paleoproterozoic snowball earth? sedimentology and geochemistry of a huronian glacial cycle
abstract
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.