STRATIGRAPHIC AND SEDIMENTOLOGIC INSIGHTS FROM SCHOHARIE, NY: THACHER MEMBER, MANLIUS FORMATION (UPPER PRÍDOLÍ, HELDERBERG GROUP)

Several large road cuts along I-88 near Schoharie, NY provide nearly continuous exposure of the limestones of the Helderberg Group (U. Sil. - L. Dev.). Detailed examination of the easternmost outcrop for an undergraduate class project in Sedimentary Geology has yielded new stratigraphic and sedimentologic insights into these well-studied strata.

The Thacher Member of the Manlius Formation disconformably overlies the Rondout Formation (Chrysler Member) with up to 31 cm of local erosional relief in the I-88 road cut (Minelot Falls Unconformity). Lithoclasts (up to 3 cm) of Rondout dolostone are incorporated in the basal peloidal grainstone of Thacher limestone. Stromatoporoids (Syringostroma barretti) immediately overlie this lithoclast zone. Identical associations of stromatoporoids with large clasts occur in proximity to other unconformities (e.g., sub-Cobleskill and Clockville) in the section. This consistent stratigraphic context suggests that stromatoporoids were opportunistic pioneers colonizing hard substrates on shallow portions of the ramp immediately following marine flooding events.

At Schoharie, a unit comprising thrombolitic mounds alternating laterally with dm-scale grainstones occurs in the middle of the Thacher Member. This distinctive unit may correlate with similar thrombolites a few meters above the base of the member at Cherry Valley and immediately above the base at Clockville. Such a correlation would demonstrate considerable westward onlap of the Thacher Member onto the Minelot Falls Unconformity, a surface which may represent a significant lowstand event in the mid-Prídolí.