An integrated trilobite and conodont biostratigraphy across the base of the Laurentian Whiterockian Series (lower Middle Ordovician) at its stratotype, Whiterock Canyon Narrows, Nevada

Abstract

The outcrop at Whiterock Canyon Narrows, Nevada, is the stratotype for the Middle Ordovician Whiterockian Series in Laurentia. Contrasts in the distribution of trilobites and conodonts between two parallel sections at the stratotype demonstrate the presence of an unconformity separating Ibexian faunas (the historic trilobite Zone J, the more recent restricted "Pseudocybele nasuta" trilobite Zone, the Reutterodus andinus conodont Zone) from overlying Whiterockian faunas (Zone L, the Psephosthenaspis pseudobathyurus trilobite Zone, the Orthidiella brachiopod Zone, the Tripodus combsi conodont Zone). The unconformity represents the erosional loss of a minimum of 10m of shale and bedded limestone within the upper Ninemile Formation. In comparison to more continuous section in the Ibex region, Utah, the equivalent to the thin Psephosthenaspis microspinosa trilobite Zone and, possibly, the uppermost interval of the underlying "P. nasuta" Zone are missing. Illaenus welchi new species is described from the Whiterockian faunas of the upper Ninemile and Antelope Valley formations.