CARBON-ISOTOPE STRATIGRAPHY OF THE GALWAY FORMATION IN SARATOGA COUNTY, NEW YORK: A CONTINUING SEARCH FOR STEPTOEAN (UPPER CAMBRIAN) SHALLOW MARINE STRATA IN THE NORTHERN APPALACHIANS

Dolomitic sandstones and arenaceous dolostones of the Galway Formation in Saratoga County of east-central New York overlie the Potsdam Sandstone and underlie the Hoyt Limestone. This Upper Cambrian succession unconformably overlies the Precambrian crystalline basement. Previous documentation of the Elvinia Zone trilobites in the Galway strata at the Greenfield railroad cut (about 5 km west-northwest of Saratoga Springs) indicated their early Franconian age. This suggested that these strata might contain a record of the large positive global Steptoean carbon-isotope excursion, which has not been documented yet from the shallow marine deposits of the northern Appalachians. The excursion began between the Crepicephalus and the Aphelaspis Zones (Dresbachian Stage), peaked at the Dresbachian-Franconian boundary, and ended during the Elvinia Zone. The maximum excursion (δ¹³C=+4 to 5 ‰ VPDB) is commonly associated with indicators of a sea level fall, which produced a craton-wide unconformity or the Sauk II-III sequence boundary.

The δ¹³C values of the Galway dolostones at the Greenfield railroad cut range between –1.41 and –0.57 ‰, indicating that the Steptoean excursion is not recorded at this locality. This suggests that these strata likely belong to: 1) the post-excursion Elvinia Zone of the early Sunwaptan age; and 2) the transgressive lower portion of the Sauk III sequence. The question remains whether any strata from the Sauk II sequence and the Sauk II-III sequence-bounding interval are present in this area. If so, they should be stratigraphically below the interval examined here, which is in agreement with the Dresbachian-Franconian boundary placement at or near the Potsdam-Galway contact. Future efforts to document the Steptoean excursion in these sandstone-dominated strata will depend on finding adequate exposures and samples to analyze. It is also very likely that the shallow-marine record of the excursion is highly condensed or absent because of the sea-level fall at the Sauk II-III boundary. This is supported by elevated δ¹³C values (up to +3 ‰) in continental slope deposits in Dutchess County of southeastern New York at the locality that has previously yielded the only documented post-Crepicephalus and pre-Elvinia Zone fossils in New York.