Pacific Section AAPG Convention

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Using 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, Madre De Dios Region


The modern east-flowing Amazon River system is geologically recent, resulting from major drainage reorganization in the Miocene to Pliocene. The timing of this reorganization is contentious, and there is little agreement as to the degree of marine (estuarine/tidal) influence in the foreland basin successions deposited before the changes in drainage. Models and hypothesis of the geologic history of Amazonia are mainly based on local and regional studies, where in contrast, studies have argued for a single basin hypothesis where the deposition of the Madre de Dios Formation records these basinal changes. The Madre de Dios Formation (Units A-C), crops out along ∼700km of the Madre de Dios River in the Amazon foreland of Peru with successions that compete in depositional setting; with fluvial, tidal and lacustrine models. In this study, the muddy components (mudstones) of three sections, Cerro Colorado, Piedras River and Candelaria were described using smear slide techniques to document mineralogy and fauna in order to help discriminate depositional environments (fluvial vs. lacustrine vs. tidal estuarine) during drainage reorganization. A total of 146 samples from Cerro Colorado, Piedras River and Candelaria sections were made into smear slides and reviewed for authigenic and biogenic debris. Assessment for mud indicates no compositional evidence for diagnostic microfossils, indicating no marginal marine or brackish water influence. No tephra fragments or horizons were found, to aid in fingerprinting chronology of the depositional successions. Instead, clay and minor sand components could be consistent with a source that is fluvial and/ or lacustrine in origin.