ONLAPPING UNITS AND CONVERGING UNCONFORMITIES: STRATIGRAPHIC RELATIONSHIPS IN THE LOWER HELDERBERG GROUP ON THE NORTHWESTERN MARGIN OF THE APPALACHIAN BASIN

The stratigraphy of the lower Helderberg Group (Rondout Fm., Thacher and Olney members of the Manlius Fm.) has been reported as complexly interfingering in western portions of the New York outcrop belt (Rickard 1962, 1975). Regional tracing of three unconformities, the informal Green Vedder Mbr. of the Manlius Fm. and a distinctive zone of thrombolitic mounds within the Thacher Mbr. of the Manlius Fm. have clarified these relationships.

Throughout the Helderberg outcrop, the Mine Lot Falls Unconformity (MLF) marks the top of the Rondout Formation. Near Schoharie, the MLF is overlain by 9.3 meters of the Thacher Mbr. which is separated from the overlying Green Vedder Mbr. (3.4m thick) by the Clockville Unconformity (CVU). The thrombolitic zone occurs 7.1 m above the MLF. The Green Vedder is truncated by the Terrace Mountain Unconformity (TMU) which marks the base of the Dayville Mbr. of the Coeymans Fm. A similar sequence occurs at Cherry Valley, where the Thacher is 11.2 m thick and the thrombolitic zone is only 2 m above the MLF. Beds below the thrombolite horizon have been eliminated via onlap onto the MLF and higher portions of the Thacher have been added. At Cherry Valley, the Green Vedder is only 0.8 m thick and comprises beds similar to the upper half of the member at Schoharie.

The Green Vedder thickens westward (max = 5.6 m, Oriskany Falls) as the Thacher Mbr. thins below the CVU. Farther west, at Clockville, the CVU has removed all but 1-2 m of the Thacher which is almost entirely comprised of thrombolitic mounds. These thrombolites rest directly on the MLF. The Green Vedder (4.0 m) is unconformably (TMU) overlain by the Olney Mbr. of the Manlius Fm.

Near Syracuse, the Green Vedder is absent and nearly all of the Thacher has been cut out by the combined CVU and TMU. A few cm of thrombolitic boundstone (remnant Thacher Mbr.?) occur in solutional (?) depressions along the MLF. For all practical purposes, the Olney directly overlies the Rondout here, a relationship that persists to the westward limit of the outcrop belt.

Westward onlap of the MLF by members of the Manlius was interrupted by several erosional episodes. Although the Olney laps farthest westward, greatest depths were recorded by the Green Vedder Mbr. Convergence and stacking of unconformities indicates minimal subsidence along the edge of the Appalachian Basin.