STRUCTURAL GEOLOGY, STRATIGRAPHY AND PETROLOGY OF THE ELKHORN RIDGE ARGILLITE, IN THE SUMPTER AREA, NORTHEASTERN OREGON
COWARD, ROBERT IRVIN
Doctor of Philosophy
The Elkhorn Ridge Argillite exposed in the Bourne-Sumpter area of eastern Oregon is a broken formation of metamorphosed (greenschist facies) hemipelagic sediments, reefal carbonates, rare clastics, mafic volcanics and intrusives that may have accumulated in a late Triassic-Early Jurassic subduction zone complex or marginal basin. The structural fabric of this unit is best explained by a model of repeated homoaxial folding and simple and/or pure shear during progressive deformation. Deformation began before the sediments were completely lithified and tight to isoclinal mesoscopic folds developed with and without axial-plane cleavage. A predominant spaced cleavage formed by a combination of dissolution-redeposition processes and microfaulting. Grain boundary sliding and movement along microfaults are important additional deformation mechanisms that operated during flexural slip folding. The lithology and structural style of the Elkhorn Ridge Argillite are characteristic of deformed sequences in subduction zone complexes and marginal basins and can not be used to discriminate paleotectonic settings. New faunal evidence suggests that substantial disruption of structural stratigraphic units such as the Elkhorn Ridge Argillite have taken place during the Late Triassic to late Jurassic. Coeval blocks of similar lithology and fauna are located in the Miller Mountain area within the John Day inlier of central Oregon and in several terranes distributed along the western Cordillera from California to Canada. Other blocks of oceanic lithic assemblages of various ages in intermediate areas show significant differences in structural and sedimentological history and together are interpreted to lie within a diffuse fault zone of amalgamated suspect terranes.