LATE LAST-GLACIAL SHORELINES IN NORTHWEST WASHINGTON STATE

Northwest Washington has a modest suite of fossil shorelines related to retreat of the Cordilleran ice sheet. Formed by wave action and deltaic deposition along the margins of ice-marginal lakes and the Salish Sea, they record progressive, spatially varying, isostatic rebound combined with global ocean-volume change. Formation of discrete shorelines requires a pause in water-level change or a burst of wave action and (or) deposition. For lakes, if the outlet is close to the rebound focus, ongoing rebound results in rising lake level and shorelines away from the outlet; conversely, an outlet far from the rebound focus does not raise lake level and causes shorelines to fall away from the outlet. If a water body is bounded by retreating ice, shorelines will young and thus show less uplift in the direction of retreat.

Glacial Lake Russell, impounded by the Puget Lobe, had its outlet to the south. Most preserved shorelines are well fit by a single deformed plane and show uplift to the NNE. A slight concave-up profile argues against significant time transgression, consistent with other evidence for a short-lived (a few decades?) lake. Northernmost Russell shorelines appear to have been further uplifted by the 900-930 AD Seattle fault zone earthquake.

Younger marine shoreline elevations have an overall trend less steep than predicted by Lake Russell observations. This reflects their youth, ongoing rebound, and stepwise retreat of the ice sheet. Shorelines at multiple elevations show progressive rebound more rapid than global sea level rise; their numbers (>20 in places) suggest they reflect individual storms. The local highest marine shoreline is commonly best developed despite having formed during most-rapid rebound; likely this reflects increasing landscape stability as it reforested.

Slowing of local rebound and increase in volume of the world ocean combined to produce a late Pleistocene marine low-stand shoreline (MLS) that is presently submerged. Rebound rates likely were spatially variable, thus the MLS is unlikely to be isochronous. Near Seattle the MLS is at ~50 m depth, which requires formation no later than 11 ka when global sea level was last this low. Local uplift of the MLS is similar to that of the 900-930 AD shoreline, clear evidence of only one large earthquake on this part of the Seattle fault zone after 11 ka.