ABSTRACT: Morphology and sedimentary processes of microtidal embayments, Beaufort Sea coast, western arctic Canada.

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Abstract

The Canadian Beaufort Sea coast is a marine-transgressive, seasonally ice-covered, fetch-limited, microtidal environment. Coastal embayments have formed by valley flooding, barrier growth, and transgressive breaching of thermokarst lake basins formed by thawing of massive ground ice. This paper presents data from two contrasting embayments: (1) an estuarine barrier lagoon receiving runoff and sediment from a 5000 sq. km catchment but relatively little sediment from the marine side; and (2) a system of breached thermokarst lake basins with negligible direct runoff but strongly affected by the turbid freshwater plume of the Mackenzie River. The lagoonal estuary of the Babbage River (Yukon coast) is typically <1.5 m deep, is partially protected by a low spit, and has a wide (2000 m) baymouth opening. The system is dominated by freshwater discharge from the river, particularly snowmelt runoff in spring; most of the fluvial sediment supply is carried into the estuary at this time and approximately 40% is exported. Positive storm surges of 0.7 m almost double the water volume in 20 hours, but the sediment introduced during these events is a small fraction of the fluvial input and resuspension from the shallow bay floor is important. Sediment budget estimates and other observations suggest mean accumulation rates as high as 8 mm/a averaged over the estuary (rates vary from <1 to >50 mm/a at individual sites). The breached-lake embayments of Richards Island, adjacent to the Mackenzie Delta, include a complex of more than 20 connected basins, some as deep as 10 m. Large parts of the system are shallow (<1.5 m) with narrow channels (up to 3 m deep) in constricted areas. Openings to the ocean and between basins vary from <10 to 1000 m in width. Sediment sources include Pleistocene sands and muddy till eroded from embayment shores or the outer coast and fine silt and clay (grain sizes <10 micro m) from the Mackenzie River plume. Summer and autumn storm surges can import water volumes of the order of 10**7 cu m over a few hours and tidal currents of to 0.6 m/s or more occur in channels. Pre-breach deposits in lake basins are massive to faintly laminated. Overlying post-breach sediments are finely stratified. Preliminary 137Cs determinations show an increase in sedimentation rates from <1 mm/a in closed lakes to <=17 mm/a after breaching, suggesting that mud from the Mackenzie plume is the dominant component of the sediment budget in these embayments.

Bibliography of Canadian Geomorphology