Diagenetic ferroan carbonates in modern organic-rich deltaic muds

Conference · Fri May 01 04:00:00 EDT 1987 · AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States)

OSTI ID:6188570

Elliott, S S; Ferrell, R E

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. delta/sup 13/C ranges from +7.40 to -10.00 per thousand PDB and generally decreases near the nodule margin. Incipient nodules have delta/sup 13/C from about -3 to -5 per thousand, 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 CO/sub 2/ and CH/sub 4/. Very low dissolved sulfate concentrations limit sulfate reduction. Methanogens preferentially reduce /sup 12/CO/sub 2/, so pore-water carbonate becomes enriched in /sup 13/C 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 /sup 13/C-depleted CO/sub 2/ and H/sub 2/S 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, delta/sup 13/C, and volume-percent carbonate reveals a record of pore-water chemical changes with burial and original depositional environments.

• Research Organization: Louisiana State Univ., Baton Rouge
• OSTI ID: 6188570
• Report Number(s): CONF-870606-
• Conference Information: Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States) Journal Volume: 71:5
• Country of Publication: United States
• Language: English

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