Substrate quarrying and subglacial till deposition by an early Proterozoic ice sheet: Evidence from the Gowganda Formation at Cobalt, Ontario, Canada

Abstract

Mapping at 1:2000 scale in the Cobalt area has resulted in a reinterpretation of the glaciogenic deposystem responsible for the basal 180 m of the lower Gowganda Formation (Coleman Member) of the Huronian Supergroup. Deposition of the Huronian strata at Cobalt probably occurred on a subsiding shelf, developed on the south-facing margin of the Superior Craton. The unconformity surface, which separates subhorizontal beds of the Coleman Member from underlying, steeply dipping stratified Archean rocks, exhibits a paleotopography with less than 100 m relief. Evidence of plucking and lee-side quarrying of this surface suggests southward movement of a grounded ice sheet. Basal diamictites occur as an extensive matrix-supported unit which generally blankets the unconformity surface, and which contains rare lenses of clast-supported diamictite. The latter generally are confined to local paleotopographic depressions that overlie breccia of subjacent derivation. Several lines of evidence indicate that the diamictites were deposited subglacially, both as an extensive orthotill (possibly melt-out till) and as restricted occurrences of lee-side till covering regolith. These conclusions, supported by studies of the lower Gowganda Formation in other areas, which also indicate either substrate glacial erosion or subglacial till deposition, suggest that recently proposed floating-ice-shelf depositional models for the Gowganda Formation are at least partially incorrect. This study also demonstrates: (1) the difficulty in identifying tillites in the rock record, indicating the need to examine many criteria before a till origin is accepted or rejected, and (2) the importance of detailed field examinations in seeking evidence of subglacial activity.