The Black Canyon of the Gunnison: Evidence of the creation of Colorado Located on the North American Plate, the basement rock of the canyon provides evidence that Colorado has oceanic crust; ~1.7 Ga, plate tectonics brought an oceanic volcanic arc to collide with the continental shores of Wyoming.
The heat- and pressured-tempered offspring of that union are metamorphic Gneiss and Schist interlaced with pinkish igneous pegmatite dikes of muscovite, potassium feldspar and quartz minerals (see photo- Painted Wall, Black Cayon of the Gunnison, Colorado -right).
Between the Entrada Sandstone and Precambrian rocks exists a Great Unconformity where millions of years of geologic history have vanished.
How the Black Canyon came to be:
The Gunnison basin was uplifted (72 Ma) and later covered with ~4,000 feet of debris from explosive, basaltic volcanism from ranges in the area; cinder and ash fall along with ash and mud flow built up the West Elks and forced the stream between the two uplifted areas. The rushing water of the Gunnison river had no choice but to cut over the hard-to-erode Precambrian basement.
A combination of a steep river gradient, glacial melt and debris, river erosion, mass wasting, impressive seasonal flooding, and no other choice led to the glorious steep Black Canyon of the Gunnison.
References:
To find a 1976 map of known faults in Colorado visit: http://ngmdb.usgs.gov/Prodesc/proddesc_15251.htm
Other:
http://www.cliffshade.com/colorado/black_canyon/
http://www.cliffshade.com/colorado/tectonics.htm
http://www.nature.nps.gov/geology/parks/blca/index.cfm
No comments:
Post a Comment