CARBON-ISOTOPE STRATIGRAPHY: TOWARDS BETTER STRATIGRAPHIC RESOLUTION AND CORRELATION OF UPPER CAMBRIAN STRATA IN THE APPALACHIANS

High-resolution stratigraphy of Upper Cambrian strata throughout the Appalachians is hindered by the lack of well-exposed and laterally extensive outcrops of thick sedimentary successions. Dense vegetation and thick soil cover limit exposure to small, isolated and widely spaced outcrops. In addition, strata at these outcrops are commonly poorly fossiliferous, highly dolomitized, tectonically deformed, and in many cases metamorphosed. Characteristic geochemical signatures, especially stable isotopes of carbon, can provide invaluable new information in regional stratigraphic studies of these strata. To be an effective tool in such studies, carbon-isotope stratigraphy is used in conjunction with all available biostratigraphic, lithostratigraphic, and sequence stratigraphic information.

A large, global, positive carbon-isotope excursion (δ¹³C values of up to +5 ‰ VPDB), recorded in marine carbonate rocks deposited during the late Dresbachian and early Franconian or the Steptoean stage of the Late Cambrian, provides a useful chemostratigraphic marker. The excursion began at the base of the Aphelaspis zone of the Dresbachian, peaked at the boundary between the Dresbachian and the Franconian, and ended during the Elvinia zone of the early Franconian. The maximum excursion is associated with indicators of a sea level fall, which produced a craton-wide unconformity (or the Sauk II–Sauk III sequence boundary) on the Laurentian continent. Large unconformities were not developed in all areas along the subsiding passive-continental margin of eastern Laurentia. Instead, in carbonate platform successions of the Appalachian region this interval is commonly characterized by condensed sections and the presence of coarse-grained siliciclastic detritus. The record of the Steptoean positive carbon-isotope excursion is a valuable marker for recognition and regional correlation of this stratigraphic interval beyond the resolution possible by any other means. Examples from throughout the U.S. Appalachians (including Tennessee, Maryland, Pennsylvania, New York, and Vermont) will be discussed to demonstrate the application and usefulness of carbon-isotope stratigraphy in studying Upper Cambrian sedimentary successions.