Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin

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Abstract
New biostratigraphic, isotopic, and well log data from exploration wells on the outer continental shelf and uppermost Amazon deep-sea fan, Brazil, reveal that the Amazon River was initiated as a transcontinental river between 11.8 and 11.3 Ma ago (middle to late Miocene), and reached its present shape and size during the late Pliocene. Prior to the late Miocene the continental shelf was a carbonate platform that received moderate siliciclastic sediment supply from the Proterozoic basement in eastern Amazonia. Average sedimentation rates on the Amazon Fan show three stages of development: (1) 11.8-6.8 Ma ago, low sedimentation rates (0.05 m/ka) prevailed on the fan, because the Amazon River was not yet entrenched and some sediments were partially trapped in continental basins; (2) 6.8-2.4 Ma ago, sedimentation rates (0.3 m/ka) increased, the river entrenched, and deposition fully migrated onto the Amazon Fan; (3) 2.4 Ma ago to the present, very high sedimentation rates (1.22 m/ka, with peaks of 11 m/ka) prevailed on the fan and the modern Amazon River developed. All these paleogeographic and depositional events are closely related to Andean tectonism (late Miocene-Pliocene) and were exacerbated by global cooling and sea-level fall during the late Miocene