ABSTRACT: Ice-bonded sediments and massive ground ice in a transgressive barrier lagoon and delta complex, Yukon coast of Beaufort Sea, western Arctic Canada.

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Abstract

The barrier-lagoon estuary of the Babbage River is predominantly less than 1.5 m deep (the approximate thickness of winter ice). It is partially enclosed by a spit 4 km long averaging 60 m wide. There is a 2 km wide baymouth opening at the end of the spit opposite the delta front. The system receives runoff and sediment from Deep Creek and Babbage River, which drain part of the coastal plain and mountains to the south. The rate of relative sea-level rise exceeds the rate of delta-plain sedimentation, the delta front. A number of shallow boreholes drilled in winter to depths from 10 to 36 m showed icebonded sediments beneath the surrounding high ground, delta plain, tidal flats, and bottomfast ice in the lagoon. In about 2 m water depth seaward of the spit, the seabed sand and gravel and upper part of underlying mud were unfrozen to a depth of 10 m and underlain by ice-bonded silt and clay with thin lenses of massive ice. Farther seaward in 8 m water depth, the sediments were unbonded to at least 22 m below seabed. Barrier-beach sand and underlying silt beneath the spit were ice-bonded to 9 m at one site (11 m at another), below which a 4 m thick talik of unfrozen silt was found above the contact withunderlying ice-bonded silt and clay. The unbonded layer at depth beneath the spit and thicker units of unbonded sediments beneath the backbarrier lagoonsuggest downward refreezing of shallow estuarine sediments as the washover-dominated spit migrates landward into the lagoon. The delta plain is a low-relief surface with numerous shallow ponds and very subtle levees along channel margins. We hypothesize that anomalous higher surfaces of chaotic microtopography may be elevated in part by massive ice growth fed from taliks beneath adjacent deep channels. A borehole through one such surface near the delta front, adjacent to an 8 m deep thalweg scour depression in the main distributary channel, was underlain by 3.5 m of massive ice over ice-bonded gravel with 20% ice by volume. Salt water under pressure was encountered at 23 m. Salinities as high as 60 psu or higher have been measured in water confined below winter ice in the delta. These resultsconfirm preservation in shallow coastal waters of massive ice and ice bonding developed below formerly subaerial surfaces which have subsequently been transgressed. Observations of ice-wedge polygon terrain below shallow lagoon margins elsewhere along the Beaufort Sea coast support similar conclusions. Downward thaw of seabed sediments is initiated in the nearshore seaward of the spit as it migrates landward.

Bibliography of Canadian Geomorphology