Inverting Stratigraphy for Marine Sediment Transport Dynamics over Geologic Time: Examples from the Orange Basin, Southern Africa
Résumé
Source-to-sink (S2S) studies seek to explicitly link the denudation of continents with the building of basin stratigraphy in an effort to infer tectonic and climatic drivers of surface change. Quantitative models for S2S systems must incorporate geomorphic processes at both source and sink, yet more effort has been devoted to developing landscape evolution models in source terranes than equivalent models for sedimentation in marine basins. In particular, most marine sedimentation models use local linear diffusion approximations for sediment transport, which have been shown to yield reasonable stratigraphy in shallow marine environments but struggle to reproduce diagnostic features of deep marine deposits. The lack of model predictive power in deep marine environments precludes the full closure of S2S sediment budgets.
We present a model for marine sedimentation with two simple modifications allowing non-local sediment transport: 1) a mechanism for sediment bypass on steep topographic slopes, and 2) a parameter allowing long-distance transport over vanishingly gentle slopes. We use Bayesian inference techniques to constrain four model parameters against the stratigraphy of the Orange Basin in southern Africa. We compare modeled against observed stratigraphy over 130 Ma of margin evolution. Our best-fit simulations capture the broad structure of the observed record, and imply non-negligible roles for both non-local model elements: sediment bypass at steep slopes and long-distance runout over gentle slopes. Residual misfit between our best-fit simulations and the stratigraphic data indicate that additional components of transport dynamics—likely hemipelagic sedimentation, grain size variations, or ocean bottom currents—might be required to achieve the longest transport distances observed in the sedimentary record. Results suggest that full closure of Earth’s sediment mass balance for S2S studies requires moving beyond local diffusion approximations, even at the longest timescales. Relatively simple modifications to modeled transport dynamics can lead to better agreement between modeled and observed stratigraphy, and may enable improved inference of landscape perturbations from the stratigraphic record.