The 1.90-1.89 billion year old Rocknest Formation in the Northwest Territories is a west-facing, passive-margin carbonate terrace in the foreland of Wopmay orogen. Initial outbuilding of an accretionary stromatolite rim over downslope facies was followed by upbuilding of the rim, local backstepping of the rim, and terminal subduction-related drowning of the entire shelf. The rim was flanked to the west by deep-water slope rhythmite and breccia, and on the east by a carbonate-shoal complex, separating the ocean from a broad (100-200 km wide) lagoon with a siliciclastic eastern shoreline. Concurrently, the shoal complex underwent repeated eastward expansion over the lagoon to form about 150 shoaling-upward cycles (1.25 m thick), consisting of carbonate tidal-flat tufa that verlies storm-dominated, mixed carbonate-siliciclastic lagoonal facies. Correlation of cycles for over 200 km parallel with and 100 km across depositional strike shows that cycle boundaries abut facies boundaries, indicating that complete shoaling of the lagoon to sea level was not required to induce the next submergence increment, suggesting an allocyclic rather than autocyclic mechanism. Radiometric constraints bracket cycle periodicity between 25,000-40,000 yr/cycle. These values are within the range of known earth orbital cycles (periods at 19,000, 23,000, 41,000, and 100,000 yr), the likely cause of Pleistocene glacio-eustatic sea level oscillations, and possibly Rocknest cyclicity. Rocknest cycles can be modeled using period and amplitude of sea level oscillation, and subsidence an sedimentation rates as variables. Resulting computer-generated cyclic stratigraphies are compared to actual Rocknest cyclic stratigraphy in order to constrain variables responsible for cycle development.

End_of_Article - Last_Page 260------------