Pedosedimentary records of Holocene paleoenvironments in a dryland alluvial fan system in Sonora, NW-Mexico

In this work we present a detailed micromorphological analysis of a sequence of palaeosols and sedimentary units at La Playa archaeological site. This sequence is from the Late Pleistocene (Marine Isotopic Stage 2) to the Late Holocene. Complementing the micromorphological study, a geomorphological analysis is provided, in order to establish the origin of the landscape units. The results show that according to the geomorphology, La Playa constitutes an alluvial fan, formed by a complex, braided channel. The older unit, the San Rafael Palaeosol (SRP) is the most developed and has a set of features associated with the changing paleoenvironmental conditions such as weathering of the primary minerals, clay illuviation (evidencing humid conditions of the Terminal Pleistocene), pedogenic carbonate accumulation in the A horizon (related to dry environments of Altithermal). The SRP is buried by fluvial sediments, which include different facies interpreted as: floodplain, sandbars, and channels. These sedimentation events are associated with the end of the Altithermal period and evidence more active geomorphic processes, probably due to strong storms causing flooding and the migration of the main tributary. Above the sedimentary unit, the Boquillas Palaeosol (BOP), formed during the Late Holocene, is derived from fluvial-alluvial deposits causing a syn-sedimentary soil, with a more incipient development as indicated by micromorphological features.