AN ALTERNATIVE TO THE MUD CRACK ORIGIN FOR SAND-FILLED CRACKS AT THE BASE OF THE COCONINO SANDSTONE, GRAND CANYON, ARIZONA

Since their first mention in the literature 70 years ago, deep sand-filled polygonal cracks just below the base of the Coconino Sandstone have been interpreted as mud cracks. The contact between the Permian Coconino Sandstone and the Hermit Formation was studied along seven trails on the South Rim and three trails on the North Rim of the Grand Canyon. Other areas were observed with a small telescope. The most notable cracks were widest (up to 25 cm) and deepest (up to 10 m) along the Bright Angel Fault on the South Rim. The fault has some of its greatest vertical displacement (about 62 m) at this location. Along the trails studied in the canyon, cracks were always present near major faults. They became narrower, shallower and were sometimes lacking altogether as horizontal distance from faults increased and vertical displacement along faults decreased. Vertical laminations within the cracks, “U”-shaped cracks, cracks that dissipated upwards, irregular spacing of cracks, widely spaced vertical cracks that turned and connected laterally with each other, and cracks that connected to lateral sand bodies within the Hermit Formation make the mud crack origin suspect. No horizontal layering was ever found within any of the cracks, which might be expected if wind-blown sands had accumulated in open mud cracks of the Hermit Formation. Because of their direct association with faults and because of features which are difficult to explain by mud cracking, the sand-filled cracks might be better explained as clastic dikes which originated during tectonic activity after the deposition of the Coconino Sandstone.