GIANT LANDSLIDES AND RURAL DEVELOPMENT, APPLICATIONS OF STATEMAP GEOLOGIC MAPPING

Recent STATEMAP funded mapping of the Coburg Quadrangle in the southern Willamette Valley has identified a series of major landslides originating from the nearly 800m high escarpment of the Coburg Hills. Although much of the area is currently timberland, the proximity of the rapidly growing Eugene Urban area will make the hills a target for future rural residential development. Understanding the landslide hazards of the area will be crucial to managing this development. The geology of the Coburg Hills consists of a thick sequence of Oligocene basalt flows overlying Eocene and Oligocene volcaniclastic and marine sedimentary rocks. A silicic ash flow interbedded with the basalt flows appears to be the main failure plane for major landslides, probably because it has relatively low permeability. Severely weathered and altered basalt horizons also serve as failure planes. The typical landslide covers .5-1.5 km3, and extends over an elevation range of 300-500 m. The slides typical coalesce into large complexes. From head to toe the slides consist of steep walled arcuate scarps, zones of bench and scarp topography, and toes that consist of debris fans that resemble large alluvial fans. We interpret the history of these slides to begin with a catastrophic failure the produces a major debris avalanche resulting in the formation of the fan-like toes, followed by continued slow failure by block gliding and slumping in the headwall and scarp and bench regions. This has significant implications for future development. It is probably unreasonable to restrict development of the debris avalanche deposits at the base of the slide, because these areas are not likely to slide further, and not likely to be subject to future debris avalanches. The upper reaches of the slides on the other hand are clearly hazardous for development, because of the abundance of small block slides and slumps perched on steep slopes. Areas between slides, where the slopes are fairly steep may actually present the greatest risk to life safety, as they are the most likely areas for future catastrophic failures and debris avalanches.