Algal-Metazoan Bioherms of Lower Ordovician Age--St. George Group, Western Newfoundland: ABSTRACT
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Bioherms are common in the St. George Group, a sequence of shallow-water carbonate rocks deposited on the western continental shelf of Iapetus Ocean. The cores of these bioherms are composed of thrombolites (unlaminated, branching, columnar stromatolites), "calcareous algae," and corals. On the basis of framework-building components, three types are distinguished: (1) thrombolite mounds, (2) Lichenaria-thrombolite mounds, (3) thrombolite-Lichenaria-Renalcis reefs. Associated with these structures is a diverse fauna of burrowing End_Page 767------------------------------ invertebrates, trilobites, nautiloids, pelmatozoans, brachiopods, gastropods, rostroconchs, and archaeoscyphiid sponges. Thrombolite mounds are circular in plan, up to 2 m in diameter and thickness, with an estimated depositional relief of 0.3 m at most. Individual mounds commonly coalesced from circular and linear patch reefs, or banks with grooved margins. Large archaeoscyphiid sponges and Pulchrilamina (encrusting sponge?) contribute in a minor way to the framework in scattered horizons. Rare small mounds are composed of an intergrowth of thrombolites and Lichenaria corals. Large thrombolite Lichenaria-Renalcis reef complexes, up to 12 m thick, with an estimated depositional relief of up to 1.5 m, occur in the lower part of the St. George. One particularly well developed complex is composed of vertically superimposed reef stages composed of Lichenaria, thrombolites, and the "calcareous alga" Renalcis. The framework is surprisingly complex, with abundant cavities and a demonstrably uneven growth surface. Cavity walls are commonly coated by algalaminites and internal sediments are burrowed. Some cavities are sediment conduits. Renalcis occurs as free-standing heads of varying shapes, as encrusting walls on small thrombolite mounds, and as manes in cavities under corals. These bioherms span a critical time gap in the development of reefs, the transition period from algal-dominated bioherms of the Precambrian and Cambrian to the metazoan-dominated bioherms of the Middle Ordovician and remaining Phanerozoic. End_of_Article - Last_Page 768------------