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Abstract: Paleoenvironmental and Taphonomic Evidence of Diverse Bioclast Sources for Southwestern Louisiana Cheniers

Laurie C. Anderson, Mark R. Byrnes, Randolph A. McBride

Southwestern Louisiana cheniers contain macrofossil assemblages that vary in taxonomic composition and taphonomic signature, indicating diverse sources for these assemblages. The most landward cheniers, Little Chenier and Chenier Perdue, are capped by alternating coarse and fine coquinas dominated by Crassostrea virginica. The estuarine fauna shows poor preservation with significant rounding, incipient carbonate precipitation, and moderate to extensive bioerosion. More seaward, Front Ridge, the most extensive chenier complex of the chenier plain is predominantly sand with shells occurring as stringers or in decimeter thick tabular beds. Mulinia lateralis dominates assemblages, although small Ostrea equestris, Abra aequalis, and Anadara transversa are also common. Preserva ion of this upper shoreface assemblage is very good and bioclasts typically show minimal rounding, breakage, bioerosion, or encrustation. Some shells retain original color and/or gloss. The most seaward relict beach ridge complex at Hackberry Beach consists of planar to low-angle cross-stratified coquinas with lens and lenticular beds of coarser shell material and thin, laterally extensive layers of macerated plant material. The assemblage is characterized by Mulinia lateralis and Anadara ovalis, and other arcids are common. Such assemblages are typical of the shoreface. Bioclasts are moderately well preserved; minor rounding and loss of gloss are common.

Differences in paleoenvironmental setting and taphonomic condition may indicate diverse bioclast sources. Assemblages of all cheniers, however, show evidence of size-sorting, alternating high and low energy conditions, and fabrics indicative of tractive transport. Oyster shell coquinas of more landward cheniers resemble beach deposits of rapidly transgressing coastal environments where brackish-water shell assemblages are reworked from estuarine sedimentary deposits. Shelly sands of Front Ridge and the eroding beach ridges of Hackberry Beach contain shallow marine, probably upper shoreface, assemblages.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana