A MODEL FRAMEWORK FOR DELTA EVOLUTION THAT ACCOUNTS FOR ORGANIC SEDIMENT DYNAMICS

Sedimentary basin models based on the Exner equation have proved to be a useful approach for modeling the average dynamics during basin formation and evolution. Such models consist of a balance between inorganic sediment supply, base-level, and tectonics. An important missing factor in the balance, however, is the formation and decomposition of organic sediment, which has been recently highlighted as a potential control in the evolution of sedimentary deltas. Thus we extend current Exner models, in particular a simple geometric prism model, to explicitly account for organic sediment accumulation. The key assumption is that the processes that control the burial of organic sediment always operate to preserve the geometric shape of the basin. In addition, we recognize the first order effect of salinity on the rate of decomposition of organic sediment and split the domain into two regions: a near-shore saline region, and an inland region with a higher fresh water presence. The model is constructed in such a way that the boundary between the regions, which is defined geometrically, is controlled by subsidence and sea level changes. Within this model framework we are able to look at a number of scenarios including differing specifications of subsidence, sea-level and peat accumulation rates in each region. The model is able to explain the observed coupling between the average accommodation rate and the quality of buried peat deposits. Further analysis of the model suggests that peat dynamics can significantly influence delta evolution, and in particular an increased presence of soil organic accumulation leads to an enhancement of shoreline regression/transgression.