Abstract

Mapping at 1:2,000 scale in the Cobalt area has allowed detailed interpretation of the glaciogenic deposystem for the basal 180 m of the Coleman Member, lowermost unit of the Gowganda Formation, Huronian Supergroup. Deposition at Cobalt occurred on the subsiding shelf of a south-facing passive margin. The unconformity that separates subhorizontal Coleman Member lithofacies from underlying Archean rocks exhibits a subdued topography and less than 100 m of relief. Evidence of plucking and lee-side quarrying of this surface support southerly movement of a grounded ice sheet. Massive basal diamictites were deposited subglacially as extensive primary basal till and as restricted lee-side till that infilled paleotopographic lows and covered preglacial regolith. Coarsening-upward rhythmite/sandstone/clast-supported diamictite associations were deposited as prograding subaqueous outwash fans in front of a slowly retreating wet-based tidewater ice sheet grounded in 50 to 300 m of water. Rhythmite characteristics indicate a distal change from underflow to interflow/overflow sediment-transport, supporting an ice-contact, marine-basin interpretation. Differences in dropstone abundance in the rhythmites suggest variable rates of ice-front calving and iceberg transport. Interfan areas received sediment from melt water and from abundant sediment gravity flows off over-steepened fan slopes and collapsing ice-marginal banks. An upper diamictite association contains dropstone diamictite, poorly laminated mudstones, diamictite with random clast orientations, and thick diamictite sequences characteristic of deposition from sediment gravity flows. These features suggest transition from a grounded to a partially floating ice sheet before deposition of the uppermost Coleman Member strata preserved at Cobalt.

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