STRATIGRAPHIC CHRONOLOGY OF THE MITCHELL AREA
QUATERNARY DEPOSITS Includes alluvium and colluvium, Mazama ash (7700 years BP),
terrace gravels, talus aprons, and landslide debris (chiefly Pleistocene).
RATTLESNAKE IGNIMBRITE A late Miocene rhyolitic welded tuff of widespread extent
from a source vent near Harney Lake (7.05 Ma).
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).
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).
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).
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).
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 metacarbonates. Petroliths were probably part of a tectonically introduced late