The Eel Canyon (shown in the inset above, adapted from Clarke, 1992) is located about 20-30 km northwest of the Mendocino Triple Junction (MTJ) and Cape Mendocino in northern California. The canyon is of Miocene age (possibly younger); however, most of sediment received on the margin has been delivered there in the last 200-300 ka. Within the last few hundred thousand years, the area just to the east of the MTJ has been undergoing a dramatic acceleration in uplift. Uplift on land causes sediment supply to the ocean to increase.

In the ocean, sediment acts to sculpt topography in a similar manner as liquid water does on dry land. This is because the negative buoyancy added to the water column by sediment drives many of the flows near the ocean bottom. These flows are sometimes called sediment gravity flows. To investigate these processes, Beth Mullenbach and I ran a multibeam-bathymetry (mapping) survey of the lower Eel Canyon. A few months before, Beth Mullenbach and Chuck Nittrouer ran a complementary cruise using the ROV Ventana to directly explore the upper reaches of the canyon. They extensively cored the canyon and the sidewalls. In the course of these observations, we discovered several pieces of evidence of recent incision. The most obvious signature of erosion was the nearly vertical mud “cliffs” observed in the uppermost reaches of the canyon (shown above). In the image below you can see the subtle long wavelength “furrows” commonly associated with erosional sediment gravity flows. The furrows appear as bands of high reflectance in the sonar image. The sonar scanned a radius of approximately 100 m.

The most interesting features we found (in my opinion) on the mapping cruise were the “step-pools” shown below. These step-pools appeared as a distributary originating from the main channel nearly 10 km landward from the base of slope. Using analysis developed for subaerial landforms, we were able to identify certain flow characteristics that have caused the features observed.

Jeff Parsons’ Research UW-Oceanography