LANDSLIDING AND CHANNEL DIVERSION AT FISHTAIL CANYON, GRAND CANYON, ARIZONA

Fishtail Canyon is a tributary of the Colorado River in the Grand Canyon. Recent field work suggests it was blocked by a previously unmapped bedrock landslide, likely during the late Pleistocene or Holocene. The landslide slid from the west side of Fishtail Canyon on a log-spiral shaped failure surface developed in the Bright Angel Shale. The infilled channel of Fishtail Creek is well-preserved, between 0 and 45 m above the present bed. The creek is incising a new channel around the blockage between 0 and 150 m east of its former position. A thick sequence of well-indurated lacustrine sediments intermixed with debris flow deposits is preserved upstream of the slide blockage. The Fishtail Creek channel profile is a classic example of a landslide dam, with a steep gradient through the slide debris and an anomalously low gradient upstream of the blockage.

The Fishtail Slide exhibits many characteristics typical of Toreva block megalandslides previously described in the Grand Canyon, most of which floor in the fissile Bright Angel Shale. Toreva blocks are typically characterized by back-rotated headscarp grabens. Although this graben is highly dissected by erosion, it contains fine-grained sediments which may be lacustrine in origin. Much of the slide is highly brecciated and cemented by travertine. Smaller sliver slumps and extensive talus testify to active regression of the main headscarp. Approximately 610 meters of stratum have been displaced by the slide and initial estimates suggest the volume of the slide is at least 340 million cubic meters.