INNER SHELF CLINOFORM DEVELOPMENT FROM FLUID-MUD DEPOSITION: CHENIER PLAIN COAST OF LOUISIANA, USA

For the past half-century, the Chenier Plain coastline of southwest Louisiana has been undergoing rapid and episodic progradation at rates up to 50 m/y. Sediment is supplied via a coastal current system from the Atchafalaya River, a Mississippi River distributary located 100 km east of the Chenier Plain, and is deposited in the form of fluid-mud event layers that are transported shorewards during cold-front passage. The most obvious manifestation of fluid-mud deposition is the formation of extensive intertidal mud flats. However, recent analysis of cores, sidescan sonar data, chirp subbottom profliles, and high-resolution bathymetric data from the inner shelf indicate that extensive subtidal deposition is building mud clinoforms that extend at least to the ~5 m isobath (7-10 km offshore) on this extremely low-gradient coast. In water depths of 0.5-2.5 m, the seabed possesses a smooth depositional surface with a very low gradient (~0.5m/km), but steeper seabed gradients and scarps indicative of slope failure occur farther seaward, to the 5-m isobath. In this region, seaward clinoform edges are locally eroded by wave action. Scours and cavities produced by erosion and slides are filled with acoustically transparent fluidized mud. High long-term accumulation rates (4-6 cm/y from Pb210/Cs137 geochronology) across the clinoforms are strongly influenced by deposition of seasonal event layers up to 25 cm thick.

Clinoform morphology has been analyzed in the context of a new analytical model for shelf evolution developed by L.D. Wright and colleagues (Mar. Geo.175: 25-45), and was modeled as an equilibrium depositional feature under the combined influences of high sediment flux, wave shoaling, and gravity-driven hyperpycnal flow. Modeled and measured seabed slope are reasonably comparable shoreward of the 7-m isobath, implying that the innermost shelf gradient is in equilibrium with sediment flux and wave climate. Failure scarps observed on sidescan sonar records occur in regions with the steepest gradients (~1m/km). Wave attenuation across the muddy seabed is an important factor, however, that is not accounted for by linear wave theory used in the model.