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Evolution of Early Proterozoic Passive-Margin Carbonate Platform, Rocknest Formation, Wopmay Orogen, Northwest Territories, Canada
John P. Grotzinger (2)
The superbly exposed Rocknest Formation, Wopmay Orogen, Northwest Territories, Canada, is an early Proterozoic (1.89 Ga) carbonate platform that formed along the eastern passive margin of Slave craton (Archean). The palinspastically restored shelf sequence is an eastward-thinning prism, 0-1,100 m thick, extending for over 220 km parallel to strike, and over 200 km perpendicular to strike. It contains, from west to east, slope, outer-shelf, shoal-complex, and inner-shelf facies. Slope facies are carbonate rhythmites, megabreccias, and platy rhythmite breccias. Outer-shelf facies include stromatolitic reefal boundstone and backreef ooid/intraclast grainstore. Shoal-complex facies are shallowing-upward cycles of tufa, cryptalgalaminite, laminated dolosiltite/lutite, overlain by tepees, b eccias, and pisolite; these formed a paleotopographic high, restricting an eastern "lagoon" from western ocean. Inner-shelf facies are dominantly asymmetric, shallowing-upward cycles of "lagoonal" mixed carbonates and siliciclastics, that grade into intertidal stromatolitic dolomites; cyclic facies pass eastward into noncyclic siliciclastics of central and eastern "lagoon." The Rocknest "lagoon" spanned the width of slave craton and was bounded by a western carbonate shoreline (shoal-complex) and by a tectonically active eastern siliciclastic shoreline.
The Rocknest carbonate shelf developed on an underlying siliciclastic shelf. An initial ramp rapidly evolved into a progradational accretionary rim and shoal complex, followed by vertical aggradation of the rim. Concurrently, the shoal complex underwent numerous west-to-east progradational events to produce many shallowing-upward cycles in the shelf interior. Cyclic sedimentation was interrupted during three events of incipient shelf drowning associated with local backstepping of the rim and noncyclic sedimentation over most of the shelf, probably caused by fourth-order eustatic sea-level oscillations. The shelf was terminally drowned and overlain by deep-water synorogenic siliciclastics during attempted subduction of the margin. Carbonates capping the shelf are stromatolitic patch re fs which overlie a possible unconformity, suggesting emergence of the shelf prior to drowning.
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