Testing depositional models of the Utica Shale, Mohawk Valley, eastern New York State

The Mohawk Valley of eastern New York State has been of interest to geologists for decades. Untangling of the Taconic Orogeny required workers to study the area nearly as much as the mountains themselves (Bird and Dewey, 1970; Bradley and Kidd, 1991; Bradley and Kusky, 1986; Dewey and Bird, 1971; Stanley and Ratcliffe, 1985). Recently, research has shifted focus towards the potential natural gas reservoir of the Utica Shale, which is one of the major units in the Mohawk Valley. It is also present in the subsurface from Ontario to West Virginia, and as far west as Indiana (Patchen et al., 2006).;We aim to track the change in basin geometry during the Middle to Late Ordovician. Faults in the basin were active during deposition, but the rate and magnitude of motion is in some cases unknown. Hoffmans Fault is one such fault, and is the area I examined.;Detailed stratigraphic analysis of cores in the study area was the primary source of data. The cores were also sampled and analyzed for Total Organic Carbon (TOC). Core diagrams were analyzed using ImageJ to determine the percent of sandstone in 76.2cm (2.5 ft.) bins, and these values compared to the TOC data for comparable depths. No correlation between sandstone and organic carbon was present. Although a multitude of K-bentonite beds are present throughout the Mohawk Valley, none was found in two of our studied cores. Dating of sections was accomplished using graptolite biostratigraphy. These data were acquired by other workers. Two of the cores bracket Hoffmans Fault, a large-offset normal fault on the eastern end of the Mohawk Valley. The locations of theses cores provide tight control on depositional changes across the fault. We mapped this fault and found Corynoides americanus Zone graptolites (Late Ordovician) on the downthrown block, directly across the fault from Cambro-Ordovician Beekmantown Group limestones. Unit thickness increased dramatically from the western side of the fault to the eastern, downthrown block. TOC does not vary across the fault, and remains around 2-3% with a few spikes to 5%. Lithologic unit variation indicates a preferential deposition in the locally deep graben of the downthrown block. That area must have been filled more completely before rapid deposition progressed to the western block.