Conor Watkins And J. David Rogers
Grand Canyon Research
Landsliding In The Vicinity of 205-Mile Canyon

Several megalandslides in the vicinity of 205-Mile Canyon are among the largest and least studied landslide events in Grand Canyon.  Although the large landslide on river right is obvious, a much larger landslide block is located just downstream of 205-Mile Canyon, on river left.  The immensity of this block makes is hard to recognize at river level.  The Granite Park Fault, a west-dipping lystric normal fault, runs through this area and may be related to the landsliding.

During March 2005 we conducted a reconnaissance of the area while participating in a research trip led by Dr. Robert Webb of the USGS in Tucson, AZ.  During the trip, the two largest slides in the vicinity of 205-Mile Canyon were investigated.

The 205-Mile River Right Landslide is easily discerned from river level.  The Colorado River channel curves around the toe of this landslide, suggesting that the slide once blocked the river.  If an old river channel is present under the landslide, it has been obscured by secondary landsliding and talus.

View of the 205-Mile River Right Slide taken from the opposite side of the canyon atop the 205-Mile River Left Landslide.  Secondary retrogressive slumps line the right bank of the channel, where the toe of the main slide mass is being undercut by the Colorado River.  Talus cones along the headscarp are fresh, indicating the scarp is still raveling.

Talus cones along the headscarp of the 205-Mile River Right Landslide appear young, suggesting that the scarp is continuing to ravel.

Much of the 205-Mile River Right Landslide consists of brecciated Redwall Limestone.  This texture is common to other landslides in Grand Canyon, including the much-studied Deer Creek Slide.

The massive 205-Mile River Left Landslide is best viewed from within 205-Mile Canyon, not from the river.  It consists of a single back-rotated block with several secondary failures along its toe.  This is the largest single landslide block identified in Grand Canyon to date.  Rough estimates indicate a volume of about 2 billion cubic meters at the time of failure.  Erosion has dissected the slide mass, removing much of the material.  The scarp of this landslide parallels the strike of the Granite Park Fault, suggesting the fault likely played a significant role in triggering the enormous slide.

This view of the 205-Mile River Left Landslide was taken from atop the 205 Mile River Right Landslide.  This enormous landslide block spans more than two miles and is outlined here in red.

Secondary slumps along the front of the 205-Mile River Left Landslide are clearly visible from the river.  Without further investigation, it is difficult to discern the much larger parent landslide block.

T he large secondary slump off the 205-Mile River Left Slide is fairly easy to discern from atop the River Right Slide.

Another secondary slump around RM 207 has calved off the front of the parent landslide block.  This view was observed from atop the 205 Mile River Right Landslide.

The toe of the 205 Mile River Left landslide is experiencing continual regression along the river channel.  The oxidized color of the parent slide debris suggest an old age, likely Pleistocene.

This view looking up the headscarp of the 205-Mile River Left Landslide shows the large back-rotated block (right) against its parent cliff (at left).  This scarp closely follows the strike of the Granite Park Fault.

The Granite Park Fault and a zone of clayey gouge is clearly exposed in 205-Mile Canyon.  Faults usually create low permeability zones with low shear strength, promoting secondary displacements that occur in response to surface processes and loading.  Visually, this gouge resembles the Bright Angel Shale, but further analysis will be required to verify this observation.

A small thrust has been preserved in a large Muav boulder in the narrows of 205-Mile Canyon, just downstream (west) of the Granite Park Fault.

The Granite Park Fault and/or the 205-Mile River Left Landslide appears to have altered the course of 205-Mile Canyon.  The canyon has carved a striking narrows through the Muav Limestone on the downdropped (west) side of the fault, but is much wider on the uplifted (east) side developed in the more resistant Tapeats Sandstone.  This suggests the Muav Narrows are a geologically young feature.  This side canyon also follows the Granite Park Fault until the narrows, where it turns abruptly to the west.  Some reddish sediments are preserved upstream of the narrows.  Most of these appear to be debris flow related, but some fine lenses may represent lacustrine deposition that were collected behind a landslide dam.

These Muav Narrows near the mouth of 205-Mile Canyon are a geologically young formation and seem to indicate that the course of the canyon has been altered by the faulting and/or landslippage.

205-Mile Canyon opens up and becomes much wider east of the Granite Park Fault.  This part of the canyon is developed in more resistant Tapeats Sandstone, so it is likely much older than the Muav Narrows.

Since the formation of the Muav Narrows in 205-Mile Canyon, displacement on the Granite Park Fault has continued.  The mouth of 205-Mile Canyon appears aggraded and choked with sediment to an unknown depth.

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