EVOLUTION OF AN EARLY PROTEROZOIC FORELAND BASIN CARBONATE PLATFORM, LOWER PETHEI GROUP, GREAT SLAVE LAKE, NORTH-WEST CANADA

The Taltheilei, Utsingi, McLean and Blanchet formations form a 175 390 m thick carbonate platform-to-basin succession in the lower part of t he PaleoProterozoic Pethei Group, preserved in the eastern arm of Grea t Slave Lake. Carbonates accumulated along the south-east margin of th e Slave Craton within a foredeep formed during the collision of the Sl ave and Churchill Cratons. The rocks include eight, predominantly micr obial, carbonate facies that comprise five facies associations represe nting (1) shallow-water rimmed shelf, (2) shallow-water open shelf, (3 ) shallow-water ramp, (4) upper slope and deep ramp, and (5) lower slo pe and basin plain environments. Microbialite facies grew by organical ly mediated precipitation of spar and micritic cement and trapping and binding of lime mud. These wholly subtidal facies typically reflect p rogressive shallowing and changing geometry of the lower Pethei sea fl oor, from ramp, to open shelf, to shallow rimmed shelf, with associate d slope and basin plain deposition. Repeated relative sea-level change s influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying mag nitude. Antecedent topography and the size of the preceding drowning e vent strongly influenced the initial growth of each interval. This rep eated pattern is attributed to interaction between (a) the inherent te ndency of microbial carbonates to aggrade vertically, (b) changing sed imentation rates and (c) readjustments of relative base level. The low er Pethei succession is one of few PaleoProterozoic examples of carbon ate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.