Abstract
Middle to Upper Cambrian platform deposits of the Port au Port Peninsula in western Newfoundland are characterized by three large-scale, 100-250-m (330-820-ft) thick, sedimentary cycles or Grand Cycles. The lower half of each cycle consists of recessive-weathering, thinly interbedded lime mudstone, shales, and argillaceous dolostones. These sediments grade into a resistant-weathering, upper half-cycle of thick-bedded ooid and skeletal grainstones and laminated dolostones. Meter-scale, shallowing-upward cycles are developed throughout the Grand Cycles and are generally punctuated by stromatolite and thrombolite mounds and storm-derived deposits, such as flat-pebble conglomerates.
The Port au Port sequence records cyclic migrations of a wide muddy tidal flat on the inner platform (recessive half cycle) and a peritidal carbonate sand shoal complex on the outer platform (resistant half cycle). These large-scale cycles are the products of environmental variations resulting from a complex interplay of differential rates of eustatic sea level change and basin subsidence.
Grand Cycles are characteristic of stable, mixed carbonate-siliciclastic shelf environments. These cycles provide a framework for depositional and diagenetic models of many Cambrian shelf deposits, such as the Cambrian of the Great basin and the Canadian Cordillera. The Grand Cycles of the Port au Port Peninsula are the first to be studied in detail from the Appalachian orogene and provide additional insight into early Paleozoic shelf evolution.
