FLUID VENTING STRUCTURES ON THE 

 NORTHERN OREGON CONTINENTAL SHELF 



LaVerne D. Kulm 1 , Erwin Suess 1 , and Parke D. Snavely, Jr. 2 



-'-College of Oceanography 



Oregon State University 



Corvallis, OR 97331 



2 U.S. Geological Survey 



Branch of Pacific Marine Geology 



Menlo Park, CA 94025 



ABSTRACT 



The Oregon subduction zone is actively venting pore fluids 

 from the accretionary prism which produce vent sites consisting 

 of one or more of the following features: (1) benthic animal 

 communities harboring chemosynthetic bacteria, (2) carbonate 

 deposits, and (3) anomalously high methane concentrations. 

 Carbonate deposits, which are apparently formed by precipitation 

 from oxidized methane and C02-charged fluids, occur on the upper 

 continental slope and outer continental shelf off northern 

 Oregon. Using the submersible Mermaid and the ROV Recon-IV , 

 extensive areas of carbonate crusts, slabs, chimneys, and 

 irregular edifices were mapped on the seafloor in water depths 

 ranging from 92 to 382 m. These carbonate structures are 

 covered with abundant living benthic organisms, especially 

 crinoids. The ROV television camera photographed one circular 

 chimney structure situated within the substrata of the seafloor. 

 It rises 15-20 cm above the seafloor and has a sediment-free 

 vertical opening of 15-20 cm in diameter, suggesting that it is 

 an active vent site. It is similar to a circular chimney, with 

 an open plumbing network, that was recovered by commercial 

 fishermen in a trawl net from the same area. In addition, one 

 200 lb, irregular carbonate edifice was recovered by the support 

 vessel Aloha in a trawl in the vicinity of the ROV trackline. 

 The lower portion of the chimney-like structure was buried in 

 the sediment and the upper portion contains a profusion of 

 cemented tubes which are exceedingly complex in shape, but 

 clearly indicate a plugged plumbing system. The bulk of the 

 chimney as well as the walls of the plumbing system consist of 

 aragonite and/or Mg-calcite cemented glauconitic sand, quartz and 

 minor amounts of clay minerals. Glauconite is common in the 

 surrounding shelf sediments. The irregular carbonate edifice is 

 highly depleted in kl3C (-48.27 to -53.31 PDB ) and has positive 

 kl80 (+4.19 to +4.84 PDB) values. These chimney structures 

 apparently grow both within the bottom sediment and above the 

 sediment-water interface depending upon the rate of clastic 

 sedimentation surrounding the structures. 



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