Geoscience Reference
In-Depth Information
Fig. 2 Cultural chronology for Arizona and northern Mexico from Watson (
2010
). Used with
permission of the author
period
and
later
Formative
period
Trincheras
tradition
(1800-500 cal BP)
(Carpenter et al.
2005
).
Geologic/Geomorphic Setting
The La Playa site is located along the boundary between the Arizona Upland and
the Lower Colorado River Valley subdivisions of the Sonoran Desert (Shreve
1951
). The site is adjacent to the Cerro Boquillas, which are northwest-southeast
trending hills of Cretaceous metasedimentary rock of the Pozo Duro Formation,
consisting of sandstone, siltstone, and conglomerate (McLaurin
2008
). Although
satellite imagery suggests that the La Playa site might result from alluvial fan
deposition, the surface sediments are fine-grained silt and lack coarse sediment
typical of most alluvial fan deposits. The overall surficial site stratigraphy,
originally described by Carpenter et al. (
2005
), is characterized by a lower,
reddish sandy paleosol overlain by a tan, homogenous silt. The paleosol is
observed in the main channel of the Rio Boquillas and across the La Playa site
where modern arroyo incision has exposed the unit. The paleosol is Pleistocene
in age (*14,500-10,500 cal BP), based on the assemblage of Rancholabrean
megafauna which includes bison, camel, mammoth, deer and tortoise (Carpenter
et al.
2005
).
The paleosol was formed during climatic conditions that were cooler and
considerably wetter than the current Sonoran Desert climate (Mabry
1998
). The
overlying silt unit is tan-brown, homogenous, and generally lacks internal
stratification and well-defined sedimentary structures. The silt is informally
designated as 'La Playa silt'. It is considered Holocene in age, although the
detailed geochronology is still being developed. The paleoenvironmental inter-
pretation of the silt is debatable, but is generally considered to represent fluvial
deposition across a large alluvial floodplain. It is within this unit that most of the
artifacts and human burials are observed. The unit is reworked and is locally
interbedded with sandier deposits exhibiting ripples and cross-lamination.
In some areas of the site the boundary between the silt and overlying sandy
deposits is marked by the presence of an artifact layer, which is the focus of this
study.
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