Abstract

Mesozoic lithic sandstones in central Oregon were deposited in a complex forearc basin within the eugeosynclinal western part of the Cordillera. The sequence in the John Day inlier of the Blue Mountains province includes about 15,000 m of Mesozoic turbidites and subordinate associated strata of other facies. Chert-rich clastic detritus in lower horizons was derived mainly from an uplifted melange ridge associated with the nearby subduction zone. Volcaniclastic debris from an active volcanic arc is dominant at middle horizons. Mixed lithic detritus derived from multiple sources exposed in a mature and dissected arc terrane prevails at upper horizons. The chief types of lithic grains in the rocks are chert, felsite, microlitic volcanic rock fragments, argillite, and slate or shale. Fragments of metamorphic quartz-mica tectonite and microgranular grains of aggregate quartz and hypabyssal igneous rocks are also present. Monocrystalline quartz grains are minor constituents of most sandstones and absent from some. In melange-derived sandstones, polycrystalline quartzose grains, mainly chert, are the principal constituents with volcanic rock fragments next in abundance; feldspar grains and metasedimentary lithic fragments are subordinate. Volcaniclastic sandstones are composed mainly of volcanic rock fragments and plagioclase grains in that order of abundance, and commonly contain more clinopyroxene than quartz. Volcanic rock fragments and feldspar grains in that order are also the most abundant grains in sandstones of mixed provenance, but sedimentary rock fragments, K-feldspar and quartz grains, and mica flakes are all more abundant than in the older, strictly volcaniclastic strata. These suites of lithic sandstones derived mainly from supracrustal eugeosynclinal sources differ markedly from arkosic or quartzose sandstones derived from uplifted or recycled cratonic sources, and also contrast with more quartzo-feldspathic rocks in other forearc basins where much of the detritus was derived from eroded granitic plutons. The characteristic compositions of the distinct suites of lithic sandstones in central Oregon can serve as guides to interpretations of provenance for similar rocks in parts of orgenic belts where geologic relations are not so well displayed.

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