CARBONATE MINERALOGY AND PARAGENESIS INVOLVED WITH THE PRESERVATION OF AMMONOID CEPHALOPODS IN UPPER MISSISSIPPIAN CONCRETIONARY SHALES, SOUTHERN OZARK REGION, NORTHERN ARKANSAS

Concretionary shales in the Upper Mississippian succession of the southern Ozark region, northern Arkansas, represents an unconformity-bounded, transgressive-maximum flooding-regressive, third order cycle referred to the Fayetteville Shale and the Imo Member, Pitkin Formation. The lower horizon in the Fayetteville Shale is associated with rapid initial transgression that developed anoxic conditions below effective wave base, with notable absence of benthonic fauna, and contains the maximum flooding interval. The upper Fayetteville reflects more oxygenated bottom conditions, with a benthonic assemblage, developing during highstand and the initiation of regression. With regression, the water column cleared, and a carbonate platform developed over the region as the Pitkin Formation. In the western Ozarks, the top of the Pitkin limestone is the Mississippian-Pennsylvanian unconformity, but in the central and eastern Ozarks of Arkansas, its Imo Member is a shallow-water, concretionary shale that concludes Mississippian deposition.

Both the Fayetteville and Imo shales preserve prolific assemblages of ammonoid cephalopods. Based on thin section petrography, the initial step in ammonoid preservation in both intervals was the filling of the living chambers by sideritic concretions that incorporate apparent fecal pellets cemented by non-ferroan calcite. The concretions may enclose the entire conch, and are penecontemporaneous with deposition of both the marine shales. After burial, the uncrushed phragmocones of the ammonoids were filled by a mosaic of calcite and dolomite suggesting a paragenetic sequence of non-ferroan calcite to ferroan calcite to ferroan dolomite identified by standard staining techniques. Later diagenesis includes replacement of the conchs by calcium phosphate and pervasive pyritization of the conchs, phragmocone filling, and the cement and pellets within the living chambers. The Imo ammonoids experienced a similar history, but exhibit pseudomorphs of calcite after aragonite within the phragmocones that preceded the other carbonate minerals. The precipitation of aragonite in the ammonoid phragmocones is consistent with the shallower water setting interpreted for the Imo compared to that of the Fayetteville.