active venting sites in the vicinity of the carbonate chimneys 

 recovered earlier. Due to inclement weather conditions and 

 equipment failures only three dives, one off northern Oregon and 

 two off central Oregon in the vicinity of Heceta Bank (Fig. 1), 

 were completed during the nine-day dive program. However, four 

 deployments of the ROV were made successfully with the Recon-IV , 

 two on the northern Oregon shelf and two in the vicinity of 

 Heceta Bank. The ROV deployments were limited to moderate sea 

 conditions because of the lack of an acoustic tracking system 

 which determines the position of the ROV relative to the surface 

 vessel . 



GEOLOGIC SETTING 



Deformation and uplift of the North American plate during 

 the past 60 my, which is caused by the converging Juan de Fuca 

 plate, has developed a structurally and stratigraphically complex 

 continental margin (Kulm and Fowler 1974; Snavely et al . 1980). 

 Clastic sediments are scraped off from the subducting oceanic 

 plate forming an accretionary prism which is comprised of a 

 series of fold and thrust ridges, with intervening basins, 

 striking subparallel to parallel to the Oregon-Washington margin 

 (Fig. 3; Kulm et al . 1973; Kulm and Fowler 1974; Barnard 1978; 

 Snavely et al . 1980). The most recently deformed ridges (<2 Ma) 

 lie farthest seaward at the toe of the continental slope (Fig. 4; 

 Kulm and Fowler 1974; Kulm and von Huene et al . 1973a) and the 

 ridges become successively older and more complexly deformed 

 farther landward. 



Submarine banks occur along the outer edge of the 

 continental shelf (Kulm and Fowler 1974; Snavely et al . 1980) 

 and form some of the most strikingly folded and faulted areas of 

 the continental margin. The chimneys described in this paper 

 are located on Nehalem Bank, on the outermost edge of the shelf 

 (Fig. 3). An accretionary prism Oligocene to Miocene in age 

 underlies this portion of the bank and underthrusts the Eocene 

 volcanics to the east (Snavely et al . 1980; Peterson et al . 

 1986). Miocene to Pleistocene sedimentary deposits overlie the 

 prism and contain unconformities that are late Miocene to late 

 Pliocene in age (Kulm and Fowler; 1974). 



RELEVANT PREVIOUS STUDIES 



Continental Slope 



In 1984 sites of fluid venting were observed on the lower 

 continental slope off central Oregon using the submersible Alvin 

 (Kulm et al . 1986). Each site is located on the crest of a 

 Pleistocene thrust ridge about 110 km to the south of the shelf 

 chimney field at a water depth of 2036 m (Fig. 4). Vent-type 

 animals (harboring chemosynthetic bacteria), authigenic carbonate 

 chimneys and slabs, and anomalous concentrations of methane 



155 



