U-Pb zircon ages of volcanic beds from the Upper Miocene Palo Pintado Formation, Salta Province, Argentina: constraints on the timing of sedimentation

GALLI, C.; COIRA, B.; MATTEINI, M.

Medellín, Colombia

Simposio; VIII SSAGI; 2012

Servicio Geológico Colombiano

Stratigraphical and sedimentological investigations were undertaken at the Palo Pintado Formation (Payogastilla Group, Late Miocene:, southern Calchaquí Valley. Strata comprising 1,387 m of clastic deposits interspersed with some pyroclastic beds are well exposed in the quebrada El Estanque. The paleoenvironmental interpretations based on stratigraphic and sedimentological features, suggest that this sediments deposited in a transitional system type between low and high sinuosity rivers forming a wandering fluvial systems sand-gravel with semipermanents small lakes (Galli et al., 2010). The cycles are composed of conglomerates and sandstones, which are channels and overbanks deposits and claystones in the floodplain deposits and in small lakes. The upper Miocene deposits of Palo Pintado Formation were derived from upper Neoproterozoic basement and Ordovician granites and pegmatites. The clay mineral assemblage of these source-area rocks is mostly smectite, interstratified illite-smectite, illite and kaolinite. The weathering of these materials under high temperature and rainfall favored the condition of the clay minerals hydrolysis and the kaolinite formation. Clay associations reflect both the influence of the climate and the composition of source areas. The clay minerals have been generated during a long period and under stable climatic conditions, under warm and humid state with low seasonal. In order to obtain new constrains on the timing of the sedimentation, two distal fall deposits at about 1000m from the base of the Palo Pintado Fm were dated. In situ U-Pb analyses on zircons from the two samples, using a LA-MC-ICP-MS (Neptune) at the Geochronology Laboratory of Brasilia University, provided two ages of 6.3 ±.0.1 and 6.8 ± 0.06 Ma respectively. These new ages of Palo Pintado Formation together with the other available ages of the base (10.29 ± 0.11my) and the top (5.27 ± 0.28 my), gives an average sedimentation rate of 0.27 mm/y.