The Eel River basin of northern California contains a relatively thick (> 4 km) section of Miocene, Pliocene, and Quaternary fine-grained sedimentary rocks that extends more than 200 km northward from Eureka along the shelf and adjacent Klamath plateau. The east margin of the basin is a fault contact with the coastal belt Franciscan assemblage; the west margin is defined by a zone of uplift along the outer plateau-upper slope. This uplifted zone is characterized by a series of north-south trending ridges that rise as much as 200 m above the adjacent seafloor and against which Quaternary sediments thin.

Seismic reflection profiling and coring studies of these ridges have shown them to be diapiric. Quaternary sediments thin against the eastern flanks and are generally absent on the upper flanks and crests of the ridges. Many of the ridges are bounded by one or more faults showing large vertical separation. Seismic reflection records show internal structure of ridges to be homogenous and acoustically opaque, or to consist of faulted and deformed strata. Ridge crests are irregular in surface topography and are presumed to be highly deformed. Shallow cores from ridge crests contain stiff clayey silts of Pliocene age.

A 2-m core from ponded sediments on the crest of one of the diapir-like ridges contained significant amounts of gasoline-range hydrocarbons as well as anomalously high quantities of gas in the methane to butane range. We infer that these hydrocarbons are derived from sediments deeper in the Neogene sedimentary section and are being released through fractures in the ridges. Deformation of young surface sediments in ridge areas indicates that uplift is presently occurring along the 200-km long lineament defined by the ridges. These diapir-like ridges may prove useful targets for evaluating the hydrocarbon potential of the offshore Eel River basin.

End_of_Article - Last_Page 706------------