Geology Reference
In-Depth Information
flying reptiles or Pterosaurs. No dinosaur remains have
been recovered in the state even though there are
abundant dinosaur fossils in coastal plain or shallow
marine sediments from Montana, Utah, Colorado, and
Wyoming. Because similar coastal marine environments
have been identified at several locations across Oregon,
it seems to be only a matter of time before diligent
fossil hunting will unearth dinosaur remains. Once the
large terrane blocks had been annexed to the mainland
by the end of the Cretaceous period, the sea had
withdrawn to the west, and erosion began to attack the
newly arrived rocks.
Cenozoic
With the beginning of the Cenozoic era, 65
million years ago, the Blue Mountains area of Oregon
was above sea level undergoing uplift and subsequent
erosion as well as ongoing, but intermittent, volcanic
activity. The Cascades and Coast Range had yet to
form, and the ocean shoreline lay east of where the
Cascades are today. Because Paleocene rocks are not
known from the Blue Mountains, it is assumed that
this epoch was mostly one of erosion. Tertiary volca-
noes of the Clarno and John Day periods as well as
flows of the later Columbia River group covered the
region with widespread, thick deposits of lava and ash
interspersed with fossiliferous sediments containing
remains of plants and animals.
Convergence, collision, and translation model for
aquisition of the Wrangellia terrane by North Ameri-
ca. The North American plate, moving in a northwest-
erly direction, captures and sweeps up the Wrange-
llia terrane prior to translating it along faults to Alaska
(modified after Moore, 1991)
North America. After this final stage of convergence,
deposits of the later Tertiary, which covered the region,
are more or less homogeneous from east to west. The
same formations are found in the same sequence over
the area. Following the tectonic activity of the middle
Mesozoic, shallow Cretaceous seas spread over wide
expanses of Oregon. The ancient shoreline during this
time was a meandering diagonal line from the Blue
Mountains southeast into Idaho. Divers molluscs living
in the Cretaceous seas included many species of coiled,
shelled cephalopods or ammonites which would eventu-
ally prove useful in age dating these rocks. Other
inhabitants preserved in rocks of the Cretaceous ocean
in central Oregon were the fish-like ichthyosaurs and
Distribution of Clarno Formation volcanics, mudflows,
and vents in eastern Oregon (after Walker and
Robinson, 1990)
