Geology Reference
In-Depth Information
Because of the relative youth and thickness of
the Columbia River basalts, rocks immediately below
the Miocene lavas are still poorly known. In southeast-
ern Washington small exposures of displaced rocks of
the Wallowa terrane imply that this terrane extends far
north of its main exposures in the Blue Mountains.
Similarly, inliers of the basalt-covered Baker terrane
occur well to the west of major exposures in the Blue
Mountains. The extent and thickness of Cretaceous
rocks as well as Tertiary Clarno and John Day forma-
tions, where they disappear under the southern edge of
the plateau lavas, may be an indication that they too
are much more widespread than presently known.
Distinctive rock fragments found imbedded in flows of
the Deschutes Formation may have been picked up by
the lava as it worked its way to the surface. Although
this evidence is tenuous, it suggests that accreted
terrane rocks, as old as preCambrian, may be buried
beneath the Cascade Range and Deschutes Basin.
Despite the thickness and extent of the lavas,
the plateau is not entirely featureless. Over most of its
western section, low wrinkles and shallow dimples dot
the surface. One of the largest of these is the C-shaped
Deschutes basin reaching into the southwest corner of
the province. During the early Miocene, the basin
received lavas and ash from vents of the Columbia
Plateau and adjacent volcanoes of the Western Cas-
cades. Intermittent volcanic activity sent lavas flooding
over the vast Deschutes basin, disrupting the flow of
streams and rivers. Sediment-choked streams covered
100 square miles of the basin with thin layers of
waterlain tuffs, sandstones, and mudstones of the
Simtustus Formation. Sedimentation was rapid, and the
shallow basin filled with Simtustus deposits up to 250
feet thick. Once designated as part of the Deschutes
Formation, volcanic sediments of the older Simtustus
Formation are lithologically distinct from the coarse
conglomerates of the younger Deschutes. Simtustus
rocks are highly significant as they were laid down
during emplacement of the Columbia River flows 16 to
12 million years ago.
Structural aspects of the Deschutes-Columbia
Plateau (after Smith, Bjornstad, and Fecht, 1989)
