main cavity extends the length of the chimney. Skeletons of 

 encrusting corals and sponges are still attached to the chimneys. 

 Dolomite (69 -89% carbonate), Si02 (14-22%), and A1203 (4-6%) 

 are the chief components of the shelf chimneys ( Schroeder et al. 

 1987). Tests of foraminifera as well as scattered quartz and 

 feldspar grains occur within the carbonate matrix. 



The stable isotopic composition of the three shelf chimneys 

 is quite similar (Table 3). Negative kl3 C values (-16.9 to - 

 21.9 °/oo) show that the carbon in these carbonates is moderately 

 depleted in carbon-13. Interestingly, the large positive kl80 

 values (+7.62 to +7.87 °/oo) are much heavier than those found in 

 other authigenic carbonates on the lower and middle continental 

 slope on the Oregon margin ( Ritger et al . 1987), suggesting a 

 different source of fluids within the accretionary prism. 



RESULTS 



Field Observations and Sampling: Northern Oregon Shelf 



One submersible dive, two ROV deployments, and two otter 

 trawls were conducted on the northern Oregon shelf and upper 

 slope in the vicinity of Nehalem Bank (Fig. 8). Collectively, 

 these observation and sampling devices outlined extensive areas 

 of carbonate deposits which occur in a great variety of sizes 

 and shapes. The first ROV traverse (#1) covered the southern 

 side of the uplifted bank from water depths of 382 to 92 m. This 

 traverse showed the frequent occurrence of flat-lying carbonate 

 slabs and crusts on the seafloor and a perfect example of a 

 cylindrical chimney rising above the seafloor. Many irregular 

 carbonate edifices, the size of large boulders, were also 

 encountered along the traverse. These features are all 

 covered with abundant benthic organisms, especially crinoids (see 

 Carey et al. 1988) . 



The crusts and slabs are generally several centimeters to 

 tens of centimeters thick, have an irregular shape and are about 

 one meter across at the widest dimension. The slabs clearly rise 

 above the surrounding sediment for several tens of centimeters. 

 They are whitish in color along their edges which strongly 

 suggests they are composed of carbonate. Several of these 

 structures have holes on their surfaces and are free of sediment 

 while others appear to have a thin covering of sediment. One 

 slab was recovered with the submersible. Crinoids are attached 

 to most slabs and crusts in great numbers. 



One perfectly circular chimney was clearly identified in 

 ROV traverse #1 (see Table 1, shelf chimney). It rises from 15 

 to 20 cm above the surrounding seafloor and is 25 to 30 cm 

 across. This chimney has a vertical opening of 15 to 20 cm in 

 diameter which is completely free of sediment. It appears to be, 

 at least partially, embedded within the sedimentary strata of the 



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