--> Facies Variability in a Large-Scale Mississippi River Point Bar, False River, Louisiana, USA

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Facies Variability in a Large-Scale Mississippi River Point Bar, False River, Louisiana, USA

Abstract

Although a basic understanding of how point bar deposits has been clear for some time these studies have been mostly based on small and medium-scale examples. In contrast, understanding how the largest-scale point bar depositional packages (∼30 m thick) are assembled is less developed and cannot be readily up-scaled to predict observed high degree of lateral heterogeneity. Characterization of equivalent-scaled modern examples of such fluvial systems has the greatest potential to link processes to their true sedimentary products. We have examined the point bar related to the False River oxbow lake on the Mississippi in Louisiana through a campaign of coring, hydraulic profiling, electrical conductivity and gamma ray logging, together with reflection seismic profiling in order to characterize the internal architecture and variation in facies. A high resolution DEM of reveals that the bar developed from a deposit within a simple elliptical arc to a 80 km2 bar within compound meander bend with multiple apices measuring 13 km in cross valley width and 8 km in down valley dimension. Initial results suggest that the head of the bar comprises massive sand while the tail is more heterogeneous in having silt and mud layers interleaved within a sandy background all the way from the top to the base where this bar sits on reworked Pleistocene deposits. Although a number of major reorientation surfaces are visible in the topography these are harder to clearly define in the subsurface. In preliminary seismic profiles, whereas shallow (∼6m) clear reflectors can be projected surface-ward they do not necessarily coincide with changes in the surface lithology. In general the point bar grain sizes fine downstream, like the smaller examples, but may also preserve some deposits related to the abandonment phase ((c. 1720) close to its final apex.