Deciphering Passive Margin Stratigraphy to Reveal Seascape Evolution Processes
Résumé
One way to decipher the stratigraphic record to reveal information about surface processes is through inverse analysis using forward models. In this approach, a forward surface process model that generates stratigraphy is run many times with varying parameter values, and the output stratigraphy is compared against observations. The parameter values that generate best-fit stratigraphy may then tell us something about surface processes. Inverse analysis of forward models is often used to infer past forcings to landscapes, but it can also be used to make inferences about process dynamics.
We use inverse analysis of forward models to ask: How should we model seascape evolution and the development of passive margin stratigraphy over geologic time? Understanding the evolution of passive margins is essential because they contain Earth’s longest and most complete records of landscape change on adjacent continents. Yet there is as yet no consensus about the process representations that should make up a simple but complete seascape evolution model. Should margin evolution be governed by processes where downslope sediment flux depends only on local slope? Or should nonlocal processes where sediment flux depends on factors other than local slope, such as the history of slopes experienced along a transport pathway, play a major role?
We conduct a data-driven inversion exercise using seven stratigraphic sections from the Southeast Atlantic Margin to assess the importance of nonlocal transport processes to building passive margin stratigraphy. We compare two mathematical models for seascape evolution: one that is purely local and one that incorporates nonlocal transport. The two models can converge through parameter changes alone, such that data-driven inversion can determine quantitatively the optimal process representation. Producing observed stratigraphy requires significant contributions from nonlocal processes like slope bypass and long-distance sediment transport; this result is true regardless of whether only the modern bathymetry or the full stratigraphic record is used to constrain the inversion. Results suggests that nonlocal processes dominate the development of passive margin stratigraphy and are essential ingredients in models used to infer past landscape dynamics from the stratigraphic record.