The study discusses new sedimentological and geochemical results for the Paleoproterozoic Bruce glacial cycle, which represents the second of three glacial cycles within the Huronian Supergroup. This Bruce glacial event is unique for its cap carbonate, the Espanola Formation, which directly overlies glacial diamictites of the Bruce Formation. Cap carbonates are ubiquitous overlying glacial sediments in Neoproterozoic successions and are believed to be precipitated as a direct result of deglaciation. An investigation of the Bruce glacial cycle has led to a summary of the depositional history, wherein the Bruce ice sheet was grounded as it entered the Huronian basin and overrode outwash of the Mississagi Formation. It quickly delaminated, and deposited a uniform blanket of diamict before retreating, followed by the development of a glacial rebound sequence within the Espanola and Serpent Formations.
The upper Mississagi Formation directly underlies the glacial diamictites of the Bruce Formation and consists predominantly of planar and trough cross-stratified sandstones. These sandstones were deposited in a distal braided fluvial to deltaic system immediately prior to the advent of the local Bruce ice sheet advance. The Bruce ice sheet was grounded as it entered the basin, which is indicated by the typically sharp contact with the Mississagi Formation and the erosional uptake of Mississagi sands into the base of the Bruce. Delamination of the Bruce ice sheet and development of a floating ice shelf is illustrated by the dominance of massive, laterally continuous diamictites and evidence for current winnowing and iceberg rafting. These units, which constitute the majority of the Bruce Formation, were deposited beneath floating ice. Partial digestion of a carbonate-rich laminated dropstone lithofacies in the uppermost Bruce Formation for δ13Ccarb analysis produced highly negative values that are derived from a methane seepage signature. These same samples also have REE patterns with consistent negative Eu anomalies that are the result of remobilizing reduced Eu2+ via the highly reducing fluids sourced from methane seepage.
The Espanola Formation, the cap carbonate which overlies the Bruce Formation glacial deposits, is typically separated from the underlying Formation by a 0.5-3m thick laminated rain-out siltstone unit. Overlying this unit, the stratigraphy of the Espanola Formation records a gradual shallowing upwards sequence, with the lower, offshore facies being dominated by laminated carbonate and siltstone with abundant contortion and slumping, and the middle and upper stratigraphy dominated by ripple and hummocky cross-lamination and intraformational conglomerate beds deposited in a higher energy nearshore environment. An upward trend, from ~-4‰ to -2.5‰ over approximately 110m, occurs in the δ13Ccarb of the carbonate fraction of the middle and upper stratigraphy in drill hole E150-2. These samples produce patterns that have a ‘hat-shaped’ distribution with middle REE-enrichment that is similar to patterns of some Neoproterozoic cap carbonates. These patterns are suggested to be a reflection of influence by both seawater and freshwater signatures.
The stratigraphy of the upper Espanola Formation gradually transitions into the lower Serpent Formation, another sandstone-dominated Formation that represents the completion of the glacial rebound cycle. Environments of deposition for the lower Serpent Formation are variable, but range from storm- tide- and wave-influenced nearshore settings to fluviodeltic with varying tidal influence. This sequence concludes the glacial rebound cycle of the Bruce ice sheet in the Huronian Supergroup.