DEPOSITIONAL HISTORY AND CARBONATE DIAGENESIS OF AMMONOID CEPHALOPODS FROM UPPER MISSISSIPPIAN (CHESTERIAN) CONCRETIONARY SHALES, SOUTHERN OZARKS, NORTHERN ARKANSAS

The Upper Mississippian (Chesterian) stratigraphic succession in the southern Ozarks, northern Arkansas, represents a single, unconformity-bounded, third-order, transgressive-regressive cycle. The Mississippian Hindsville/Batesville Formations represent a coeval, platform carbonate and delta complex resting unconformably on the Moorefield Formation (Meramecan) or older strata and mark the initiation of the Chesterian marine transgression. Continued transgression and maximum flooding characterize the succeeding lower Fayetteville Shale, which reflects anoxic conditions below effective wave base with fossils limited to ammonoid cephalopods. High-stand and regression in the upper Fayetteville Shale established more oxygenated bottom conditions demonstrated by a diverse fauna of bivalves, gastropods and other benthic invertebrates associated with ammonoid cephalopods. The succeeding Pitkin Formation reestablished platform carbonates over the region. The Imo Member, Pitkin Formation, is a shallow-water shale facies with a diverse pelagic and benthic faunal assemblage that concludes Chesterian deposition and is capped by the Mississippian-Pennsylvanian unconformity.

Ammonoid cephalopods occur in three horizons of concretionary shales representing the lower and upper Fayetteville and Imo intervals. The ammonoids preserved within the lower and upper Fayetteville concretions display imploded septa indicating burial in a deep water environment. In contrast, Imo concretions contain ammonoids displaying intact septa suggesting burial in a shallower water environment. Ammonoids from all three environments have phragmacones filled by carbonate minerals, while pelleted concretions fill their living chambers. The paragenesis of the Fayetteville ammonoids suggests the sequence: non-ferroan calcite to ferroan calcite to ferroan dolomite. Later replacement of the conchs by calcium phosphate was followed by pervasive pyritization of both the conch and concretionary living chamber filling. Preservation of Imo ammonoids follows a similar sequence, but includes calcite after aragonite pseudomorphs not found in those from the Fayetteville. Precipitation of aragonite in the Imo ammonoid phragmacones is further support of its shallower setting compared to the Fayetteville.