Quaternary Stratigraphy of the La Playa Archaeological Site (SON F:10:3), Northern Sonora, Mexico - Reconstructing Human-Landscape Interactions: Interpreting Desert and Fluvial Environments - page 8

lithologies on the Quickbird imagery base within GIS allowed the delineation of
map units and spatial distribution of depositional and cultural deposits.
The extensive erosion that serves to expose and destroy human burials and
artifacts at La Playa also provides an opportunity to examine the site stratigraphy.
When describing and classifying these sediments, it is critical to distinguish sed-
iments that are naturally deposited with those that result from cultural processes
including modern disturbances. The natural deposits that define the landscape of
La Playa can be divided into eight units. A composite stratigraphic column
illustrating the basic site stratigraphy is shown in Fig. 4 . The age and exact
stratigraphic relationships of these units are not yet clearly defined and are
undergoing additional study. The surficial map of the La Playa site (Fig. 5 )
delineates both natural deposits and areas of cultural modification. The surficial
units, described in more detail below, are: (1) Paleosol and associated sediments
(Qp), (2) bajada deposits (Qbd, Qbdd), (3) alluvial gravels/pavements (Qg),
(4) silt (Qs), (5) modern Rio Boquillas bar sands (Qrbbs) (6) modern Rio Boquillas
channel deposits (Qrbcd), (7) fire-cracked rock concentrations (Qfcr), (8) minor
channel sands (Qmcs), (9) dissected/reworked silt (Qdrs), and (10) modern

disturbances (Qmd).

Paleosol and Associated Sediments: Qp

The paleosol at La Playa is the deepest exposed stratigraphic horizon and is
primarily confined to localities west of Los Entierros and Hornos Alineados. It is
considered to be Pleistocene in age based on the fossil assemblage of Rancho-
labrean fauna (Carpenter et al. 2005 ). There is a small area of exposed paleosol
adjacent to the Rio Boquillas in the northern part of El Canal/Los Entierros. In
addition, in the channel of the Rio Boquillas, the transition between the paleosol
and overlying silt is exposed along a *400 m section. The paleosol is red in color
and the maximum thickness exposed is approximately 4 m (Fig. 6 ). Upper inter-
vals of the paleosol are sandy with minor gravel and a variable silt/clay content
from 9 to 50%. The sand fraction is concentrated in the fine to medium-grained
sizes. Early stage pedogenic carbonate formation is indicated by the presence of
calcium carbonate nodules and root casts. Overall, the top of the paleosol is a
planar surface with relief up to 50 cm. In several areas of the site, including the
Rio Boquillas channel, deposits of gravels within the paleosol are observed. These
gravel intervals are 4 m thick and occupy up to 30 m wide zones within the
paleosol (Fig. 7 ). The gravels are calcite cemented with reddish sand and contain a
diverse assemblage of rounded clasts of schist, gneiss, volcanics, plutonics, and
sedimentary compositions. The clasts are in the cobble size range (up to 10 cm)
although some clasts exceed 25 cm. Clast imbrications of the gravels indicates a
west to southwesterly paleoflow direction.