GRAPTOLITE BIOSTRATIGRAPHY AND K-BENTONITE TEPHROCHRONOLOGY FROM A CORE SECTION THROUGH THE UTICA, TRENTON, AND BLACK RIVER GROUPS NEAR BALSTON SPA, NEW YORK

The Utica Group is a thick sequence of black shale that is the maximum drowning interval of the Appalachian Basin Taconic Supersequence in the northeast United States and eastern Canada. The age of the upward-deepening Black River, Trenton, and Utica succession in relation to regional patterns of sequence development and basin expansion provides critical data for constraining the timing of Taconic thrust loading in the region. These data are useful for petroleum exploration in the region, as well. Use of graptolite biostratigraphic zones allows for the temporal correlation of like units and tephrochronology permits correlation with events in the orogenic belt. We report here preliminary data from core 75-NY2. The Knox Unconformity is directly overlain by a relatively thin Black River-Trenton succession: 7.9 m of nodular, black wakestones, followed by a 4.6 m crinoidal grainstone unit, and capped by an interval of shale and tempestite interbeds in the upper 6 m. The Trenton-Utica contact is sharp and marked by a pyrite-rich, lag horizon. The overlying Utica is fissile, organic rich, and contains abundant graptolites, including Diplograptus molestus and an unfamiliar species of Lasiograptus. These two taxa were previously unknown in this region, however D. molestus is common in the equivalent of the C. bicornis Zone in Baltoscandia. These relations indicate that the base of the Utica here is substantially older than it is farther west in the central Mohawk Valley, where its base lies within the succeeding C. americanus Zone. The K-bentonites that occur in the boundary interval (one in the top-most levels of the Trenton Group and 12 in the overlying 24 m of the Utica Group) provide an opportunity to test this interpretation. These beds may represent the Hagan Complex and may include the Deicke or Millbrig K-bentonite beds. We are processing the Black River and Trenton limestones for conodonts, which will provide further biostratigraphic data that bear on these correlations. We anticipate that these efforts will improve our understanding of the regional facies and temporal relations of the examined units and their integration with the Upper Ordovician geochronology.