Tuesday, May 27, 2014

The Grand Mesa Colorado Mudslide Beneath Sugarloaf Peak - Updated June 23

UPDATE - On June 20, NASA's Earth Observatory web site showed before and after pictures from space of the Colbran, CO landslide. They are excellent views of the area and you can look at the images here. As mentioned briefly below, there curiously is a gas well located at the head of the slide. No one seems to be talking about this. Anyway, I mention the NASA site for two reasons, to see the before and after shots but also to highlight that on that site there is a link to my posting. It is the highlighted text in paragraph 3, telling readers to check out "these diagrams." Glad to know the folks at NASA EO are watching Earthly-Musings!   

A massive landslide on the north slope of Grand Mesa, Colorado let loose on May 25 and three people are missing and feared dead. You can view aerial photos of the earth failure here. The images show the massive amount of earth that let loose and slid down the hillslope.

The landslide appears to have let loose beneath Sugarloaf Peak at an elevation of 9,400 feet. It "toes out" at about 7,400 feet (yep - landslides have toes). The map coordinates are N39.19078° W107.85518°. A view of the topography of this area can be viewed on Topoquest here. The road and aqueduct that were disrupted can also be seen on the map. Thanks to Chuck L. for this information.

It appears from Google Earth images that a previous slide had occurred on this slope. This slide is in the geologic past but most of the slides likely were active in the late Pleistocene Period, during the last Ice Age when conditions were much wetter here. There is some chatter on the web about the influence that fracking may have had on this, however it is apparent that slippage has occurred here long before fracking. Time will tell if this was a contributing cause or not.

The earth movement appears to be of a distinctive type called a Toreva landslide. These were first described in 1937 and named after the small Hopi Indian village of Toreva. The original article that describes these features can be accessed here. It makes for fascinating reading. See a complete treatment of Toreva blocks on Conor Watkins and J. David Rogers web site here.

The slide likely originated in the Green River and/or Wasatch formations, two of the younger rock units of the northern Colorado Plateau. These deposits are around 55 to 50 million years old and record deposition in ancient lakes and rivers. Here is a map to show the ancient setting of the deposit.

Image from "Ancient Landscapes of the Colorado Plateau" by Ron Blakey and Wayne Ranney
The soft mudstones, claystones, and marls that are responsible for the earth fasilure were laid down along the shores of this ancient lake.

The landslide mass is approximately two miles long and 3/4's of mile wide near the top, thinning to about 1/2 mile wide in the middle. It covers about 700 acres. Jonathan White of the Colorado School of Mines was quoted on May 27 that a concern is that water is accumulating on top of the back-rotated block near the head scarp of the slide. This ponded water could further destabilize the huge mass of rock that has already slipped about 100 feet down.

Dan M. of Washington also sent me some really good geologic information on this particular area. You can access a USGS report on landslide studies of Grand Mesa here. Within the report are these two diagrams, showing how these features develop.

From Baum and Odum, 1996


From Baum and Odum, 1996


5 comments:

Dan McShane said...

Thanks for the link. The Unita Shale and some other young weak rock seems to be a problem. The adjoining Grand Junction map has most of the slope below the Mesa mapped as landslides.

John Hodge said...

"As mentioned briefly below, there curiously is a gas well located at the head of the slide. No one seems to be talking about this."

The gas well is actually located at the toe of the slide. Since drilling in this area is into the Williams Fork or Isles Formations, it's extremely unlikely that drilling had anything to do with the slide.

The size of the slide is unusual, but landslides around the Grand Mesa are very common. From my home in Grand Junction I can look up and see several scarps on the west side of the Grand Mesa.

(The comments process is not being the most cooperative so I may have posted a comment regarding this already; if so, I apologize.)

Joe in Durango said...

Just curious, when it initially broke loose, was it like the geologic equivalent of a deep slab avalanche, that is, even as the upper part was beginning to slide, did the whole thing move as a unit, did the path lower down simultaneously break apart due to some kind of liquefaction, sucking the three men in where they stood, or did they get overrun by debris rushing down from above?

Thanks - Joe

John Hodge said...

In response to Joe in Durango:
Since my previous comments, which I posted quite some time ago, the Grand Junction Geological Society had a special meeting with multiple guest speakers who described what is known about the slide so far. I've had the special, and somber, opportunity to visit and actually stand upon the slide, courtesy of Colorado Mesa University and the property owners. So whereas there's more to discover, and nothing is yet published to my knowledge, this is what I recall as best I can:
The slide is best described as a rock avalanche. Slides are very common in the areas surrounding the Grand Mesa, but the size and extent of the slide are very unusual; there are no other slides of that extent in the area.

The material was dominantly or exclusively old slide material, which itself was derived from Green River Formation; in fact, all of the slide material that I saw near the toe was all derived from Green River Fm. The failure surface appears to have been a previous slide failure surface, so it could be described as essentially a reactivation.

Based on seismology data, the slide occurred in a total of at least four pulses that were mere seconds apart (or less), and took about two minutes for the whole thing to take place. The idea that I took away was that the lower portion of an older slide, which had been creeping for some time, failed, thus removing support for portions higher up, so in a sort in a sort of domino effect, waves of material collapsed.

As for the slide victims, they were probably overrun by a slide moving at over 100kph. They never stood a chance.

Although snow melt and water from recent rains had saturated the upper part of the slide, the bulk of the slide was dry and no mudflows emanated from the toe of the slide; in fact, to this day, water does not drain from the slide. Water is still being infiltrating the slide at the top but the slide is absorbing this water (although small ponds are appearing on the slide).

This has caused some concern since such saturation could cause a reactivation of portions of the slide, particularly in the upper reaches. However, it's also possible that the slide will merely creep. It's very unlikely that the toe will propagate much further since the slope angle at the toes is quite shallow; the slide made it as far as it did downslope by virtue of its momentum. Still, property owners below the toe have not been allowed to reoccupy their homes.

Looks for reports to be published from the Colorado Geological Survey, the USGS, and School of Mines on various aspects of this slide.

Wayne Ranney said...

Many thanks to John Hodge for this excellent update and evaluation of the Grand Mesa rock avalanche of May 25, 2014.