NEOGENE IMPACT GLASS LAYERS OF THE SOUTHERN PAMPAS, ARGENTINA (Invited Presentation)

The southern Pampean plain of Argentina is covered by a vast Neogene sedimentary mantle made up of loess and loess-like deposits of Andean source (volcaniclastic material) with an average thickness varying from 100 to 200 m. The outcrops are reduced to discontinuous exposures of very limited extension which together with its rather homogeneous lithology are major hindrances to correlate and analyze the stratigraphy of the succession, mostly based on its fossil vertebrate assemblages. The accumulation of the deposits, interpreted as sinorogenic material related to the late Miocene Andean orogeny, started circa 12 Ma following the marine regression of the Paranense sea. Paleosols are common throughout the succession along with carbonate accumulations of varied morphology. A unique and remarkable feature is the occurrence of vesicular and glassy fragments, known as escorias, which were reported since the first general studies in the late XIX century. Their genesis was debatable being attributed either to natural or human induced fires, diagenetic processes or volcanic activity. Studies developed during the last 15 years have demonstrated an impact origin for the escorias as revealed by their geological context together with their petrological and geochemical characteristics. Although originally thought to be distributed throughout the succession, detailed stratigraphic analysis permitted to identify three impact glass layers dated by ⁴⁰Ar/³⁹Ar at 3.27 Ma ± 0.08 Ma (Chapadmalal), 5.28 ± 0.04 Ma (Bahía Blanca) and 9.21 ± 0.08 Ma (Chasicó). The stratigraphic record of each event is characterized by the presence of a primary impact glass layer composed of large individuals (0.5m, 1m) as well as concentrations (up to 2 m in diameter) of angular fragments of variable sizes. These primary layers are followed by sedimentary units including much smaller, less abundant and usually rounded fragments of reworked impact glass. Thus, the primary layers of impact glass have become useful benchmarks which provided an independent tool to chronologically calibrate the late Miocene and Pliocene biostratigraphic units (Chasicoan, Huayquerian, Montehermosan and Chapadmalalan land mammal ages), as well as the two main cycles of the Neogene sedimentation process.