Use of Sedimentary Petrology and Provenance to Resolve Questions Regarding Neogene Fluvial/Lacustrine Drainage Development and Potential Marine Influence in the Southern Rim of the Amazon Basin

The modern east-flowing Amazon River system is geologically recent, resulting from a major drainage reorganization in the Miocene to Pliocene. Different depositional environments (fluvial/ deltaic/ lacustrine/ tidal-marine) have been proposed for the Andean foreland just prior to the drainage reorganization as recorded in the Madre de Dios Formation exposed in the upper reaches of the Amazon River catchment. The objective of this study is to constrain the stratigraphy, depositional environment and drainage evolution in southern Amazonia through petrographic analysis and provenance of the sand and mud fractions of the three recognized members of the Madre de Dios Formation. Samples were examined from three stratigraphic sections measured previously along riverbank outcrops: Cerro Colorado, Piedras River and Candelaria. Petrographic analyses of 144 smear slides made from muddy samples show the major silt components to be quartz, feldspar and mica, with no identifiable microfossil or pyroclastic debris. Petrographic analyses of thin sections of sand separates from 25 sandy samples show that the sand components include abundant monomineralic (Qm) and polymineralic (Qp) quartz. Sedimentary (Lsa and Lsi) and metamorphic (Lmp, Lmt, Lmf) lithic fragments are common to abundant. Muscovite, potassium feldspar and sodium-plagioclase are less abundant. All of the sand samples are lithic arenite in composition with a recycled orogen provenance. Mineralogical maturity increases up-section, but slightly higher feldspar in member B is likely a function of a change in source area or depositional environment. Sand composition in member C is similar to the modern river sand composition, consistent with the recycling of Madres de Dios Formation sand into the modern river system. There are six main colors of mud in the Madre de Dios Formation and these characterize distinct stratigraphic intervals from I at the base to VI at the top of each section that do not correspond directly to member boundaries. There are no significant changes in mud composition among the three members. Red to orange to brown mudstones, like those that dominate Madre de Dios Formation members A and C, are formed in oxidizing environments generally in continental settings (e.g., fluvial systems). None of the 144 smear slides or the 25 thin sections contained microfossils, and the lack of marine or marginal marine biogenic debris appears to rule out tidal influence for member B. Based on its light olive gray color, interval (IV) in member B may have been deposited in a more reducing environment such as a lacustrine-deltaic setting. Overall, the Madre de Dios Formation exhibits up-section compositional and thickness trends that are indicative of changes in provenance and depositional environment from fluvial (member A) to lacustrine/deltaic (member B) to fluvial (member C).