THE SOURCE OF ASBESTIFORM CHRYSOTILE AT SWIFT CREEK WA: GEOLOGY, MINERALOGY AND SEDIMENT TRANSPORT

Intense sedimentation in Swift Creek and Sumas River, located in the Cascades foothills in NW Washington (48°55′N, 122°16′W), results from erosion of the oversteepened, unvegetated toe of a large (0.55 km²) active landslide. The bedload and suspended sediment in the creek are a direct result of the weathering of the serpentinitic bedrock in which the landslide is rooted. The ultramafite is unconformably overlain by Eocene Huntingdon Formation conglomerate and glacial till. The conglomerate strata are brittle, relative to the underlying sheared serpentinite, and break into large blocks that are carried by the landslide. Historical photos and tree-ring analysis suggest the slide has been active since the 1930’s. The landslide is failing by rotational movement at the head of the slide, transitioning to a flow toward the toe. Aerial photos over the last 30 years indicate that the toe of the landslide has enlarged steadily in both thickness and area. Repeat GPS surveys from 2004-2008 show that the main mass of the deep-seated Swift Creek landslide moves 4-5 m/y. Time-lapse photographic monitoring confirms the shallow flow of the toe area can exceed 50 m/yr. Storm-generated debris flows transport sediment through the high gradient section of the creek downstream from the earthflow.

Broad sampling and analysis of the earthflow material reveals that the serpentinite weathers to asbestiform chrysotile (not smectite) with minor amounts of chlorite, illite and hydrotalcite, all of which occur in clay seeps on the unvegetated surface of the landslide. The chrysotile fibers average 2 mm in length and make up at least 50%, by volume, of the suspended load transported in Swift Creek. The suspended sediment concentration of Swift Creek ranges from 0.02 g/L during low summer flows to >40 g/L during winter flows. Based on combined bedload and suspended load estimates the erosion rate for the Swift Creek watershed is 11 mm/yr and 158 mm/yr for the Swift Creek Landslide alone.