Geology Reference
In-Depth Information
Each of the terranes which make up the Blue
Mountains contains its own distinctive suite of rocks
and fossils. The combination of these characteristics in
the province represents a wide variety of ancient
environments from marine to nonmarine and from
tropical to temperate. Fossils from the terranes as well
as the orientation of the magnetic minerals in the rocks
suggest that many of these blocks may have formed
well out in the western Pacific Ocean as far south as
Mexico City at 18 degrees north latitude before moving
north and eastward to attach to North America. Today
in the Blue Mountains these exotic terranes are ex-
posed in linear strips that extend in a southwest to
northeast trend.
the forearc basin between the island archipelago and
the trench of the subduction zone, the Baker terrane,
a deep ocean floor crust environment, the Grindstone
terrane from a shallow ocean backarc basin, and the
Wallowa volcanic archipelago terrane.
The Olds Ferry terrane extends northeast by
southwest in a curved alignment of isolated exposures
from Ironside Mountain in Malheur County through
Huntington in Baker County, up to the Cuddy Moun-
tains of western Idaho. Older rocks of this terrane,
designated the Huntington Formation, include a thick
sequence of volcanics and sediments of a type today
associated with volcanic island chains or archipelagos
like those in the north and western Pacific. A variety of
volcanics, pyroclastics, and ash, as well as silica-rich
rhyolites, and lesser amounts of basalt and andesite,
make up the formation. Between the volcanic layers,
fossils from thin marine beds of sandstones, siltstones,
Individual Terranes
Five major terranes have been recognized in
the Blue Mountain province. These are the Olds Ferry
island archipelago of volcanics and sedimentary rocks,
the Izee terrane, an assemblage of layered rocks from
Distribution of major exotic terranes in the Blue Mountains (after Silberling and Jones, 1984)
