River runners have a different perspective on Grand Canyon geology. Remarkably, the put in at Lee’s Ferry is above the entire Grand Canyon sequence, and all the strata seem to emerge in turn from the river.
The Kaibab Limestone that visitors to both the North and South Rims stand on to admire the canyon emerges from the river at the end of the first mile. The six highest strata, from the Kaibab to the Redwall emerge, follow the river a while, and then disappear to the heights as the canyon deepens. They are never seen at river level again.
The Muav Limestone emerges from the river, disappears to the heights, but comes back down to form the Muav Gorge before climbing back up for the rest of the canyon. The Bright Angel Shale and Tapeats Sandstone both make two complete entrances and exits from the river. The Vishnu Complex of granite intruded bedrock actually makes four complete entrances and exits. The Supergroup is just too weird. It is a mile of sediments ranging from 1.25 billion to 725 million years old that overlies the Vishnu (1.75 to 1.68 billion years old) but suddenly disappears without explanation or further ado. The river encounters only a small part, briefly. The Supergroup will be the subject of a future post.
The 280 miles typically floated through the Grand Canyon basically makes a giant “W”. The first segment from Lee’s Ferry to Unkar, the second from Unkar to Dubendorf Rapid, the flattened middle segment from Dubendorf to Parashant Canyon, a segment from Parashant to Diamond Creek, and a final leg from Diamond Creek to the Grand Wash Cliffs.
In the first graphic we stretched the “W” into a straight line. It can be seen from the graphic that the river seems to cut through at least three ancient mountain ranges founded on elevated portions of Vishnu Complex. The first of these ranges corresponds very closely to the second segment of the “W” from Unkar to Dubendorf. This can be seen as the dark, forested region in the Google Earth image above where the highest rim elevations of the canyon are found. The roads to both the North and South Rims reach the canyon in this area, and it is commonly called the Kaibab uplift.
A second dark forested area can bee seen in Google Earth corresponding to the elevated area where the Kaibab through Hermit strata end, the Hurricane Fault Complex, and the beginning of the Lower Granite Gorge. It seems clear that ancient structures affect the course of the river.
It is peculiar that the ancient substructure telegraphs up through the younger strata. The current thinking is that the Vishnu (and Supergroup) were worn flat before the Cambrian Tapeats was laid down after 540 million years ago. Near Sockdolager Rapid, the Tapeats sits on the Supergroup right next to the Vishnu at the same altitude!
We are thinking that the recent uplift of the Colorado Plateau lifted the Vishnu cores of the ancient mountains more than other areas. Just another Grand Canyon mystery we will be exploring before returning to Radiative Altitudes.
Geological maps used for elevation Data:
Billingsley, G.H., and Priest, S.S., 2010, Geologic map of the House Rock Valley area, Coconino County, northern Arizona: U.S. Geological Survey, Scientific Investigations Map SIM-3108, scale 1:24,000.
- Title: Geologic history and paleogeography of Paleozoic and early Mesozoic sedimentary rocks, eastern Grand Canyon, Arizona
- Author(s): Blakey, R.C., and Middleton, L.T.
- Publishing Organization: Geological Society of America
- Series and Number: Special Paper 489, p. 81
Huntoon, P.W., Billingsley, G.H., Sears, J.W., Ilg, B.R., Karlstrom, K.E., Williams, M.L., and Hawkins, David, 1996, Geologic map of the eastern part of the Grand Canyon National Park, Arizona: Grand Canyon Association, , scale 1:62,500
Billingsley, G.H., and Huntoon, P.W., 1983, Geologic map of Vulcan’s Throne and vicinity, western Grand Canyon, Arizona: Grand Canyon Association, , scale 1:48,000
Billingsley, G.H., Clark, M.D., and Huntoon, P.W., 1981, Geologic map of the Hurricane fault zone and vicinity, western Grand Canyon, Arizona: Grand Canyon Association, , scale 1:48,000