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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 45 (1995), Pages 21-29

Paleoenvironmental and Taphonomic Evidence of Diverse Bioclast Sources and their Implications for the Depositional Settings of Southwestern Louisiana Cheniers

Laurie C. Anderson (1), Mark R. Byrnes (2), Randolph A. McBride (2)

ABSTRACT

Southwestern Louisiana cheniers contain macrofossil assemblages that vary in taxonomic composition, taphonomic signature, and bioclast fabric, suggesting diverse bioclast sources and/or depositional environments. The most landward cheniers, Little Chenier and Chenier Perdue, contain alternating coarse and fine coquinas dominated by Crassostrea virginica and Ostrea equestris. This estuarine fauna shows fair to poor preservation. More seaward, Front Ridge, part of the most extensive chenier complex of the chenier plain, is predominantly sand with shells scattered, in stringers, in lenses, and in decimeter-thick tabular beds. Mulinia lateralis dominates assemblages, and preservation of the upper-shoreface-derived fauna is very good. The truncated beach-ridge complex at modern Hackberry Beach consists of planar to low-angle cross-stratified fine-grained coquinas with lens and lenticular beds of coarser shell and thin, laterally extensive layers of macerated plant material, mud, and shell hash. Assemblages are shoreface-derived, dominated by Mulinia lateralis and Anadara ovalis, and moderately well-preserved.

Differences in bioclast source, taphonomic condition, bioclast fabric, and stratigraphy of chenier-plain deposits indicate that cheniers did not form in a single depositional setting. Oyster-dominated coquinas of Little Chenier and Chenier Perdue resemble beach deposits of rapidly transgressing coastlines where brackish-water assemblages are reworked from older estuarine deposits exposed on the lower foreshore or shoreface. Front Ridge also represents foreshore deposition, but shells are less abundant and assemblages are derived from the upper shoreface, features indicating that the shoreline was regressive during Front Ridge formation. Coquinas of the truncated beach-ridge complex at Hackberry Beach are also shoreface derived but high shell density and taphonomic condition indicate significant winnowing. Bioclasts of this beach-ridge complex were probably derived from older beach-ridge deposits, which is analogous to present conditions at Hackberry Beach. Paleoenvironmental, stratigraphic, and taphonomic data show, therefore, that many but not all cheniers are transgressive.


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