Daryl M. Wightman

A discontinuous glaciomarine outwash terrace on the north shore of the Minas Basin and ice contact stratified drift north of Parrsboro were mapped. Detailed sedimentological studies on some the deposits provide insight into deglacial depositional processes.

The outwash terrace consists predominantly of marine Gilbert type deltas with molds and casts of Portlandia arctica in the bottomset beds. The topset beds and glaciofluvial gravel inland of the deltas were deposited by braided meltwater streams with sustained velocities (inferred) of 1 to 4 m/sec and with flow depths of 1 to 2 m. The predominant crude bedding of the gravel is a result of rapid downstream sediment movement. Shifting of the streams produced a planar, erosion unconformity between the topsets and finer grained foresets. Bed load sediment avalanched down the foreset slope while fine sediment was carried in suspension onto the bottomset slope. In the winter, the supply of meltwater ceased and clay and fine silt settled out of suspension, producing bottomset varves where coarse silt and sand had been deposited. Infrequent large slumps of the foresets produced turbidity currents which deposited coarse sand in the bottomsets. Foreset progradation caused instability and failure of the bottomsets which occasionally submerged parts of the delta and caused secondary foreset deposition.

Pebble counts show that as the receding valley ice crossed bedrock boundaries, different rock types were supplied to the delta. During and after deltaic deposition, postglacial rebound exceeded sea level rise and the deltas became emergent, with emergence increasing steadily to the west. Meltwater then eroded terraces in the glaciomarine and inland glaciofluvial sediments. The northward receding ice in the Parrsboro Gap built a small recessional moraine that forms the present drainage divide between Parrsboro River and River Hebert at Gilbert Lake. Holocene sea level rise and subsidence of the land have resulted in submergence of the east end of the terrace and substantial erosion of the deposits. Eroded sediments are subaerially preserved only at Advocate Harbour.

Radiocarbon dating of organic sediment in kettles and lakes is inconclusive. The pollen assemblage and stratigraphy of the Leak Lake core suggest correlation with outwash deposits in southwestern New Brunswick, dated at 13,000 to 13,325 yr BP. Glaciomarine sediment, which appears to be an offshore equivalent to the outwash terrace, in the Minas Basin is about 14,000 years old, suggesting that the terrace is slightly older than was previously thought.

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Pages: 440
Supervisor:  H. B. S. Cooke

Bermuda 1978