Geoscience Reference
In-Depth Information
twenty-first
century
have
served
to
initiate
arroyo
cutting
and
the
gradual
destruction of the La Playa site.
Conclusions
The paleoenvironmental and depositional history of the La Playa site reflects the
climatic shift from the terminal Pleistocene into the Holocene. Wetter conditions
were present during the last glacial maximum to around 9,000 years BP in
northern Mexico (Metcalfe et al.
2000
). At La Playa the paleosol reflects
depositional conditions that were fluvially dominated as suggested by poorly
sorted, rounded cobbles within channel fill of a sandy floodplain. The cobbles
were transported by high energy river systems. These cobbles are composed of
diverse lithologies from a large drainage basin that tapped source areas of
volcanic, plutonic, and metamorphic rock. Gravel imbrications show that the
paleoflow in the Pleistocene was westerly and consistent with the direction of
modern flow of the Rio Boquillas. These rivers that traversed the La Playa area
either were a single, frequently avulsing channel, or possibly a multi-channel
braided system. Individual channels were at least 30 m wide. Climatic conditions
during the Pleistocene were clearly adequate to support a diverse biota as sug-
gested by the Rancholabrean fauna (Carpenter et al.
2005
). The transition to the
overlying silt records a period of landscape stabilization as indicated by the
gravel sheet and desert pavements that overlie the paleosol. This period of
stability possibly records a climatic shift from a much wetter environment
capable of supporting high energy fluvial systems to one that is drier. The La
Playa silt was previously described as a cienega or swamp-type depositional
system, but many of the characteristics of the silt seem more typical of the eolian
deposition of loess. The tan-brown clayey silt is extremely fine-grained and does
not exhibit sedimentary structures that are consistent with alluvial deposition.
For the silt to have been deposited in such an environment would have required
very quiet depositional conditions where the environment was isolated from
major flooding events. If La Playa represented a typical alluvial floodplain, there
should be coarser channel deposits and crevasse splay sands that are interbedded
with the silt and are a result of flooding events or avulsion. A floodplain
interpretation for the silt that can explain the lack of alluvial features would
depend on a stable river system that was incising rather than frequently mean-
dering across its floodplain. Clearly, more detailed studies are necessary to
answer this question.
The description and mapping of the surficial units at La Playa provides a
fundamental stratigraphic framework for understanding landscape changes within
the larger context of climate change. The striking differences between the
paleosol and overlying silt are due to the highly variable nature of their
depositional systems. This model of significant shifts in climate during the
Pleistocene-Holocene transition at La Playa may, in the future, prove to be
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