Abstract
The western Louisiana inner shelf along the Chenier Plain coast has experienced fluid mud deposition in response to increased fine sediment supplied by the Atchafalaya River since ~1950’s. The goal of this project is to assess the characteristics comprising the sedimentary strata and stratigraphic architecture that result from mud deposition on the inner shelf. Recent time-series cores collected from the inner shelf along the Chenier plain coast show the region is subject to transient fluid mud deposition, leading to high long-term accumulation rates. Sediment cores were collected in May 2001, March and May 2002 from the inner shelf landward of the 10 m isobath, 100 km west of Atchafalaya Bay. Porosity, granulometry, X-radiography, and 7Be, 210Pb and 137Cs geochronology indicate the presence of high-porosity event layers 2-25 cm thick composed of clay with basal silt laminations. These event layers appear to concentrate around a depocenter located 95-110 km west of the Atchafalaya River, landward of the 7 m isobath, but can be ephemeral features on a seasonal time scale.
The combination of high-energy benthic hydrodynamics and sufficient fine sediment can result in cross-shelf gravity-driven flows (on very low slopes) that can blanket hundreds of square kilometers to thicknesses exceeding 10 cm. The sedimentary fabric that results from gravity-driven flows consists of a stacked pattern of predominantly fine-grained, fining upward packages. The resulting morphology of the shelf may be a clinoform, with maximum deposition occurring on the foreset (convex upward) region. The observations from the western Louisiana inner shelf (mud/silt couplets that comprise the fine-scale stratigraphy of the region, ephemeral 7Be deposits, and clinoform morphology exhibiting erosional features in sidescan and chirp data) are consistent with the wave-enhanced gravity-driven flow model. These results indicate that wave-enhanced gravity driven flows may be an important component in the dispersal of Atchafalaya River sediment.
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