THE INTERACTIONS OF SEA-LEVEL CHANGE, TERRIGENOUS-SEDIMENT INFLUX, AND CARBONATE PRODUCTIVITY AS CONTROLS ON UPPER CAMBRIAN GRAND CYCLES OFWESTERN NEWFOUNDLAND, CANADA

The Upper Cambrian stratal record in western Newfoundland is complex a nd reflects the interplay and feedback between (1) changes in third-or der sea level, (2) episodic incursion of terrigenous fines onto the pa leoshelf, and (3) productivity levels of the ancient carbonate factory . Mixed carbonate and terrigenous clastic rocks of western Newfoundlan d are configures in large-scale, third-order stratal patterns called ' 'Grand Cycles'' (such cycles are tens to hundreds of meters in stratig raphic thickness and appear to span millions of years). Grand Cycles a re divided here into ribbon half-cycles (composed principally of ribbo n rock) and oolitic half-cycles (dominated by oolite). Stratigraphic d istribution of shale in these rocks, however, is not everywhere coinci dent with large- and small-scale patterns in carbonate lithofacies, an d it suggests that the incursion of terrigenous clay and silt onto the ancient platform must be evaluated separately, in part, from the dyna mics of carbonate accumulation.Areal and stratigraphic distribution of rock cycles record shifts in facies belts on the ancient platform, pr esumably in response to third-order changes in accommodation potential . Meter-scale cycles in carbonate lithofacies are of two types: periti dal cycles that fine upward, and shelf cycles that coarsen upward. Thi ck successions of shelf cycles composed of ribbon rock are interpreted as highstand deposits and are separated (by a sequence boundary marke d by peritidal cycles) from lowstand deposits comprising thick success ions of oolite 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 o f platform inundation, influenced the health of the platform carbonate factory. During times of low sea level, the platform was bathed in wa ter more saline than open-ocean sea water, and it had a robust non-ske letal carbonate factory that was resilient to poisoning by episodic in flux of terrigenous fines onto the shelf. In contrast, the carbonate f actory during highstands was less resilient to terrigenous poisoning a nd 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 terr igenous poisoning.