Spatial variations in distributive fluvial system architecture of the Upper Cretaceous Marília Formation, SE Brazil

Conceptual models developed from modern distributive fluvial systems (DFSs) show a predictable spatial variation of facies progression from the margin to the interior of continental sedimentary basins. The Upper Cretaceous Serra da Galga Member of the Marília Formation has recently been interpreted to be a result of DFS deposition in the intracratonic Bauru Basin. This paper describes the spatial variations in stratigraphic architecture from the proximal to medial regions of the studied DFS and presents the implications of the recorded changes for fluvial reservoir characterization. The proximal areas are dominated by amalgamated channel-fill deposits and weakly developed, well-drained palaeosol profiles with only sparse floodplain deposits. The medial areas display a reduction in channel-fill deposits, an increase in the preserved thickness of floodplain deposits and the occurrence of sheetflood deposits and well-developed, poorly drained palaeosols. This spatial trend is accompanied by changes in the net : gross ratios of the geological bodies, forming possible high net : gross reservoirs in the proximal areas and reservoirs with very variable net : gross ratios in the medial areas. This down-system change in preserved stratigraphic architecture is evidenced for the first time in the studied sedimentary succession and follows the same pattern of facies progression described from modern examples of DFSs.