Structurally controlled hydrocarbon seeps on a glaciated continental margin, Hecate Strait, offshore British Columbia

Abstract

This paper describes structurally controlled gas seeps and their influence on seafloor morphology at sites within Queen Charlotte Basin (QCB); a sedimentary basin situated offshore British Columbia, western Canada. Numerous surficial and shallow sediment features related to both ice cover and shallow gas have been identified in high-resolution multichannel seismic, Huntec deep-towed seismic, multibeam bathymetry and backscatter, and piston core data that were acquired over a 10 km by 10 km grid in an eastern part of QCB. Pockmarks, seafloor mounds and iceberg ploughmarks are visible in the multibeam data; small-scale folds, vertical blank zones and acoustic turbidity are observed in the Huntec data. The multichannel seismic data show a faulted-anticline, phase-reversed bright spots, zones of low instantaneous frequency and velocity pull-ups. One seafloor mound lies directly above a shallow fault that offsets an asymmetric fold. The remaining mounds lie either above or in close proximity to asymmetric folds that are faulted. Bright spots are observed along some fault traces and within a cut and fill sequence at two-way travel times between 0.42 and 1.0 s. Velocity pull-ups are observed in the multichannel data below gas expansion voids present in piston cores. Backscatter data show that seafloor mounds are highly reflective compared to the surrounding seafloor. These observations indicate that gas has been migrating upward through the rock column and expelled into the shallow sediments. The shallow gas has been biologically mediated to form carbonate sediments within and around seafloor mounds; shallow gas has also deformed surficial sediments into positive relief mounds and negative relief pockmarks.