Geology Reference
In-Depth Information
The Columbia River Basalt Group develops its
maximum thickness of nearly 15,000 feet under
the Columbia Plateau near Yakima, Washington
(after Reidel and Hooper, 1989)
unable to push through the mound of hardened, rough
rock along the edges, comes to a stop, essentially
dammed up by its own cooled crust.
The relationship between the Columbia-Desch-
utes Plateau and the surrounding provinces is unclear
because thick lavas obscure the deep crust which has
not been well-studied. Crustal thickness in this province
has been estimated between 15 to 20 miles. In the
1980s, a series of four wildcat wells were drilled
through the lavas in the vicinity of Yakima, Washing-
ton, by Shell Oil Company. Targeted were Cretaceous
and Tertiary rocks lying below the basalt along older
anticlinal structures that were judged to have sufficient
organic content and porosity to produce gas. Although
the potential reservoir rocks were dry, rocks of the
predicted formations were encountered by drilling deep
beneath the basalt.
Of the four main formations making up the
Columbia River lavas, the oldest is the Imnaha Basalt,
followed by the Grande Ronde Basalt, the Wanapum
Basalt, and finally the youngest Saddle Mountains
Basalt, with the Grande Ronde composing more than
85% of the formations by volume. The Grande Ronde,
Wanapum, and Saddle Mountains were formerly
lumped together and designated the Yakima Basalts,
but they are now distinguished by subtle differences in
mineralogy and texture. Variations in magnesium oxide,
titanium oxide, and phosphorus are also used to
characterize individual flows.
One model for the movement of lava in an Aa
flow
