Biostratigraphic analysis of core 75NY-2 from near Ballston Spa, New York
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The Ordovician Utica Group is a thick sequence of black shales that forms the maximum drowning interval of the Taconic Supersequence in the Appalachian Basin in the northeast United States and eastern Canada. The age of the upward-deepening Black River, Trenton, and Utica successions in relation to regional patterns of sequence development and basin expansion provide critical data for constraining the timing of Taconic thrust loading in the region. The age of the base of the Utica Shale is especially interesting as it lies on top of an unconformity, and appears to be older than sections previously examined. I analyzed of the stratigraphy and biostratigraphy of core 75NY-2 from near Ballston Spa, NY and identified graptolite and conodont biozones. These data are supplemented by links based on three geochemically fingerprinted K-bentonite beds. Absence of the Thruway disconformity within this core is determined by the lack of a distinct disconformity, despite deformed intervals within the top of the Dolgeville-like interval and in the overlying Frankfort Formation. I have determined that the Dolgeville Formation is not present within this core, replaced instead by the Utica Shale. Dolgeville-like facies present within the lower D. spiniferus Zone interval in the core are younger than the Dolgeville Formation of the western Mohawk Valley and probably not physically continuous with that unit. Use of graptolite biostratigraphic zones allows for the temporal correlation of similar units and tephrachronology permits correlation with events in the orogenic belt. These data also allow for correlations between the New York biostratigraphy and that of many other regions including Texas, Idaho, Australia, and Scotland. The K-bentonites that occur in the boundary interval (one in the top- most level of the Trenton Group and 12 in the overlying 24 m of the Utica Group) produce three distinct correlations to K-bentonites elsewhere. Conodonts from the Black River and Trenton limestones provide further biostratigraphic support for these correlations. These findings indicate that the base of the Utica Shale within core 75NY-2 is slightly older than core and outcrop sections found in other parts of the region. This section lies within the uppermost Climacograptus bicornis Zone, rather than the Corynoides americanus Zone.