GEOCHRONOLOGICAL CONSTRAINTS ON THE PERINS PEAK LANDSLIDE COMPLEX NEAR DURANGO, COLORADO

The vulnerability of human settlement to mass wasting events calls for the continued monitoring of active events and a more comprehensive understanding of past activity. Constraining the age and extent of ancient mass wasting deposits improves our ability to evaluate the reactivation risk of existing deposits and determine the danger posed by cliffs and steep terrain, both adjacent to those deposits and in similar geologic settings.

This study focuses on a previously studied but poorly understood mass wasting deposit, the Perins Peak landslide complex. This fragmented Pleistocene unit drapes more than 6.3 km² of dissected ridges and hillslopes west of Durango, Colorado. It formed from the collapse of the Cretaceous Mesa Verde Group sandstone sequence, which overlies the weaker Cretaceous Mancos Shale, and is characterized by unsorted tan to yellow sandstone clasts in an oxidized yellow-brown matrix of silt and clay. The time of stabilization within the north flank of the complex is constrained by two bounding deposits: an underlying 640 ka Lava Creek B ash bed and an overlying glaciofluvial deposit from which a luminescence date was acquired.

The steep northwest flank of a bedrock bench east of the complex (known as College Mesa) exposes outcrops of several Pleistocene units of similar age as the Perins Peak landslide complex. These include mixed glaciofluvial deposits that bury a mass wasting unit that may correlate with the complex. The minimum age of this mass wasting unit is constrained here by dating the overlying deposits with OSL and IRSL at the Utah State University Luminescence Lab. Furthermore, field observations and the GIS analysis of a LiDAR-derived elevation dataset yield longitudinal profiles and cross-sections of the study area, allowing for the reconstruction of former slip planes and the identification of different generations of landslide development.

In addition to constraining the age of the Perins Peak landslide complex, this research may provide new insights into the landscape evolution of southwest Colorado by refining the incision history of the Animas River.