West Salt Creek, Colorado, Sunday, May 25, 2014
For photos and discussion of the slide, please see the AGU Landslide Blog post Grand Mesa: an unusually large and mobile mudslide and photos at the Denver Post.
Slide dimensions: 2.7 miles long (straight line distance), 0.5 miles wide (maximum)
Elevation: top of scarp, ~2975 m (9760'); end of toe, 2290 m (7513')
Overview:
A major landslide occurred on Sunday, May 25, along West Salt Creek about 3 miles WSW of Vega Reservoir and 7 miles ESE of Collbran, Colorado. Data from seismic stations at several locations indicates the main event took place at about 5:44 PM, although newspaper reports indicate some small-scale movement may have occurred earlier in the day. Intermittent rain fell in the area throughout the day, and rainfall amounts totaled approximately an inch in the slide area. Three local area men were in the area apparently checking on problems with an irrigation ditch when the slide occurred and have not been located.
Seismic recordings of the slide from two stations in the CMU seismic network indicate the duration of the slide was approximately 2 minutes. The seismic traces are shown here. The closest station is Rapid Creek (RCCO) is near Palisade, Colorado, approximately 24 miles west of the slide area. The trace from that station shows an irregular increase in wave amplitude for about 35 seconds, fluctuating but high amplitude for about 40 seconds, and generally decreasing amplitude for another 40 seconds or so. This pattern may represent initial flow acceleration, maximum velocity, and then deceleration. If material from the upper end of the flow moved the entire distance to the toe, an average minimum flow velocity can be estimated at 72 mph. Parts of the flow may have traveled at significantly higher velocities. For locations of the seismometers, visit the CMU Seismic Network homepage.
Causes:
The West Salt Creek debris flow resulted from a classic combination of steep slopes, weak and unconsolidated regolith, and abundant moisture. The upper end of the flow is a steep area on the north flank of Grand Mesa, and the flow followed an existing drainage (West Salt Creek). The upper end the drainage, where the flow initiated, receives significant snowfall. Snowmelt maximum is typically reached in late May, depending on total snowfall and weather conditions. The underlying material on the flanks of Grand Mesa in this area is a mixture of weak, Green River Formation mudstones, weak upper Wasatch Formation mudstones, and pre-existing, unconsolidated landslide, slump, and debris flow material.
Debris flows in areas such as this are sometimes triggered by heavy rain, but also can occur without any apparent specific triggering mechanism. This flow did occur on a day of moderate rainfall (about an inch spread over much of the day) that may have contributed to the timing of the event although the heavy runoff from snowmelt was almost certainly as much if not more important. There is no evidence that an earthquake or "fracking" triggered this event.
Regional geomorphology:
All sides of the upper flanks of Grand Mesa comprise a mix of rotational slumps, slides, rockfalls, and debris flows that result in an irregular topography with poor drainage and numerous lakes. In many places, recent to ancient debris flow lobes are clearly visible in aerial images. On the ground, active scarps are common in many places and indicate slow movement of slumps. In fact, this upper part of the flanks of Grand Mesa is essentially dominated by nothing but active and relict mass wasting and "landslide" type deposits.
This particular flow is larger than most that are visible, but certainly not unprecedented in size. For example, the well-know Slumgullion slide at Lake City, Colorado, is about 50% larger than the West Salt Creek slide.
Predictability:
Unfortunately, geologists are not able to predict the specific TIMING of debris flows such as this.
For further information:
References for more detailed information:
Baum, R.L., 1997, Retrogressive slumping at Grand Mesa, Delta and Mesa Counties, Colorado: Association of Engineering Geologists program with abstracts, 40th annual meeting, p. 82.
Baum, R.L., and Odum, J.K., 1996, Geologic map of slump-block deposits in part of the Grand Mesa area, Delta and Mesa Counties, Colorado: U.S. Geological Survey Open-File Report 96-017, 12p., 2 oversize plates, 1:24,000.
Baum, R.L., and Odum, J.K., 2003, Retrogressive slumping at Grand Mesa Colorado, in Boyer, D.D., Santi, P.S., and Rogers, W.P., eds., Engineering geology in Colorado contributions, trends and case histories: Association of Engineering Geologists Special Publication No. 15 and Colorado Geological Survey Special Publication 55, (CD-ROM), 17 p.
Baum, R.L., Lidke, D.J., and Hart, M. 2007, Grand Mesa landslide complex, in Noe, D.C., and Coe, J.A., eds., Field Trip Guidebooks, 1st North American Landslide Conference: Association of Environmental and Engineering Geologists Special Publication 21 and Colorado Geological Survey Special Publication 56.
Baum, R.L., Odum, J.K., and Savage, W.Z., 1996, Evidence for tensile failure and forward rotation during early stages of slumping at Grand Mesa, Mesa County, Colorado: Geological Society of America Abstracts with programs, v. 28, no. 4, p. 2.
Cole, R.D., 2011, Significance of the Grand Mesa basalt field in western Colorado for defining the early history of the upper Colorado River, in Beard, L. Sue, Karlstrom, Karl E., Young, Richard E., and Billingsley, George H., CREvolution 2-Origin and Evolution of the Colorado River System, Workshop Abstracts: U.S. Geological Survey Open-file Report 2011-1210, p. 55-61
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Day, W.C., and Bove, D.J., 2004, Review of the geology of western Colorado, in Bankey, V., ed., Resource potential and geology of the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests and vicinity, Colorado: U.S. Geological Survey Bulletin 2213-B, p. 11-36.
Yeend, W.E., 1969, Quaternary geology pf the Grand and Battlement Mesas area, Colorado: U.S. Geological Survey Prof. Paper 617, 50 p.
Yeend, W.E., 1973, Slow-sliding slumps, Grand Mesa, Colorado: The Mountain Geologist, v. 10, no. 1., p. 25-28.