Incision and aggradation phases of the Amazon River in central-eastern Amazonia during the late Neogene and Quaternary

Abstract

The development of the Amazon River as a transcontinental drainage has been narrated in terms of the sedimentary record in western Amazonia and in marine offshore areas. However, little is known about the landscape changes in central-eastern Amazonia, which is considered a former water divide prior to the establishment of the Amazon as a transcontinental drainage. In this study, we carried out geomorphological and sedimentary facies analyses of fluvial deposits associated with the Amazon River in central-eastern Amazonia. Additionally, extended range luminescence ages and sediment provenance data are used to help to improve the chronostratigraphic framework of the study area. Erosive scarps bound the Amazon River valley in the study area, where higher elevation upland terrains (locally called ‘terra firme’) are sustained by older and resistant bauxitic-lateritic paleosols that cap the Cretaceous sandstone bedrock. Two abandoned fluvial terraces (T3 and T2), representing lower elevation terra firme, and the active floodplain (T1), collectively record the incision and aggradation phases of the Amazon River in a NE-SW oriented valley since the Late Miocene. The T3 is intensely dissected and has weathered deposits with surface elevations between 30 and 50 m above sea level (a.s.l.). These deposits are composed of gravel with boulders, coarse sand, and locally grey mud rich in organic matter. Quartz thermally-transferred optically stimulated luminescence (TT-OSL) dating allows us to determine the oldest Quaternary ages reported for fluvial terraces associated with the Amazon River. Depositional ages for the T3 deposits range from 944 ka to 2.3 Ma. The T2 deposits form a flat area with surface elevations between 25 and 30 m a.s.l. These deposits comprise fine to medium sand interbedded with mud and locally coarse sand and mud alternations forming inclined heterolithic stratification. Feldspar post-infrared infrared (pIRIR290) luminescence ages indicate an incision and aggradation phase leading to deposition of the T2 from 557 to 140 ka. The T1 represents the active floodplain with a surface elevation lower than 25 m a.s.l. These deposits are composed of moderately to well-sorted silty fine sand and greyish mud. OSL sensitivity measurements suggest that deposits from all terraces are formed by a mixture of Andean and shield-cratonic sediments, which are similar to the sediments transported by the modern Amazon River. The fractures in the Paleozoic to Cretaceous bedrock that comprises the higher elevation terrain incised by the Amazon River in central-eastern Amazonia lead to differential fluvial erosion, shaping the current NE-SW and NW-SE valley orientation. The T3 and T2 terraces record the progressive incision of the Amazon River valley since the Late Miocene, leading to poor preservation of fluvial deposits in comparison with western Amazonia. Here in central-eastern Amazonia, fluvial activity has been locked into a narrow fluvial valley in comparison with a wide area in western Amazonia. The stratigraphic scenario with a limited accommodation space favoured the recycling of terraces within the Amazon River valley. The restricted distribution of deposits associated with the Amazon River in central-eastern Amazonia is attributed to the uplift behaviour and high erosional resistance of the Paleozoic and Mesozoic bedrock forming a high elevation plateau. During the Pleistocene, the T3, T2 and T1 deposits record at least three main phases of incision, which promoted the abandonment of floodplains and formation of new lowland terra firme areas.