During May-June 2004 we conducted a detailed reconnaissance of Fishtail Canyon, where a large deposit of Quaternary terrace gravels had been mapped previously (see geologic map below).  Field work revealed this is actually a large bedrock landslide block, which slid on the Bright Angel Shale, blocking Fishtail Creek. This enormous block appears much younger than those recognized in the Surprise Valley area a few kilometers upstream.  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.

Portion of the Fishtail Mesa 7.5' Quadrangle showing landslides near the mouth of Fishtail Canyon.  Note smaller slides emanating from both sides of the side canyon.  The stream profile is diagnostic of a landslide dam, with below normal gradient upstream of the blockage and acute gradient through the obstruction.  Well-indurated lacustrine sediments are preserved upstream of the largest block in lower Fishtail Canyon.

Channel profile of lower Fishtail Creek exhibits a classic choke point formed by the landslide dam that diverts the channel.

Portion of "Geologic Map of the Grand Canyon 30' x 60' Quadrangle, Coconino and Mohave Counties, Northwestern Arizona" published by the USGS in 2000.  This map inherited previous interpretations which assumed the lower Fishtail and Cranberry areas were blanketed by Quaternary terrace deposits. We have observed terrace gravels east of the mouth of Cranberry Canyon well above the present channel, and along the lower portion of this landslide. Billingsley (2005) believes that these gravels were deposited during an outwash flood caused by the failure of the landslide dam at Deer Creek or river mile 235 on Cogswell Butte, indicating that the Fishtail Landslide predates whichever event placed the cobbles.


Main Fishtail Landslide block as it appears from mouth of Fishtail Canyon along the Colorado River.  This jumble of disaggregated debris appears similar to the Deer Creek Landslide.

The mouth of Fishtail Canyon is incised in the Bright Angel Shale, the unit normally associated with fomenting large scale landslippage in the Grand Canyon.  The channel profile and deflected channel plan are diagnostic of a breached landslide dam.  The red line indicates the approximate position of the pre-slide channel, within 45 m of the present bed level.

This mouth of Fishtail Canyon is deeply incised in the fissile Bright Angel Shale, testifying to rapid entrenchment by the diverted overflow channel.

Looking downstream at the Fishtail Landslide debris infilling the pre-slide channel of Fishtail Creek. The channel has been diverted, creating a narrow gorge through the Bright Angel Shale, between 0 and 150 m east of the old channel.  The similarities between the mouth of Fishtail and the Deer Creek Narrows are striking.

Fishtail Canyon opens up upstream of the slide block, but is mantled by what appear to be retrogressive sliver slumps, which may have been triggered by rapid drawdown when the main landslide dam was breached.  Prior to breachment the side canyon appears to have been infilled with colluvial talus and lacustrine sediments mixed with debris deposits. This shows the chaotic jumble of house-sized blocks littering the channel of Fishtail Creek, making passage difficult and clumbersome. 

   
These indurated lacustrine sediments are present upstream from the main slide block in Fishtail Canyon.  Unlike the extensive talus/colluviual sediments, these materials were deposited with near-horizontal inclination, likely in some sort of impoundment. They exhibit alternating fine and coarse grained materials, suggesting cyclic deposition during high and low energy events.  The boulders choking Fishtail Canyon in the background are the size of small houses.

View looking upstream from summit of the Fishtail Landslide block, looking east towards the Deer Creek Slide Complex.  We created a photorealistic render of a similar scene using a 10 m digital elevation model (DEM) with the software package Terragen.

A zoomed in view of the Deer Creek Slide from the top of the Fishtail Landslide.

A similar view of the Deer Creek Landslide created using Terragen, a photorealistic scene generator that uses digital elevation models (DEMs) to render scenes.  Note the similarities between the prototype scene and the artificial rendering.  This image was generated using a 10 meter DEM.  Although the resolution is only 10 m, renderings can be useful forensic tools to ascertain where old photographs were imaged.  Images depicting the Martian landscape using Terragen appeared on the cover of the January 2004 National Geographic Magazine.

Another view from the summit of the Fishtail Landslide block looking NNE into the landslide graben, with Fishtail Canyon visible at upper right.  Thick talus fans obscure the headscarp evacuation scars.

The landslide evacuation scarps are punctuated by secondary sliver slumps, which are mantled by talus.  This secondary slump is barely recognizable beneath the talus.  Similar secondary slides are well documented in Surprise Valley, but also typify the Deer Creek Landslide Complex. 

Sediment filled depression in remnant of headscarp graben of the Fishtail Landslide, looking SW. Erosion has reduced the graben to a silty-filled saddle between the main slide block and the talus covered scarp.  This graben likely formed a closed depression that ponded water and sediment before dissection.  These fine grained materials may be of lacustrine origin, but have not been sampled yet.

Beds of indurated lacustrine sediment deposited in lake formed by the Fishtail Landslide Dam, NNE of the landslide graben.  These are located about 45 to 60 m above the floor of Fishtail Creek.

Detail view of the alternating bands of coarse and fine materials preserved in these indurated sediments.   The fine grained nature of these interbeds is indicative of lacustrine deposition.

Lacustrine sediments were collected from this indurated lacustrine unit for palynology assessment.  Fine grained sediments were sampled near the basal contact with the underlying slide breccia.

A large capstone which fell from the landslide headscarp protected the underlying lacustrine sediments from post breakout flood erosion.

Graphic contact between the Fishtail Landslide block and the proto Fishtail Creek channel, excavated in the Bright Angel Shale. This view is looking downstream, towards the Colorado River. The axis of the proto channel bed appears zero to 150 m west of the present channel, which was presumably excavated during a breakout flood via overtopping.  

A closer view of the buried channel showing the striking contact with the Bright Angel Shale.  Note the brecciated character of the slide fill.  Some channel cobbles are preserved beneath the slide debris.  

Ground view of the chaotic blocks choking Fishtail Creek upstream of the main slide block.  The house-size blocks are comprised of travertine-cemeted slide breccia and intact blocks of Muav and Redwall Limestone.

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Questions or comments on this page?
E-mail Dr. J David Rogers at rogersda@umr.edu
or Conor Watkins at cwatkin@umr.edu.