Seabed processes on the northeastern Grand Banks of Newfoundland; Modern reworking of relict sediments

Abstract

On the northeastern edge of the Grand Banks of Newfoundland, between 140 and 70 m depth, a thin Holocene sediment veneer unconfortmably overlies Tertiary siltstones and sandstones. These surficial sediments show a depth-controlled facies distribution. Below 90 m the facies range from continuous fine sand, through lag gravel and sand ribbons with arcuate sand waves and megaripples normal to the axis of the ribbons. Above 90 m depth alternating sand ridges up to 3 m in height and lag gravels occur. Megaripples and arcuate sand waves (400 m wave length) with megaripples on their stoss side are developed on the terrace. The regional geology suggests that many of the surface sediments were inherited from Pleistocene deposits subsequently reworked in coastal environments by an early Holocene transgression. Side-scan sonar surveys, bottom photography and submersible observations demonstrate intermittent sediment mobilization of recent material. In particular, photographs show asymmetrical ripples in well-sorted sand after seasonal storms. Side-scan sonar coverage in the Hibernia area shows fields of two- and three-dimensional megaripples (5 m length) which appear to be migrating across trawl marks left by recent fishing operations. These sedimentary bedforms are attributed to unidirectional storm driven currents, ocean currents and extreme waves. The larger bedforms may reflect paleoceanographic conditions while the small bedforms appear to be modern.