13C-rich marbles from the Neoproterozoic McCoy Creek Group having δ13C values up to +12% have been discovered at three separate localities in eastern Nevada and western Utah. Detailed isotopic profiles measured across individual amphibolite facies, metre-thick marble layers have extremely uniform 13C/12C ratios, which probably represent depositional values. Because individual carbonate horizons have distinctly different but abnormally high values of δ13C, the discovery permits the use of 13C/12C ratios for stratigraphic correlation within the generally nonfossiliferous Neoproterozoic sedimentary sequences of western North America, even where these rocks have been strongly metamorphosed. The anomalous values add to a growing array of data from western North America and elsewhere delineating a large and geographically widespread Neoproterozoic carbon isotope anomaly.

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