NEW CONODONTS FROM THE UPPER DEVONIAN – LOWER CARBONIFEROUS DYER FORMATION, CHAFFEE GROUP, NORTHWEST COLORADO, USA

The Dyer Formation, part of the Chaffee Group, is subdivided into the Broken Rib Member and overlying Coffee Pot Member. In the White River Plateau of northwest Colorado, the Broken Rib Member is primarily biorudstone and biowackestone that was deposited in an open marine shelf setting. The Coffee Pot Member consists of clay-rich to sandy dolomitc mudstone that is intermittently interbedded with biowackestone and intrarudstone; rarely these are stromatolitc, cherty, or karsted. These features point to intermittent subaerial exposure and suggest a more restricted depositional setting compared to the Broken Rib Member.

At Crane Park Quarry, a diverse conodont fauna occurs in the Broken Rib Member and lower Coffee Pot Member. The Broken Rib fauna contains Apatognathus varians, Bispathodus ac. aculeatus, Bi. ac. plumulus, Icriodus costatus darbyensis, Pelekysgnathus inclinatus, Polygnathus communis ssp., Po. experplexus, Po. semicostatus, and Pseudopolygnathus primus new subspecies, indicative of the aculeatus Zone into possibly the ultimus Zone. The lower Coffee Pot fauna contains Bispathodus stabilis vulgaris, Icriodus costatus darbyensis, Pandorinellina cf. P insita, Patrognathus donbassicus, Pelekysgnathus inclinatus, and Polygnathus cf. Po. symmetricus. The overlapping ranges of these taxa after the first occurrence of Bi. aculeatus ssp. is indicative of the aculeatus Zone to ultimus Zone. Conodont fragments from one horizon at nearby Bison Lake, as well as carbon and strontium isotopic data, suggest that the upper Coffee Pot Member is Carboniferous. The Broken Rib Member conodont fauna is typical of the Polygnathus-Icriodus biofaices, the Coffee Pot Member conodont fauna is also typical of the Polygnathus-Icriodus biofacies with elements of deeper water – Bispathodus, and shallower water – Pandorinellina and Patrognathus.