DETERMINING WATER DEPTH OF THE UTICA SHALE, MOHAWK VALLEY, NEW YORK STATE

The Ordovician age Utica Shale is a natural gas producing black shale which outcrops in the Mohawk Valley of eastern New York State, and is of recent interest in the Ohio region. The environment of deposition is traditionally interpreted to be deep water anoxia, but recent work is suggesting a shallow water model. The current shallow water model places the Utica in 50m of water and occasionally subaerial. This model is based strongly on the presence of the Thruway Disconformity which separates the Indian Castle shale above from the Dolgeville Limestone below, as well as the Knox Unconformity separating the Utica Group (Flat Creek Formation) from the underlying Glens Falls Limestone. A series of syndepositional northeast-southwest trending normal faults confine grabens and corresponding thickened section of Utica Group deposits. Total Organic Carbon has been measured in core from the Knox Unconformity to the surface on the upthrown and downthrown sides of several of these faults.

Detailed core logs have revealed thick (several meters) silt sections on the upthrown side of Hoffmans Fault, near the eastern boundary of the basin. On the downthrown block of the same fault, shale is interbedded with younger, fine to medium-grained cross bedded sandstones. Towards the middle of the basin to the west, the Flat Creek Formation displays thick sections of silt with intermittent, 2-5cm thick, very fine sandstones. Typically, massive sand contacts are basally sharp and grade up to silt over ten cm. This sequence is often cyclic, occurring five to ten times over two to three meters. Although the Flat Creek is calcareous in many intervals, there are also significant intervals in which no carbonate is present. Furthermore, Dolgeville Formation-like ribbon carbonate facies appear to be absent from the eastern portion of the basin. These differences from the western and central Mohawk Valley succession may indicate a change in provenance or high sedimentation rate dilution of carbonate material, likely influenced by basin geometry.