Abstract
The Pre-Cordillera in the northwest of Argentine is characterized by geomorphological features formed during Miocene Andes Cordillera uplifting. The arid weather and compressive tectonic regime propitiated the origin of countless sedimentary basins filled by alluvial fans and playa-lakes deposits. Because of the outcrops abundance and their qualities these basins are important sites for direct observation of depositional alluvial architectures and facies analyses. The description of some deposits in the Rio Jáchal valley (neighborhood of San Jose de Jáchal), and Pie de Palo desert alluvial plain in San Juan, allowed the recognition of main depositional mechanisms in these environments. The analyses were based in the longitudinal cross sections that show intertonguing of the alluvial fans, playa-lakes and eolian deposits. It was characterized the internal and external geometries of the bodies by hierarchical surfaces and macroforms analyses to make the interpretation of the sedimentary process and the depositional dynamics. The facies variation from proximal to distal deposits recorded the evolution of confined or unconfined non-cohesive debris flow to turbulent and laminar flows, resulting by the transport stream energy decrease of ephemeral torrential currents associated with storm periods. Due to the annual low pluviosity (70 mm/y), the sedimentation rate is very low, mainly suggested by the vegetation and soil developed. Although this study has been made in a restrict area with a very recent sedimentary history initiated only in the Late Pleistocene to Holocene, the facies distribution and the depositional mechanisms conditioned by geomorphologic positions in the alluvial fan systems and the sedimentologic characteristics conditioned by the arid climate allow to compare these deposits with the Brazilian neocretaceous, in the Bauru Basin, that shows similar sedimentologic and architecture characteristics.
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