INTRODUCTION 



In 1985, while dragging for bottom fish along the northern 

 Oregon coast, the fishing vessel Kodiak recovered three chimney- 

 like rocks located in a water depth of about 247 m on the outer 

 edge of the continental shelf (Figs. 1,2; Schroeder et al . 

 1987). Interestingly, these chimneys are similar to carbonate 

 chimney structures observed on the lower continental slope off 

 central Oregon with the aid of the submersible Alvin in August 

 1984 (Kulm et al . 1986) at sites of fluid venting in the Oregon 

 subduction zone. The shelf chimneys were the first occurrence of 

 possible fluid venting structures in the shallow waters of the 

 continental shelf off Oregon, and they are among a growing number 

 of features for fluid venting recently reported from shallow 

 water areas of the active and passive margins of the United 

 States (Paull et al. 1984; Brooks et al. 1984; Kulm et al . 1986; 

 Childress et al. 1986), North Sea (Hovland et al. 1987) and 

 Japan (Le Pichon et al. 1987). Both the venting sites off 

 Oregon and Japan are associated with subduction zones. 



Previous experience has shown that these venting sites can 

 only be studied in detail with the use of a submersible. Direct 

 observation, sampling and experiments are necessary to conduct 

 studies of the fluid venting processes and manifestations of such 

 processes. The submersible Mermaid and a Remotely Operated 

 Vehicle (ROV) Recon-IV were utilized to survey the suspected 

 venting area off northern Oregon during August 11-21, 1987. This 

 underwater survey was the first of a two-year dive program on the 

 Oregon continental shelf. 



This was an integrated geological and biological study of 

 fluid venting sites in the oldest portion of the accretionary 

 prism on the upper slope and outer continental shelf. The 

 specific geological objectives of this study were to (1) locate 

 actively venting carbonate chimneys on the outer shelf, (2) 

 determine the configuration and position of the chimneys in 

 relation to the geologic structure of seafloor, (3) compare the 

 physical, chemical and mineralogical characteristics of the shelf 

 chimneys with those found in deeper water on the continental 

 slope, and (4) determine the nature and source of fluids that 

 flow through the shelf chimneys. The first three objectives were 

 partially completed in the fall of 1987 two months after the end 

 of the field program. The fourth objective will be the focus of 

 the second field season scheduled for late summer of 1988. The 

 biological objectives are described elsewhere (Carey et al. 

 1988). 



METHODS 



In August 1987 the submersible Mermaid and support vessel 

 Aloha were contracted by NOAA ' s Office of Undersea Research to 

 survey the northern Oregon continental shelf area to locate 



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