UPPER TRIASSIC CORALS AND REEF-FACIES FROM THE BLUE MOUNTAINS PROVINCE (OREGON) LINK THE WALLOWA AND OLDS FERRY TERRANES

Late Triassic carbonate reef facies are distinctive elements in the Wallowa terrane, one of two island arc complexes of the Blue Mountains Province (BMP), referred to as the Olds Ferry and Wallowa terranes. The island arcs are separated by the Baker terrane, representing a subduction complex, and are overlain by a sedimentary complex, the Izee Terrane. Plate tectonic models of terrane accretion during the early Mesozoic in the BMP have been unclear on the timing of amalgamation of the two island arcs. Dateable corals and reef facies are present in the Wallowa terrane in the Carnian-Norian Martin Bridge Formation. However, such fossils and facies have been conspicuously missing from the nearby Olds Ferry terrane, which in contrast to the Wallowa terrane, is mostly represented by volcanic and volcaniclastic rocks. Field investigations in the Olds Ferry Terrane yielded limestone conglomerates bearing the first examples of Upper Triassic corals and reef facies. They are found in the Upper Huntington Formation at a site called Willow Spring and closely resemble carbonate rocks from the Martin Bridge Formation in the Wallowa terrane. The Martin Bridge Formation contains scleractinian corals, sponges, and rare reef-like structures, which date closely to the Carnian-Norian boundary. Norian reef-like assemblages in the Martin Bridge Formation exhibit strong similarities with newly discovered fauna in the adjacent Olds Ferry Terrane as well as the western Tethys. Currently, three reef taxa have been identified from the Willow Spring site in the Olds Ferry Terrane, Paracuifia magnifica, Kuhnastraea cowichanensis, and Spongiomorpha ramosa. This constitutes the first occurrence of reef facies and reef taxa from the Olds Ferry Terrane. These detrital faunas are present in limestone conglomerate of the Upper Huntington Formation, which is composed of rounded carbonate clasts that vary from pure limestone to mixed carbonate-volcaniclastic lithologies. The presence of similar shallow-water, tropical to subtropical Late Triassic reef faunas and microfacies in the Olds Ferry and Wallowa terranes that are comparable to Tethyan counterparts provides a new link between these two terranes and suggests that they were in close proximity during Late Triassic time.