EARLY FORELAND BASIN DEPO-SYSTEMS IN THE CENTRAL-SOUTHERN ANDES OF ARGENTINA, 32˚S -35˚S

The Andes Mountains are the type-example of an active Cordilleran style margin with a retroarc fold-thrust belt and foreland basin. Upper plate shortening, crustal thickening, and arc processes related to subduction of oceanic plates under South America led to uplift of the Argentine Andes. Total shortening is highly variable along the length of the Andes, ranging from ~ 350 km in Bolivia to < 20 km in the northern Patagonian. Timing of initial shortening and foreland basin development also seem to vary along strike. The early foreland basin stratigraphy provides a record of the timing of earliest uplift and the location of the thrust front along the length of the orogen.

Correlative stratigraphic sections from the Malargüe and Aconcagua fold-thrust belts (35˚S and 32˚S, respectively) in south central Argentina record a facies change between marine evaporites of the Huitrín Formation (~125 Ma) and fluvial units of the Diamante Formation (100-80 Ma). In the Malargüe area a 25-30 myr unconformity between the Huitrín and Diamante formations is interpreted to represent the passage of the flexural forebulge. A change in provenance and a transition from west to east directed paleocurrents within the Diamante Formation, may reflect a late Cretaceous transition from forebulge derived sediment in the distal fordeep to sediment derived from the thrust belt to the west; deposited in the proximal foredeep. In the Aconcagua area, there is no break in sedimentation between the Huitrín and the Diamante formations; instead there is a continuous coarsening upward trend, from mixed evaporites and carbonates in the Huitrín Formation to dominantly ripple-cross-laminated sandstone with minor evaporite beds in the Diamante Formation. Growth structures in the upper Diamante Formation mark the transition from proximal foredeep to wedge-top deposition. Although stratigraphic sections exposed in the Malargüe and Aconcagua fold-thrust belts are currently in similar locations, with respect to the modern volcanic arc, basin reconstructions, taking into account Cenozoic shortening in the two areas, suggest that the Aconcagua area was located ~100 km west, and significantly closer to the Cretaceous thrust front, than the Malargüe area.