Seafloor Mapping Group
A 21-day field operation was conducted during August and September, 1998 in the estuaries and on the inner continental shelf off the northern Oregon and southern Washington coast to generate reconnaissance maps of the seafloor geology and the shallow subsurface stratigraphy. Sidescan sonar data were collected on the innermost shelf to document changes to the sea floor since 1997. Sediment samples, bottom photographs, and video images were collected to document the sea floor geology. Seismic-reflection profiles were collected to fill in gaps in the 1997 shelf-mapping program and to extend the framework geology from the shelf into the estuaries. The Columbia River has provided huge amounts of sediment to this shelf during the Holocene transgression. Previous studies have mapped the distribution and thickness of this modern deposit on the middle and outer shelf (Nittrouer and Sternberg, 1981; Wolf and others, 1997), however, its distribution on the inner shelf and its link to the adjacent estuarine systems were not as clearly understood because of incomplete data coverage in these areas. Anthropogenic activity within the Columbia River drainage basin (agriculture, logging, construction of dams, and dredging of the estuary mouth) may have altered the sediment discharge from the river which, in turn, may be affecting sediment distribution on the shelf and in adjacent coastal environments. The seismic-reflection data in concert with the sidescan-sonar imagery and sediment samples will provide an overall synthesis of the seafloor environment including the Holocene stratal geometry, the distribution of different sediment types, the distribution of mobile bedforms, outcrops of Pleistocene and older strata, and the locations of faults. The goal of this project is to provide a regional synthesis of the offshore geology to support a wide variety of management decisions and to provide a basis for further process-oriented investigations. The project is conducted cooperatively with the Washington State Department of Ecology and several universities including Portland State University, University of Wisconsin Eau Clair, University College of the Fraser Valley, Grays Harbor College, University of South Florida, and Oregon State University. Entity_and_Attribute_Detail_Citation Nittrouer, C.A., and Sternberg, R.W., 1981, The formation of sedimentary strata in an allochthonous shelf environment: the Washington continental shelf: Marine Geology, V. 42, p. 201-232. Wolf, S.C., Hamer, M.R., and McCrory, P.A., 1997, Quaternary geologic investigations of the Continental Shelf offshore southern Washington and northern Oregon: U.S. Geological Survey Open-File Report 97-67.
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