PERITIDAL CARBONATE PLATFORM GROWTH AND CYCLICITY IN AN EARLY PROTEROZOIC FORELAND BASIN, UPPER PETHEI GROUP, NORTHWEST CANADA

The upper Pethei Group is a 170-235 m thick, largely limestone, succes sion located in the east arm of Great Slave Lake, northwest Canada. St rata accumulated in a Paleoproterozoic foreland basin during the late stages of marine deposition. Rocks include nine predominantly stromato litic facies that are grouped into five facies assemblages: (1) margin al peritidal complex; (2) back-barrier lagoon; (3) shelf margin; (4) s ubtidal 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, ooid s, oncoids, peloids, and intraclasts). The upper Pethei platform is in terpreted to have evolved through three basic platform geometries, per itidal rimmed shelf, subtidal rimmed shelf, and subtidal open shelf, i n response to changes in the rate of creation of accommodation space. The peritidal complex and rim behaved in a manner consistent with mode ls of Phanerozoic reefal rims, and alternatively, kept up, gave up, or caught up to sea level in response to fluctuations in the rate of cha nge of relative sea level and lateral variations in sediment productio n and accumulation. Three scales of cycle are recognized: (1) overall, formation-scale upward-deepening of the platform; (2) laterally trace able, decameter-scale shallowing-upward cycles separated by marine flo oding surfaces; and (3) laterally discontinuous, meter-scale shallowin g-upward peritidal cycles. The first two cycle types are interpreted t o 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 Pethei platf orm is similar to other Paleoproterozoic foreland-basin carbonate plat forms. Important differences, however, include (1) laterally discontin uous peritidal cycles, (2) lack of widespread sand shoals inboard of t he shelf rim, (3) a thick, predominantly carbonate, slope and basin su ccession, (4) moderate to low platformal micrite content, (5) extensiv e lagoonal ooid sands, and (6) low contents of terrigenous clastics.