CLAY MINERALS AND WEATHERING OF THE VERA MEMBER, SARMIENTO FORMATION, PATAGONIA, ARGENTINA

The Vera Member of the Sarmiento Formation in Southern Argentina contains the only continuous fossil record across the Eocene-Oligocene Transition (EOT) in the Southern Hemisphere. Marine records indicate that the EOT was a time of dramatic climate change; plant fossils (macrofossils, pollen) also suggest cooler environments with a transition from subtropical to temperate forests at middle latitudes during this interval. Phytoliths from the Vera Member, however, indicate a potentially wetter climate and relatively stable plant communities, and oxygen isotope data from tooth enamel also suggests minimal climate change across the EOT. Here we use clay mineralogy to constrain the paleoclimate in which the Vera Member was deposited, giving a context for plant and animal evolution. X-ray diffraction (XRD) patterns from the Vera Member <2µm fraction are at least partially consistent with several clay minerals, including smectite, illite, and possibly sepiolite. Variations in the clay mineralogy and degree of crystallinity correlate with other sedimentary properties such as carbonate content, particle size, and magnetic properties, and likely reflect changes in climate. Smectite and sepiolite in particular do not appear in intervals that show other signs of extreme aridity.