LANDSLIDE DAMS IN TAPEATS CAVE CANYON, GRAND CANYON, ARIZONA

Tapeats Cave Canyon is structurally controlled by the Tapeats Fault, which also controls Tapeats Cave and Spring, the largest spring in Grand Canyon (48 mgd). Recent field work suggests that the Tapeats Spring was blocked by previously unmapped bedrock landslides, likely during the late Pleistocene or Holocene. Rotational slump block slides emanate from both sides of the channel along basal failure surfaces developed in the Bright Angel Shale. The Tapeats Spring branch channel profile suggests at least two recent episodes of landslide dams, characterized by a steepened gradient through the slide debris and an anomalously low gradient upstream of the blockages. The steady flow of water from Tapeats Spring has removed much of the debris and it is possible other dams once blocked the channel. A thick sequence of indurated lacustrine sediments intermixed with debris flow deposits and talus from the canyon wall is intermittently preserved upstream of the slide blockages, with the best example just west of Tapeats Spring. The outlet of Tapeats Spring is also choked with landslide debris. Because of this obstruction, the spring emerges as a series of separate pools and waterfalls coalescing to form the massive flow of Tapeats Spring. The Tapeats Cave Canyon Landslides exhibit many characteristics typical of Toreva block megalandslides previously described in the Grand Canyon, most of which floor in the fissile Bright Angel Shale.