CONODONTS FROM THE UPPER DEVONIAN – LOWER CARBONIFEROUS COFFEE POT MEMBER OF THE DYER FORMATION, CHAFFEE GROUP, COLORADO

The Dyer Formation is of interest because it is prolifically fossiliferous and spans the end-Famennian mass extinction event. The unit consists of shallow water limestones, dolostones, and rare cherts deposited on a tropical carbonate platform that extended from present day Nebraska to Idaho. The diverse biota in the Dyer Formation disappears close to the end of the Famennian, just above the transition between the Broken Rib Member and overlying Coffee Pot Member. To explore the cause(s) of this faunal and lithologic change, we logged and sampled the unit for conodont biostratigraphy and chemostratigraphy at three localities in the White River National Forest, including Bison Lake, Monument Lake, and Crane Park Quarry.

Preliminary sampling of the uppermost Broken Rib Member at Monument Lake reveals the presence of Mehlina strigosa and Polygnathus semicostatus, indicative of lower/middle expansa Zone, typical of earlier reported conodonts from this interval. Two horizons in the Coffee Pot Member yielded conodonts from the Bison Lake locality. Conodonts are rare, 2-3/kg, and specimens were highly fragmented. The lower horizon, in the middle Coffee Pot Member, yielded Polygnathus perplexus? – suggestive of the Middle expansa Zone. The upper horizon, in the upper Coffee Pot Member, ~15 m above P. perplexus?, contains fused clusters of fragmented conodonts of Siphonodella, Polygnathus, and sand grains. The fauna is suggestive of the Lower Carboniferous duplicata Zone and lies above limestones that exhibit unusually high (5-7 per mil) δ¹³C values, interpreted as the Hangenberg Excursion. This excursion characterizes uppermost Devonian strata worldwide and suggests the Devonian-Carboniferous boundary is within the upper Coffee Pot Member. Regionally extensive karst and dissolution features in the Coffee Pot Member may represent several conodont zones.