Documenting Erosion of the St. Bernard Delta: Past and Present

The Chandeleur Islands lie off the coast of eastern Louisiana and are thought to have formed as a barrier- island arc at the maximum extent of the St. Bernard Delta Complex of the Mississippi River between 2500 and 3000 yr BP. The relatively planar, deltaic stratigraphy of this region represents a unique opportunity to approximate the volume of sediment removed since formation and identify erosional processes driving the ongoing evolution of the delta complex. Detailed mapping of the stratigraphic facies of this delta complex, which indicates the presence of distributary sands, interdistributary muds, delta-front sandy muds, prodelta clays, and barrier-island sands, was interpreted from a dense network of geophysical (seismic-reflection, sidescan-sonar, and swath-bathymetry) and vibracore data collected offshore of the Chandeleur Islands in 2006 and 2007. Truncation of deltaic units on the modern shoreface and inner continental shelf indicates that the delta-front has been eroded significantly since the islands began retreating landward. At a finer scale, subcircular and linear depressions on the modern shoreface may be indicative of recent, event-driven erosion. The subcircular features occur in delta-front deposits, range from 7 to greater than 300 meters in diameter, and are commonly associated with gas-escape structures on seismic profiles. The linear features have lengths greater than 500 m, widths less than 300 m, and occur where distributary-channel deposits crop out on the seafloor. These features suggest that different delta facies respond uniquely to landward retreat of the shoreface and oceanographic processes acting on the ravinement surface. Although, the extent of geomorphologic control imposed on the modern barrier system by the location of distributary channels in the coastal zone is not well defined, the barrier islands, in part owe their continued existence to the reworking of sand from the continental shelf by erosional processes and subsequent alongshore sediment transport.