Pre-Tertiary metamorphic rocks, Jurassic granitic intrusions,
and Eocene basalts are exposed along the North Fork of the John Day
River at its confluence with Granite Creek. Geochemical and textural
evidence suggest greenschist-metamorphosed, strongly sheared,
volcanogenic rocks originated in an island-arc environment. These
greenstones were apparently intruded during the Late Permian by a
silicic pluton that is similarly metamorphosed and brecciated. South
of this arc terrane, tectonically disrupted ophiolitic rocks are
exposed. This east-west-trending belt of melange contains blocks of
chert, metagabbro and metabasalt in a serpentinite matrix.
Titanaugite indicates the original basalt may have been alkalic.
Paleozoic or Triassic Elkhorn Ridge Argillite underlies much of the
thesis area and consists mostly of contorted chert and argillite.
Graywackes, greenstones and limestones are intercalated with Elkhorn
Ridge Argillite. Regional metamorphism is lower greenschist facies.
Two relatively fresh granitic stocks may be satellites of the
Upper Jurassic Bald Mountain batholith exposed nine kilometers to the
east. An intrusive sequence ranging from mafic quartz diorite to
granite comprises the larger stock, exposed along Granite Creek. This
pluton contains mostly quartz diorite and tonalite. A 0.5 kilometer
wide stock of porphyritic tonalite intrudes argillite on the north
side of the North Fork, John Day River canyon. Mineral assemblages in
the contact metamorphic aureoles around the two stocks are
characteristic of hornblende hornfels facies.
Tertiary dark gray basalt overlies the Mesozoic and Paleozoic
rocks at a profound unconformity. Geochemistry suggests the
olivine-bearing, vesicular basalt is equivalent to the Clarno
Formation exposed farther to the west.