Searching through the CFR (Title 14, Part 91) brings up Special Federal Aviation Regulation No. 50-2 - Special Flight Rules in the Vicinity of the Grand Canyon National Park, AZ.
There is also Sub-part U of Part 93.
The Sub-part U regulations seem to be the most current applicable regulations (please correct me if I am wrong).
The rule applies to everything under 18,000 feet MSL within an area around the park (like the Grand Canyon Sectional Chart says).