The interactions of sea-level change, terrigenous-sediment influx, and carbonate productivity as controls on Upper Cambrian Grand Cycles of western Newfoundland, Canada

摘要

The Upper Cambrian stratal record in western Newfoundland is complex and reflects the interplay and feedback between (1) changes in third-order sea level, (2) episodic incursion of terrigenous fines onto the paleoshelf, and (3) productivity levels of the ancient carbonate factory. Mixed carbonate and terrigenous clastic rocks of western Newfoundland are configured in large-scale, third-order stratal patterns called "Grand Cycles" (such cycles are tens to hundreds of meters in stratigraphic thickness and appear to span millions of years). Grand Cycles are divided here into ribbon half-cycles (composed principally of ribbon rock) and oolitic half-cycles (dominated by oolite). Stratigraphic distribution of shale in these rocks, however, is not everywhere coincident with large- and small-scale patterns in carbonate lithofacies, and it suggests that the incursion of terrigenous clay and silt onto the ancient platform must be evaluated separately, in part, from the dynamics of carbonate accumulation. Areal and stratigraphic distribution of rock cycles record shifts in facies belts on the ancient platform, presumably in response to third-order changes in accommodation potential. Meter-scale cycles in carbonate lithofacies are of two types: peritidal cycles that fine upward, and shelf cycles that coarsen upward. Thick successions of shelf cycles composed of ribbon rock are interpreted as highstand deposits and are separated (by a sequence boundary marked by peritidal cycles) from lowstand deposits comprising thick successions of ootite shelf cycles. The former mid-Grand Cycle transitions of previous workers are thereby reinterpreted in this study as sequence boundaries. The long-term position of sea level, and thus the degree of platform inundation, influenced the health of the platform carbonate factory. During times of low sea level, the platform was bathed in water more saline than open-ocean sea water, and it had a robust non- skeletal carbonate factory that was resilient to poisoning by episodic influx of terrigenous fines onto the shelf. In contrast, the carbonate factory during highstands was less resilient to terrigenous poisoning and was susceptible to localized drowning by the reduction in the rate of carbonate-sediment production. The thick shale units are therefore interpreted to represent the coincidence of (and interaction between) (1) incursion of terrigenous fines onto the ancient platform and (2) a weakened highstand carbonate factory unable to recover from this terrigenous poisoning.