Results from physical and numerical experiments suggest that sediment cohesion influences deltaic morphodynamics by promoting the development and maintenance of channels. As a result, cohesion is thought to increase the magnitude and time scales of internally generated (autogenic) processes and the dimensions of their stratigraphic products. We test these hypotheses by examining the surface processes and stratigraphic products from a suite of physical experiments where the influence of cohesion is isolated over temporal and spatial scales important for basin filling. Given the stochastic nature of autogenic sediment transport processes, we develop and employ a range of statistical tools and metrics. We observe that (1) an increase in sediment cohesion decreases lateral channel mobility and thus increases the time necessary to regrade deltaic surfaces; (2) enhanced channelization, due to sediment cohesion, increases the time necessary for the deposits of autogenic processes to average together and produce stratigraphic products with shapes set by the generation of regional accommodation; (3) cohesion promotes the transport of suspended sediment to terrestrial overbank and marine environments, which decreases the volume of channel, relative to overbank and marine deposits in the stratigraphic record. This increase in overbank and marine deposition changes the spatial distribution of sand in stratigraphy, with higher cohesion linked to enhanced segregation of fine particles from coarse sand in the experimental deposits. Combined, these results illustrate how the cohesion of sediment is fundamental in setting autogenic spatial and temporal scales and needs to be considered when inverting stratigraphic architecture for paleo‐environmental history.

Abstract:Although the Mississippi River deltaic plain has been the subject of abundant research over recent decades, there is a paucity of data concerning field measurement of sediment erodibility in Louisiana estuaries. Two contrasting receiving basins for active diversions were studied: West Bay on the western part of Mississippi River Delta and Big Mar, which is the receiving basin for the Caernarvon freshwater diversion. Push cores and water samples were collected at six stations in West Bay and six stations in Big Mar. The average erodibility of Big Mar sediment was similar to that of Louisiana shelf sediment, but was higher than that of West Bay. Critical shear stress to suspend sediment in both West Bay and Big Mar receiving basins was around 0.2 Pa. A synthesis of 1191 laser grain size data from surficial and down-core sediment reveals that silt (4-63 µm) is the largest fraction of retained sediment in receiving basins, larger than the total of sand (>63 µm) and clay (<4 µm). It is suggested that preferential delivery of fine grained sediment to more landward and protected receiving basins would enhance mud retention. In addition, small fetch sizes and fragmentation of large receiving basins are favorable for sediment retention.

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