Most of the Grand Canyon was carved through the fairly soft Coconino sandstone, the 800 odd feet of red rock commonly seen in pictures. Coconino underlays much of Arizona and is easily fissured into layers. I have a piece of it sitting on my mantle, which has begun to split into layers over the years just by way of changing humidity.
I don't know the geology of this river but it does not shock me that a gorge could be cut through sedimentary strata quite rapidly.