Origin of bottom-simulating reflectors: Geophysical evidence from the Cascadia accretionary prism

Abstract

Research Article| May 01, 1994 Origin of bottom-simulating reflectors: Geophysical evidence from the Cascadia accretionary prism Mary E. MacKay; Mary E. MacKay 1Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822 Search for other works by this author on: GSW Google Scholar Richard D. Jarrard; Richard D. Jarrard 2Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar Graham K. Westbrook; Graham K. Westbrook 3School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom Search for other works by this author on: GSW Google Scholar Roy D. Hyndman Roy D. Hyndman 4Pacific Geoscience Centre, Geological Survey of Canada, Sidney, British Columbia V8L 4B2, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Mary E. MacKay 1Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822 Richard D. Jarrard 2Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Graham K. Westbrook 3School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom Roy D. Hyndman 4Pacific Geoscience Centre, Geological Survey of Canada, Sidney, British Columbia V8L 4B2, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (5): 459–462. https://doi.org/10.1130/0091-7613(1994)022<0459:OOBSRG>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mary E. MacKay, Richard D. Jarrard, Graham K. Westbrook, Roy D. Hyndman; Origin of bottom-simulating reflectors: Geophysical evidence from the Cascadia accretionary prism. Geology 1994;; 22 (5): 459–462. doi: https://doi.org/10.1130/0091-7613(1994)022<0459:OOBSRG>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Vertical seismic profile (VSP) data from two drill sites on the Cascadia margin show low-velocity zones, indicative of free gas, be. neath a bottom-simulating reflector (BSR). Offshore Oregon, at Ocean Drilling Program (ODP) Site 892, velocities drop from an average of 1750 m/s above the BSR to less than 1250 m/s below it. Sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the BSR at this site. Similarly, at ODP Site 889 offshore Vancouver, velocities range from 1700 to 1900 m/s in the 100 m above the BSR and drop abruptly to 1520 m/s in the 15 m just beneath it. The low velocities observed beneath the BSR are strong evidence for the presence of 1%-5% free gas (by volume). The BSR at these two sites results from the contact between gas-free sediments containing a small quantity of hydrate above the BSR and a low-velocity free-gas zone beneath it. Although the BSR is associated with the base of the hydrate stability field, hydrate appears to account for relatively little of the velocity contrast that produces the BSR. Velocity above the BSR at Site 889 is only about 100 m/s greater than that expected for sediments of similar porosity. Sediments above the BSR at Site 892 appear to have normal velocity for their porosity and may, contain little hydrate. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.