New biostratigraphic constraints for the Martin Bridge Formation (Upper Triassic, Wallowa terrane, Oregon, U.S.A.)

The stratigraphic relationships between the thick carbonate deposits of the Martin Bridge Formation and contiguous clastic lithologies are poorly understood. Biostratigraphic markers are almost absent in the shallow-water Triassic limestones and, in places where deeper, fossiliferous-rich carbonate rocks occur, incomplete exposures, metamorphism and structural complexity have obscured most stratigraphic contacts. We have investigated two deep-water localities in the eastern and southern Wallowa Mountains, in which transitional contacts are preserved. There, rich assemblages of Halobia bivalves have been found, including H. ornatissima, H. radiata or H. septentrionalis, and Halobia spp. aff. H. beyrichi. The short temporal range of these taxa designate a precise timing constraints of lithological transitions, at the base and at the top of the Martin Bridge Formation