SEQUENCE STRATIGRAPHY AND THE CORRELATION OF SELECTED MIDDLE ORDOVICIAN (MID-MOHAWKIAN) K-BENTONITES BETWEEN THE OZARK DOME AND CINCINNATI ARCH

Numerous Middle Ordovician K-bentonites have been documented throughout eastern North America. Of these, the mid-Mohawkian Deicke and Millbrig K-bentonite beds appear to be particularly widespread. Published stratigraphic work correlates the Deicke and Millbrig from their type sections along the Ozark Dome eastward though the subsurface of the Illinois Basin to outcrops along the Cincinnati Arch where the Deicke is suggested to be correlative with the Pencil Cave-B-T3 K-bentonites and the Millbrig with Mud Cave-A-T4 K-bentonites. However, a detailed sequence stratigraphic analysis of the Upper and uppermost Middle Ordovician section of the Illinois Basin indicates that the Deicke correlates with the Mud Cave-A-T4 and that the Millbrig is not present along the Cincinnati Arch. Conversely, the Pencil Cave-B-T3 of the Cincinnati Arch area is not present along the Ozark Dome.

The sequence stratigraphic analysis of the Deicke and Millbrig-bearing section impacts the correlation of these K-bentonites by recognizing two important discontinuities and their stratigraphic significance. One of these discontinuities is a regional unconformity (sequence boundary) located at the base of the Kimmswick Limestone along the Ozark Dome that can be traced eastward to the base of the Curdsville Member of the Lexington Limestone along the Cincinnati Arch. The second discontinuity is a marine flooding surface (systems tract boundary), located at the top of the Castlewood Member of the Decorah Formation along the Ozark Dome. It is important to note that the unconformity and the marine flooding surface define the boundaries of a genetically related, sequence stratigraphic unit (Upper Decorah Systems Tract) consisting of practically all the Decorah Formation along the Ozark Dome and includes the Millbrig K-bentonite. Critically, this sequence stratigraphic unit is progressively truncated beneath the unconformity towards the east and is not present in the Cincinnati Arch area. The Deicke is located a few feet below the marine flooding surface and is locally preserved beneath the unconformity along the Cincinnati Arch as the Mud Cave-A-T4. The underlying Pencil Cave-B-T3 is largely preserved from erosion along the Cincinnati Arch but pinches out to the west in the subsurface of the Illinois Basin.