MEGATURBIDITES IN LAKE CRESCENT, WASHINGTON—SEDIMENTARY RECORD OF REPEATED CATASTROPHIC EVENTS TRIGGERED BY SURFACE RUPTURE OF THE LAKE CREEK–BOUNDARY CREEK FAULT

Lake Crescent, situated in Olympic National Park in western Washington, is a deep, glacially carved lake surrounded by steep slopes. Large Holocene landslide scarps surround the lake, and swath bathymetric data demonstrate that these failures deposited large masses of material into the lake itself. High resolution seismic reflection data and a suite of piston cores reveal that these landslides have produced four lake tsunami/seiche events during the past 8500 years, the last around 3000 years ago. The penultimate landslide-tsunami pair, with a median calibrated radiocarbon age of 4015 cal yr BP was responsible for the separation of lakes Crescent and Sutherland. These events locally caused meters of erosion into the lakebed, and deposition of “megaturbidites” comprising normally graded, faintly laminated sand overlain by thick, massive silt with dispersed plant debris, and clay caps. The megaturbidite beds are traceable over the entire 20 km² lake basin and exceed 1 m thickness over much of the deep lake floor. The 3000-yr BP deposit has an estimated volume of about 4.8 x 10⁶ m³. Petrologic data from the basal megaturbidite sands indicates that the sediment was derived from sources around the entire lake margin, as opposed to a single point source, and was likely remobilized from shallow subaqueous environments by the tsunami waves. Offset of the megaturbidite layers visible in seismic reflection profiles indicates that their deposition is related to ruptures of the Lake Creek–Boundary Creek fault, an oblique strike-slip fault that partially accommodates northward convergence and clockwise rotation of the northern Cascadia forearc and that runs directly beneath Lake Crescent. Results from Lake Crescent provide insight into the paleoseismic history of this important structure, and into potential hazards associated with future seismic events.