EPISODIC SEDIMENTATION AND THE STRATIGRAPHIC RECORD—A LEGACY OF BOB DOTT’S MUSINGS, AND AN EXAMPLE FROM HOLOCENE LACUSTRINE SEDIMENTS IN WESTERN WASHINGTON

In his 1982 SEPM Presidential Address, Bob Dott pondered the nature of the stratigraphic record, including the role of episodic sedimentation and the amount of time missing. Bob was a pioneer in thinking about the processes that produce and preserve events layers, and with his students, his work included investigation of storm-worked conglomerates in Wisconsin, turbidites and debris flow deposits in Chile, and hummocky stratified sands in southwestern Oregon. Many of his academic offspring have gone on to contemplate such deposits as well, with applications ranging from reconstructing paleogeography and past climate to predicting the future impacts of tsunamis and earthquakes.

The Holocene sedimentary fill of Lake Crescent, a deep, glacially carved lake located on the Olympic Peninsula in western Washington, contains a remarkable record of episodic disturbance, including two types of event layers attributed to earthquake shaking. Four, roughly two-meter-thick layers termed megaturbidites record ruptures in the past 7200 years along the Lake Creek-Boundary Creek Fault zone, an oblique-slip crustal fault system that runs directly beneath the lake. These earthquakes each triggered rockslides that entered the lake and caused displacement waves (lake tsunamis) and seiches, and reworking of lacustrine sediments. In addition, the strata that have accumulated since the last crustal fault rupture around 3100 years ago include eleven decimeter-scale turbidites formed by remobilization of unconsolidated, surficial sediments from the lake’s subaqueous slopes. The most recent of these layers overlaps in age with the well-documented AD1700 Cascadia subduction earthquake, and many of the earlier layers similarly overlap with Cascadia megathrust ruptures inferred from regional evidence. In total, earthquake deposits constitute approximately 75% of Lake Crescent’s middle-late Holocene strata. Given that the shaking from moderate-large size earthquakes typically lasts for seconds to minutes, we estimate that three-quarters of the 7200-year-long record was produced by events that cumulatively spanned one hour or less. In accordance with Bob Dott’s observations, the Holocene stratigraphy of Lake Crescent is dominated by “short periods of terror” in an otherwise tranquil lacustrine setting.