Conodonts and the Devonian–Carboniferous transition in the Dyer Formation, Colorado

Abstract

Conodont assemblages from shallow platform carbonate strata of the Dyer Formation in northwestern Colorado indicate that the unit spans the Devonian–Carboniferous transition, and include four new taxa, described herein. The underlying Parting Formation and most of the Broken Rib Member of the Dyer Formation are in the Palmatolepis expansa Zone. The upper portion of the Broken Rib Member and the lower part of the Coffee Pot Member of the Dyer Formation are in the Bispathodus aculeatus Zone to the Bispathodus ultimus Zone, and associated with an ~6‰ positive δ13C shift identified as the end-Devonian Hangenberg excursion. Upper Coffee Pot Member conodonts are equivocally Tournaisian, and cap the δ13C excursion. Collectively, these observations suggest that the Devonian–Carboniferous boundary is in the upper Coffee Pot Member of the Dyer Formation, and thus the overlying Gilman Sandstone is Carboniferous.