MOVEMENT OF A LARGE LANDSLIDE BLOCK DATED BY TREE-RING ANALYSIS, TOWER FALLS AREA, YELLOWSTONE NATIONAL PARK, WYOMING

Tree-ring analysis can be a valuable tool to date geomorphic events in regions lacking long historical records. In this study, the latest movement of a section of a large landslide block in the Tower Falls area of Yellowstone National Park, Wyoming, was dated by tree-ring analysis of Douglas-firs (Pseudotsuga menziesii var. glauca). The movement tilted many of the trees and damaged their root systems. In August 2000, thirteen old, tilted Douglas-firs, at three sites, were sampled within the section of the landslide block that moved during the life of these trees. In addition, ten young, upright, undisturbed Douglas-firs were also sampled at the sites in order to establish a minimum age for the movement. The oldest of the ten young, upright trees had an age of about 135 years, indicating that the latest movement of the landslide block occurred prior to 1865 A.D. The youngest of the 13 old, tilted trees germinated in about 1630 A.D., providing a maximum age for this latest landslide movement. The majority (11) of the large, tilted Douglas-fir sampled on the section of the landslide block near Tower Falls recorded an abrupt reduction in annual ring width that began in 1694 A.D and lasted for 2 to 16 years. As no other period in the tree-ring record between 1865 and 1630 A.D. revealed such an abrupt reduction in annual ring width, landslide movement is thought to have occurred sometime between the end of the 1693 A.D. growing season and during the 1694 A.D. growing season. Attribution of this part of the tree-ring record to landslide movement is further supported by the absence of similar tree-ring reductions in chronologies within the region. A tree-ring chronology of Douglas-fir at a site near Gardner, Montana (Drew, 1975), about 30 km northwest of Tower Falls, as well as trees at two other nearby sites, do not display any significant climate-related reduction in tree-ring width during the 1690s A.D. Because Yellowstone National Park is within the Intermountain Seismic Belt, a zone of pronounced seismic activity, movement of the landslide block may have been caused by an earthquake at that time.