HR: 1340h
AN: OS33B-0582    [Abstracts]
TI: Marine Geology of the Southwestern San Juan Islands: New Insights From Multibeam Imagery and Processed Aeromagnetic Data
AU: * Tilden, J E
EM: jtilden@mlml.calstate.edu
AF: Moss Landing Marine Laboratories, 8272 Moss Landing Rd., Moss Landing, ca 95039 United States
AU: Greene, H G
EM: greene@mlml.calstate.edu
AF: Moss Landing Marine Laboratories, 8272 Moss Landing Rd., Moss Landing, ca 95039 United States
AU: Blakely, R J
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, ca 94025 United States
AB: The San Juan Islands, located in the seismically active northern Puget Sound, have a complicated and not yet fully understood geologic history. This study is among the first marine geologic mapping efforts within the San Juan Islands, filling an important gap in an otherwise well-studied region. Existing geologic and geophysical data were combined with interpretations of new multibeam bathymetry and backscatter seafloor imagery to construct a seamless onshore - offshore geologic map of the southwestern San Juan Islands. Simrad EM1002 (95 kHz) and Reson 8101 (240 kHz) multibeam bathymetry and backscatter data were collected between October 2000 and November 2003 within Haro Strait, northeastern Strait of Juan de Fuca, and San Juan Channel. Sun-shaded images of the processed data reveal a complex network of faulted and fractured bedrock exposures, deep glaciated channels, Pleistocene glacial sediments, and dynamic bedforms. Distinct slump morphologies in multibeam and backscatter imagery suggest active slumping of recent sediments at the mouth of San Juan Channel. A number of previously inferred offshore geologic structures were extended and constrained based on distinct linear bedrock features visible in the multibeam imagery. Aeromagnetic data collected by the U.S. Geological Survey in 1997 were processed to accentuate short-wavelength, presumably shallow, magnetic sources. The resultant derivative aeromagnetic map reveals a number of areas with distinctive anomaly patterns. Gradients in magnetic anomalies often corresponded with fault traces identified in high-resolution multibeam imagery and may reflect slight magnetic susceptibility contrasts across fault contacts. Aeromagnetic data also constrain two tectonostratigraphic terrane boundaries not identified in multibeam imagery: 1) the Buck Bay fault, which separates the Lopez Structural Complex and Decatur terrane from the underlying Constitution Formation, and 2) the Haro fault separating the Deadman Bay terrane of the San Juan Thrust system from the Wrangellia terrane on Vancouver Island.
DE: 8010 Fractures and faults
DE: 3005 Geomagnetism (1550)
DE: 3045 Seafloor morphology and bottom photography
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting