--> ABSTRACT: Diagenetic Ferroan Carbonates in Modern Organic-Rich Deltaic Muds, by Sarah S. Elliott and Ray E. Ferrell; #91038 (2010)

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Diagenetic Ferroan Carbonates in Modern Organic-Rich Deltaic Muds

Sarah S. Elliott, Ray E. Ferrell

Carbonate modules and bands are forming in lower delta plain muds of Terrebonne Parish, Louisiana. X-radiographs show diagenetic carbonate accumulating within 50 cm of the sediment surface. The precipitation of the carbonate is a result of bacterial activity and signals the onset of the first stages of pore water evolution and diagenesis.

The early diagenetic carbonates are calcic siderites. Some nodules are zoned, as XRD, petrography, and microprobe analyses reveal calcite, dolomite, and pyrite around their margins. ^dgr13C ranges from +7.40 to -10.00 ^pmil PDB and generally decreases near the nodule margin. "Incipient nodules" have ^dgr13C from about -3 to -5 ^pmil, reflecting early fractionation of organic carbon isotopes in methanogenic microenvironments. Volume-percent carbonate decreases from nodule centers to margins. These variations reflect marine water intrusion associated with compaction and subsidence of deltaic sediments. In anaerobic freshwater sediments, methanogenic bacteria produce CO2 and CH4. Very low dissolved sulfate concentrations limit sulfate reduction Methanogens preferentially reduce 12CO2, so pore-water carbonate becomes enriched in 13C relative to its detrital source. Early carbonates are ferroan with higher Mn:Fe ratios than later precipitates. Marine water introduces dissolved sulfate, and sulfate-reducing bacteria generate 13C-depleted CO2 and H2S to form calcite and pyrite later.

This study provides a modern analog for many occurrences of carbonate concretions in organic-rich mudstones. Determining mineralogy, composition, ^dgr13C, and volume-percent carbonate reveals a record of pore-water chemical changes with burial and original depositional environments.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.