The western Grand Banks of Newfoundland: Transgressive Holocene sedimentation under the combined influence of waves and currents

Abstract

The surficial sediments on the western Grand Banks of Newfoundland have been reworked from older deposits (primarily late Wisconsinan glacial and glaciomarine sediments) during the post-glacial sea-level fall and Holocene rise. Winnowed medium to coarse sands and lag gravels that are generally less than 0.5 m thick chracterize the areas above the lowstand elevation at −110 to −120 m. The Holocene sediments which have accumulated in the deeper interbank channels, isolated basins and on the upper continental slope are thicker and consists of the fine sands and muds that were removed from the banks. The deposits below the lowstand elevation coarsen upward because little deposition occured during the sea-level rise. In general, the Holocene succession becomes thicker and finer grained to the south. Active sediment movement occurs at all depths less than about 110 m. Wave ripples composed of sandy gravel form to depths in excess of 100 m in response to long-period (15 s) surface waves, but the predominant bedforms consist of current-generated sand ribbons, megaripples and sandwaves which exhibit unidirectional migration azimuths toward the southwest, even though the storm-current directions are highly variable and the maximum sediment-transport rates are directed toward the east or southeast. The bedform orientation and resultant transport are instead parallel to the southerly and southwesterly storm currents which occur during the prolonged, post-storm spin-down period, and to the weak (<0.1 m s −1) Labrador Current and the orbital motion associated with the largestwaaves. Thus, the interaction of the oceanic current and wave-orbital motion with the directionally-variable strom currents is responsible for the consistent orientation of the current-generated bedforms, and for the distribution of Holocene deposits.