Sedimentation and Seismic Stratigraphy of Sable Island Bank

Ron Boyd, Shirley A McLaren

The seismic stratigraphy of Sable Island Bank provides a model for continental shelf sand-body accumulation. The Holocene Sable Island sand body is located 227 km offshore from Halifax, Nova Scotia. It covers an area of approximately 6,500 km² on the outer Scotian Shelf and contains a sand volume of 98 km³. This region at 44°N lat. experiences both high-energy North Atlantic storms and high-velocity tidal currents. Relative sea level has risen from depths of at least 50 m 11,000 years ago and continues to rise at 33 mm/year today.

The Holocene shelf sand body was derived from the transgressive reworking of Pleistocene proglacial sediments that were deposited during the most recent late Wisconsinan lowstand of sea level. The subaerial Sable Island sand body is lens shaped, 50 km long, 2 km wide, and up to 51 m thick. The island extends subaqueously into asymmetric spits, each over 30 km long and up to 45 m thick. South and west from sable Island lies a system of shoreface-connected and shoreface-detached ridges that result from storm reworking.

Sediment transport in the early phase of transgression resulted from shoreface retreat and shoreface ridge reworking within Sable Island Bank. Transport was directed northeast to accumulate in the Sable Island region, forming a coarsening-upward sand body. Modern reworking results from both tidal and storm processes. Shoreface ridges migrate southeastward and continue to rework Pleistocene sediments south and west of Sable Island. Further reworking of the Holocene sand body is also taking place now by the migration of shoreface ridges. Sediments are transported northeast toward East Bar, northwest toward Northern Spur, and north over West Bar. Tidal and storm currents rework sediments arriving on both East Bar and West Bar, producing bidirectional sediment transport to both the north nd south.

The major Sable Island sand accumulation developed during the Holocene in an isolated outer shelf setting. It illustrates the importance of sea level and transgressive reworking to sand-body accumulation. The variety of morphologic and stratigraphic features present in the sand body indicate the complexity of sand transport processes in the continental shelf setting.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.