The Pan-Amazonian Ucayali Peneplain, late Neogene sedimentation in Amazonia, and the birth of the modern Amazon River system

The Madre de Dios Formation has three members, the oldest of which documents a short-lived, high energy depositional environment followed by a moderate-energy depositional environment, both occurring at a time when drainage from the basin was unobstructed. The upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed, disorganized, and took place over long distances with extremely low gradients. The sedimentology of the Madre de Dios Formation, particularly the thick, massive beds of clay, and the widespread presence of paleodeltas and associated geomorphic features on the Amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake, Lago Amazonas, or, more probably, within a complex series of interconnected mega-lakes that occasionally united to cover most or all of lowland Amazonia to a shallow depth from the latest Miocene until similar to 2.5 Ma. The presence of the Ucayali Unconformity and the relatively uniform lithostratigraphy basin-wide of the fluvial, fluviolacustrine, and lacustrine sediments of the upper Neogene Madre de Dios Formation are consistent with the hypothesis that the Amazon Basin acted as a single, undivided sedimentary basin in the late Neogene. The biostratigraphic correlation across important modem drainage divides of both micro- and macro-sized, late Miocene fossil vertebrates recovered from basal conglomerates of the Madre de Dios Formation, and the absence therefrom of fossil vertebrates of any other age, is also consistent with this hypothesis. Two 40Ar/39Ar dates on ash deposits within the Madre de Dios Formation corroborate the upper Miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper Pliocene age for the youngest sediments of the formation. The Madre de Dios Formation has three members, the oldest of which documents a short-lived, high energy depositional environment followed by a moderate-energy depositional environment, both occurring at a time when drainage from the basin was unobstructed. The upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed, disorganized, and took place over long distances with extremely low gradients. The sedimentology of the Madre de Dios Formation, particularly the thick, massive beds of clay, and the widespread presence of paleodeltas and associated geomorphic features on the Amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake, Lago Amazonas, or, more probably, within a complex series of interconnected mega-lakes that occasionally united to cover most or all of lowland Amazonia to a shallow depth from the latest Miocene until similar to 2.5 Ma. The presence of the Ucayali Unconformity and the relatively uniform lithostratigraphy basin-wide of the fluvial, fluviolacustrine, and lacustrine sediments of the upper Neogene Madre de Dios Formation are consistent with the hypothesis that the Amazon Basin acted as a single, undivided sedimentary basin in the late Neogene. The biostratigraphic correlation across important modem drainage divides of both micro- and macro-sized, late Miocene fossil vertebrates recovered from basal conglomerates of the Madre de Dios Formation, and the absence therefrom of fossil vertebrates of any other age, is also consistent with this hypothesis. Two 40Ar/39Ar dates on ash deposits within the Madre de Dios Formation corroborate the upper Miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper Pliocene age for the youngest sediments of the formation. The Madre de Dios Formation has three members, the oldest of which documents a short-lived, high energy depositional environment followed by a moderate-energy depositional environment, both occurring at a time when drainage from the basin was unobstructed. The upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed, disorganized, and took place over long distances with extremely low gradients. The sedimentology of the Madre de Dios Formation, particularly the thick, massive beds of clay, and the widespread presence of paleodeltas and associated geomorphic features on the Amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake, Lago Amazonas, or, more probably, within a complex series of interconnected mega-lakes that occasionally united to cover most or all of lowland Amazonia to a shallow depth from the latest Miocene until similar to 2.5 Ma. The presence of the Ucayali Unconformity and the relatively uniform lithostratigraphy basin-wide of the fluvial, fluviolacustrine, and lacustrine sediments of the upper Neogene Madre de Dios Formation are consistent with the hypothesis that the Amazon Basin acted as a single, undivided sedimentary basin in the late Neogene. The biostratigraphic correlation across important modem drainage divides of both micro- and macro-sized, late Miocene fossil vertebrates recovered from basal conglomerates of the Madre de Dios Formation, and the absence therefrom of fossil vertebrates of any other age, is also consistent with this hypothesis. Two 40Ar/39Ar dates on ash deposits within the Madre de Dios Formation corroborate the upper Miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper Pliocene age for the youngest sediments of the formation.