SUBFOSSIL FORESTS AS RECORDS OF PAST EARTHQUAKES, VOLCANIC ERUPTIONS, AND LANDSLIDES IN THE PACIFIC NORTHWEST—AN OVERVIEW OF DATING TECHNIQUES, SELECTED SITES, AND RECENT RESEARCH

Use of radiocarbon dating along with dendrochronology to study subfossil forests can provide valuable clues to the history and impacts of postglacial volcanism, fault movements, landslides, and flooding. Submerged sites along coastal Washington, in Puget Sound, in the Columbia River, and in lakes (landslide-, and fault-dammed) contain subfossil snags that record the timing of geologic events including paleo-earthquakes. Studies of submerged forests in Puget Sound by various researchers suggest multiple episodes of abrupt local tectonic subsidence related to shallow crustal faults.

Ages of more than 30 landslides in Washington and Oregon have been estimated by radiocarbon dating of associated subfossil wood and(or) by tephrochronology—Fewer than 25 percent of these dates yielded high-precision radiocarbon ages that constrain the calendric age of tree mortality to within a few decades. Further sampling could yield outer wood of old-growth victim trees with enough rings for wiggle-match radiocarbon dating. Other tree-bearing lakes have been discovered, but not sampled, and more than two dozen candidate sites lack reconnaissance—Some could record paleo-earthquakes. Collecting samples using scuba diving in landslide-dammed lakes can yield more accurate dates and additional samples for development of more robust floating chronologies for crossdating.

Buried forests throughout the Cascade Range record the timing of volcanic events. Further dating of trees buried by volcanism is important because past volcanic disturbances buried extensive riverine landscapes downstream of volcanoes and likely destroyed and/or severely disrupted pre-Euro-American-settlement human communities on floodplains that are now more densely populated. Lahars (volcanic debris flows) and laharic flooding that severely and repeatedly aggraded the Nisqually, Puyallup, White, Skagit, Duwamish, Stilliquamish, and Nooksack Rivers also caused delta progradation that dramatically altered the coastline of the Puget Lowland.

Combined use of radiocarbon ‘wiggle matching’ and dendrochronology on subfossil trees along with new collection tools and techniques, such as scuba diving, could greatly improve dating precision and our understanding of the magnitude and frequency of the associated geologic events.