Cliff Erosion along the Oregon Coast: A Tectonic-Sea Level Imprint Plus Local Controls by Beach Processes

Abstract

Sea cliff erosion along the Oregon coast threatens communities that have been built on uplifted marine terraces. There is considerable spatial variability in the rates of cliff retreat, including a coast-wide pattern that is likely due to tectonic activity causing differential uplift rates. The highest rates of uplift are found along the southern half of the coast and near the Oregon-Washington border in the north, areas where the uplift exceeds the present rise in sea level. There is evidence for past cliff erosion in those areas, and it is hypothesized that it was initiated about 300 years ago when a subduction earthquake abruptly lowered those stretches of coast, but that subsequent uplift has halted the continued erosion. Along the northcentral Oregon coast, the global rise in sea level exceeds the tectonic uplift. Cliff erosion is continuing within this stretch of coast, but the rates are highly variable due to local effects. There are differences between the littoral cells isolated between headlands, largely due to contrasting capacities of the respective beaches to act as buffers between the waves and cliffs. There also are marked differences in the nature of the erosion processes depending on the composition and stratigraphy of the bluff. In the Newport area, the cliffs consist mainly of seaward dipping Tertiary mudstones that are susceptible to landsliding, and this process dominates cliff retreat. In the Lincoln City littoral cell the cliffs are entirely Pleistocene sands that tend to erode uniformly with minimal development of landsliding. In this latter cell, there is a longshore variation in beach sediment grain sizes, ranging from a coarse-sand reflective beach to a fine sand dissipative beach. Rip current embayments are more important to cliff erosion on the reflective portion of the beach, producing bluff retreat that has a high degree of spatial variability and is extremely episodic. Wave runup during major storms is important on the dissipative portion of the beach, but the waves generally act to remove only the accumulated talus brought down by subaerial processes. Excluding the landslide areas, cliff retreat along the Oregon coast is generally progressing at a low rate largely because of tectonic uplift, but should another subduction earthquake occur, rapid coast-wide erosion would be renewed.