EARLY DIAGENETIC MINERALS AND VARIABLES INFLUENCING THEIR DISTRIBUTIONS IN 2 LONG CORES (GREATER-THAN-40 M), MISSISSIPPI RIVER DELTA PLAIN

Two long (> 40 m) continuous push cores of sediments from the Mississi ppi Delta Plain, one at a northern freshwater site and one at a southe rn marine-influenced site, have provided samples of individual concret ions and homogenized bulk samples for a study of early diagenesis. Ear ly (< 10,000 y) diagenetic minerals are abundant and include: siderite > pyrite > calcite approximate to dolomite > vivianite approximate to hematite, Siderite formed postburial and contains bands of elastic mi nerals and accessory diagenetic calcite and dolomite, as well as Ca, M g, and Mn substituted in the lattice, Values for delta(13)C increase w ith increasing depth to +11% (PDB) at a depth of 40 m, reflecting the increasing role of methanogenesis with depth. From analyses of bulk sa mples, pyrite concentrations are higher in organic-rich horizons, do n ot vary appreciably with depth below one meter when adjusted for organ ic-carbon control, and are generally higher in the marine-influenced c ore than at the freshwater site. Conversely, concentrations of siderit e are higher in fine-grained siliciclastic horizons, increase with dep th to about 20 m, and do not vary with the extent of marine influence in these cores. To a first approximation, distributions of pyrite appe ar to be controlled by variations in organic carbon concentration in s ulfate-sufficient systems that formed through marine incursion into or ganic-rich layers that cap sediment packages in the deltaic system. Re sidual pyritic S is sulfate-controlled and lower in the northern, more freshwater organic rich sediments. Siderite formation appears to be f ocused in sulfate-deficient systems that formed when siliciclastic wed ges that constitute the central parts of sediment packages prograded i nto lacustrine or bay environments. Low sulfate levels in these horizo ns in both cores facilitated similar levels of siderite, A juxtapositi on of marine versus freshwater diagenetic mineral suites in adjacent h orizons resulted from these interactions between physical and geochemi cal processes.