PENNSYLVANIAN TO EARLY JURASSIC FAUNAS FROM INLIERS OF OPHIOLITIC MELANGE LINK THE BAKER AND OLDS FERRY TERRANES OF EAST-CENTRAL OREGON

New stratigraphic and faunal data indicate that the Baker terrane (Elkhorn Ridge Argillite) and adjacent Olds Ferry terrane (Weatherby and Huntington Formations) in the Blue Mountains of eastern Oregon are depositionally related and that both represent parts of a single allochthonous terrane. The flyschoid Lower and Middle Jurassic Weatherby Formation depositionally overlies the contact between the mélange-rich Pennsylvanian to Upper Triassic Baker terrane on the north and Upper Triassic volcanic arc rocks of the Huntington Formation (Olds Ferry terrane) on the south.

Sedimentary rocks in the Baker terrane are characterized by a tectonic mixture of Pennsylvanian to Triassic limestone and chert that are typically altered to marble and metachert. Baker terrane lithologies commonly exhibit tectonic shear fabrics and regional greenschist and higher metamorphic grades indicating partial subduction in an accretionary complex. Faunas from the western part of the Baker terrane include middle Permian and Late Triassic radiolarians and middle Permian fusulinids from mélange blocks in the Strawberry Mountain area near John Day. Faunas in the central (Aldrich Mountains) and eastern (Virtue Hills) parts of the Baker terrane include middle Permian (Guadalupian) and Late Triassic (Carnian and Norian) radiolarians, Pennsylvanian to Late Triassic conodonts (CAI's of 5-6), middle Permian fusulinids and tabulate corals, and rare middle Permian and Late Triassic trepostome bryozoans.

The Lower and Middle Jurassic Weatherby Formation (Olds Ferry terrane) is composed largely of volcanic wackes, siltstones, and argillites. Inliers of ophiolitic mélange containing Pennsylvanian and lower Permian limestone and Permian (Wolfcampian to Guadalupian) and Upper Triassic (Carnian) radiolarian-bearing chert and argillite occur as olistoliths in the Baker terrane. Compression of the Weatherby basin resulted in imbrication of Weatherby strata as well as faulting and thrusting of the accretionary complex (Baker terrane) over the Weatherby along the Connor Creek fault. The upper age limit for Baker mélange formation is constrained to latest Triassic and Early Jurassic on the basis of Late Triassic radiolarians in chert from the Huntington Junction mélange inlier and Early Jurassic (Sinemurian) ammonites in the overlying Weatherby Formation.