LARGE LANDSLIDES IN THE MOUNT PITTSBURG QUADRANGLE, EL PASO, FREMONT, AND TELLER COUNTIES, COLORADO

The Mount Pittsburg 7.5-minute quadrangle is located in El Paso, Fremont, and Pueblo Counties, at the southern margin of the Colorado Front Range. Landslides are prevalent around Table Mountain, Wild Mountain, and Timber Mountain where resistant Dakota Sandstone is stratigraphically above several less-competent shales and mudstones including the Purgatoire, Morrison, and Lykins Formations. The Dakota Sandstone is highly jointed at these locations, allowing water to seep into and mobilize the underlying shales and mudstones. Some of the failures are rotational slumps where failure typically occurs along a vertical or nearly vertical fracture that intercepts a gently dipping shear plane within the shale and mudstone formations. The headscarps can be extremely tall; some reach 100 ft in height on the southeast side of Table Mountain. Pressure ridges and terracettes may reach heights of 10 ft. The two large landslide masses on the southeast side of Table Mountain cover an area of roughly 2.3 mi² with a conservative estimated depth of 10 ft, resulting in over 635,000,000 ft³ of mobilized debris. Landslides surrounding Wild Mountain and Timber Mountain are also rotational slumps but the mobilized material covers a much smaller area when compared to the slumps surrounding Table Mountain. Translational and rotational slides occur along Patton Canyon where mesas are separating along laterally extensive pull-apart fissures that reach widths of 200 ft. Small earthflows of clayey material occur at the toes of many of the landslides in the quadrangle. Soil profiles examined in the landslide deposits on the south side of Timber Mountain have multiple buried A-horizons indicating repeated movements during the late Quaternary and Holocene. Landslides on the south side of Table Mountain have various soil profiles; some locations have little or no pedogenesis while other locations have thick Bt-horizons. This suggests either some of the masses have not moved since the middle to late Pleistocene or the soil profile may predate landslide movement.