MIXED CARBONATE-VOLCANICLASTIC FACIES OF THE OLDS FERRY TERRANE, EASTERN OREGON AND WESTERN IDAHO

The Olds Ferry terrane of eastern Oregon and western Idaho represents a volcanic island arc succession of Middle to Late Triassic age including mafic to silicic hypabyssal and extrusive volcanic beds and interbedded coarse to fine volcaniclastic strata of the Huntington Formation. A unit of thin-bedded limestone within the Huntington Formation is discontinuously exposed along the Snake River north and south of the town of Huntington, Oregon. Ammonite and bivalve age assignments indicate that the limestone unit spans the late Carnian to early Norian. Exposed sections contain a suite of four lithofacies interbedded at the decimeter scale. Graded Skeletal Packstone and Mudstone Facies is interpreted to record episodic, distal, nonchannelized deposition from turbidity currents in a low-oxygen, slope to toe-of-slope setting. Tuffaceous Peloidal Grainstone Facies is interpreted to represent deposition from volcaniclastic-carbonate turbidity currents in a slope setting. The composition of volcanic lithic grains and the presence of shallow-water carbonate skeletal debris indicates that sediment was sourced from erosion of subaerial volcanic deposits, remobilization of subaqueous basaltic glass, and arc-proximal, shallow-water carbonate zones. Peloidal Skeletal Packstone Facies is interpreted to represent winnowing and sediment redistribution by large storm currents. Graded Lapilli-Tuff Breccia Facies is interpreted to represent subaqueous suspension-fall of ash and lapilli following either submarine or subaerial pyroclastic eruptions. Lithofacies are interpreted to represent deposition on a fine-grained carbonate slope apron adjacent to shallow-water carbonate shoals and intermediate-depth crinoid thickets. During times of tectonic and storm quiescence the slope accumulated silicilastic and lime mud, pelagic ammonite and bivalve shells, and radiolarian tests via suspension settling. The apron surface was extensively reworked and winnowed during times of low relative sea level or by large storms which effectively lowered storm wave base. Storms also swept the shallower ramp, mobilizing carbonate and volcaniclastic sediment which was subsequently reworked, winnowed and deposited on the apron surface