CARBON-ISOTOPE STRATIGRAPHY OF THE LOWER GORGE FORMATION (UPPER CAMBRIAN), NORTHWESTERN VERMONT

Previous biostratigraphic studies by others have suggested that the strata of the lowermost Gorge Formation exposed at Highgate Falls in northwestern Vermont belong to the Aphelaspis and Dunderbergia trilobite zones of the Dresbachian Stage (Steptoean, Upper Cambrian). These studies have also suggested that there is an unconformity because the uppermost Dresbachian, Franconian, and lower Trempealeauan strata are not present at this locality. Marine carbonate rocks deposited world-wide during the Late Dresbachian and Early Franconian have d¹³C compositions of up to +5 ‰ VPDB. This large, global positive carbon-isotope excursion began at the base of the Aphelaspis zone and peaked near the Dresbachian/Franconian boundary. The goal of this study was to determine if the Gorge Formation records the start of this excursion, and if the unconformity abruptly terminates the record of the excursion near its maximum.

A 22 meter-thick section of the Gorge Formation was measured, described and sampled. The section consists of sandy and cobbley dolostones, dolomitic quartz arenites and shales interpreted as carbonate-platform slope deposits. Sixty samples of dolomitic matrix were analyzed for their stable isotope compositions. The d¹³C values range from -0.57 to +0.39 ‰ VPDB, indicating that there is no distinct record of the positive carbon-isotope excursion. The results question the age for the lowermost portion of the Gorge Formation at Highgate Falls and suggest that more of the Dresbachian strata may be missing along the unconformity than previously recognized. This study demonstrates the potential of using carbon isotopes in stratigraphic studies of highly dolomitized and poorly fossiliferous strata.