Depositional Environment and Early Diagenetic Controls on Mineralogy of Kittanning Formation, Allegheny Group, Eastern Ohio

Thomas Brocculeri, Annabelle M. Foos

This study centers on the mineralogy and petrography of shales, siltstones, and associated siderite concretions from the Pennsylvanian Kittanning Formation, Allegheny Group of eastern Ohio. Marine units are distinguished from nonmarine units on the basis of mineralogic differences. Semiquantitative XRD analyses show an increase in the amount of illite and a decrease in kaolinite/chlorite, and petrographic analyses show an abundance of pyrite and a decrease in siderite in marine relative to nonmarine sediments and concretions.

Clay mineralogic variations are most likely the result of two processes: (1) the differential flocculation and sorting of clay particles during transportation and deposition, and/or (2) the conversion of illite to kaolinite during deposition under slightly acidic conditions within low-lying nonmarine swamps and marshes.

Early diagenetic pyrite and siderite occur as disseminated stringer between clay-rich laminae and in well-developed concretions among shale beds.

These cements formed under sulfidic and nonsulfidic anoxic conditions, respectively. Shales and concretions from marine units contain greater amounts of pyrite due to a higher concentration of dissolved sulfate in pore waters that was available for pyrite formation prior to siderite cementation. In contrast, nonmarine pore waters contained a significantly lower concentration of sulfate, which limited the precipitation of pyrite cement. In the nonmarine environment, complete consumption of sulfate early in the diagenetic history of these sediments resulted in nonsulfidic conditions, which facilitated abundant siderite cementation. These mineralogic differences between marine and nonmarine units are the result of variations in paleosalinities that reflect fluctuations in ion concentrati ns associated with periodic marine invasions during Kittanning deposition.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.