Into the abyss: Passive margin stratigraphy reveals seascape evolution processes

This is a presentation given at the 2022 Geological Society of America annual meeting (October 9-12, 2022). It summarizes Shobe et al. (2022; Basin Research). Abstract: Passive margin stratigraphy holds time-integrated records documenting the evolution of landscapes that are long gone. It is probably possible to read the stratigraphic record by inverting coupled landscape evolution models and stratigraphic forward models (SFMs) for past landscape boundary conditions (e.g., climatic/tectonic perturbations). One hurdle is that there is not broad agreement on the optimal form of the SFMs we use to simulate marine sediment transport and the development of the offshore stratigraphic record over geologic time; there has been a lack of tests against observed stratigraphy in well-constrained environments. Specifically, it is not clear whether SFMs should be governed primarily by local (sediment transport rate depends only on local bathymetry) or nonlocal (transport rate depends on the history of bathymetry experienced along a transport pathway) sediment transport processes. Here we develop a simple nonlocal SFM, which incorporates the long-term average effects of nonlocal processes like turbidity currents and marine debris flows, and test it against a commonly used local model to determine which approach provides the best fit to stratigraphy in a well-constrained natural experiment. We invert for optimal model form by comparing calibrated implementations of both models against seven seismic sections from the Southeast Atlantic Margin representing 130 Ma of sediment accumulation. Results suggest that observed stratigraphy is not compatible with the local SFM and that fitting observed stratigraphy requires strong contributions from slope bypass and long-distance transport processes. This outcome holds true in cases where the model is only compared to the observed modern bathymetric surface, as well as cases where all seismic reflectors are considered. Processes of sediment bypass and long-distance transport may be essential to modeling realistic passive margin stratigraphy, and incorporating such processes into our inversion models may therefore clarify source-region dynamics inferred from the sedimentary record.