THE ROLE OF ORGANIC SEDIMENT DYNAMICS ON THE EVOLUTION OF FLUVIAL DELTAS: COUPLING NUMERICAL MODELING AND FIELD OBSERVATIONS

In-situ organic matter accumulation via plant growth generally accounts for a significant volume fraction of modern and ancient fluvio-deltaic plains but has been largely unexplored. To fill this knowledge gap, we extend an existing geometric model for the longitudinal profile evolution of a fluvio-deltaic system to account for a variety of sea-level curves, including sea-level cycles. Such geometry includes two key interconnected environments: a fluvial surface or topset, and a subaqueous region or forest. These environments are in turn delimited by three moving boundaries: the alluvial-bedrock transition, which separates the bedrock from the topset, the shoreline, which separates the topset from the foreset, and the delta toe, where the foreset intersects with the bedrock. A key assumption of the model is that sedimentological processes operate to preserve a linear geometry for the delta plain or topset. Specifically, when the rate of organic matter accumulation exceeds the rate of accommodation, the excess is either rapidly oxidized or eroded away. In contrast, if the organic matter accumulation rate is below the accommodation rate, the shortfall is filled with inorganic sediment supply from the river when available. We calculate the fraction of organic sediments preserved as the ratio between the organic and total sediment volume in the deltaic system. Under sea-level rise, the maximum organic fraction occurs when the organic matter accumulation rate matches the rate of accommodation, a result consistent with field observations from coal geologists. Additionally, as we increase the organic sediment fraction model, the shoreline progrades farther and faster. Under high enough rates of sea-level fall, the delta plain degrades and the delta front is forced to prograde. We assume that the majority of the sediments that contribute to building the delta front in this scenario are of inorganic nature as organic sediment are rapidly eroded and either decomposed or transported beyond the delta toe. Consequently, the higher the organic sediment fraction is during sea-level fall, the lower the rate of shoreline progradation. Overall, these results suggest that organic sediment dynamics can play a significant role on delta dynamics.