The Geology of the Bras d'Or Lakes, Nova Scotia

Abstract

The evolution of the Bras d’Or Lakes since the retreat of the last ice sheets c. 15 ka (thousands of radiocarbon years before present, where present is defined as 1950) is inferred from multibeam bathymetry, seismic reflection profiles, and sediment cores. The thickness of stratified sediment in the Lakes overlying glacial till shows that there was a step-like retreat of ice towards a late ice centre in the western part of the Bras d’Or Lakes. As ice retreated, a lake formed in the area of the modern Bras d’Or Lakes and probably drained through Little Bras d’Or Channel. Ice retreat and sea level change on the continental shelf off south-eastern Cape Breton are inferred from multibeam bathymetry that shows proglacial subaerial river channels and suggests that sea level was perhaps 50 m lower than present about 15 ka. Relative sea level appears to have risen subsequently, so that marine conditions existed in Bras d’Or Lakes basin at 10 to 9 ka. Sea level may have risen to -15 m (below modern sea level) before falling again in the early Holocene. This falling early Holocene relative sea level resulted in the creation of freshwater lakes, with a prominent erosion surface at -25 m marking the lake level in some areas. Rising sea level then resulted in a return to marine conditions in the Lakes by 4 to 5 ka.