Medial and distal portions of distributive fluvial system: Case study of the Rio do Rasto Formation (Parana<acute accent> Basin, south Brazil)

In distributive fluvial systems (DFS), architectural variations occur from proximal to distal regions, characterized by a decrease in channel size, depth, and fluvial flow downstream. This study aims to elucidate these downstream trends in facies and facies associations across the distal and medial zones of a DFS. A detailed characterization was conducted on deposits from the Rio do Rasto Formation in southern Brazil. A total of 1218 m from six W-E oriented cores was analyzed, focusing on facies, facies transitions, surfaces, architectural elements, and facies associations. Seven distinct facies associations were identified and interpreted: fluvial channels, floodplains, proximal and distal terminal splays, wave-dominated lacustrine delta, eolian dunes, and eolian sand sheets. Two stratigraphic intervals were identified. The lower interval, characterized at the base by a gradual transition from the Teresina Formation to the Rio do Rasto, represents a more distal portion of the DFS. The upper interval, marked by an unconformity, suggests a reorganization in the fluvial equilibrium profile and represents a more intermediate portion of the DFS. Analysis of these temporal intervals reveals a general progradation of the unit with the depocenter shifting eastward toward Namibia. This study enhances the understanding of the stratigraphic record of DFS and provides insights into predicting facies and architectural complexity in such systems.