Cyclicity and paleoenvironmental dynamics, Rocknest platform, northwest Canada

Abstract

The superbly exposed Rocknest Formation, Northwest Territories, Canada, contains 140 to 160 shallowing-upward cycles (1 to 15 m thick) that contain distinctive marker beds and can be correlated for >200 km parallel to strike and >120 km perpendicular to strike. Cycles are grouped according to cycle base lithology, which reflects paleogeographic position on the platform. Shale-based cycles of the distal inner shelf pass westward into dolosiltite-based cycles, then into tufa-based cycles of the shoal complex, and then into grainstone-based cycles of the outer shelf. Cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex. Cycles formed in response to small, asymmetric (rapid rise/slow fall) eustatic oscillations in sea level (∼10 m). During sea-level rise, rapid transgression and submergence of tidal flats occurred along the 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. A glacio-eustatic model for cyclicity accounts for the data, favoring asymmetric, eustatic sea-level oscillations; subaerial exposure and vadose diagenesis of platform during sea-level falls; arrested progradation and incomplete shoaling of many cycles; and cycle periods of 20,000 to 100,000 yr (18,000 to 30,000 yr for Rocknest). Inner-shelf slopes ranged from 8 cm/km (average) at high sea-level stand to 2 cm/km (average) at low sea-level stand. Tidal-flat progradation rates were ∼7 to 9 km/1,000 yr. Interaction of inner-shelf slope, sea-level fall rate, and sedimentation rate generates “complete,” “condensed,” or “expanded” cycle profiles. One-dimensional computer modeling reveals that sedimentation rates of shoal-complex facies were ∼250 cm/1,000 yr, that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2,000 yr on the distal inner shelf, and that subtidal sedimentation rates may have ranged from 80 cm/1,000 yr for the proximal inner shelf to 20 cm/1,000 yr for the distal inner shelf.