POCKMARKS, SEEPS, AND CARBONATES ON HECETA BANK AND ITS SEAWARD SLOPE

Dives with the ROPOS remotely operated vehicle in the summers of 2001 and 2002 on Heceta Bank and its seaward slope off central Oregon discovered 10 seep sites in water depths from less than 100 meters to 500 meters. These can be grouped into four types: (1) Circular "pockmarks"100-200 in diameter and up to5 meters deep within the mud zone on the north and northwest seaward flanks of the bank in depths of 220 to 300 m; (2) streams of gas bubbles at the top of the bank (<100 m) emanating from mudstone outcrops; (3) A 2 x 0.5 km-area of acoustically reflective material south of Heceta Bank in 500 meters of water characterized by extensive carbonate pavements and patches of Calyptogena pacifica (S. Goffredi, pers. commun.), and (4) a 100-m long carbonate ridge oriented NW-SE parallel to the regional left-lateral strike slip direction described by Goldfinger et al. (1992). Most of the pockmarks had active areas characterized by microbial mats and gas bubbling. Solemya sp. valves and a live specimen of Conchocele bisecta were recovered from a pockmark on the upper continental slope northwest of the bank. In addition to the more obvious seeps characterized by carbonate pavements, endosymbiotic fauna and (in some cases) gas bubbles, there were numerous other sightings of small patches of (apparent) microbial mats. The origin of the methane is problematic. However, Collier and Lilley (1992) reported carbon isotope values from a methane sample from a shallow water pockmark on the southern Oregon shelf consistent with a deep thermogenic source. The overall impression is that methane seepage is very common in the Heceta Bank and that it could be an important component of the geologic and biologic processes of the upper slope and outer shelf region.