Abstract Substantial advances by numerous researchers over the past 20 years have made it possible to develop a composite biochronological scheme for the Triassic based on the bivalves Claraia , Peribositria , Enteropleura , Daonella , Halobia , Eomonotis and Monotis . These bivalves exhibit temporal durations nearly equal to ammonoids and conodonts. Widely distributed across the Tethys, Panthalassa and Boreal regions, these bivalves occur in a wide variety of marine facies and water depths, but are most notable for their thick shell accumulation in deeper-water oxygen deficient environments. They were most likely resting or reclining benthos, may have housed chemosymbionts, and were part of episodic opportunistic palaeocommunities in or near oxygen deficient settings. A new biochronological zonation for bivalves is presented that encompasses the entire Triassic and is integrated with standard ammonoid schemes. The Lower Triassic is characterized by 2–3 zones of Claraia , most notably from the eastern Tethys representing the entire Induan and lower portion of the Olenekian. Later in the Olenekian, and most notably from the Boreal realm, species of Peribositria (included by some workers within Bositra ) provide useful zonal indexes. The Middle Triassic is well represented by Enteropleura (Middle Anisian) and Daonella (Upper Anisian through Ladinian) in the Tethys and North America with significant occurrences throughout the circum-Pacific and Boreal realms. The Upper Triassic can be subdivided into 8–13 bivalve zones based on the succession of Halobia , Eomonotis and Monotis sensu lato species with best representation in the Tethys, Boreal and eastern Panthalassa regions.

The Upper Triassic (Carnian–Norian) Martin Bridge Formation of northeastern Oregon, southeastern Washington, and western Idaho is characterized by rapidly shifting depositional processes within a tropical volcanic island arc setting. Martin Bridge sequences in the Hells Canyon and northern Wallowa Mountains document shallow-water peritidal evaporitic sediments that are succeeded by deeper and predominantly subtidal deposits. This indicates drowning of the carbonate platform and a transition to deeper-water turbiditic sedimentation before a late Triassic transition into the overlying mid-Norian to Jurassic Hurwal Formation. At the type locality in the southern Wallowa Mountains, dysaerobic shales, carbonate debris sheets, and turbiditic sediments indicate distal slope and basinal environments while other facies at other sites in the Wallowa Mountains and Hells Canyon areas indicate reef and shallow-water platform settings. In this paper we formally recognize the name Martin Bridge Formation and reinstate the type locality in the southern Wallowa Mountains as the principal unit strato-type. An additional reference section is given at Hurricane Creek in the northern Wallowa Mountains. The Martin Bridge is formally divided into four members: the Eagle Creek and Summit Point Members are introduced and formally proposed herein and the BC Creek and Scotch Creek Members also are elevated to formal status. A partial reconstruction of the Wallowa terrane during deposition of the Martin Bridge Formation suggests a north-south (or northeast-southwest) trending platform margin facing a forearc basin situated to the east (or southeast). The lithofacies and paleontological characteristics of the Martin Bridge can be put into the framework of a depositional and a tectonic model to help better explain many of the stratigraphic and paleontologic problems previously encountered. We believe that the Wallowa terrane provides one of the best and most complete examples yet known for shallow-water carbonate depositional patterns in an oceanic island arc setting.