A very large landslide that translated into a very very big mud flow took place in western Colorado last Sunday. Based on news report images and a helicopter flyover footage, I figured out the location and did some old report reading and geology searching. The fly-over is below:
I very roughly estimated the extent of the landslide on the following GoogleEarth image.
Perhaps the most remarkable thing about this slide is that a portion of the mud flow flowed up and over a ridge that was on the order of 200 feet above the base of the valley.
An oblique view of the headwall of the slide location shows that there was a classic down dropped block.
The image also shows some shallow surface failures or erosion on the block down slope side of the block. The landslide images appear to indicate that pre existing block moved again, perhaps dropping 200 to 300 feet and possibly expansion to the sides.
Baum and Odom (1996) provide a description of geology units and mechanisms of similar slides along the outer perimeter of Grand Mesa, the high (10,000-foot) plateau in western Colorado where the slide took place. A geology map (http://ngmdb.usgs.gov/Prodesc/proddesc_9117.htm) of the area to west of this site showing the perimeter of the Grand Mesa covered with large areas mapped as landslides. (If you look at the map, the Grand Mesa is on the southwest corner of the map.) In addition to the deep-seated failure mechanism in the underlying clay formations, this is an area that was glaciated - the Grand Mesa is over 10,000 feet.
Wayne Ranney has some great links on his blog of pictures and a further broader and interesting discussion of these types of landslides with links to the papers.
The slide appears to fit in well with the mechanisms discussed by Baum and Odom as a large intact rotational block of coherent bedrock over very soft Green River and Wasatch Formations of silts and clays. I have done some work in the Green River Formation clays and they are really a messy formation to try to walk around on when wet. There is a very strong translational component to these slides post block failure and seeing the extent of the mud flow perhaps a bit easier to understand why these slides are so extensive and rotated blocks end up so far from the rime area of the mesa.
While this area has been mapped as consisting of landslide deposits over broad areas, the Baum and Odom paper suggests much of the sliding is very old. This slide might be one of those low frequency but very large events that will provide the math inclined lots of opportunity to study run out distances.
I'm not sure we have anything quite like these types of landslides in Washington State. The closest might be the weak clay units in the Ringold Formation in south central Washington's Franklin County or failures within sub basalt units underlying the Columbia River Flood Basalt. Deep rotational slides that then turn into translational slides - future posts.