The Greenhorn Mountains contain a tectonic ally disrupted
ophiolite and both arc-derived and pelagic sediments. Age of major
sedimentary units within and bordering the thesis area is Early
Permian, based upon dates of conodonts and fusulinids from contemporaneous
but allocthanous limestones. Sediments near the south
boundary of the thesis area previously considered Triassic to Jurassic
are re-assigned to the upper-Early Permian Metamorphic grade
varies in metasediments from prehnite-pumpellyite in the southern,
proximal arc sediments to incipient (disequilibrium) upper greenschist
facies in the pelagic series. Pillow lavas associated with Elkhorn
Ridge Argillite of the thesis area are only partly spilitized and have
original alkalic affinities, probably representing a seamount.
High pressure, incipient blueschist metamorphism which retrograded
with time to lower greenschist facies is recognized at Bennett
Creek, and is correlated with the Mine Ridge Schist. Microprobe analyses of amphiboles and other phases from Bennett Creek, Mine
Ridge Schist near Hereford, Oregon, and the lawsonite blueschist of
Mitchell, Oregon indicate that in all units pressure decreased and
temperature increased with time. Bennett Creek and Mine Ridge
Schist metamorphism may be associated with tectonic overpressures
followed by burial and increased temperatures with approach of the
North American plate, or may be related to subduction. The Mitchell
blueschist appears to be a product of subduction.
Metagabbro and alpine peridotites of the ophiolite are structurally
and lithologically equivalent to the Canyon Mountain Complex,
and were tectonically emplaced during Late Triassic time. Late
Jurassic intrusives similar to the Bald Mountain batholith range from
norite to two-mica granodiorite. Seven discrete intrusions were recognized.
Eocene olivine basalt and basaltic andesite exposed in the thesis
area are alkalic in affinity and are equivalents of the Clarno Formation.
Overlying andesites and dacites are probably Oligocene and
may represent the equivalent of John Day volcanism. The Columbia
River group consists of two Picture Gorge flows in the western-most
portions of the thesis area.
Extensive alpine glaciation during the Pleistocene incised deep,
U-shaped valleys in the western thesis area. Morainal deposits are
common at lower elevations in the area. Deposits of Mazama ash
up to six feet in thickness occur along streams of the area.