EOCENE SEDIMENTATION WITHIN THE ARGENTINE PUNA: IMPLICATION FOR EARLY PLATEAU DEVELOPMENT

Timing of deformation and resulting sedimentation patterns in the Altiplano/Puna Plateau of the southern Central Andes are the subject of ongoing debate. In the Bolivian Altiplano sedimentation into a foreland basin commenced during the Paleocene. In the Puna region, a lack of data precludes to such an interpretation, although it has been suggested that sedimentation into a wide foreland basin commenced during the Eocene-Oligocene. Here, we present new data from the southern Puna that document early deformation and sedimentation modes. These data demonstrate that deformation and basin compartmentalization was already occurring in the Eocene. In the Salar de Pozuelos over 1.5 km of Eocene fluvial rocks (Geste Formation; GF) record tectonic deformation and unroofing of ranges within the plateau. The lower part of the GF consists of interbedded massive and trough cross-stratified sandstone and siltstones, conglomerate channels, and limited palaeosols. The upper part comprises coarser facies composed of imbricated, horizontally stratified conglomerates intercalated with massive, horizontally stratified sandstones. Paleocurrent data document a uniformly eastward flow. Sandstone petrography shows quartzitic to lithic-feldspatic compositions. Clast lithologies show a source composed of phyllites, quartzites, pure quartz, and quartz-mica schists. Phyllites are typical of western Ordovician sources. Quartzite and pure quartz are more difficult to trace to present day proximal sources, but are typical of Cambrian rocks, which today are present more than 100 km to the north. However, an unequivocal eastward paleocurrent direction, and proximal facies clearly show that these sediments cannot have been derived from such a source. Instead, our data suggest that Cambrian rocks originally outcropped to the west but have been completely eroded. The GF therefore records a response to range deformation within the plateau in the Eocene. Thermochronological data along the Puna margin show that a contemporaneous proto-Eastern Cordillera was also being exhumed. Combined, these data show that deformation in this part of the Andes had already commenced by the Eocene, and that a compartmentalized foreland, similar to that observed today in the Sierra Pampeanas (27^o-33^o), existed by this time.