RIVER CAPTURE AND TRANSIENT INCISION ACROSS LITHOLOGICALLY CONTROLLED TOPOGRAPHIC BARRIERS IN AMAZONIA'S EASTERN GUIANA SHIELD

Major reconfiguration of the drainage network in the Amazon region has been ongoing since the Amazon river established its transcontinental flow in the late Miocene. While most studies are concentrated in the western Amazon, little is known about past landscape changes in the eastern Amazon. Here, numerous examples of anomalous river patterns, wind gaps, asymmetrical drainage divides and steep topographic escarpments point to a complex drainage reorganization history. In this study, we used longitudinal river profiles and topographic analyses to identify multiple erosive fronts propagating into the Guiana Shield across a topographic barrier. This barrier is up to 800 m tall, is composed of highly resistant Ordovician sandstones, and marks the southward conformable contact between the crystalline shield rocks and Amazon's paleozoic sedimentary basin rocks. On the topographic barrier, wind gaps preserved between 250 m and 300 m point to former southward barrier-crossing rivers. The modern south-flowing river networks contain different river levels with equilibrium-like concave morphologies bound by two sets of knickpoints at approximately 200 and 350 m elevation contours. These chi-clustered vertical-step knickpoints are not always associated with lithological changes, suggesting they are likely upstream propagating erosive pulses. Basins in the central plateau region near the bedrock transition reveal slope-break knickpoints, suggesting base level fall in the lowermost river network. Reconstructed paleo-outlet elevations are consistently around 250 m to 300 m, consistent with intermediate knickpoint and wind gap elevations. The 250-300 m contours therefore mark the paleosurface of a former barrier-crossing river network, captured by the modern rivers. The upstream location of knickpoints are farthest from the topographic barrier in the edges of the plateau, suggesting longer time since river capture. These analyses suggest the current landscape results from multiple river excavation events and progressive river captures. The pace of these river captures were possibly regulated by large lithological contrasts in the Guiana shield. River capture and base level fall are important mechanisms in the erosion of elevated topography over cratonic landscapes.