Subsurface gas offshore of northern California and its link to submarine geomorphology

The northern California continental margin contains evidence of abundant su bsurface gas and numerous seafloor features that suggest a causative link b etween gas expulsion and geomorphology. Analyses of seismic reflection, sid escan sonar, and high-resolution multibeam bathymetric data show that the o ccurrence of subbottom gas and the migration processes beneath the shelf di ffer from those beneath the slope. Subsurface gas, inferred from enhanced r eflectors and other geophysical indicators, is spatially variable and relat ed more to total depth and stratigraphy than to underlying structure, with the exception of one band of gas that follows the regional structural trend . Shallow depressions on the seafloor (pockmarks) are used to infer expulsi on sites. The largest zone of acoustically impenetrable subsurface gas occu rs between water depths of 100 m and 300 m, where expulsion features are ra re. The upper slope (water depths 400-600 m) has a high concentration of po ckmarks (diameter 10-20 m), in contrast to a near-absence of pockmarks at w ater depths shallower than 400 m. Of nearly 4000 pockmarks observed on side scan sonar records, more than 95% are located in water depths deeper than 4 00 m. Bottom simulating reflectors (BSRs) on some seismic reflection profil es indicate the possible presence of gas hydrate. We find that gas and pore -fluid migration in the offshore Eel River Basin is: (1) correlated to surf ace morphology; (2) a contributor to seabed roughness; (3) a significant mo de of sediment redistribution on the upper slope; and (4) potentially a fac tor in large slope failures. (C) 1999 Elsevier Science B.V. All rights rese rved.