Early Paleogene climatic conditions at mid latitude Southern Hemisphere: Mineralogical and paleobotanical proxies from continental sequences in Golfo San Jorge basin (Chubut, Patagonia, Argentina).

RAIGEMBORN, M., BREA, M., ZUCOL, A AND S. MATHEOS.

Geologica Acta

Año: 2007 vol. 000 p. 1 - 48

We present results from clay mineral and paleobotanical analyses of sediments and fossils from the lower and middle section of the Río Chico Group (Peñas Coloradas and Las Flores formations, respectively). These units represent a non-marine succession, about 120 m thick, in the Golfo San Jorge basin, southeastern Chubut, central Patagonia, Argentina. Both units were probably deposited during part of the Paleocene-Eocene Thermal Maximum (PETM); a period of transient and intense global warming that had a deep effect on plant groups in high latitudes. Clay minerals were identified on the basis of X-ray diffraction analysis. Smectite (S1 assemblage) dominates the Peñas Coloradas Formation, while smectite with variously abundant kaolinite (S2 assemblage) is present toward the Las Flores Formation; kaolinite (S3 assemblage) dominates the Las Flores Formation. These changes in clay mineral composition indicate a different weathering process and could be to a change in the climatic regime. The paleobotanical data, based on fossil woods from the Peñas Coloradas Formation and phytolith assemblages from the Las Flores Formation, suggest that vegetation in the Golfo San Jorge basin during the Early Paleogene was variable with significant changes in 2 composition and diversity through time. These results constitute the first evidence of a taphoflora in these units during the Early Paleogene in southern Chubut. In combination, the clay mineral composition and palaeovegetation assemblages suggest that during the deposition of the Peñas Coloradas and the Las Flores formations a change of temperate-warm and humid conditions, with highly seasonal precipitation in the lower term were replaced by subtropical-tropical conditions with more year-round rainfall.