Lecture 37 (Friday, April 26, 2002): Geology of the Western US
Brief outline of this lecture:
Overview of the Western US
Timing of Rocky Mountain uplift
Structure of the Basin and Range
Main points of this lecture:
The western US can be divided into eight geologic provinces, almost all of which can be related to Mesozoic and Cenozoic plate-tectonic convergence and subduction.
The Western Interior (roughly the topographic Great Plains) is underlain by Cretaceous marine sedimentary rocks relfecting high sea level and Tertiary terrestrial sediments shed off the Rockies.
The Rockies and Black Hills consist of bodies of rock uplifted and deformed most recently in the Tertiary. The Rockies actually consist of several different mountain ranges.
The Colorado Plateau consists of flat-lying Phanerozoic sedimentary rocks that have been uplifted but otherwise not deformed in the building of the Rockies.
Much of the Pacific northwest consists of volcanic rocks, either volcanoes in the Cascades or flood basalts in the Columbia Plateau and Snake River Plain. Yellowstone is the eastern limit of the latter.
The Basin and Range has undergone extension and so consists of mountain ranges and sediment-filled valleys, where the valleys have been dropped by normal faults.
The Sierra Nevada mountains are mostly granitic rocks formed over the subduction zone in the Jurassic to Cretaceous.
The Pacific Coast Ranges are a series of small mountain ranges generated by convergence and subduction.
Calfornia's Central Valley lies between the Sierra Nevadas and the Coast Ranges.
Figures and slides used in this lecture:
A very simple geologic map of the western US, with labels
A very simple geologic map of the continental US, without labels
The Black Hills and Bighorns
A geologic model for the Black Hills
Hogbacks and the Front Range of the Rockies west of Denver, Colorado.
Simplified geology of the Basin and Range
Mount Hood, Mount Jefferson, etc. in the Cascades - a landscape view looking south.
The Cascade volcanoes in a set of MODIS images.
The Sierra Nevadas
Flood basalt on Snake River Plain east of Craters of the Moon National Monument, Idaho.
Sequence of flood basalts exposed in Snake River cut at Twin Falls, Idaho.
Reading assignment: "Raising the Rockies" on the USGS's Rocky Mountain System webpage. .
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UGA Geology Department web page