PALEOPEDOGENESIS AND SEDIMENTATION IN LA PLAYA, SONORA AS EVIDENCES OF THE PALEOENVIRONMENTAL CONDITIONS FROM: MACRO AND MICRO SCALES OF OBSERVATION

In this work, we present a detail micromorphological analysis of a sequence of palaeosols and sedimentary units, in La Playa archaeological site. This sequence is formed since the Late Pleistocene (Marine Isotopic Stage 2) to the Late Holocene. Together with 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 constitute an alluvial fan, constituted by a complex, braided channel. The older unit, the San Rafael Palaeosol (SRP) is the most developed and has a set of features associated to the environmental conditions such as weathering of the primary minerals, clay illuviation (evidencing humid conditions), pedogenic carbonates accumulation both micritic and sparitic, hypocoatings and nodules (related to dry environments). The SRP pedogenesis is interrupted by the occurrence of alluvial sediments, which includes different facies: floodplain, sandbars, channel. This sedimentation event is associated to the end of the Altithermal period and evidences more active geomorphic processes, probably due strong temporal storms causing flooding and the migration of the main distributary. Above the sedimentary unit, the Boquillas Palaeosol (BOP), formed during the Late Holocene, is derived from alluvial deposits causing a syn-sedimentary soil, which development is more incipient as its micromorphological features indicate.