Mio-Pliocene geology of the Southern Puna Plateau Margin, Argentina

The Mio-Pliocene geologic record is investigated along the southeastern margin of the Puna plateau in northwestern Argentina. The Puna plateau is the southernmost extension of the high elevation, internally drained, Central Andean plateau. A series of intermontane basins at the plateau margin preserve thick stratigraphic sections spanning ~10-3 Ma. The strata in these basins were investigated between 25°30?S and 28°S latitude, with geochronological and paleoenvironmental objectives. The stratigraphy, composition, and age of volcanic ash beds provide age control. Fossil vertebrates and soils interbedded with these volcanic ash beds provide material for stable isotopic proxies of environment. This approach permits analysis of spatial and temporal patterns in the tectonic and climatic evolution of the landscape at the Puna margin. Stable isotope analysis of fossil tooth enamel from the 2.5 km thick section at Puerta de Corral Quemado documents the late Miocene expansion of plants using the C4 photosynthetic pathway. Tooth enamel was analyzed by conventional methods, and by laser ablation which incorporates small bodied taxa in the study. These results indicate the presence of C4 plants in the region by at least 8.5 Ma and a shift to C4 diets among fossil notoungulates between 7-5.5 Ma. Fossil rodents exhibit a less dramatic diet change across this interval, and all taxa document transient enrichment in 13C and 18O approximately coincident with the Miocene-Pliocene boundary. Interbasin correlation of ash beds demonstrate that conglomeratic deposits are conformable with stratified sections and initiated between 4-3 Ma. These deposits lag uplift of basin margin mountain blocks by several million years and precede contractional deformation of Mio-Pliocene strata. This sequence is characteristic, but diachronous between basins. This Mio-Pliocene pulse of deformation typifies the evolution of a broken foreland at the plateau margin. Interbasin comparison of isotopic proxy data from fossil soils identifies regional 18O enrichment concurrent with topographic growth at the plateau margin. Carbon isotope data from fossil soils demonstrate strong ecological gradients at the landscape-scale and at local scales. The record of C4 expansion in fossil soils is significantly influenced by the growth of complex topography and possibly by regional reorganization of precipitation systems.