ESTABLISHING TEMPORAL AND SPATIAL RELATIONSHIPS OF TRIASSIC REEFAL CARBONATES: WALLOWA TERRANE, NORTHEAST OREGON AND WESTERN IDAHO

The Martin Bridge Formation is the only distinctive carbonate deposit in the Wallowa terrane, Blue Mountains Province (BMP), northeastern Oregon and adjacent Idaho. Following Permian to middle Triassic volcanic activity, the terrane existed during Late Triassic time as a tropical island arc in the Panthalassa Ocean and contains Tethyan-type reef development. Although isolated and adversely affected by post-accretionary processes, depositional environments of platform, slope and basin have been reconstructed. These developed around the shallow-water setting of an island arc. Shoal/barrier and lagoon facies contain patch reefs and diverse Upper Triassic foraminifers, corals, thalamid sponges, brachiopods, mollusks, echinoderms and calcified algae as well as microcoprolites and problematic microfossils. Over 20 foraminifer genera from 17 families and over 21 genera of reef building corals, comprising predominantly Tethyan species have been identified. An international collaborative effort has focused on comparative study of paleoecology, carbonate microfacies analysis, carbon isotope sampling and dating of the sequences and comparisons with counterparts in Japan and the Tethys. Foraminifers, along with the reef organisms distinguish patch reefs, lagoonal and deeper water carbonates. New foraminifers, in concert with ammonoids and conodonts, constrain the shallow-water reefal sequences to the late Carnian and assemblages show striking similarities with coeval associations from Tethyan localities in the northern Tethys and differ from examples in Japan. A carbonate ramp model developed for the Martin Bridge initiated during Late Carnian-Early Norian time, immediately following cessation of arc volcanism. Onset of drowning of the reefal facies occurred soon after the start of Norian time, as revealed by the upper Martin Bridge and succeeding non-carbonate Hurwal Formation, which display downslope blocks of older reef limestone. This phase coincided with tectonic accretion with other terranes of the BMP.