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Autogenic Progradation and Retrogradation Patterns in Deltaic Strata Generated by the Interplay Between Discharge and Basin Geometry

Abstract

Autogenic Progradation and Retrogradation Patterns in Deltaic Strata Generated by the Interplay Between Discharge and Basin Geometry

Lexie Stodden, Brady Z. Foreman

Deltaic stratigraphy records the interaction of subsidence, eustasy, sediment flux, and autogenic processes. These parameters commonly lead to a hierarchy of progradational, retrogradational, and aggradational stratal patterns that broadly reflect the ratio of accommodation to sediment supply. Increased water discharge can result in progradation due to greater sediment capacity. However, the effects of water discharge on base level are commonly viewed as negligible (i.e., the marine basin is orders of magnitude larger than the volume of water input from distributary channels). Herein we perform a series of physical experiments that evaluate the scenario wherein fluvial discharge into the basin significantly impacts base level. The experiments were performed in a closed 50 cm x 167 cm x 45 cm flume using a mixture of silica sand (D50 = 120 µm) and anthracite coal (D50 = 300 µm). In each experiment the ratio of sediment flux to water discharge was held constant and the experimental delta allowed to freely evolve and fill the initially empty basin. We observe an initial progradation phase, followed by aggradation, and retrogradation phases of delta deposition, and we characterize these deposits in three-dimensions using computed tomography. The position of the shoreline, stratigraphic thickness, and geometries of geo-bodies appears to be a function of water discharge volume relative to the basin accommodation space. The experiments represent a special case of basin evolution, but illustrate the generation of complex stratigraphic patterns in the absence of external forcings.