CHAPTER 2: LITHOSTRATIGRAPHY, MULTI-TAXA BIOSTRATIGRAPHY AND SEQUENCE STRATIGRAPHY OF THE HELDERBERG GROUP (PŘÍDOLÍ – LOCHKOVIAN) IN NEW YORK STATE

The new Devonian of New York volume includes major revisions to the stratigraphy of the Siluro-Devonian Helderberg Grp. (Dhg) in New York State. Three new formations and four new members are recognized. Developments in the sequence stratigraphy and biostratigraphy of the Dhg are presented as well.

Unconformable contacts predominate between formations and members of the Dhg. The stratigraphic architecture of the Dhg is strongly controlled by episodic, tectonically driven subsidence produced by crustal loading during a pre-Tectophase I episode of Acadian orogenesis. During deposition of the Thacher Mbr. maximum subsidence occurred near Cherry Valley, NY. A low arch developed during deposition of the overlying Green Vedder Mbr., separating the basin into two sub-basins. This arch disappeared during deposition of the Dayville Mbr. of the Manlius Fm. and maximum subsidence was centered near Columbia Center, NY. The arch was reactivated and broadened to extend between Cherry Valley and the West Berne area during deposition of the Elmwood through Jamesville members of the Manlius Fm. The arch disappeared as subsidence increased during deposition of overlying Dhg units. Post-Manlius formations document eastward migration of the axis of maximum subsidence and later bathymetric axis of the basin as well. This pattern reversed during deposition of the lower part of the Becraft Fm. in which deeper water environments existed in the Schoharie region and shallower environments developed in the Hudson Valley. This transposition was short-lived as maximum subsidence migrated into the Hudson Valley region once again and persisted during deposition of the upper Dhg formations.

The Silurian – Devonian boundary, marked by the co-occurrence of scyphocrinitid loboliths and a positive δ¹³C excursion (Klonk Event), occurs in the Green Vedder Mbr. of the Manlius Fm. The Judds Falls Bentonite Bed in the New Scotland Fm. is constrained to the middle Lochkovian (upper bohemica zone) by chitinozoans. The Becraft and Alsen formations are late Lochkovian age (lata zone) in age. The Lochkovian – Pragian boundary likely occurs in the Port Ewen Fm. Pragian conodonts from the Deansboro Fm. (new rank) represent a maximum age for the bioherms that have been grouped with the Deansboro. The overlying Buckley Mill Fm. (new) is also likely Pragian in age.