AMMONOID TAPHONOMY AND ITS RELATIONSHIP TO UPPER CHESTERIAN SEQUENCE STRATIGRAPHY IN NORTHERN ARKANSAS

Ammonoid cephalopods preserved in the carbonaceous shales of both the Fayetteville Shale and the younger Imo Member, Pitkin formation, exhibit unique taphonomy that contributes to the identification of the maximum flooding, highstand, and regressive intervals within this Chesterian (Late Mississippian), third-order, eustatic cycle. The interpreted age and depositional environment for the interval is based on faunal and to an extent, floral biostratigraphy, whereas the rate of deposition, permeability and porosity, and identification of the productive “sweet spot” of petroleum resources have been determined by petrography and lithostratigraphy.

The ammonoids of Emstites fayettevillea and Anthracoceras discus from the Fayetteville Shale and Imo Member of the Pitkin Limestone, respectively, are preserved in sideritic concretions. Fayetteville individuals typically exhibit imploded septa infilled by coarsely crystalline calcite and minor dolomite. In contrast, Imo specimens do not exhibit imploded septa, and their chambers are also filled by calcite and dolomite, but preserve pseudomorphs of calcite after aragonite, not found in the Fayetteville specimens. Both the taphonomic features of septal condition and mineralogy of chamber in-filling indicate preservation in a deep water setting for the Fayetteville occurrences, but above carbonate compensation depths. In the Imo specimens, lack of imploded septa, and the more complex mineralogy of chamber infilling clearly indicate deposition in much shallower water, confirming a regressive Upper Mississippian (Chesterian) depositional history. This interpretation is further supported by the presence of outer shelf, obviously deeper water, trilobites (Brezinski, 2008) preserved in the Imo Shale, associated with many other invertebrates as well as botanical remains, likely reflecting transportation and deposition by tempestites.