The upper Pethei Group is a 170-235 m thick, largely limestone, succession located in the east arm of Great Slave Lake, northwest Canada. Strata accumulated in a Paleoproterozoic foreland basin during the late stages of marine deposition. Rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages: (1) marginal peritidal complex; (2) back-barrier lagoon; (3) shelf margin; (4) subtidal shelf; and (5) slope and basin. Carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae, water-column precipitation of carbonate mud, and generation and transport of nonskeletal grains (stromaclasts, ooids, oncoids, peloids, and intraclasts). The upper Pethei platform is interpreted to have evolved through three basic platform geometries, peritidal rimmed shelf, subtidal rimmed shelf, and subtidal open shelf, in response to changes in the rate of creation of accommodation space. The peritidal complex and rim behaved in a manner consistent with models of Phanerozoic reefal rims, and alternatively, kept up, gave up, or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation. Three scales of cycle are recognized: (1) overall formation-scale upward-deepening of the platform; (2) laterally traceable, decameter-scale shallowing-upward cycles separated by marine flooding surfaces; and (3) laterally discontinuous, meter-scale shallowing-upward peritidal cycles. The first two cycle types are interpreted to have been allogenic, controlled by changes in eustasy and subsidence rates. Meter-scale cycles are interpreted as mainly autogenic, having formed in an aggradational tidal island model. The upper Petrel platform is similar to other Paleoproterozoic foreland-basin carbonate platforms. Important differences, however, include (1) laterally discontinuous peritidal cycles, (2) lack of widespread sand shoals inboard of the shelf rim, (3) a thick, predominantly carbonate, slope and basin succession, (4) moderate to low platformal micrite content, (5) extensive lagoonal ooid sands, and (6) low contents of terrigenous clastics.

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