The Modern Atchafalaya Shelf Mud Clinoform and Its Overlooked Implications for Large River Diversions

Process and framework stratigraphic studies conducted on the inner shelf off the Atchafalaya River over the last decade have documented the presence of a progradational clinoform offlapping older strata out to 8 m water depth. This 3-m-thick feature, deposited since the 20th century onset of major Mississippi sediment delivery down the Atchafalaya pathway, has peak sediment accumulation rates of about 3 cm/yr, which equates to a facies progradation of about 30 m annually. Foreset progradation progresses, at least in part, driven by wave-supported hyperpycnal (fluid mud) flows generated by cold front passage, and coinciding with the loading of fresh river sediments onto the topset area. The resulting deposit is indistinguishable in core section from descriptions of “prodelta” muds in earlier Holocene Mississippi shelf delta deposits. This suggests that this mechanism was likely active in forming the distal (muddy) stratigraphic section of these earlier lobes, but has not been recognized to date due to its absence in the modern shelf-edge Plaquemine-Balize lobe. It is further suggested that this type of deposit would be expected to form seaward of any large Mississippi River diversions designed to discharge directly onto the shelf (e.g., Breton Sound). The rheology of this soft mud deposit has major overlooked implications for storm surge and wave height suppression in hurricanes.