CYCLIC INPUT OF SILICICLASTIC SEDIMENTS IN THE NEW SCOTLAND FORMATION REVEALED BY GAMMA RAY SPECTROMETRY

The New Scotland Formation (Helderberg Group, Lower Devonian) comprises rust brown weathering, thick to massive, siliceous wackestones interbedded with thinner laminated shales, minor skeletal packstones and nodular black chert. A stratigraphic section and gamma ray profile were constructed for an outcrop of the New Scotland Formation on Route 23 in Jefferson Heights. (Catskill), NY. Concentrations of Thorium (ppm), Uranium (ppm), and Potassium (%) were collected using a gamma ray spectrometer at 0.5m intervals throughout the 36.0m section. Raw gamma ray data were converted to API units to construct a synthetic gamma ray log for the outcrop. Elemental ratios of Thorium/Uranium, Thorium/Potassium, and overall gamma radiation in API units reveal three and a half cycles of increasing gamma radiation and increasing Thorium relative to both uranium and potassium followed by generally thicker intervals of decreasing values. We interpret these cycles as representing variations in the supply of siliciclastic sediments relative to in situ production of carbonate sediment. There is no sedimentologic evidence to suggest a eustatic origin for these cycles.

Predominantly oxidizing conditions during deposition of the New Scotland Formation in the Jefferson Heights area are indicated by Th/U that are generally above 2.0. Only a few minor fluctuations below 2.0 recording brief episodes of anoxia. The Th/U ratio increases to above 3.0 in the upper part of the New Scotland Formation as it grades into the overlying Becraft Formation. Several spikes in API suggest the presence of tephra beds manifested as thin, deeply weathered horizons of fine-grained sediments mainly in the lower third of the formation.

Gamma radiation in the New Scotland Formation at Jefferson Heights averages 10 API units higher than the average for the formation at I-88 near Schoharie, NY and 20 API units higher than the average for the section at Route 20 north of Cherry Valley, NY (Larsen et al. 2019). The formation is also thicker in the east than in western outcrops. This trend in API data suggests that the Jefferson Heights area was more proximal to the source of siliciclastic sediments entering the Appalachian basin during the Early Devonian. This trend in proximality indicates an Acadian source for the siliciclastic sediments of the New Scotland Formation.