High-resolution stratigraphy and correlation of Cambrian strata using carbon isotopes: an example from the southern Appalachians, USA

Abstract

This study presents an example of using well-constrained stratigraphic trends in carbon isotope composition of marine carbonate deposits as a tool for high-resolution correlation of successions that lack prominent biomarkers. A large, positive carbon isotope excursion is recorded within the Steptoean (Furongian, Upper Cambrian) strata of the southern Appalachians in Tennessee. A coeval excursion (also known as SPICE) has been reported for the western United States (the Great Basin area), China, Kazakhstan, and Australia, indicating that this is a global phenomenon. The anatomy of this excursion was determined through extensive sampling of homogenous micrite and dolomicrite from strata at the Thorn Hill locality in northeastern Tennessee. The δ13C values show an increase from the upper Nolichucky Shale into the overlying Maynardville Formation (Conasauga Group). The most positive δ13C values (+4 to +5 ‰ VPDB) correspond to the transition between the Maynardville and the Copper Ridge Dolomite (Knox Group). The δ13C values decline in the lower part of the Copper Ridge. This characteristic carbon isotope record is compared with the δ13C record from the Lee Valley locality, which is about 30 km away and contains a coeval succession of strata that represent similar carbonate platform depositional environments. A comparison of the δ13C records shows a remarkable similarity and provides the means for detailed correlation of these two stratigraphic successions. Next the results are compared with those from the Tazewell locality, which is in the northwesternmost outcrop belt of lower Paleozoic rocks in Tennessee. This section is separated from the Thorn Hill and Lee Valley sections by three thrust faults and is truncated at the base by a thrust fault. The strata at the Tazewell locality differ from those at Thorn Hill and Lee Valley in that they were deposited more closely to the carbonate platform margin. Despite these differences, carbon isotope records proved very useful for correlation among the three successions. The results of this study encourage application of similar approaches in correlation of Steptoean strata at more widely spaced outcrops within and beyond the Appalachian basin.