In the Bermejo basin of western Argentina, semi-arid, sandy, low-gradient sedimentation has accompanied deformation of the Frontal Cordillera and Precordillera since the mid-Miocene. Utilizing the precise geochronology established for strata at Sierra de Huaco, we document an immediate basinward migration of facies coincident with thrusting in the central Precordillera (8.7 Ma). An earlier shift in environment (10.3 Ma) is related to a change in drainage patterns produced by more distant deformation in the western Precordillera. Evidence from Sierra de Huaco suggests that depositional environments are largely independent of the net subsidence rate. Sedimentologic features observed at Sierra de Huaco bear directly on the nature and scale of the processes that operate in sandy, ephemeral settings: 1) lateral accretion was the dominant process within an ephemeral braidplain (the Jarillal Formation); 2) fining-upward cycles in sandflat deposits were a consequence of the dynamics of unconfined flows coupled with lobe switching on alluvial fans (the Huachipampa and Quebrada del Cura Formations); and 3) unconfined flow deposition in the Bermejo basin occurred (and still occurs) over distances on the order of 50 km from point sources at alluvial fan heads.

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