Large-scale sedimentary cycles, or Grand Cycles, are the most obvious features of the Port au Port Group in southwestern Newfoundland. Each cycle is interpreted to represent deposition in outer-platform, muddy tidal flats (lower shaly half-cycle) and ooid shoal complexes (upper carbonate half-cycle). Unpredictable metre-scale assemblages of oolite and carbonate laminite, constituting the carbonate half-cycles, are attributed to frequent migration of the shoal complexes in response to hydrographic factors such as tidal fluctuations and storms. Predictable metre-scale, shallowing-upward cycles of parted limestone and shale in the shaly half-cycles were probably controlled mainly by variable rates of carbonate sedimentation; a eustatic mechanism, however, cannot be discounted.
Comparison with other North American Grand Cycles suggests a eustatic mechanism for Grand Cycle formation, possibly involving variations in the rate of sea-level rise. The interpreted sequence of formation, established on the basis of the Port au Port Group, involves (1) a rapid sea-level rise, which flooded the platform with shoreward-derived siliciclastic muds and resulted in the development of muddy tidal flats, and (2) the gradual development of ooid shoal complexes, which encroached onto the tidal flats, with decreasing rates of sea-level rise. In contrast to western North American examples, Newfoundland Grand Cycles are composed almost entirely of peritidal sediments; they are also thinner and have a smaller areal extent than do those in the west. These differences may be due to such factors as narrower shelves, slower rates of sedimentation, and lower amplitudes of sea-level rise on the present northeastern margin of the North American craton than on the western margin.