Geoscience Reference
In-Depth Information
Bajada Deposits: Qbd, Qbdd
Bajada deposits include colluvium derived from the adjacent Cerros Boquillas.
These deposits occur on the north side of the Rio Boquillas (not mapped) and on
the eastern side of the site. The slope in these areas is steeper and overall has
higher elevations than other areas at the site. Slopes are covered by cobble-size
angular clasts of Cerros Boquillas lithologies derived from the bedrock exposures.
These deposits are a mixture of sandstone, siltstone and conglomerate clasts.
In some areas, these Bajada deposits have been incised and dissected (Qbdd) by
younger drainage systems. These drainages, depending on depth of incision, may
expose some of the bedrock lithologies beneath the colluvial cover.
Alluvial Gravels/Pavements: Qg
A layer of thin gravel lies atop the paleosol in the northern and southwestern areas
of the site (Fig.
8
a). These gravels are dominated by clasts derived from the
sedimentary rocks and quartz veins of the Cerros Boquillas. The gravels
also contain clasts of granite, volcanic, and metamorphic rocks. These clasts are
well-rounded and up to 9 cm in diameter and appear to have been reworked
significantly. The gravels often form a pavement-like surface containing slightly
varnished cobbles, particularly in the southwestern areas of the site (Fig.
8
b).
Silt: Qs
The silt that covers much of the site, herein termed the La Playa silt, possesses
sedimentological properties that are different from other archaeological sites in the
southwestern U.S. and northern Mexico that are interpreted as alluvial. The silt
stratigraphically overlies the paleosol and is often exposed in the cut banks of
modern arroyos and channels that have incised into the landscape. The age of the
silt is unknown, but is assumed to be Holocene in age. For mapping purposes the silt
unit is considered undissected silt (Qs) and covers those areas where the silt has not
yet been incised by gully formation. The silt is tan-brown in color and lacks
definable stratigraphy or sedimentary structures (Fig.
9
). The unit is largely barren
of fossils compared to the underlying paleosol. However, terrestrial and aquatic
gastropods were previously described within the La Playa silt by Drake (
1960
,
1961
). The exposed thickness of the silt is at least 2 m thick, but there is no
complete section that preserves the entire silt succession on top of the paleosol.
Grain size distribution within the silt is fairly consistent with no particles larger
than very fine sand. Dry and wet sieve analysis of four samples of the silt show
remarkable consistency in their grain size trends with each sample approximately
98% clayey silt and 2% very fine sand size. Additional grain size determination,
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