Discussion and reply: 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming Discussion

Smith et al. (2003, hereafter referred to as Smith et al.) present seven new isotopic ages from the Green River Formation in the greater Green River Basin. These data provide welcome new information in the ongoing effort to develop a coherent and reliable geochronological framework for this large and stratigraphically complex basin. Several of their main conclusions are based on a revised age estimate for the Wasatchian-Bridgerian North American Land Mammal Age boundary. For instance, Smith et al. use the revised age estimate for the Wasatchian-Bridgerian boundary to argue that the onset of maximum temperatures during the early Eocene climate optimum (EECO) could not have caused the faunal turnover that marks the boundary. Since no vertebrate fossils are known from the stratigraphic sections presented in Smith et al., these important conclusions rest entirely on their reinterpretation of a magnetostratigraphic correlation presented in Clyde et al. (2001, hereafter referred to as Clyde et al.). Unfortunately, the authors overlook several impor-