2021 CSDMS meeting-132


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The role of organic sediment dynamics on paleo-sea level reconstruction from fluvio-deltaic stratigraphy: Insights from a geometric model

Norjmaa Khosbaatar, Montclair State University Montclair New Jersey, United States. nkhosbaatar@gmail.com
Jorge Lorenzo Trueba, Montclair State University Montclair New Jersey, United States.

Coastal-plain depositional systems such as fluvial deltas are archives of past external (allogenic) forcing, such as sea-level variations, and their evolution can be described by two geomorphic boundaries: the alluvial-basement transition or upstream boundary, and the shoreline or downstream boundary. Patterns of landward/seaward migration of the shoreline (i.e., transgression/regression) and the alluvial basement transition (i.e., coastal onlap/offlap) in the rock record are often used for reconstruction of past sea-level changes. Theories for stratigraphic interpretation, however, need to be adapted to deal with internal (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 accounting for a significant volume fraction in most fluvio-deltaic plains and likely affect their response to sea level variations. To fill this knowledge gap, we develop a geometric model for the long-profile evolution of a fluvio-deltaic environment that accounts for sea-level cycles and organic sediment dynamics. The model assumes that sedimentological processes (i.e., inorganic and organic sedimentation) operate to preserve a linear geometry for both the delta plain or topset, and the subaqueous offshore region or forest. Changes in topset length can occur via shoreline transgression/regression, or coastal onlap/offlap, and the magnitude and timing of these changes can be directly related to the amplitude, phase and frequency of the sea-level variations. The model predicts that the maximum organic fraction occurs when the organic matter accumulation rate matches the accommodation rate, an observation consistent with field observations from coal geology. Further, we find that organic matter accumulation during the topset aggradation and organic matter erosion and decay during topset degradation generally results in substantial increase in the coastal onlap/offlap amplitude, which can result in an overestimation of the sea-level variations. These results are consistent with the discrepancy in sea-level amplitude reconstructions between sequence stratigraphic models and geochemical models over the Cretaceous.