SEABED CRATER SIZE: A POSSIBLE RELATIONSHIP TO RESERVOIR DEPTH?

Over 4000 seabed craters (pockmarks) are observed on the upper continental margin offshore of western North America in the Eel River basin. The sizes of the craters appear to be correlated to specific geographic locations, and may be related to fluid reservoir depth. Craters that range from less than 10 m diameter to about 25 m diameter exist exclusively on the upper continental slope near an active anticline exposed at the seafloor. These craters exist in a relatively narrow range of water depth, from 400-600 m, coincident with the upper limit for gas hydrate stability in this region. Larger craters, about 40-100 m diameter, are present only in a region of extensive gullies. Craters that are greater than 200 m diameter are observed only near the headscarp of a local seafloor slide. In seismic reflection profiles across cratered regions of the seafloor, anomalous acoustic responses suggest the subsurface presence of gas hydrate, gas, or overpressured fluid zones. These acoustically anomalous areas are possible reservoirs of fluid, and vary in subsurface depth. The largest seafloor craters appear to be correlated with acoustic anomalies (possible fluid reservoirs) that are more than 100 m subsurface. In contrast, the smallest craters may form from shallow fluid reservoirs generated by disassociating near-surface (less than 10 m subsurface depth) gas hydrate. Video of an actively venting crater suggests that at least some of the craters form by rapid fluid expulsion from the seafloor.