continental slope with the aid of the submersible Alvin. Two 

 time series measurements of fluid flow were completed using the 

 chemical changes of the captured fluids ( Suess et al. 1987). On 

 one of these deployments, fluid flow was measured with a flow 

 meter attached to the water sampler. It is highly probable that 

 an equally successful deployment could have been achieved had the 

 weather conditions been better and the technical layout and 

 equipment performance of the Mermaid and Aloha been optimized. 



Formation and Origin of the Carbonate Chimneys 



Two hypotheses had been proposed for the origin of the 

 chimney structures: (1) upward growth of the chimney above the 

 seafloor, and (2) chimney formation below the seafloor within the 

 sediment column with later exhumation by uplift and erosion 

 (Ritger et al . 1987; Schroeder et al. 1987). Based upon the 

 present study, we now think that the chimney grows upward 

 approximately at the same rate as the clastic sediments are 

 deposited at the sediment-water interface around the chimney. 

 The continuing fluid flow essentially keeps the precipitating 

 carbonate walls free of fine clastic debris, although the three 

 chimneys recovered from the shelf do contain sand and silt- 

 sized quartz, glauconite and feldspar. If the rate of clastic 

 sedimentation is slow and fluids continue to flow through the 

 chimney, it will grow upward well above the sediment-water 

 interface producing a much longer cylindrical chimney structure 

 (i.e., similar in length to the one recovered by the fishing 

 vessel Kodiak , Fig. 7). On the other hand, if the rate of 

 sedimentation is fast, the upward growth of the chimney might not 

 keep pace with the clastic sedimentation and the structure will 

 be buried. In this case, the escaping fluid may find a more 

 circuitous route to the surface and in the process forms the 

 complexly intertwined plumbing structure noted in the irregular 

 carbonate edifice recovered in this project (Fig. 9). It is 

 clear that some of the shelf chimneys were formed within the 

 seafloor and that they probably represent relatively recent 

 venting of methane and dissolved carbonate-bearing fluids from 

 the underlying Oligocene to Miocene accretionary prism. 



CONCLUSIONS 



1 . Extensive carbonate deposits were discovered on the 

 seafloor on the upper continental slope and outer continental 

 shelf off northern Oregon. This carbonate deposition is 

 occurring in a depositional environment dominated by clastic 

 sedimentation, which implies a unique set of geochemical 

 processes in the area 



2. These carbonate deposits include a large variety of 

 structures, such as crusts, slabs, chimney and irregular 

 edifices, which are covered with abundant living benthic 

 organisms. The lack of terrigenous sedimentation on many of the 



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