CONICET | Buscador de Institutos y Recursos Humanos

The Salta Group was deposited in an intracontinental rift, the Salta Basin that evolved from the lower Cretaceous to the middle Palaeogene, and is subdivided intothe Pirgua, the Balbuena and the Santa Barbara Subgroups. The MaízGordo Formation (MG Fm) that is 200 meters thickis the middle unit of the Santa Bárbara Subgroup, corresponding to late post-rift sedimentation. These deposits mainly consist of sandstones, mudstones and limestones, plus severalpaleosol levels. We studiedthe mineralogy of fine-grained levels of the MG Fmby XRD and SEM in order to examine the connection between vertical changes in clay mineralogy in alluvial sediments and paleosol levels and globalpaleoclimatic changes registered during the Paleogene. The samples were collected in two stratigraphic sections,the Tin Tinand Obeliscosites, located in the west of the Salta Basin.XRD revealed marked vertical changes in clay mineralogy and kaolinite/muscovite (Kln/Ms)ratios that according to Chamley (1989) can be used as a palaeoclimatic indicator. In the basal level of the MGFm at the Tin Tin section illite-mica isvery abundant whereas smectite relative abundance is around 25%, and kaolinite is absent (Kln/Ms=0). The nextfine-grained level, 25 meters up section, depicts a sharp increase in kaolinite content together with a strong decreasein smectite and illite-mica abundances (Kln/Ms=0.68). Upwards kaolinite abundances gradually increase at the expense of illite-mica for the next 30 meters, except for slight fluctuations, reaching a maximum Kln/Ms ratio of1.3. Then, kaolinite contents decrease sharply and Kln/Ms ratio decrease to 0.8 in few (five) meters. Towards the top of the unit another three cycles of increase/decrease in kaolinite relative abundances occur in shorter intervals,depicting, respectively, maximum Kln/Ms ratios of 2.7, 5.2 and 3.9. On the other hand, in fine-grained levels of the MG Fmat the Tin Tin site, smectite contents remain low, except for one bed with a peak value of 38%. At the Obeliscosite the clay assemblages of the MG Fmshow a similar trend asat Tin Tin, with successive cycles of increase/decrease in kaolinite contents. Likewise, smectite contents remain less than 10% with the exception of a level in which smectite has a peak value of 29%. SEM showedeuhedral kaolinite plates forming well-developed booklets and smectite packets. Both clays grew from altered K-feldspar fragments indicating their authigenic origin.For the paleosol levels occurring in the upper 15 meters of the studied sections organic matter carbon isotope data that records three negative carbon isotope excursions (CIE) were recently obtained, one of them was correlatedbythese authors with the Paleocene to early Eocene Thermal Maximum. It is remarkable that the three CIEs match very well with the three levels depicting the highest Kln/Ms ratios mentioned above. The cyclic increase in kaolinite abundance and Kln/Ms ratio indicate several climatic changes implying progressively higher hydrolyzing conditions. In fact, the increase in kaolinite relative abundances in the upper part of the profiles is in agreementwith the rise in paleoprecipitacion and paleotemperature indicated by different proxies [2]. Thus the cyclic changes in kaolinite relative abundances in the study sections of the MG Fmconstitute strong evidence of the occurrence ofseveral short-lived hyperthermals during Paleocene-early Eocene time in the Southern Hemisphere coincident with well-establishedwarm episodes in the Northern Hemisphere.