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Subaqueous deltaic formation on the Atchafalaya Shelf, Louisiana Ciara F. Neill1

Summary: Subaqueous deltaic formation on the Atchafalaya Shelf, Louisiana
Ciara F. Neill1
, Mead A. Allison*
Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, USA
Received 26 January 2004; received in revised form 7 October 2004; accepted 5 November 2004
The Atchafalaya River in Louisiana shares the third largest drainage basin in the world with the Mississippi River. Sediment
cores and seismic profiles were used to examine the development and impact on land accretion of an early-stage subaqueous
delta accumulating on the shallow (b25 m water depth) continental shelf seaward of the Atchafalaya River mouths in the period
(~100 years) since the Atchafalaya has captured a significant fraction of the overall Mississippi discharge. The subaqueous
clinoform is muddy (70100% finer than 63 Am) and extends approximately 2126 km seaward of the shell reef (to 8 m water
depth) across the mouth of the Atchafalaya Bay, with a discontinuous, and, in places, mobile modern mud layer b20 cm thick
covering a relict deltaic shoal area further seaward. The sigmoidal clinoform has a topset surface that steepens from east to west
(1:2500 to 1:1600), a foreset with maximum slopes of about 1:550, and a limited bottomset region (b0.5 km wide). 210
Pb and
Cs geochronology show maximum sediment accumulation rates (N3 cm/year) correspond to the foreset and bottomset region,
with rates decreasing to as low as 0.9 cm/year on the shelf topset region and its extension inside Atchafalaya Bay. Seven
sedimentary facies are observed in the subaqueous delta, with differences created by degree of biological destruction of physical
stratification, which is inversely related to sediment accumulation rate, and by the proximity of an area to the riverine sand


Source: Allison, Mead - Institute for Geophysics, University of Texas at Austin


Collections: Geosciences