A VERTICAL GEOLOGIC STRIP MAP AND LANDSLIDE INVENTORY OF THE TULALIP TRIBES RESERVATION SHORELINE, PUGET SOUND, WASHINGTON

In support of a federally-funded project to inventory and assess nearshore marine salmonid and shellfish habitat and water quality, we are mapping the geology and landslides of the Tulalip Tribes Reservation shoreline. Mapping objectives are to identify and characterize sediment input sources and processes along the approximately 25 km of coastal bluffs. Geologic units are differentiated and described by their grain size, observed hydrologic characteristics, and depositional environment, then correlated to regional geology and depositional history. Landslides are classified by mode of failure according to Varnes (1978). Detailed stratigraphy aids in classification and determining causal mechanism.

Tulalip Reservation bluffs expose a sequence of sediments suggesting late-phase non-glacial deposition, grading upward into advance, full glacial, and recessional environments of the late Pleistocene Vashon Stade of the Fraser Glaciation. The lower portion of the exposed section is represented by dense, thinly-bedded silts and clays. Advance sands and silts dominate mid-section, and till and recessional gravel cap the section. Landslides are ubiquitous along the shoreline, and vegetation removal and development is occurring within designated critical areas in the uplands and at the base of the bluffs. The dense, water-perching basal silt/clay aquitard appears to be the principal stratigraphic control on deep-seated landsliding. Shallow surface failures are common in the advance sands, and toppling failures are common in the overlying till.

Given the nature of these near-vertical bluffs, which commonly exceed 100 m in height, accurate mapping must be carried out at large scales with detailed topography. LiDAR data are available and being used with GPS-linked laser survey data to locate geologic contacts and landslide boundaries with one to two meters of horizontal accuracy. Final map products will include a vertical “strip map” linked to landslide inventory data, photographs, geologic cross sections, and previously-interpreted well logs. The intended users of the information are tribal and county planners, developers, and regulators from county, state, and federal agencies. The new information will enable better-informed decisions regarding land-use and habitat management.