Morphodynamic Preservation of Fluvial Channel Belts

Abstract

The fluvial sedimentary record is largely composed of deposits from relatively common flow events, rather than more catastrophic scour-and-fill events. At the scales of bedforms, such deposits are preserved within the stratigraphic record because they rapidly accumulate within, and are protected by, morphodynamic topographic depressions that occur naturally in the fluvial system as a result of feedbacks between flow, sediment transport, and topography. Examples include the preservation of ripples in front of dunes, dunes in front of bars, and bars within channels. Here, we used 3D seismic data that images preserved channel belts to test the hypothesis that alluvial-ridge basins, morphodynamic depressions formed between raised channel beds due to decreasing sedimentation rates away from channels in alluvial settings, are a source of topography driving channel-belt-scale preservation. Using the 3D seismic data, we measured the stratigraphic positions of channel belts, as well as their lengths, widths, sinuosities, and centerline orientations in the 3D seismic dataset. Results are consistent with well-preserved channel belts steered by alluvial-ridge-basin topography. Further, the thickness of the channel-belt interval exceeds the relief of any one alluvial-ridge basin, suggesting the volume records the filling of multiple alluvial-ridge basins and that the process is common. Characterizing the stratigraphic signature of alluvial-ridge basins is necessary for understanding contrasting fluvial architectures where external forcings prevented their formation.