If you follow I 17 north a bit, you get to Black canyon city, at the south western end of a river system with much darker rock in it than most of the surrounding area.

Based on the erosion patterns, and the spot height readings, I think I know why. The hills around are significantly higher than the area of Black canyon, and appear to be actively eroding, implying something quite soft.
The area bordering Black canyon appears fairly flat, with much less pronounced drainage patterns, implying it's reasonably hard, supported by the formation of typical mesa type cliffs. This would imply the Black rock is some softer material. The most likely kind of sequence to explain this is marine sediments. A quick look at web references shows Arizona did have a sequence of marine transgressions at about 300 ma

Top:
Poorly cemented ( ie softer) Sandstone
Well cemented ( ie harder) Sandstone
Shale ( organic matter rich mudstone)
Bottom.


It's amazing that we can get this kind of information so quickly and easily, and for free.