UPPERMOST SILURIAN STRATIGRAPHY IN THE TYPE HELDERBERG REGION: NEW INSIGHTS FROM SURFACE OUTCROP AND SUBSURFACE CAVE EXPOSURES

The Helderberg region of southern Albany County is the type area for the Upper Silurian/Lower Devonian of eastern North America. The Upper Silurian strata of the Heldeberg region sit upon a disconformable erosion surface above the underlying Upper Ordovician flysche deposits of the Taconic Orogeny. The Helderberg region sits at the bend in the outcrop belt of the Upper Silurian/Lower Devonian in NYS, with outcrops treading westward and southward from the exposures near the Indian Ladder at Thacher Park. Tracing of the uppermost Silurian units from the west and south along the outcrop belt towards Thacher Park indicates that the underlying Upper Ordovican strata reach a topographic high under the unconformity roughly aligned with surface exposures at Thacher Park.

The uppermost Silurian strata in this area consist of the Brayman (pyritic, dolomitic, green sandy-shale), Cobleskill (fossiliferous, dolomitic-limestone), Rondout (lamiated dolostone), and Manlius (micritic limestone) formations in ascending order, with the lower three units showing marked thinning approaching the exposures at Thacher Park. At their type area, ~30km west of Thacher, the Brayman is 15m thick and Cobleskill 3m thick; the Rondout here is ~12m thick. The easternmost, exposure of the Brayman and Cobleskill is just north of Gallupville, ~17.5km west of Thacher. Here, the Brayman has thinned to ~6m, the Cobleskill to ~1.5m, and the Rondout to ~6.5m. Sitting upon the Upper Ordovician strata at the Indian Ladder is a ~0.6m pyritic, green sandy unit above which is ~1.5m of dolomitic strata equated to the Rondout. Rickard has stated that he does not correlate the lower sandy unit here to the Brayman and the Cobleskill is thought to be absent here. Approaching Thacher Park from the south, these units display a similar pattern of thinning to the north. At Feura Bush, ~13km southeast of Thacher, the Brayman is ~3m thick, the Cobleskill again thought to be missing, and the Rondout ~2m thick.

Examination of strata exposed within Knox Cave, ~10km west of Thacher has revealed the presence of the Brayman, Cobleskill, and Rondout units below the Manlius Fm. Passage within the cave exposes at least 3.5m of Brayman, ~0.3m of Cobleskill, and ~2.5m of Rondout. The presence of definitive Brayman strata at Knox Cave suggests that the unit under the Rondout at Thacher is the Brayman.