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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.

High-resolution seismic and side-scan sonar surveying, combined with coring, of the Fraser Delta slope seaward of the main fluvial distributary has revealed a complex suite of morphologic units which include undisturbed delta slope sediments, shallow rotational slides, relict and active slope channels, debris accumulations and ponded turbidites. Sediment transport through the main submarine channel is thought to be primarily by debris flows, rather than by turbidity currents. Although this is a seismically active region, the instability related features described here are probably due primarily to rapid sediment deposition on the upper delta slope at the river mouth.

The northwestern Canadian continental shelf, including Dixon Entrance, Hecate Strait and Queen Charlotte Sound, can be divided into four surficial geological units all overlying Tertiary bedrock within the Queen Charlotte Basin. They include: (1) glacial till, deposited primarily in the troughs between banks and considered to be late Wisconsinan in age; (2) glaciomarine mud, deposited in the troughs as ice retreated from the shelf; (3) Queen Charlotte sands and gravels, outwash deposition in the north and sublittoral sand deposition in the south during sea level low stand and subsequent reworking during transgression; and (4) Queen Charlotte muds, deposited within the troughs of the region from late Wisconsinan times until present. These units are a result of a glaciation ending 15,000–13,000 years BP followed by relative sea level lowering by approximately 100 m, that persisted until 10,500 years ago, and a subsequent transgression that occurred over approximately 1500 years in a series of steps.

Icebergs drifting in ocean currents over the Labrador continental shelf may contact and move through seabed sediments to create characteristically curvilinear features, commonly referred to as scours, which average 1–2 m in depth and about 30–40 m in width. Such scours may continue for several hundreds of metres or several kilometres. The period of seabed interaction may last from a few minutes to several months and results in modification of both the seabed sediments and the iceberg keel. When observed from submersible soon after their formation, scours exhibit morphological characteristics not seen in old, degraded scours. The scour trough, between two berm ridges, is generally flat-bottomed but is characterized by the presence of ridge-and-groove microtopography, with amplitudes up to 0.3 m developed parallel to the scour axis. These features are formed at the trailing edge of the keel by clastic material embedded in the ice and by open fissures in the ice. In places along the inner berm margins, ridges and grooves may be developed at an angle to the scour axis, reflecting lateral displacement of material towards the berm as the iceberg moves forward. Voids up to 1 m deep and 2 m wide occasionally truncate the ridges and grooves. Voids are formed following the dissolution of small (a few m 3 ) masses of debris-laden ice that are mechanically broken off from the base of the keel and pressed into the seabed by the scouring iceberg. Initially low areas within the scour trough may preserve seafloor that has not been affected by ice-seabed interaction. In these regions deposition of bulldozed sediment from the surcharge at the leading edge of the keel may partially fill the narrow spaces beneath the keel in areas of initially low seabed elevation. Scour berms consist of in situ fractured but intact blocks of material on the inner flanks, and disarticulated blocks 1–2 m high along the berm crest. The outer berm slopes generally consist of pieces of larger blocks spalled from the berm crest resting in relatively finely comminuted, reworked material. The reworked material originates in the leading edge surcharge before being displaced to either side of the keel. Scour berms have irregular topography ranging in height from a few centimetres to as much as 6 m above the seabed. Isolated irregularly-shaped mounds beyond the outer berm may represent pieces of cohesive sediment that originated at the berm crest as overhangs and which subsequently collapsed.

Bioherms dominated by hexactinellid sponges occur in the three major troughs which cross the north-central continental shelf of British Columbia, Canada. The preferred substrate of the bioherms is till, glaciomarine sediments or bedrock in 150 to 250 m water depth. These bioherms began to develop in the early Holocene and have expanded since that time to discontinuously mantle more than 700 km 2 today. The sponge colonies form mounds and sheet-like structures up to 15 m thick and several kilometres wide and accrete new material to the bioherm through the removal of silty clay from suspension by the baffling of bottom currents. The sedimentologic and morphologic characteristics of the bioherms are in part due to fused siliceous mainframe skeletons of the sponges that provide the framework for the construction of the bioherm. The locus of sponge habitation on the surface of the bioherms changes with time in response to sediment accretion and microtopographic changes which affect delivery of sediment and nutrients entrained in bottom currents.