THE PHI KAPPA FORMATION IN THE TRAIL CREEK REGION (CENTRAL IDAHO): INTEGRATED BIOSTRATIGRAPHY OF ORDOVICIAN TO SILURIAN GRAPTOLITE AND CONODONT FAUNAS

Carter and Churkin (1977, USGS Prof Paper 1020) used the graptolite succession in the Ordovician-Silurian Phi Kappa Formation of the Trail Creek region (Central Idaho) to establish a set of graptolite zones, now in general use as a standard reference succession for western North America. We have recollected a number of sections in the Trail Creek region that provide important new information on the graptolite biostratigraphy, but also yield abundant bedding plane conodonts. The Phi Kappa Formation can now be integrated into a regional composite that spans the Lower Ordovician (Chewtonian 2) to Lower Silurian (Llandovery). None of the sections covers the entire interval, however, and the combined information from all sections reveals a few minor regional gaps. Conodonts from the Archiclimacograptus riddellensis Biozone at Little Fall Creek and the Nemagraptus gracilis Biozone at the Trail Creek Summit section include the index species of Pygodus. Collections from the creek section at Trail Creek yield a diverse graptolite assemblage of the Paraorthograptus pacificus Biozone including the poorly known Diceratograptus mirus. At this site the lowermost Silurian collection, 1.5 m above the Ordovician-Silurian boundary, produces a graptolite fauna that includes Paraclimacograptus innotatus and various slender monograptids and normalograptids suggestive of the Rhuddanian, possibly the Coronograptus cyphus Biozone. Previously it was believed that the Silurian graptolite succession starts only after a considerably larger gap in the late Aeronian Lituigraptus convolutus Biozone. The Silurian succession continues into the Wenlock with faunas bearing Cyrtograptus species in the Trail Creek Formation. The Phi Kappa composite section represents one of the longest and most complete exposures of Ordovician to Lower Silurian rocks in the world. Its graptolite succession can be compared easily to successions from South China and Australasia. The conodont/graptolite co-occurrence data improves considerably the biostratigraphic correlation of deep-water graptolite and conodont assemblages.