Cretaceous Salta Basin – Argentina. Ricardo Ruiz (1,2), Robert Ondrak (2), Brian Horsfield (2), Eduardo Rossello (3) (1) Institute of Earth and Environmental Science, Universität Potsdam, Germany, (2) Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences, Section 3.2: Organic Geochemistry, Potsdam, Germany, (3) CONICET-FCEN, University of Buenos Aires, Argentina Depositional Environment Interpretation from Organofacies Characterization of Yacoraite Fm. Outcrop Samples, Cretaceous Salta Basin – Argentina. Introduction The Yacoraite Formation (Maastrichtian-Danian) is one of the main source rocks in the Salta Rift Basin. It was deposited following Lower Cretaceous rifting. Marine depositional environment has interpreted (Marquillas et al., 2007) although locally evidence has been found for a lacustrine depositional setting (Terra et al., 2012; Schmidt et al, 2017). The depositional environment seems to have been complex, involving the coexistence of lacustrine environments associated marine ones as a consequence of the last Cretaceous marine ingression. Such complexity in depositional environments would explain the variety of kerogen types and organic richness (up to 7% TOC). Goals Interpret depositional environments for Yacoraite Fm. based on biomarkers in rock extracts, and provide geochemical indices for organofacies that can also be used in source-oil correlations studies. Organofacies: Mainly Kerogen Type II and II/III mixtures, indicative for marine organic matter input. Kerogen Type III, indicating terrestrial organic matter input. Source rocks have oil and some gas generation potential. Thermally immature (<0.5% Ro) and in oil window (0.5 - 1%Ro). Oxidation did not affect samples during or after deposition as shown for low Oxygen Index values (Inset in fig. 2). Reducing environments during the deposition in which highly anoxic conditions prevailed in agreement with low Oxygen Index values from Rock-Eval analysis favorable conditions for organic matter preservation (Fig. 3, 4). Shales Stromatolites Pisolite Shales Stromatolites Pisolite Shales Stromatolites Pisolite Fig. 3: Ratios of n-alkanes vs. Pristane and Phytane were used to interpret redox conditions Fig. 4: Ratios of Pristane/Phytane vs. Homohopanes were used to better define redox conditions during deposition. Fig. 1: Study area showing sampling location. Salta subbasins: MA: Metán-Alemanía, TC: Tres Cruces Fig. 2: Tmax vs. Hydrogen Index (HI) kerogen classification in whole rock samples. Red lines mark maturity isolines, black lines separate kerogen types. Marine vs. lacustrine Paleoenvironment Most of the samples show geochemical evidence for marine environments as interpreted from independent biomarkers e.g. steranes distribution C27, C28, C29 (Fig. 5) and hopanes (Fig. 6, 7). Some samples from Tres Cruces subbasin (26, 32, 39) clearly plot in lacustrine zone (Fig. 7). While deposition of Yacoraite Fm. both marine and lacustrine environments were present in different subbasins. Shales Stromatolites Pisolite Shales Stromatolites Pisolite Shales Stromatolites Pisolite Carbonate Fig. 6. (Left) Tricyclic Terpanes ratio (C26/C25) vs Hopane ratio (C31R/C30 Hopane) showing marine environment for most of the samples. However some samples do not plot in a distinct region but more between marine and lacustrine zones. Inset in fig. 6 shows that it is likely that different depositional environments were connected allowing mixing of fresh and brackish water reflected in low variability of Gammacerane Index (Gam Index) (10 X Gammacerane/ (Gammacerane + Hopane) Fig. 7: (Left) Hopane ratio (C29/C30) vs. Hopane ratio (C31R/C30 Hopane). The Hopane ratio indicates more clearly a lacustrine depositional environments for samples 26, 32, 39 collected in Tres Cruces subbasin. In addition for sample 25, 31 and 43 a lacustrine depositional environment is indicated.  Marine Lacustrine Fig. 5: Regular steranes distribution %C27, %C28, %C29 indicates a marine depositional environment. Conclusions Complex depositional environments involving marine and fresh water coexisted during the deposition of Yacoraite Fm. Low Oxygen Index, Pristane/Phytane ratios, Hopane (34S/C35S) ratios indicate anoxic conditions during deposition and absence post depositional alteration of organic matter favored its preservation. Differences in lacustrine and marine environments during deposition of Yacoraite Fm. cause the observed organofacies variability. References Marquillas et al . 2007. Carbon and oxygen isotopes of Maastrichtian–Danian shallow marine carbonates: Yacoraite Formation, northwestern Argentina .JSAES, 23 304-320. Peters et al. 2005. The biomarker guide, volume 2: Biomarkers and isotopes in petroleum exploration and Earth history. Cambridge University Press, UK, 674 p. Schmidt et al. 2017. Strategy Proyect 3.1. Testing long-term controls of sedimentary basin architecture, Tres Cruces subbasin, Jujuy, Argentina Terra et al. 2012. Salta Basin, Argentina: A Good Analog for Phanerozoic Lacustrine Microbialite-Bearing Reservoirs, AAPG Hedberg Conference Microbial carbonate reservoir characterization, Houston, TX.