POST-JAMESVILLE MEMBER (MANLIUS FORMATION, HELDERBERG GROUP) STRATIGRAPHY AND THE AGE OF THE MOSQUITO POINT REEF, MUNNSVILLE, NEW YORK

Strata overlying the Jamesville Mbr. of the Manlius Fm. in central NYS have been assigned to the Deansboro Mbr. of the Coeymans Fm. and correlated with the Ravena Mbr. of the Coeymans Fm. (Lochkovian, zone of woschmidti group). However, Rickard (1962, p.77) stated that the Deansboro was “somewhat younger.” A coralline buildup known as the Mosquito Point reef (Munnsville, NY) occurs in the Deansboro and was referred to as “early Early Devonian” (Oliver 1960) in age (i.e. Lochkovian). Recent fieldwork at Mosquito Point has revealed a more complex stratigraphy. Preliminary conodont biostratigraphy indicates a much younger age for the Mosquito Point reef.

Overlying the Jamesville Mbr. at Mosquito Point are beds that include Lepocrinites gebhardi and Gypidula coeymanensis, an association that characterizes the uppermost Ravena Mbr. in east-central NYS. The presence of the gebhardi epibole indicates that the uppermost portion of the Ravena Mbr. has lapped westward onto the Howe Cave Unconformity at the top of the Jamesville Mbr. Crinoidal grainstones of the Deansboro Mbr. abruptly overlie this westward extension of the Ravena Mbr.

Samples of the sub-reef Deansboro Mbr. have yielded pyrite encrusted conodont elements of Pelekysgnathus serratus brunsvicensis and an icriodid of the steinachensis group. Using the original definition of the Pragian Stage (Barrandian area, Czech Republic), these conodonts signal an early to middle Pragian age for the sub-reef strata. However, the problematic Zinzilban GSSP for the base of the Emsian Stage (kitabicus zone) that drastically shortens the Pragian Stage makes the sub-reef strata at Mosquito Point late Pragian in age. In either case, the Mosquito Point reef is clearly younger than the rest of the Helderberg Group (Lochkovian), and the Deansboro is a much younger unit of unknown affinity.

Strata above the Deansboro, including the Meristella arcuata epibole, have been referred to the Kalkberg Fm. (Rickard 1962). However, the Kalkberg Fm. is Lochkovian in age as indicated by the presence of conodonts of the Icriodus postwoschmidti group (Kleffner et al. 2009). Therefore, assignment of the supra-Deansboro beds to the Kalkberg Fm. is inappropriate. This unit must be of Pragian, or possibly early Emsian age. Preparation of samples of this unit from Oriskany Falls is currently in progress.