ANATOMY OF A LATE TRIASSIC REEF AND CARBONATE PLATFORM FROM THE WALLOWA TERRANE, NORTHEASTERN OREGON AND WESTERN IDAHO

The Wallowa terrane, one of several accreted terranes in the Blue Mountains Province, northeastern Oregon and adjacent Idaho, originated as a tropical island arc in the eastern reaches of Panthalassa. During the Late Triassic to Jurassic it amalgamated with other terranes of the Blue Mountains Province before accretion to the craton. It is well-dated with foraminifers, halobiid bivalves, ammonoids, conodonts and radiolarians. Late Triassic shallow to deeper-water limestone and calcareous siliciclastic rocks belong to the Martin Bridge and Hurwal formations (late Carnian to Early Jurassic in age) and were deposited abruptly after cessation of volcanism. Based on study of over 50 outcrops from 35 localities with more than 1,000 thin sections, we present a new model of deposition to revise the paleoecology, paleogeography and sedimentological history of Late Triassic carbonate rocks. A marginal reef facies formed barriers, providing abundant debris with olistostromal blocks transported into the basin. The upper part of the Triassic succession records deepening and drowning of the platform. New benthic foraminifers as well as diverse corals, sponges, mollusks and algae, assist dating, facies analysis and interpretation of the paleoecology. A distinctive reef facies with patches of framework-building corals, solenoporacean algae and thalamid sponges, surrounded by bioclastic debris characterized the shelf edge. Fifty species of scleractinian corals (30 species and 16 genera from the reef) reveal dominantly Tethyan affinities with 25% terrane-endemic species. Foraminifers are far more diverse and useful than previously thought (30 genera and 60 species) of which about 15% are endemic. The well-dated reef and carbonate platform from this part of Panthalassa increase the accuracy of paleoecologic and paleobiogeographic comparisons with better-known counterparts from the distant Tethys.