Evolution of early Proterozoic passive-margin carbonate platform, Rocknest Formation, Wopmay Orogen, N.W.T., Canada

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Virginia Polytechnic Institute and State University

The superbly exposed Rocknest Formation, Wopmay Oroqen, N.W.T., Canada, is an early Proterozoic (l.B9 Ga) passivemargin carbonate platform. It is an eastward-thinning prism, 0 to 1,100 m thick, extending for over 220 km parallel to strike, and over 200 k.m perpendicular to strike. It contains from west to east, slope, outer-shelf, shoaloomplex, and inner-shelf facies. Slope facies are rhythmites, megabreccias and rhythmite breccias. Outershelf faciea 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, breccias and pisolite; these formed a paleotopographic high, restricting an eastern "lagoon" from western ocean. Innershelf facies are dominantly asymmetric, shallowing-upward cycles of "lagoonal" mixed carbonates and siliciclastics, that grade up into intertidal stromatolitic dolomites; cyclic facies pass eastward into non—cyclic siliclastics of central and eastern "lagoon". The shelf was terminally drovmed during attempted subduction of the margin. Individual cycles are correlatable for over 200 km parallel to strike and over 120 Ion perpendicular to strike. Cycles formed in response to small, asymmetric (rapid rise/slow fall) eustatic oscillaticns in sea-level (= 10 m). An alpine glacio-eustatic model accounts for the origin of cycles best. During sea-level rise, rapid transgression and submergence of tidal flats occurred along eastern margin of the shoal-complex, followed by eastward expansion of the shoal—complex, and slow progradation of tidal flats over lagoonal facies during sea-level fall. Computer modelling of Rocknest cycle types across the platform provides important information on absolute l sedimentation rates and lag times.