLUVIAL FLOODPLAIN AT LA PLAYA ARCHAEOLOGY SITE, SONORA, MEXICO

La Playa is a large (12 square km) Early Agricultural period (1500B.C.-A.D.200) site located on a floodplain of the Río Boquillas in northern Sonora, Mexico. Interpretation of archaeological features and an extensive canal system has been hampered by a limited understanding of the site’s alluvial history due in part to extensive historic erosion and dissection of the floodplain, as well as local complexity in geomorphic features. This study uses geoarchaeological methods to reconstruct the formation processes and the depositional environment of the site before, during, and after occupation. This paper focuses on the area of Los Montículos, where the stratigraphic record appears different from that of the rest of the site, and tests the hypothesis that the stratigraphy of this area reflects a more varied and energetic geomorphic history due to its proximity to the river.

Before and during occupation by Early Agricultural groups, sediments on the floodplain were deposited by low-energy overbank deposition. This was possible due to the stable location of the river channel through much of the Holocene. The stable floodplain surface would have been an attractive place for canal agriculture due to slow and steady deposition of fine-grained sediments and low frequency of high intensity floods. This resulted in multiple, weakly developed buried soils in Los Montículos nearer to the channel, and fewer but more strongly developed cumulic soils farther from the channel. Sometime after occupation by Early Agricultural groups, there was a shift to higher-energy deposition of sandier sediments. Higher-energy flows resulted in multiple episodes of erosion and deposition in the area of Los Montículos, and nearly all archaeology in Los Montículos now exists in an eroded and reworked context.