Geology Reference
In-Depth Information
Cascades Stratigraphy (after Priest and Vogt, 1983)
sediments that they built up limestone lenses marking
quantities of the Columbia River Grande Ronde
the old shoreline on the eastern margin of the Willam-
basalts began to flow from fissures near La Grande
ette Valley. By Oligocene time numerous eruptions of
about 17 million years ago. These massive flood basalts
andesitic lavas and siliceous tuffs of the Little Butte
blanketed the landscape filling in depressions to
Formation were interspersed with oceanic sediments
produce an even, flat platform of cooled lavas. The
along the eastern margin of the valley.
eruptions were synchronous in part with volcanic
activity taking place in eastern Oregon in the Strawber-
Western Cascades
ry Mountains as well as in Steens Mountain. Concur-
A period of tilting and folding during the
rently in the Western Cascades violent eruptions from
middle Miocene was followed by renewed outpourings
composite cones active between 13 and 9 million years
of lavas throughout Oregon accompanying the growth
ago comprise the Sardine Formation.The combined
of the Western Cascades volcanic arc. Even with a
amount of all this volcanic material has no modern
large volume of volcanic debris, the growth of the
counterpart. By 7 million years ago the belt of active
range was modest as the accumulation sank almost as
volcanoes had narrowed to an area only slightly wider
fast as it piled up. About the same time voluminous
than the present High Cascade Range.
