BETTER UNDERSTANDING LANDSLIDE TRIGGERS: WHAT ARE THE CAUSE(S) OF LARGE, ROTATIONAL LANDSLIDES ALONG THE WESTERN SHORELINE OF CENTRAL WHIDBEY ISLAND, WASHINGTON?

On the morning of March 27^th, 2013 a deep-seated, rotational landslide of 150,000 cubic meters of unconsolidated glacial sediment was displaced towards the westerly shoreline of central Whidbey Island, WA. This recent landslide, known as the Ledgewood-Bonair Landslide (LBL), is a part of much larger, prehistoric landslide complex that extends 3 kilometers along the central Whidbey Island shoreline. The stratigraphy in this central portion of Whidbey Island is the result of repeated glacial-interglacial cycles during the late Quaternary. The sediment record is inherently complex in this region because of numerous unconformities related to subglacial erosion, relative sea level change and rapid facies changes that occur within the sediment record in a glacial marine environment.

Detailed geologic mapping (surficial and substrate) of the exposed westerly shoreline bluffs and proximal well logs provide important information relevant to understanding the LBL’s spatial extent and its underlying cause(s). Detailed mapping of the landslide complex and adjacent shorelines (north and south of the LBL complex) indicate that a paleo-glacial trough existed in this region, which was subsequently infilled with glaciomarine sediment during a readvance cycle of the Puget Lobe during a prior (pre-Vashon – before about 15,000-13,000 yr B.P.) glaciation. The axis of this paleo-trough appears to be broadly consistent with the axis of the LBL complex. The glacial retreat record of the most recent (Vashon) glaciation near Penn Cove, WA provides an excellent proxy for the paloeotopography and sediment records observed in the LBL substrate geology.