Late Cambrian (Steptoean) sedimentation and responses to sea-level change along the northeastern Laurentian margin: Insights from carbon isotope stratigraphy
Carbon isotopes are applied as tools for stratigraphic correlation of poorly fossiliferous Upper Cambrian carbonate strata in the northern U.S. Appalachians. Upper Cambrian (Steptoean) marine carbonate rocks record a significant global positive carbon isotope excursion (δ 13C = +4‰ – 5‰ Vienna Peedee belemnite [VPDB]), the timing of which is well documented in fossiliferous sections elsewhere. The Steptoean excursion peaks at a sea-level lowstand that produced the Sauk II– Sauk III sequence boundary on the North American craton. In this study, this excursion is documented for the first time in the northern U.S. Appalachians in poorly exposed limestone debris flow and olistolith deposits interbedded within 20 m of continental slope shales of the Schodack Formation. These deposits contain the only reported pre– Elvinia zone Steptoean fauna in New York and record δ 13C values of up to +3‰ . The slope carbonate sediment was mainly derived from the shelf margin and is mixed with common coarse-grained silici-clastic material. These deposits reflect a seaward migration of the siliciclastic source area (exposed craton), suppressed carbonate platform sedimentation, and shelf bypassing during the Sauk II– Sauk III sea-level fall. Nonfossiliferous dolostones and dolomitic marbles of the proposed carbonate platform correlatives (the Pine Plains Formation in southeastern New York and the Stockbridge Formation from western Massachusetts) also contain common coarse siliciclastics ; however, sampled sections do not record elevated δ 13C values, indicating that these strata are probably not of Steptoean age. This suggests that Steptoean time is represented in the carbonate platform to slope succession of the northeastern (present-day) Laurentian margin by an extremely condensed stratigraphic interval or even a hiatus.