ORDOVICIAN GRAPTOLITES AND CONODONTS OF THE PHI KAPPA FORMATION IN THE TRAIL CREEK REGION OF CENTRAL IDAHO: A REVISED, INTEGRATED BIOSTRATIGRAPHY

The Ordovician Phi Kappa Formation in the Trail Creek region of Central Idaho consists of weakly metamorphosed, black, siliceous shale that has yielded biostratigraphically important graptolites for nearly a century. The Trail Creek Summit section has served as a standard reference for western North America, although the graptolite identifications and biozonation are in need of revision. We have recollected the Trail Creek Summit, Little Fall Creek, Trail Creek road, and Trail Creek (creek) sections, updated the graptolite identifications from our new and the older collections, and revised the biozonation. In addition to providing important new graptolite information, our collections have also yielded abundant conodonts on some bedding plane surfaces, which have never been previously reported from the Phi Kappa. One of these conodont assemblages belongs to the key Upper Ordovician index species Amorphognathus tvaerensis.

Our analysis of these new collections indicates that 1) the Phi Kappa Formation at these sections can be integrated into a regional composite that spans the Lower (Chewtonian Ch2) to Upper Ordovician (Bolindian Bo3) with only minor gaps; 2) the graptolite succession is most similar to successions from South China and Australasia; and 3) the conodont/graptolite co-occurrence data are consistent with previous studies of zonal integration. The Phi Kappa composite section represents one of the longest and most complete exposures of Ordovician rocks in the world.