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Clay Mineral Variations in Four-Point Bayou, Terrebonne Parish, Louisiana

Penny Baxter, Ray E. Ferrell

Cores of unconsolidated sediments from the Four-Point Bayou area, representing an interdistributary basin deposit less than 1,000 years old, were examined for variations in clay mineralogy. Two specific environments of deposition, interpreted from grain-size analyses and sedimentary structures, were compared: a sand-rich natural levee deposit and an overlying clay-rich interdistributary bay.

Analyses of the < 2 µm clay fraction using Schultz's method show smectite to be the predominant clay in both environments. The relative abundance of kaolinite:illite:smectite in the natural levee, near-channel deposits is 1:3:6. In bay deposits, it is 2:3:5. Hydraulic fractionation of clays during deposition should result in a high K/I ratio in natural levees and a low ratio in bay deposits, the reverse of what was observed. Kaolinite should have been enriched in natural levees at the expense of illite.

Other processes must have been operative either at the time of deposition or during early diagenesis of these deposits. Possible mechanisms include localized redistribution and sorting or chemical equilibration with early pore fluids associated with saltwater intrusion. The mixing of clays from different sources could also account for the observed differences.

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