CARBON ISOTOPE STRATIGRAPHY OF THE DANBY AND CLARENDON SPRINGS FORMATIONS IN SOUTH-CENTRAL VERMONT

A large positive carbon-isotope excursion (d¹³C=+ 4 to 5 ‰ VPDB) is recorded in Upper Cambrian (Steptoean or Dresbachian–Franconian) marine carbonate rocks worldwide. This characteristic isotopic signature is useful in stratigraphic studies of strata without biostratigraphic indicators. This study focused on nonfossiliferous metamorphosed carbonate rocks of the Danby and lower Clarendon Springs Fms. in south-central Vermont, which have been correlated with unmetamorphosed Dresbachian to Franconian strata in eastern New York. This correlation was based on the presence of quartzite interbedded with dolomitic marbles of the Danby Fm., which may represent siliciclastic input onto the carbonate platform during a sea level fall at the Dresbachian–Franconian or the Sauk II–III sequence boundary. Maximum carbon-isotope values are associated with this boundary interval. The overlying Clarendon Springs Fm. contains clean dolomitic marbles, which may represent rising sea level at the beginning of the Sauk III sequence. If the proposed stratigraphic interpretations are correct then these strata should record the Steptoean carbon-isotope excursion. The record of such a large isotope excursion may be altered but not entirely obliterated in these metamorphosed and structurally deformed strata that retain many primary sedimentary structures.

The thickness of the Danby Fm. in Vermont has been estimated at 350m. This study investigated four small isolated outcrops of the Danby Fm. and one outcrop of the overlying Clarendon Springs Fm. The total measured thickness of strata at these outcrops is 36m and the outcrops are widely spaced over a stratigraphic interval about 440m thick. The range of documented d¹³C values, from –1 to +1 ‰, indicates that the strata examined do not record the positive carbon-isotope excursion and are not of Steptoean age. The excursion may be contained within the covered intervals between the examined sections. Alternatively, the excursion may be recorded in strata above or below the examined successions. Finally, there may be no thick strata of Steptoean age in the northern Appalachians due to suppressed deposition during the Sauk II–III sea level fall.