FURTHER CONSTRAINTS ON THE AGE OF THE TACONIC FORELAND BASE FILL IN EASTERN NY: GRAPTOLITE BIOSTRATIGRAPHY AND TEPHROCHRONOLOGY FROM CORE 75NY2, BALSTON SPA, NY

The age of the upward-deepening Black River, Trenton, and Utica succession 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. We report here preliminary graptolite biostratigraphic and tephrochronologic data from core 75-NY2, which provides a complete succession from Grenville basement into the Late Ordovician Schenectady Formation. Furthermore, this core from near Balston Spa is located close to the regional eastern limit of autochthonous Ordovician strata. The post-Knox carbonates here are only 24 m thick and deepen upward through their upper third. The Trenton-Utica contact is a starved drowning surface. The basal 7 m of Utica Shale contain abundant graptolites that suggest correlation with the uppermost C. bicornis Zone. The overlying 61 m of strata produce a sparse C. americanus Zone fauna. This interval is roughly 25% thinner than equivalent sections farther west. Dolgeville facies are absent from 75NY2. Silty grey shales of the Frankfort facies replace typical Utica fissile black shales mid way through the greatly expanded, 85 m-thick O. ruedemanni Zone. The succeeding D. spiniferus Zone fauna continues upward into sandy, Schenectady-like facies. The silty and sandy facies show prominent soft sediment deformation at numerous levels. These relations indicate that Taconic clastic wedge prograded into this region substantially earlier than in the central Mohawk Valley. K-bentonites present in the core provide an opportunity to test this interpretation. Two such beds have been successfully correlated on the basis of apatite phenocryst chemistry (especially Mn and Mg concentrations). A bed in the middle part of the C. americanus Zone in 75NY2 corresponds to a bed in a similar position in the central Mohawk Valley. A bed near the top of the Trenton Group in 75NY2 corresponds to K-bentonite bed within the uppermost C. bicornis Zone strata of the Martinsburg Fm in West Virginia, a locale that occupied a position even more proximal to the orogenic front. Thus, tephrochronology supports the biostratigraphic relations. Taconic subsidence in the central Appalachians migrated westward rapidly during latest Sandbian time (early Late Ordovician, approximately 454-452 ma).