Red Butte Erosion: An Enormous Amount of Erosion

Figure 5.1 The enormous drainage basin of the Colorado River includes parts of seven states plus Mexico.

Our attention is drawn first to the colossal quantity of material which has been removed. Figure 5.1 shows the entire drainage basin of the Colorado River. [1] It comprises an area of one quarter million square miles. Sedimentary strata, the major rocks forming the surface of the broad area known as the Colorado Plateau, have been deeply incised, destroying the original continuity of the strata. In Grand Canyon, we see the breached remnants of once-continuous strata. A simple calculation of the volume of the Canyon shows that almost 1,000 cubic miles (4,000 cubic kilometers) of sediment have been removed from northern Arizona to produce just the topographic form of the Canyon itself.

But this is not all the erosion. Beside the road, just 16 miles south of Grand Canyon Village, rises Red Butte, a prominent conical hill standing 1,000 feet above the present surface of the Coconino Plateau (see Figure 5.2). Red Butte is composed of shale of the Moenkopi Formation overlain by Shinarump Conglomerate of the Chinle Formation (the same formations outcropping in southern Utah).[2]

Figure 5.2 Red Butte, just south of Grand Canyon. (Photo by Steven A. Austin.)

The very top of the butte is volcanic rock from an ancient lava flow. This small butte stands on top of the Kaibab Limestone, which forms the present surface of the Coconino Plateau. Figure 5.3 is a geologic cross section through Red Butte, showing the small amount of Moenkopi and Chinle formations composing the butte and the connection of these formations with the outcrops in southern Utah. The top of Red Butte is capped by a lava flow which has protected the underlying shale and conglomerate from erosion. We might ask how a lava flow could cover a butte, since lava does not usually flow over hills but around them. The answer is found by postulating that the lava flowed over a vast plain that existed 1,000 feet above the present south rim of the Canyon, and that the Moenkopi and Chinle formations covered the entire surface of the present Coconino Plateau and Kaibab Plateau above the Kaibab Limestone! This distribution of Moenkopi and Chinle is sketched in figure 5.3. Red Butte is simply interpreted as an erosional remnant providing evidence of broad, sheetlike erosion of the Coconino Plateau.

Figure 5.3 Geologic cross section along a north-south line from southern Utah to northern Arizona. The extensive erosion of the plateau is indicated by the continuity of the Moenkopi and Chinle formations, which are projected from Utah over Grand Canyon to Red Butte.

This plateau appears to have been buried even deeper than the 1,000 feet indicated by Red Butte. There is evidence above the Moenkopi and Chinle formations, which have now been eroded off the south rim, that the Glen Canyon Group (Navajo Sandstone, Kayenta Formation, Moenave Formation, and Windgate Sandstone)—another 2,000 feet of strata—were present, as well. Our minds are staggered in the attempt to imagine not just 1,000 cubic miles of canyon erosion, but many times that volume, indicated by thousands of feet of erosion off the plateaus which surround the Canyon.

References
1. C. R. Longwell, "How Old is the Colorado River?" American Journal of Science 244 (1946): 817 - 835. Return to Text

2. R. J. Rice, "The Canyon Conundrum" Geographical Magazine 55 (1983): 288 - 291. Return to Text

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