A SYNTHETIC GAMMA RAY LOG OF THE ONONDAGA FORMATION AT CHERRY VALLEY, NEW YORK

Outcrops of Late Silurian to Middle Devonian strata near Cherry Valley, New York have long served as a primary reference for the stratigraphy of the Appalachian Standard Succession (Johnson and Murphy 1969). Radiometric dates (U-Pb) from K-bentonites in these outcrops played a central role in calibrating the Devonian timescale (Tucker et al. 1998). These volcanogenic units provided a portion of a comprehensive framework for correlation of upper Lower and lower Middle Devonian (Pragian through Givetian) units in New York and throughout the Appalachian Basin (Ver Straeten and Brett 1995, 1997; Ver Straeten 2001). The Cherry Valley outcrops were essential in an overall revision of the lithostratigraphy of the Late Silurian (Přídolí) to Early Devonian (Lochkovian) Helderberg Group (Ebert 2008; Ebert and Matteson 2003, 2012; Wilson, Ebert and Matteson 2011).

Owing to the importance of the Cherry Valley outcrops, a project has begun to build a synthetic gamma ray log from outcrop measurements for comparison to subsurface logs from the Appalachian Basin. Students enrolled in Sedimentary Geology (Fall, 2018) at SUNY Oneonta collected gamma ray measurements at half-meter intervals through ~26m of the Onondaga Formation on U.S. 20 near Cherry Valley. The Gamma Surveyor Vario, a 2048 channel gamma-ray spectrometer with VB21 Probe (BGO detector 347cm³), was used to measure K (%), U (ppm) and Th (ppm), which then were converted to API units. A synthetic gamma ray log was prepared from the field measurements. The Edgecliff, Nedrow, Moorehouse, Tioga Bentonite and Seneca members of the Onondaga Fm. are readily recognized in the synthetic log. The synthetic log was compared with the gamma ray log from the nearest borehole, the Consolidated Lum #1 well in eastern Otsego County, approximately 24 km (15 miles) to the south southeast of the outcrop belt. The synthetic log is consistent with the gamma ray log of the Lum #1 well and members of the Onondaga Fm. are correlated easily between the two.

Future class projects will continue this effort with gamma ray measurements to be collected from the Helderberg and Tristates groups and the Marcellus Subgroup. Eventually, a synthetic gamma ray log will be compiled for the entire Cherry Valley composite section from the upper Přídolí Rondout Formation through the lower Givetian Oatka Creek Formation.