MODELING THE EFFECT OF ORGANIC SEDIMENT DYNAMICS ON COASTAL PLAIN DEPOSITIONAL SYSTEMS UNDER SEA-LEVEL CYCLES

Theories for stratigraphic interpretation generally describe the architecture and dynamics of depositional systems in terms of sediment supply, tectonics and eustacy, neglecting important factors that could also play a significant role such as in-situ organic matter accumulation via plant growth. In order to fill in this knowledge gap, we extend an existing geometric model for the profile evolution of a fluvio-deltaic environment to account for organic sediment dynamics under sea-level cycles. The model assumes a linear topset, delimited by two moving boundaries: the alluvial-basement transition (ABT) where the accumulation of sediment separates the bedrock from the topset, and the shoreline (SH), which separates the topset from the foreset. We calculate the rate of organic matter accumulation in terms of the balance between the rates of organic sediment production and sea-level rise. We also account for the reworking and decomposition of previously accumulated organic sediment when the topset degrades. We run this model under a range of scenarios and find that the average carbon fraction, defined as the ratio between the organic and the total sediment volume in the sedimentary prism, increases as the sea-level rise rate increases and reaches its maximum when the rate of sea-level rise equals the rate of production. In contrast, under sea-level fall the carbon fraction can be reduced due to both the reworking and decomposition of organic deposits as the topset degrades and the reduction of organic matter preservation due to lack of accommodation. Under sea-level cycles, the maximum carbon fraction occurs for a specific amplitude for the sea-level oscillations below which organic matter preservation is limited by the rate at which accommodation is created. For amplitudes higher than this value the reworking, decomposition and export of organic sediments increases. Additionally, we find that by including organic sediment dynamics, the response of the ABT to sea-level cycles is amplified, whereas the response of the SH is dampened. Overall, with this modeling framework we aim to describe basin-scale profile carbon fraction trends and ABT and SH trajectories under a wide range of scenarios.