COMPARATIVE DIAGENESIS OF CEPHALOPOD PHRAGMOCONES FROM THE FAYETTEVILLE AND IMO SHALES (UPPER MISSISSIPPIAN), NORTHERN ARKANSAS

The Chesterian Series of the Upper Mississippian period is a single, third-order, transgressive and regressive sequence. The Fayetteville Shale represents the later transgressive, maximum flooding and high stand conditions in that cycle. It is a widespread, deep-water unit with deposition occurring below effective wave base. The Imo, the last unit in the regression, is a shallow water, clastic wedge that has limited aerial extent and lacks basinal equivalents in the adjacent Arkoma Basin. Both shales are black and form abundant, fossiliferous, siderite concretions that frequently contain ammonoid cephalopods.

Individual ammonoids occur loose in the section or are incorporated into sideritic concretions for both the Fayetteville and Imo Shales. Ammonoids from the two units were examined in thin section and by cathodoluminescence. As a consequence of water depth, Fayetteville ammonoids typically exhibit imploded phragmocones, while those of the Imo are intact. In both sections the living chambers are filled with pelleted siderite cemented by nonferroan calcite that is replaced by ferroan calcite and ferroan dolomite, although Imo cephalopods lack evident cementation by pyrite replacement of calcite characteristic of the Fayetteville Shale. Phragmocones from both horizons were filled initially by ferroan calcite with a variety of later diagenetic carbonate minerals seen as replacements, including ferroan dolomite and nonferroan calcite and dolomite. In addition, specimens from the Imo Shale exhibit pseudomorphs of nonferroan calcite after aragonite as well as some infilling by botryoidal fluorapatite. Under cathodoluminescence, nonferroan calcite in the living chambers from both horizons is brightly luminescent, while later ferroan calcite and dolomite are dimly luminescent. Brightly luminescent nonferroan calcite is seen in the early whorls and as a replacement of the conch and septa in the Imo specimens, but not those of the Fayetteville. These differences in diagenesis tend to reflect the different depositional settings of deep versus shallow. The bulk of the minerals infilling phragmocones in both horizons appears to be the result of groundwater after formation of the sideritic concretions.