An Insight into Environment and Provenance from Geochemistry of the Early Cretaceous Sedimentary Rocks, Upper Magdalena Valley, Colombia

The Basin of the Upper Magdalena Valley is one of the most important petroleum producing basins of Colombia and the sandstones (upper member) of the early Cretaceous Caballos Formation represents one of the well-known reservoirs of this basin. In this study, we used abundances of organic carbon, carbonate and sulfur, and concentrations of major and trace elements in all three members of the Caballos Formation in order to identify the possible provenance and infer paleo-productivity and paleo-environment during the middle Aptian-middle Albian. The sedimentary rocks (i.e., sandstone, siltstone and shale) comprises mainly of quartz, kaolinite, illite, siderite and pyrite. Calcite and dolomite are present in some samples but in scarce amounts. Both the ternary diagrams (A-CN-K and A-C-N) suggest that clastic fractions of the sedimentary rocks were sourced from a source enriched in Ca-bearing plagioclase (anorthite), similar to the andesitic-basaltic lavas of the Jurassic Saldaña Formation. However, the upper and lower members were more and homogenously altered compared to the heterogeneously altered middle member. Calculated indices of alteration (CIA and PIA) indicate extreme alterations for the lower and upper members and moderate to extreme alteration for the middle member. Different degrees of alteration of the sedimentary rocks follows the atmospheric CO2 concentrations inferred for the Aptian-Albian interval. Mineralogical and geochemical data suggest a depositional environment with pH varying between 7 and 9. Higher alteration caused mobility of Si with respect to Al and hence we observed incongruences between lithological and geochemical classifications. Different abundances of siderite and pyrite along with higher organic carbon and redox-sensible trace elements suggest in general anoxic environments of deposition. However, sedimentary rocks of the middle and upper member were deposited in a relatively more anoxic environment compared to sedimentary rocks of the lower member.