DETRITAL RECORD OF UPPER TRIASSIC REEFS IN THE OLDS FERRY TERRANE, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON, UNITED STATES

Abstract

We report a coral-sponge dominated reef lithofacies in detrital boulders of carbonate-clast conglomerate from the Olds Ferry terrane, Blue Mountains Province, Willow Spring locality, northeastern Oregon. This discovery constitutes the first report of Triassic reefal lithofacies from the Olds Ferry terrane. Corals at the site include a high-growing, dendroid-phaceloid species that acted as a framework or sediment baffle, and a low-growing, cerioid, encrusting coral and a sponge that acted as sediment binders. The co-occurrence of coral taxa in the Wallowa intraoceanic arc and Olds Ferry pericratonic arc terranes suggests that during the Late Triassic, these regions were in proximity to one another. With this report, Upper Triassic carbonate rocks are known from all of the major terranes of the Blue Mountains Province and a Tethyan affinity is established for the fauna of the Olds Ferry terrane. We suggest that the source of this Late Triassic coral-sponge reef fauna in the upper member of the Huntington Formation was intrabasinal and that the source rocks are no longer exposed in the region. Numerous possible regional source areas for these detrital limestone clasts rule out the need to call upon an exotic terrane source area.