A different path to the Negro River in the Chibanian as a window to temporalize the eastward-flowing transcontinental Amazon

Abstract

When the Amazon was transformed into an eastward-flowing transcontinental drainage is an unresolved issue of great international interest. This is mainly due to the contribution of this river to global hydrological and climatic processes. The proposed time for this event ranges largely from the late Miocene-Pliocene to Pleistocene. Advancing this issue depends on new information from the continental domain that witnessed this event. This study aims to contribute to this debate by focusing on new geomorphological interpretation of digital elevation models and a chronological framework from available optically stimulated luminescence ages. The analysis was completed with subsurface information, including stratigraphic correlation of well logs, and interpretation of a regional seismic reflection line. The interpretation of geomorphological data revealed a major, narrow, N–S elongated, C-shaped depression connected with the Negro River to the north. This depression is confined between bounding surfaces marked by regularly distributed, spoon-shaped scours related to fluvial erosion. Sediments within this depression record late Chibanian ages and have the surfaces marked by abundant ridges and swales attributed to migrating fluvial bars. These characteristics altogether led to interpret the C-shaped depression as a fluvial paleovalley, which most likely corresponds to a previous path of the Negro River approximately 100 km west of its present-day position. Notably, the southern sector of this paleovalley is sharply truncated by terraces of the Solimões River, conformed by sediments with early/middle Tarantian and Holocene ages. These data suggest that the establishment of the Solimões River across the study area occurred only after the Negro River moved eastward to occupy its modern location.