Geology Reference
In-Depth Information
Structure and distribution of exotic
accreted terranes in the Blue
Mountains (after Silberling and
Jones, 1984; Vallier and Brooks,
1986).
and limestones have been dated at late Triassic to
middle Jurassic. Significantly, the Olds Ferry terrane
complicated the interpretation of the John Day inlier.
includes volcanics that invaded the formations almost
Imbedded in the western edge of the Izee
as rapidly as they formed, suggesting the marginal
block, but quite distinct from it, is the Grindstone
ocean island archipelago was in very close proximity to
terrane. The smallest of all northeastern Oregon
the volcanic source vents. In contrast to other Blue
terranes, the Grindstone is limited to a narrow area of
Mountain terranes, Olds Ferry terrane rocks show only
eastern Crook County called the Berger Ranch where
minimal folding, which probably took place near the
a mere 200 foot thickness of Devonian limestones,
end of the Jurassic.
cherts, and argillites represent Oregon's oldest rocks.
The Izee terrane, directly to the west of the
Corals and fishtooth-like microfossil remains called
Olds Ferry terrane, is separated by a fault which runs
conodonts from these beds have been dated as lower
between the two. This terrane is exposed in what is
Devonian, 380 million years old. In the same vicinity,
called the John Day inlier, an erosional window cut
the Mississippian Coffee Creek and Pennsylvanian
through younger rocks revealing the older strata below.
Spotted Ridge formations include over 2,500 feet of
Composed of a number of formations, the Izee terrane
limestones, mudstones, sandstones, and cherts. A
represents the environment of a shallow marine forearc
shallow-water tropical invertebrate fauna of corals and
basin between a volcanic island archipelago and the
brachiopods are found in the Coffee Creek Limestone,
oceanic subduction trench. Fossils are abundant in this
but the overlying, lower Pennsylvanian Spotted Ridge
twelve mile thick forearc basin sequence in which the
nonmarine sandstones once supported coastal plain
limestones, mudstones, silts and sandstones, have been
ferns and fern-like foilage similar to those found in the
subdivided into distinct marine environments.
late Paleozoic coals of the central and eastern United
Folding of the Izee terrane began during the
States. Nine hundred feet of fossiliferous Permian age
late Jurassic and gradually intensified due to compres-
Coyote Butte limestones, cherts, and sandstones are the
sional, thrusting events. With the intrusion of lower
uppermost formation of the Grindstone terrane,
Cretaceous granites, rocks of the Izee terrane have
although isolated blocks of radiolarian-bearing cherts
experienced several episodes of folding and refolding.
in the vicinity are as young as lower Triassic. Because
The resultant picture of folds within folds has greatly
of poor exposures and the difficulty of tracing the rock
