THE PYGODUS SERRA ZONE – PYGODUS ANSERINUS ZONE BOUNDARY FROM BEDDING PLANE CONODONTS IN THE PHI KAPPA FORMATION (ORDOVICIAN), TRAIL CREEK REGION, CENTRAL IDAHO

The Ordovician Phi Kappa Formation is well exposed near the headwaters of Little Fall Creek in the Trail Creek region of central Idaho. The section consists of weakly metamorphosed, siliceous black shale and has yielded abundant graptolites and conodonts on bedding plane surfaces at multiple levels that range between the Dapingian and Sandbian in age. Between 151m and 215m in our measured section there are at least 4 levels that contain bedding plane associations and partial assemblages of P. serra. The first unequivocal occurrence of P. anserinus is at 216.5m. Distinguishing between P. serra and P. anserinus is difficult on bedding planes in this setting as preservation is rarely complete. The poor preservation is common because the shale does not split parallel to bedding due to the moderate cleavage. The use of both signal mix (SED & BSD) and 3D Roughness Reconstruction Software on the Phenom XL G2 Desktop SEM has enabled the unequivocal recognition of the 4^th row denticle that distinguishes P. anserinus from P. serra at 216.5 m. The well documented graptolite fauna in the Little Fall Creek succession allows us to place the globally recognized Pygodus serra Zone – Pygodus anserinus Zone boundary with high resolution within the Nemagraptus gracilis Zone. This provides another important and direct link between graptolite and conodont biostratigraphic schemes.