CORRELATION OF SYNTHETIC GAMMA RAY PROFILES FROM OUTCROPS OF THE NEW SCOTLAND FORMATION, HELDERBERG GROUP (LOCHKOVIAN) AT CHERRY VALLEY AND SCHOHARIE, NEW YORK

The upper Kalkberg Fm. and overlying New Scotland Fm. contain several tephra beds, some of which have yielded zircon U-Pb ages important to the Devonian time scale. An outcrop on I-88 near Schoharie, NY exhibits 22 m of the New Scotland Fm. and includes the sharp lower contact with the Kalkberg Fm. and the gradational contact with the overlying Becraft Fm.. At Cherry Valley (CV), NY, the New Scotland Fm. (11 m) is disconformably overlain by the Oriskany Ss. The basal contact is not exposed. Lithologic correlation suggests that the strata at CV correspond to the lower New Scotland at I-88 and that the CV section is somewhat condensed.

These two stratigraphic sections were measured, described, and gamma ray measurements were made at half-meter intervals through the New Scotland Fm. at CV and 37 m of the uppermost Kalkberg, New Scotland and most of the Becraft Fm. on I-88. The Gamma Surveyor Vario, a 2048 channel gamma-ray spectrometer was used to measure K (%), U (ppm) and Th (ppm), which then were converted to API units to create synthetic gamma ray logs.

The gamma ray log of the New Scotland Fm. at CV shows greater radioactivity (average 10 API units higher) with greater enrichment of U and Th than the I-88 outcrop. We interpret the Th data as indicating that CV received greater siliciclastic input and occupied a more proximal position than the I-88 section. This is supported by the presence of rounded quartz silt present in thin sections from the CV section.

The highest API peak at CV corresponds to the Judds Falls Bentonite (a.k.a. “Kalkberg” bentonite; H1-1 of Husson et al. 2015). The highest API peak at I-88 occurs in a tephra bed ~ 8 m above the contact with the Kalkberg Fm. (H2-4 of Husson et al. 2015). The shapes of the gamma ray curves suggest that these maxima do not correlate. Rather, the Judds Falls Bentonite correlates with a tephra bed approximately 2 m above the Kalkberg - New Scotland contact (H2-2 of Husson et al. 2015).

Small scale (2-8 m) cycles within the New Scotland are characterized by abrupt shifts to high radioactivity at the base which then decline upward. These cycles likely reflect onsets of dysoxia followed by a gradual increase in oxygenation. Preliminary data on vertical variations in macrofauna lend further support to the interpretation that these oxygenation cycles represent small scale (5^th or 6^th order) eustatic cycles.