RAPID RETREAT OF A POTENTIALLY UPLIFTED COASTAL TERRACE AT RIALTO BEACH, ON THE PACIFIC COAST OF WASHINGTON

A low-elevation terrace at Rialto Beach, along the northwestern coast of Washington State, separates a steep forested upland from a high-energy Pacific beach. The terrace, which hosts an established-but-dying Sitka spruce forest, forms a flat bench one to two meters above the current beach surface that is eroding rapidly toward the slope. The terrace is enigmatic - is abrupt relative sea level change required to form the terrace and establish a forest atop it? Or can the morphology and stratigraphy of the terrace be more simply explained as part of the wave-dominated delta of the nearby Quillayute River or long-term erosion due to sea level rise? The terrace can be traced intermittently from well south of Rialto Beach to as far east as Port Angeles, WA. We study the stratigraphy and coastal morphology of the terrace At Rialto Beach to evaluate its origins and possible future. Exposures of strata within the terrace are ephemeral; outcrops rising above the modern beach level include heavily oxidized sand and gravel deposits, and a wave-cut bedrock surface covered by heavily oxidized sand and gravel of a probable older beach. 14C ages on detrital wood from the inferred older beach deposits date to tens to hundreds of years after 550–660 cal yr BP. Old beach deposits exposed in the terrace are locally capped by other deposits which include paleosols, possible back-beach marsh deposits, and debris flows from the slopes above. The ages of overlying debris flow and landslide deposits are as old as ~300 cal yr BP which may be compatible with the 1700 AD Cascadia Subduction Zone earthquake. Contemporary coastal erosion is removing terrace strata rapidly - repeated high-resolution beach and terrace edge surveys show that the terrace is eroding at a rate of 1.2 meters/year over the last seven years with more recent localized rates of at least 2 meters/year. Scientists recall a significantly wider and more stable terrace and forest fifty years ago. If retreat continues at its current rate of 1 to 2 meters/year, at least part of the terrace may retreat to the toe of the slope within the next few decades leading to more landslides as the toe is undercut by waves. Investigation into the paleo-history, contemporary morphologic evolution, and landslide ages is ongoing.