Portland State University. Department of Geology
Ansel G. Johnson
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
Faults (Geology) -- Washington (State) -- Hood Canal Region, Geology -- Washington (State) -- Hood Canal Region, Seismic reflection method
1 online resource (xi, 134 pages)
The north-northeast trending Hood Canal is an elongate 75 km long, 2-5 km wide, maximum 190 m deep glacial trough that represents the western limit of Washington's Puget Sound estuary complex and eastern boundary of the Olympic Peninsula. Airgun seismic reflection data were collected in Hood Canal April 4-5, 1994 onboard the University of Washington's R. V. Thomas G. Thompson for the purpose of defining the enigmatic and relatively unstudied Hood Canal-Discovery Bay fault zone (HDF). This fault zone parallels western Hood Canal, is obscured by marine waters and thick late Quaternary glacio-marine sediments, and is defined locally by gravity data and regionally by aeromagnetic, resistivity, and teleseismic studies.
Seismic-stratigraphic interpretations (0.7 km deep; 1500 m/s two-way travel time) reveal a bedrock erosional surface overlain by two distinct seismic facies. Overlying the bedrock is a IO to 400 m-thick, heterogeneous, massive to hummocky, sub-parallel to discontinuous, and disrupted unit inferred to be Quaternary glaciomarine sediments. Shear zones within this unit are defined by near-vertical faults. Directly below the sediment-water interface, the youngest deposits are 10 to 50 m-thick, flat lying, acoustically continuous and inferred to be late Holocene ice-contact deltaic and glacial outwash deposits capped by more recent fine-grained marine sediments. The interpreted bedrock is defined by high-amplitude 20 to > 40° east dipping reflectors, asymmetric "V"-shaped valleys, complex faulting patterns, and striking vertical relief. This seismic unit corresponds to 20-65° east dipping Eocene Upper Crescent Formation along Hood Canal's western shoreline.
Major structural faulting is consistent with previously mapped fault traces and is defined by abruptly terminating bedrock reflectors. Fault structures include high relief horst and graben, classic normal faults, crustal blocks with up to 200 m ofapparent vertical displacement, pressure ridges, and probable remnant thrust fault structures. In southern Dabob Bay, a 2.0 km-wide north-northeast plunging graben structurally defines the southwestern shoreline ofToandos Peninsula. North of Quatsap Point, east-west trending bedrock topographic lows bounded by near-vertical faults may represent westward-projected fault splays ofthe Seattle Fault mapped 8 km to the east. Dextral strike-slip displacement may have formed a broad pull-apart basin south of Toandos Peninsula.
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Haug, Brian J., "High Resolution Seismic Reflection Interpretations of the Hood Canal-Discovery Bay Fault Zone, Puget Sound, Washington" (1998). Dissertations and Theses. Paper 6352.