The role of organic matter accumulation and degradation on the evolution of a coastal-plain depositional system under sea-level cycles

Theories for stratigraphic interpretation of depositional systems such as fluvio-deltaic environments need to be adapted to deal with autogenic processes that could play a significant role, but are to date largely unexplored. In particular, in-situ organic matter accumulation via plant growth has generally received little attention despite often accounting for a significant volume in the sedimentary record. We aim to fill this knowledge gap by extending an existing geometric model for the profile evolution of a fluvio-deltaic environment to account for organic sediment dynamics under sea-level cycles. A key assumption of the model is that sedimentation processes operate to preserve the geometry of the topset, which is delimited by two moving boundaries: the alluvial-bedrock transition (ABT), which separates the basement from the topset, and the shoreline (SH), which separates the topset from the foreset or subaqueous region. We account for organic sedimentation as a function of the rates of organic matter production and sea-level rise. If the rate of organic matter accumulation exceeds the sea-level rise rate, the excess is either rapidly oxidized or eroded away. In contrast, when the organic matter accumulation rate is below the sea-level rise rate, either the shortfall is filled with inorganic sediment or the system undergoes SH retreat. We run the model under different sea-level cycles and find that the average carbon fraction of the sedimentary prism increases during the sea-level rise phase and reaches its maximum when the rate of sea-level rise decreases below the rate of organic matter production, shortly before the highstand. In contrast, during sea-level fall the average carbon fraction decreases due to a reduction in organic sediment preservation (as accommodation decreases), as well as the reworking and export of previously deposited organic sediment during topset degradation. We also find that as the amplitude of sea-level oscillations increases, the amplitude of ABT response increases and the amplitude of the SH response decreases. Overall, we find that this modeling framework can be useful to study the response of the ABT and SH, as well as basin-scale profile trends in the distribution of organic matter, under a wide range of scenarios.