A REVISED ORDOVICIAN GRAPTOLITE BIOSTRATIGRAPHY OF THE PHI KAPPA FORMATION AT TRAIL CREEK, CENTRAL IDAHO

The Phi Kappa Formation exposed at the Trail Creek Summit section of central Idaho contains a graptolite succession that discontinuously spans the Early Ordovician through Middle Silurian. Previous biostratigraphic work established a set of zones that have been employed as a standard reference for the North American Cordillera. Extensive new graptolite collections and systematic reassessment of the fauna demonstrate a strong similarity to the Australasian and South China succession and necessitate a substantial revision of the Ordovician zonation. Additionally, the Trail Creek succession is more incomplete than previously thought. The first 15 meters of back shale above the base of the section contain an interrupted sequence of Lower and Middle Ordovician faunas. Collections at 1.0 and 1.8m contain Xiphograptus lofuensis, Tetragraptus reclinatus, T. serra, Didymograptellus bifidus, and Pseudotrigonograptus ensiformis, a fauna referable to Chewtonian-2. Collections at 2.15m, in contrast, contain Arienigraptus gracilis and Isograptus maximodivergens, taxa indicative of Castlemainian-4. Yapeenian and Darriwilian faunas are mostly missing from the Phi Kappa Fm. at Trail Creek (though present in other sections). The subsequent 135 meters of strata contain a more complete succession of Upper Ordovician zones. We have substantially lowered the bases of the N. gracilis and C. bicornis zones (by nearly 60 meters in the latter case, through the entire sandy interval in the middle part of the section). The upper part of the C. bicornis Zone and lowermost part of the succeeding “Passage Beds” contain a sparse early Eastonian fauna (Ea1-2) that includes Climacograptus spiniferus. The first appearance of C. tubuliferus occurs near the base of the “Passage Beds,” and is correlative with the base of the Ea3. The Dicellograptus gravis Zone (Ea4) appears at the level formerly identified at Trial Creek as the base of the C. tubuliferus Zone. Bolindian-1 faunas occur within the upper part of that interval and Bo2 faunas occupy most of the interval identified as the Pleurograptus linearis Zone. Finally, the most diverse assemblages, those of the Bo3, are present in the former Dicellograptus ornatus Zone interval. D. ornatus itself, however, appears within the Bo1 along with Climacograptus uncinatus.