Grand Canyon: Last Theory Standing

The March issue of GSA Today carries a paper on the formation of the Grand Canyon. Papers on that subject have a proud history, and the citations in this one cover three centuries. The big problem it addresses is how the Colorado River managed to get across the high Colorado Plateau to dig the Grand Canyon. There are three theories. Author Joel Pederson claims to have narrowed them down to one.

The paper reviews the three theories of how the Colorado River integrated itself; that is, how it become one stream running from the Rockies all the way to the Gulf of California. It took the combined water power of the whole integrated river to cut the Grand Canyon since 6 million years ago. The upper and lower Colorado have clear and independent histories before 6 Ma, but there is a big plateau between them with high cliffs on its western edge. The three theories are that the ancient upper river (1) ducked around the plateau to the south, (2) ducked around it to the north, or (3) headed straight onto it but ended there in the desert. In "The mystery of the pre–Grand Canyon Colorado River—Results from the Muddy Creek Formation," Pederson cites problems that rule out the first theory, then reports how he tested the second by examining rocks in Nevada that should have been deposited by the Colorado River if it had gone that way. His analysis of the rock's sedimentary sources does not show a strong signal from the Colorado, but rather signs of local sources as in most Basin and Range basins.

Pederson looks at theory 3 more closely. Most rivers that come up against a barrier simply form a lake that spills over the obstacle and cuts a channel there, but the ancient Colorado never had that much water—at least, there are no signs of lake deposits (like mudstone beds or paleo lake shores) in the right place. Nor did the lower Colorado erode headward, uphill, for hundreds of kilometers in a narrow channel. Instead, Pederson points to groundwater. There are thick beds of limestone in the area, which could have absorbed the Colorado's water and fed underground rivers and springs that accomplished what surface waters clearly did not. Signs of them crop out high on the walls of the Grand Canyon's western reach. Makes sense to me. Groundwater does these things elsewhere, including, it's thought, on Mars.

Still, there ought to be some old river sediment on the plateau top somewhere. Pederson suggests, maybe it blew away? Throwing up his arms at this stubborn Grand Canyon problem, he says, "This remains yet another conundrum."

Because the Grand Canyon is one of those things that make me go "hmm," this is my entry for The Accretionary Wedge #6.