CONODONT BIOSTRATIGRAPHY AND SR-ISOTOPE STRATIGRAPHY ACROSS THE KNOX/BEEKMANTOWN UNCONFORMITY IN THE CENTRAL APPALACHIANS

The disconformity that cuts out Middle Ordovician rocks in the Appalachian outcrop belt between TN and PA represents a hiatus that becomes progressively shorter in duration from south to north. The expression of the disconformity fades in northern VA near Strasburg where conodont biostratigraphy does not resolve a well-defined gap in time; however, there is lithologic evidence of stratigraphic omission in the paleokarst surface preserved at the Beekmantown-New Market contact. The conodont faunas from an exposure of this contact on the southbound lane of Interstate 81 (I-81) near Strasburg and at the nearby Tumbling Run section yield Curtognathus sp. and Erismodus sp. from the top of the Beekmantown suggesting a maximum age of Midcontinent holodentata Zone (lower-middle Darriwilian). The lowermost New Market contains a conodont fauna of Paraprioniodus sp., Drepanoistodus suberectus, Phragmodus flexuosus?, Curtognathus sp., Erismodus sp., and Panderodus sp. This fauna corresponds to the Midcontinent friendsvillensis Zone. If there is a gap in time represented by the paleokarst at this section, it most likely represents the Midcontinent polonicus Zone or shorter in duration. Along the westbound lane of I-70 near Clear Spring, MD, the paleokarstic expression of the unconformity is gone and the contact between the Pinesburg Station Dol. (top Beekmantown) and the St. Paul Gp (Row Park and New Market Fms) is sharp. In contrast, in the New Enterprise Quarry near Roaring Spring, PA, the contact between the Bellefonte Dol. and the overlying Loysburg Fm is gradational.

A major shift in Ordovician seawater ^87/86Sr corresponds largely to the North Atlantic serra to anserinus Zones, upper Darriwilian, in NV (Antelope Valley-Copenhagen Fm transition), and the nearly time equivalent Midcontinent friendsvillensis to sweeti Zones in OK (McLish-Bromide Fm transition). We show that this Darriwilian ^87/86Sr shift is present in VA, MD, and PA. As this Sr isotope shift becomes well-calibrated to both the North Atlantic and Midcontinent zonal schemes, it may be used to project the conodont biozonation into sections with little or no conodont biostratigraphic control. Similarly, preliminary d¹³C stratigraphy reveals a significant and continuous shift in the I-70 section near the Pinesburg Station-St. Paul contact, consistent with a relatively conformable succession.