DETERMINING ACTIVE EPISODES OF THE DONALD DUCK LANDSLIDE COMPLEX ON GRAND MESA, CO BY DENDROGEOCHRONOLOGY

Grand Mesa is the largest mesa in the world and exhibits numerous dynamic geomorphologic phenomena including ancient and continuing mass movement features. The dense basaltic mesa top is underlain by weak Tertiary rocks that are highly susceptible to slope failure, together producing movement along the mesa flanks. Recent reactivation of these ancient features is noticeable on aerial photographs and in the field but requires a time frame to contextualize magnitude and frequency. This study investigates the timing of movement on one landslide on the north flank of Grand Mesa which is of particular interest because of the periodic sediment it delivers across scenic Colorado Highway 65. The magnitude of the active complex was determined by refractive seismology. Soil samples were obtained from above and below the head scarp as well as along the complex flanks and the crests of internal lobes. Dendrogeomorphological methods were used to study the trees on the complex surface. The direction and degree of tilting of trees as well as the deformation of trunks was compared to trees growing on adjacent stable surfaces. To determine when landslide moved, tree-ring cores from tilted and damaged trees on the landslide were examined for signs of disturbance. Disturbances appear as impact scars, compressed annual rings, and/or the formation of reaction wood. These methods indicate multiple active episodes during the past 100 years. Correlations of active episodes with historical precipitation records indicate the movement is in response to increased soil moisture and saturation as well as possible human disturbance.