Morphologic Expression of Gas Hydrate Growth and Accumulation within the Seafloor of the Santa Monica Basin, Offshore California

C. K. Paull¹, W. Ussler, III¹, D. W. Caress¹, and W. R. Normark^2
1Monterey Bay Aquarium Research Institute, Moss Landing, CA, [email protected], [email protected], [email protected]
²United States Geological Survey, Menlo Park, CA

Seafloor blister-like mounds, methane migration, and gas hydrate formation were investigated through detailed seafloor surveys in Santa Monica Basin, offshore of Los Angeles, California. Two distinct deep-water (≥800 m water depth) bathymetric features were surveyed using an autonomous underwater vehicle equipped with a multibeam sonar and a chirp sub-bottom profiler, providing 1-m lateral resolution bathymetry and 0.1-m resolution seismic images of the subseafloor stratigraphy. The northerneastern mound was explored with the remotely operated vehicle Tiburon, which collected sediment core, biological, and methane gas samples. The mounds are ~10-m-high and ~100-m-wide dome-shaped features, which protrude from crests of broad anticlines (~20 m high and 1 to 3 km long) formed within latest Quaternary-aged seafloor sediment by compression between lateral offsets in regional strike-slip faults. No allochthonous sediment was observed on the mounds, except for the locally derived slumped material off steep slopes. Continuous streams of methane gas bubbles emanate from the crest of the northeastern mound, and extensive methane-derived authigenic carbonate pavements, microbial mats, and chemosynthetic communities mantle the mound surface. The large local vertical displacement needed to produce these mounds suggests a corresponding net mass accumulation has occurred within the immediate subsurface. Formation and accumulation of pure gas-hydrate lenses in the subsurface is proposed as a mechanism to blister the seafloor and form these mounds.

The high-resolution chirp sub-bottom images allow the stratigraphy to be traced 15 km across the Santa Monica Basin, from ODP Site 1015 (where the upper 100 m is primarily composed of turbidite sands) to the mounds. This tie indicates that the subsurface section within the gas hydrate stability zone and above the level of a BSR is sand-rich. This is among the few areas where a marine gas hydrate occurrence in potentially sand-dominated lithologies has been identified.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009