Robert C. Witter, formerly with Oregon Dept of Geology and Mineral Industries, Coastal Field Office, P.O. Box 1033, Newport, OR 97365;
Eileen Hemphill-Haley, Consulting Micropaleontology, 1871 Pickett Road, McKinleyville, CA 95519;
Roger Hart, Oregon Dept of Geology and Mineral Industries, Coastal Field Office, P.O. Box 1033, Newport, OR 97365;
Lindsey Gay, Oregon Dept of Geology and Mineral Industries, Coastal Field Office, P.O. Box 1033, Newport, OR 97365.
Three tsunamis triggered by great earthquakes on the Cascadia subduction zone have inundated Nestucca Bay, Oregon over the past 2000 years. The primary evidence includes layers of sandy sediment that bury tidal marshes submerged by earthquake-related subsidence. Additional tsunami evidence includes: the spatial extent of sandy deposits, clear trends in deposit thickness and mean particle size that decrease with increasing distance inland, the presence of brackish-marine diatoms within the deposit and normally graded layers within each deposit. 14C age ranges for the youngest tsunami sand span the date of the most recent Cascadia earthquake and tsunami in 1700. Sediment cores and a single tidal outcrop define the spatial limit of the 1700 tsunami deposit, which extended at least 4.4 km inland. The widespread extent of the 1700 deposits makes storm surges, and waves superimposed on them, an unlikely explanation. Physical attributes similar to beach and dune sand indicate an ocean-ward source and preclude river flooding.
Two older sandy deposits record tsunamis that inundated the bay about 1.2 ka and 1.6 ka – times that correspond to widespread evidence for great Cascadia earthquakes and tsunamis at adjacent estuaries and in offshore turbidite records. Both deposits meet multiple criteria used to infer a tsunami origin, although the physical properties of the sand resembled sediment from the sandy flats of the Little Nestucca River rather than beach or dune sand over 5.5 km away. The physical characteristics of older sandy layers reflect sediment sources along the tsunami flow path.
Stratigraphic sequences of intertidal facies beneath the margins of Nestucca Bay archive a history of relative sea level change in response to the earthquake deformation cycle as well as the effects of tsunamis, but they may not capture the complete record of great Cascadia earthquakes and tsunamis at Nestucca Bay over the past 2000 years. This inference is supported by equivocal evidence for one or two peaty horizons buried between the times of the 1.2 ka and 1700 tsunamis. These buried marsh deposits, which largely lack distinctive, laterally continuous sandy deposits, probably reflect small changes in relative sea level that may have been caused by a number of processes, including earthquakes, climate variation and changes in the configuration of the estuary (Nelson et al., 1996). In addition, older sharp lithologic contacts recognized in stratigraphic sequences along Upton Slough lack the lateral continuity and biostratigraphic evidence necessary to confidently attribute them to Cascadia events. Therefore, the evidence we have uncovered at Nestucca Bay stresses the importance of understanding the variability of creation and preservation thresholds that control whether a particular site holds complete or incomplete geologic record of great Cascadia earthquakes and tsunamis.
This study was funded by a grant from the U.S. Geological Survey’s National Earthquake Hazard Reduction Program, award number 08HQGR0076.
[poster PDF, 4.7 MB] Tracking Prehistoric Cascadia Tsunami Deposits at Nestucca Bay, Oregon, USA, by Robert C. Witter, Eileen Hemphill-Haley, and Roger Hart, 2010
GSA 2009 Meeting, Portland, Oregon, Field Trip 435. Coastal Geomorphology, Hazards, and Management Issues along the Pacific Northwest Coast of Oregon and Washington
Leaders: Jonathan Allan, Oregon Dept. of Geology and Mineral Industry Newport Coastal Field Office; Peter Ruggiero; Robert Witter.
Development along the coasts of Oregon and Washington, USA, is threatened by a variety of natural hazards, including coastal erosion, landslides, earthquakes, and tsunamis. The impacts of hazards on development have increased significantly in recent years due to poor management practices and an intensification of the physical processes that drive coastal change. This field trip will visit a number of sites that illustrate the processes that shape Pacific Northwest coastal geomorphology and create hazards, including potentially catastrophic tsunamis generated by the Cascadia subduction zone (CSZ). We will discuss what we have learned about the ocean processes that produce elevated tides and high wave runup levels, resulting in the erosion of the coast, which leads to property losses. Also examined is the human response, which includes constructing coastal engineering structures and the establishment of coastal “erosion” hazard zones. At three sites, we will discuss new research findings on tsunamis and the effects of coastal subduction caused by great earthquakes on the CSZ, including examining efforts by public officials to prepare and mitigate for future events. The field trip will conclude on the southern Washington coast at Cape Disappointment State Park adjacent to the Columbia River, where researchers will discuss the response of the coast to the construction of the Columbia River jetties. Additional discussion will focus on the role of river flow regulation and dredging and disposal activities in influencing the sediment budget of the Columbia River littoral cell as well as current sediment management practices that aim to ensure more effective decision making within this cell.