STRATIGRAPHIC CHRONOLOGY OF THE MITCHELL AREA

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QUATERNARY DEPOSITS   Includes alluvium and colluvium, Mazama ash (7700 years BP),

terrace gravels, talus aprons, and landslide debris (chiefly Pleistocene).

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RATTLESNAKE IGNIMBRITE   A late Miocene rhyolitic welded tuff of widespread extent

from a source vent near Harney Lake (7.05 Ma).

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PICTURE GORGE FORMATION   Basaltic lavas of the Middle Miocene Columbia River Group

that poured into the Mitchell Area from dike vents near Monument (15-16 Ma).

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JOHN DAY FORMATION   Chiefly red, green, and tan tuffs produced by Oligocene and early

Miocene deposition and weathering of air-fall ash from Cascade eruptions. Contains ashflow

member “A” (40 Ma), basaltic lavas and dikes (34 Ma), and Haystack member fluvial gravels,

Picture Gorge ignimbrite (29 Ma), and rhyodacitic intusive bodies(22Ma).

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CLARNO FORMATION   Western part of a calcalcaline-calcic volcanic field that covered parts

of Oregon, Washington, Idaho, and Wyoming during late Eocene and early Oligocene time.

Includes basaltic to rhyolitic intrusive bodies, lavas, and ash-flow tuffs. Volcanic mudflows

covered vast areas. (41 to 52 Ma).

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CRETACEOUS MARINE ROCKS   Dominant conglomerates and minor sandstones of the

Gable Creek Formation interbedded with dominant mudstones and minor conglomerates of the

Hudspeth Formation. Albian and Cenomainian (93.5 to 112 MA).

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PRECRETACEOUS BASEMENT ROCKS    A me`lange of low-grade metamorphic rocks

including phyllites, bedded cherts, pillowed metabasalts with mafic breccias and tuffs, serpentinites, 

blueschists, and metacarbonatesPetroliths were probably part of a tectonically introduced late

Paleozoic and early Mesozoic crust.

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