Late Triassic bivalves of the Martin Bridge Limestone, Hells Canyon, Oregon: taphonomy, paleoecology, paleozoogeography.

Richly fossiliferous lenses of silicified shells in the Norian Martin Bridge Limestone of Hells Canyon represent 'windows' of unusually excellent preservation through which can be read the taphonomy, paleoecology and paleozoogeography of Late Triassic bivalves of the Wallowa Mountains-Seven Devils Mountains volcanic arc terrane of northeastern Oregon. Taphonomic, petrographic, and stratigraphic analysis of the shell beds discloses that they represent storm lag concentrations formed by processes of winnowing and local transport of shells. Despite the inferred transport, however, the autecology of 38 bivalve species is remarkably uniform, suggesting an epifaunal, suspension-feeding association that inhabited peloidal carbonate sediment. Many of these epifaunal bivalve species (74 percent) are either new or indeterminate on the basis of present collections. Because of the high frequency of new or indeterminate species, zoogeographic conclusions based on species are necessarily preliminary. This perplexing assortment of zoogeographic affinities can best be understood in terms of Tozer's paleogeographic model of central Pacific island arcs with mixed East and West Pacific faunas. It should be emphasized, however, that Norian bivalve species of both the North American craton and the suspect terranes along its margins are at present imperfectly documented. -from Author