TRANSPORT AND ACCUMULATION OF COHESIVE FLUVIAL SEDIMENTS ONTHE INNER SHELF: FROM THE ATCHAFALAYA RIVER DOWNDRIFT TO THELOUISIANA CHENIER PLAIN

摘要

The Louisiana Chenier Plain, located 100-150 km west of the Atchafalaya River inrnsouthwest Louisiana, has been undergoing episodic progradation for the past several decades atrnrates up to 50 m/y due to coast-parallel transport of fine sediments derived from the AtchafalayarnRiver. During the past two years, we have studied the relationship between event-scale sedimentrntransport and formation of the longer-term sediment record. Box and gravity cores have beenrncollected before and after seasonal discharge maxima, and have been analyzed using 7Be, 210Pb,rnand 137Cs geochronology, granulometry, X-radiography, and multi-sensor core logging.rnHydrodynamical data have been collected from an ADCP deployed at the 3-m isobath, and fromrnthe LSU WAVCIS CSI-3 ocean observation station. In addition, a geoacoustic survey of thernstudy area was conducted using sidescan and chirp sonar.rnHydrodynamic observations document intense wave-current resuspension eventsrnproduced by passing cold fronts. Net westward and onshore flow occur in the bottom boundaryrnlayer, consistent with coastal deposition of sediment derived from fluvial sources to the east.rnCore and sonar data indicate the presence of an inner shelf/shoreface depocenter characterized byrnseasonal deposition of bedded clay/silt event layers 2-25 cm thick. Rapid deposition from arnfluvial source is indicated by the presence of high and uniform 7Be activities within fresh layers,rnand the lack of bioturbation. The prevalence of similar bedding throughout the upper 2-3 m ofrnthe seabed implies that accumulation of such event layers is linked to longer-term patterns ofrncoastal progradation on the Chenier Plain.