ORDOVICIAN GRAPTOLITES AND CONODONTS FROM THE PHI KAPPA FORMATION IN THE TRAIL CREEK REGION OF CENTRAL IDAHO: AN INTEGRATED BIOSTRATIGRAPHY

The Ordovician Phi Kappa Formation in the Trail Creek region of Central Idaho is black, siliceous, weakly metamorphosed shale that has yielded biostratigraphically important graptolites for nearly a century. The section serves as a standard reference for western North America, however the graptolite systematics and biozonation are out of date. We have extensively recollected the Trail Creek Summit, Little Fall Creek, Trail Creek road, and Trail Creek (creek) sections. In addition to providing important new graptolite information, our collections also yielded abundant conodonts on bedding plane surfaces, which have never been previously reported from the Phi Kappa. 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 2) to Upper Ordovician (Bolindian 3) 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 is 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.

The Trail Creek Summit section spans the entire interval listed above, but has several prominent stratigraphic gaps. The largest of these gaps, which comprises the upper Middle Ordovician (Yapeenian 2 to Darriwilian 4), is however, entirely represented at the Little Fall Creek section. Conodonts from the riddellensis biozone at Little Fall Creek and the gracilis biozone at the summit section include the index species Pygodus serra and P. anserinus, respectively. The Trail Creek road section yielded specimens of D. bifidus and I. victoriae, indicating ages of Chewtonian 2 and Castlemanian 2. Castlemanian 1 age strata are missing or non-fossiliferous at that section. Collections from the Trail Creek (creek) section yielded a diverse assemblage of pacificus biozone graptolites including Diceratograptus mirus, followed by a limestone-rich interval that may correspond to the Hirnantian Stage sea level lowstand. Conodont collections from the bifidus, victoriae, and morsus biozones contain bedding plane assemblages that are currently being studied.