Geology Reference
In-Depth Information
Oligocene, and 32 degrees since the middle Miocene
with an average of 1.5 degrees of rotation per 1 million
years throughout the entire 50 million year period of
movement. This second rotational motion was in
response to the extension or extreme stretching of the
Basin and Range province which pushed the Coast
Range block out to its present location. In accordance
with the concept of crustal rifting, small fragments of
the Coast Range, caught between moving plates, simply
rotated like ball bearings.
Eocene
As the seamount terrane was accreted to North
America, a trough or forearc basin, oriented southeast
by northwest and partially bounded by the ancestral
Cascade volcanoes to the east, developed along its axis.
From Eocene through the Miocene the elongate basin
was repeatedly choked with sediments before uplift of
the Coast Range brought about a shallowing and
eventual closure of the trough. Early Eocene sediments
were carried into the new marine basin by streams and
rivers, blanketing the older submarine volcanics with
sands, muds, and silts of the Roseburg, Lookingglass,
and Flournoy formations. Rock particles that make up
these three formations are lithic in nature. That is, they
are composed of pre-existing cherts, metamorphic
rocks, and heavy minerals typical of those found in the
Klamath Mountains.
Isolated pillow basalts are common in the Depoe
Bay basalts (photo courtesy of Oregon Dept. of
Geology and Mineral Industries).
Eocene tectonics and sedimentary environments of
the Oregon Coast range (after Ryberg, 1984; Heller
and Ryberg, 1983)
