The Foreslope Hills: large-scale, fine-grained sediment waves in the Strait of Georgia, British Columbia

Abstract

The Foreslope Hills are a set of 20-m-high, >5-km-long ridges at 230–350-m water depth in the south-central Strait of Georgia, occupying a >60-km2 area at the base of the Fraser River delta foreslope. They are composed of clay, silt and fine sand. Previous authors have interpreted them as: (1) a single mass-slide deposit, (2) mud diapirs, (3) in situ rotational failures, and (4) creep deformation features, but collaborative evidence for these interpretations is lacking. Seafloor surface morphological renders derived from recent multibeam sonar bathymetry data show linear, evenly spaced symmetrical ridges with ridge bifurcation, resembling wave ripples but on a much larger scale. Detailed multichannel and single-channel seismic reflection data show sinusoidal internal reflections suggesting upslope migration. The ridges occur within the uppermost portion of the Holocene section in a very high sedimentation rate environment. Consequently, they are probably less than 3000 years old. Moored current meter records for the region show reversing currents, largely oriented normal to the ridge crests, averaging 10 cm/s with a maximum of 50 cm/s. This evidence, combined with increasing documentation of large-scale sedimentary bedforms in slope and rise environments, suggests that the Foreslope Hills should be re-interpreted as sediment waves. Upslope migration is typical of many large-scale, fine-grained sediment waves. The preferred physical interpretation for the creation and maintenance of the Foreslope Hills as sediment waves is the lee-wave model, requiring a phase shift in density-stratified flow as it passes over the wavy topography. Maximum measured bottom currents are easily strong enough to entrain silt and clay delivered to the site by turbidity currents or by sediment plume fallout from Fraser River outflow. Either strong flood tides or severe biannual density intrusions, due to their intense stratification, may be susceptible to lee wave formation and the creation of sediment waves, leading to the observed pattern of upslope migration.