HILLS TO RIVER: ARTIFACT RESOURCE PROCUREMENT AT LA PLAYA, MEXICO

La Playa (SON: F:10:3) is a 12 km² archaeological site in northern Sonora, Mexico. Material remains document that the site was continuously utilized from the Paleoindian period (10,000–8,000 B.C.) to the present, with the most intensive occupation during the Early Agricultural period (1500 B.C.–A.D. 200). Artifacts recovered from La Playa include groundstone, flaked-stone, ochre, quartz crystals, and pottery. A preliminary geologic and stratigraphic framework was constructed for the adjacent Boquillas Hills to determine if artifact source materials were procured locally.

La Playa is located in the floodplain just south of where the Boquillas River exits the northwest-southeast striking Boquillas Hills. These hills are composed of >1km thick, slightly metamorphosed, fining-upwards successions (up to 80 m thick) of conglomerate, sandstone and siltstone (Altar Formation?) that dip to the southwest. Northeast dipping, bedding parallel quartz veins occur within the more competent conglomerate and sandstone lithofacies. Conglomerate intervals, up to 13 m thick, are gradationally overlain by sandstone, which also occurs as thin interbeds within siltstone units and can be hematitic. Maroon and olive-colored siltstone units are up to 40 m thick and laterally extensive. The repetition of these fining-upward packages, paleocurrent indicators, and sedimentary structures indicate deposition in a fluvial environment.

Comparison of rocks in the Boquillas Hills with artifacts recovered from La Playa indicates humans occupying the site had immediate access to source materials. Chert, quartz, and quartzite clasts from the conglomerate were used to form projectile points. Hematitic sandstones were used to produce the ochre that occurs as grave offerings and pottery paint. The maroon siltstone was a source for temper used in the production of the pottery. Clear quartz crystals were sourced from the quartz veins crosscutting the sandstone and conglomerate. Of the artifact types examined, the groundstone is composed of materials that are foreign to the site. Manos and metates were constructed from igneous (plutonic and volcanic) and metamorphic materials that were either brought in from other locations through trade or sourced from much further upstream along the Rio Boquillas.