INSIGHTS FROM CARBON ISOTOPE STRATIGRAPHY INTO LATE CAMBRIAN (STEPTOEAN) SEDIMENTATION AND RESPONSES TO SEA LEVEL CHANGE ALONG THE NORTHEASTERN LAURENTIAN MARGIN

Carbon isotopes are applied in correlation of Upper Cambrian carbonate strata from the northern Appalachians whose age is not well constrained due to poor fossil preservation. Upper Cambrian (Steptoean) marine carbonate rocks record a global positive carbon isotope excursion (d¹³C=+4 to 5‰ VPDB), which peaks at the Sauk II–Sauk III sea-level lowstand. Timing of the excursion is documented in fossiliferous sections elsewhere. Fossil evidence from the Schodack Formation and from strata overlying and underlying the non-fossiliferous Pine Plains Formation in southeastern New York suggests that parts of these formations and their proposed correlatives in the non-fossiliferous Stockbridge Formation from western Massachusetts may be of Steptoean age.

Poorly exposed limestone debris flow and olistolith deposits interbedded within 20 m of continental slope shales of the Schodack Formation contain the only documented Steptoean fauna in New York and record d¹³C values of up to +3‰. This is the first documentation of the Steptoean carbon isotope excursion in the northern U.S. Appalachians. The slope carbonate deposits contain common coarse siliciclastics, which likely reflect arrested sedimentation and shelf bypassing during the Sauk II–Sauk III sea level fall. Dolostones and dolomitic marbles of the Pine Plains and Stockbridge Formations also contain coarse siliciclastics deposited on the shallow carbonate platform during low sea level. Sampled sections, however, do not record elevated d¹³C values, indicating that these strata are not of Steptoean age. This suggests that Steptoean time is represented in the carbonate platform to slope succession by a condensed interval or even a hiatus. In conjunction with lithologic evidence, the results of this study: 1) enhance understanding of the responses of depositional environments to sea level change; 2) offer insights into timing and rates of sedimentation along the northeastern Laurentian margin during the Late Cambrian; and 3) improve stratigraphic interpretations beyond the resolution possible by biostratigraphy.