GEOMORPHOLOGY AND FIELD CHARACTERIZATION OF THE NEW PIEDRA CANYON LANDSLIDE, SOUTHWESTERN, COLORADO

Piedra Canyon is a narrow, steep-sided, fluvially-carved canyon that cuts through the Hogback Monocline (northern limit of the San Juan Basin). The Piedra Canyon Landslide (PCL) is located approximately 80 km east of Durango, Colorado (292020mE, 4131130mN, UTM Zone 13N), covers an area over 133,500 m², occupies a 15%, west/northwest-facing slope, and consists of fine-grained soils to coarse-grained, prismatic blocks of limestone (up to 5 m³). The large limestone blocks (Pennsylvanian Paradox Formation) are dislodged from the head scarp (~267 m above the canyon floor). Northeast and northwest trending joints and numerous interbedded siltstone lenses help dislodge large, intact limestone blocks.

Longitudinal cross-sections were measured in the field and entered in to the slope stability software XSTABL to better constrain volumetric estimates and to estimate subsurface rupture planes. A reconnaissance field map was generated which details the location of longitudinal and transverse displacements; mapping also revealed the presence of an older head scarp (~10 m above the modern head scarp). Aerial photographs were used to constrain the age of the slope failure from between September 15, 1989 and August 17, 1995, and information from the U.S. Forest Service (San Juan National Forest) indicates that the slope burned on September 14, 1995. Tree ring data from fir and pine saplings growing in transverse fracture zones were characterized by relatively small growth rings; all saplings were younger than 10 years old. Precipitation data for 1995-2005 also indicate that minimal precipitation occurred in this area over the last decade. X-Ray Diffraction analysis of soil samples revealed the presence of swelling clays (Montmorillonite) in samples from the toe and head scarp. PCL soil samples were classified as CH and CL-ML based on soil-water consistency (Atterberg limit) lab tests.

The PCL does not currently pose any immediate threat to buildings or structures. However, a catastrophic failure would threaten ranches, campgrounds and a state highway bridge (~12.8 km downstream where the river leaves the narrow canyon and empties on to a broad alluvial plain). Periodic monitoring during high precipitation and runoff events would be useful, however, no mitigation of the landslide, based on this preliminary study, is proposed.