QUATERNARY STRATIGRAPHY OF THE LA PLAYA ARCHAEOLOGICAL SITE (SON F:10:3), NORTHERN SONORA, MEXICO

The La Playa archaeological site is located along the Rio Boquillas, just north of Trincheras in northern Sonora, Mexico. The site contains an extensive record of human occupation beginning during the Paleoindian period with the most intense utilization of the site during the Early Agricultural period (ca. 1600 B.C. - 200 A.D.). This study is part of a continuing effort to document the site stratigraphy and geomorphology within the larger context of paleoenvironmental and paleoclimate reconstruction. Although the gross stratigraphy has been previously described, this work focused on detailed mapping and description of the stratigraphic units across the site. The oldest exposed stratigraphic unit at La Playa is a reddish, sandy paleosol. This paleosol (Pleistocene) contains a diverse Rancholabrean fauna that includes mammoth, bison, and tortoise. The exposed thickness of the paleosol is 2 m and the base of the unit has not been observed. In areas of the site, the paleosol grades laterally into gravels that contain clasts up to 25 cm in diameter. These clasts are a diverse mix of metamorphic, plutonic, volcanic and sedimentary lithologies. Overlying the paleosol is a brown, homogenous silt (middle – late Holocene?) that has a maximum thickness of approximately 3 m. This silt lacks sedimentary structures and is consistently 98% silt and clay and 2% very fine sand. The silt is barren of fossils or coarse lags except in the upper portions of the unit, where artifacts begin to appear. In most areas of the site, the silt is overlain by a 10 – 20 cm thick layer of rippled and laminated sandy silt.

The stratigraphy at La Playa is interpreted as representing climatic and depositional extremes. The paleosol and associated gravels were deposited during a time of relatively moist conditions compared to the present climatic setting. The gravels are evidence of pre- Rio Boquillas channels cutting across the landscape. The composition of these gravels indicates significant transport distance based on the occurrence of nonlocal lithologies. The overlying silt is interpreted as a product of eolian deposition and is considered to be a loess that was deposited during arid conditions. The grain size and other field characteristics of the silt at La Playa are consistent with loess deposits described in other areas of the world.