GENESIS OF AN ARTIFACT LAYER AT THE LA PLAYA ARCHAEOLOGICAL SITE (SON F:10:3), SONORA, MEXICO

The La Playa archaeological site (SON F:10:3), located north of Trincheras, Sonora, Mexico, preserves 13,000 years of human occupation and utilization with the greatest concentration of artifacts dating to the Early Agricultural period (ca. 1600 B.C. - 200 A.D.). Ongoing field research has focused on documenting climatic and landscape changes since the terminal Pleistocene. Geologically, the site is characterized by a stratigraphic succession consisting of a homogenous silt (Holocene?) overlain in places by a thin layer (2 - 6 cm) of mixed cultural artifacts (ceramics and groundstone) and rock. This artifact layer is, in turn, overlain by interbedded silts and cross-laminated and rippled, very fine-grained sands.

The goal of the study was to map the distribution of the artifact layer and overlying sediments to determine: 1) if the layer is a lag deposit resulting from deposition and concentration of artifacts by a flooding event; or 2) if it is a cultural layer and is an undisturbed, earlier level of occupation that was subsequently buried by younger deposits. A total of nineteen stratigraphic sections were measured to describe thickness and grain-size trends. Elevations were taken of each section and their positions recorded using a Trimble ProXH GPS. An elevation map of the artifact layer and an isopach map of the sediment above the layer were created to define the spatial limits and geometry of this interval. Cross sections were constructed to illustrate and correlate the distribution of sediment types and thicknesses. Results show that the artifact layer is confined to a 0.4 km² area of the site and dips to the southwest at approximately 0.5°, which is consistent with the slope of the current topographic surface. The sand and silt overlying the artifact layer is up to 1.5 m thick and has a lens-shaped geometry.

The artifact layer is interpreted as a cultural layer and is a former occupation surface that has been subsequently buried by flooding. Several lines of evidence support this interpretation including the lack of imbrication of particles, the angular shape of the particles, and the absence of significant wear and breakage of artifacts. The presence of ceramics within the cultural layer indicates that burial of the occupation surface was after 200 A.D.