Geology Reference
In-Depth Information
Structure and stratigraphy of the eastern flank of the
Wallowa Mountains from the Minam River to the
Snake River (after Smith and Alien, 1941; Walker,
1979)
The extent and volume of the Columbia River
lavas tend to overshadow important Miocene eruptive
provinces that were developing at the same time in the
southern Blue Mountains where Sawtooth Crater,
Strawberry Volcano, and Dry Mountain evolved as
three separate volcanic centers. These centers in turn
line up with a fourth volcano of the same age at
Gearhart Mountain to the southwest in the Basin and
Range province. Lavas from these four volcanic regions
were predominantly a stiff, slowly flowing andesite
variety that accompanies very explosive eruptions.
Vents in the vicinity of Strawberry and Lookout
mountains were the most numerous, extruding the
thickest and most extensive lavas. Covering a total of
1,500 square miles, the Strawberry volcanics are over 1
mile thick at Ironside Mountain in Malheur County.
By late middle Miocene time, the Oregon
oceanic shoreline had retreated far to the west near the
present day coast. Ash from late Miocene and Pliocene
volcanoes in the Cascades fell over an increasingly
cooler temperate landscape where streams and rivers
intensively eroded the older sediments. Volcanism at
this time in the Blue Mountains was limited to small
eruptions of lava along the southern and western
margins of the province as local lavas mixed with
showers of ash from emerging Cascade volcanoes to the
west. Sediments and lava flows from these events were
deposited directly atop the Columbia River basalts. Of
these, the Mascall Formation of volcanic tuffs, carried
and deposited by streams, preserved a variety of middle
Miocene fossils. A rich mammal assemblage of horses,
antelope, camels, deer, and oreodons along with
predators such as dogs, bears, weasels, and racoon
suggest an open woodland or savanah in association
with cool, temperate broadleaf plants dominated by
angiosperms.
Lineation of Miocene andesitic volcanic centers in
eastern Oregon (after Robyn and Hoover, 1982)
