CARBONATE PARAGENESIS DETERMINED BY CATHODOLUMINESCENCE IN AMMONOID PHRAGMOCONES FROM THE FAYETTEVILLE AND IMO SHALES (UPPER MISSISSIPPIAN), NORTHERN ARKANSAS

Upper Mississippian (Chesterian) concretionary black shales are well developed in the southern Ozark region of northern Arkansas. The Chesterian succession preserves a single Vail cycle, in which the Fayetteville Shale represents transgressive, maximum flooding and high stand conditions, while the Imo Shale is interpreted as a regressive sequence that trails aggrading and prograding carbonates as sea-level fell to end the cycle. Both shales are black and develop abundant, commonly fossiliferous, sideritic concretions, but Fayetteville deposition represents significantly deeper water conditions compared to the Imo Shale. As a consequence of water depth, Fayetteville ammonoids typically exhibit imploded phragmocones, while those of the Imo are intact. Ammonoid cephalopods are common in both the Fayetteville and Imo Shales. Individuals occur loose in the section or are incorporated into sideritic concretions. Ammonoids from the two units were examined in thin section and also using cathodoluminescence. Living chambers are filled with pelleted siderite cemented by nonferroan calcite that is replaced by ferroan calcite and ferroan dolomite. All carbonates may be replaced by pyrite as a final diagenetic event. 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. Under cathodoluminescence, nonferroan calcite in the living chambers from both horizons is brightly luminescent, while later ferroan calcite and dolomite are dullishly 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. The bulk of the phragmocone infilling in both horizons appears to be the result of groundwater after formation of the sideritic concretions.