Late Quaternary sedimentation and sea level changes on the inner Scotian Shelf

Abstract

The inner Scotian Shelf off the Eastern Shore of Nova Scotia forms an irregular surface that extends some 25–30 km seaward of the present coastline to water depths of about 100 m where it drops off into Emerald Basin. The distribution of Late Quaternary deposits is highly variable both along and across the shelf. These sediments preserve a record of Late Wisconsinan glaciation, ice recession, and late- and post-glacial changes of relative sea level. Glaciomarine deposits occur in a valley complex extending seaward from Halifax Harbour. East of Halifax, we observe a three-part zonation across the inner shelf. The innermost zone extends to water depths of about 50 m. It is characterized by acoustic basement (Meguma Group metasediments), either outcropping or overlain by acoustically unstratified deposits, interpreted as glacial diamict, and by a unit interpreted as stratified outwash. These units are overlain by stratified valleyfill deposits representing Holocene lacustrine and estuarine facies, which have been sampled in a number of cores. Much of the inner shelf is covered by a thin veneer of sand and gravel, generally less than 1 m thick. Further seaward, the sea floor is an erosional unconformity that truncates acoustic units interpreted as glacial diamict and stratified drift. The stratified estuarine deposits found in the inner zone appear to be absent here, but thin patches of transgressive lag deposits occur throughout the area. The outer part of the inner shelf is dominated by outcrop of acoustic basement, with very limited surficial sediment cover. This zone of rugged outcrop occupies half or more of the inner shelf width over much of the study area. The extensive outcrop is attributed to a combination of glacial and/or glaciofluvial erosion, limited recessional deposition, and reworking of any remaining sediment cover by energetic long-period surface gravity waves under lower postglacial sea levels. Seaward of the outcrop zone, there is a transitional area between the inner shelf and Emerald Basin. This zone is characterized by high relief, with exposures of acoustic basement rising up to 60 m above intervening depressions. The depressions are partially filled by stratified glaciomarine and marine deposits up to 55 m thick. Lateral transitions between stratified and unstratified facies along a morainal ridge in northern Emerald Basin suggest the presence of a partially grounded floating ice margin in this area. Late-glacial relative sea level changes remain poorly defined. Several lines of evidence suggest high relative sea level early in the process of glacial recession from the inner shelf, followed by a rapid drop resulting from glacio-isostatic rebound. Samples of estuarine and salt-marsh deposits collected in cores from the inner part of the inner shelf provide evidence of Holocene marine transgression from below −40 m at 11,000 years BP, continuing to the present.