The St. George Group (Lower Ordovician), western Newfoundland: sedimentology, diagenesis, and cryptalgal structures

Detailed stratigraphic examination was undertaken of the Lower Ordovician Isthmus Bay, Catoche, and Aguathuna Formations, St. George Group, in western Newfoundland, Lithologic descriptions were grouped into seven peritidal lithotopes, with additional descriptions of the effects of erosion and subaerial exposure, such as hardground formation, evaporite precipitation, and karstification. The vertical oscillation of lithotopes generates sequences rather than ideal shallowing upward cycles. These sequences show that the continental shelf in western Newfoundland, in Lower Ordovician time, was a patchwork of low-relief islands and banks whose character differed geographically and changed with time, as the shelf slowly subsided. Slight uplift and subaerial exposure of the shelf occurred at the end of the Lower Ordovician, resulting in irregular erosion. -- Cryptalgal structures of very diverse character are related to environmental and biological parameters that interacted in a complex way. Laminated structures, stromatolites and cryptalgal laminites, formed in inter- and supratidal areas areas characterized by the episodic addition of sediment. Thrombolite mounds were common in the subtidal zone, where sedimentation onto algal mats resulted in unlaminated microstructure. In places, sponges, primitive corals and the 'calcareous' algae Renalcis contributed to their framework. These mounds are ecologic reefs of surprising complexity, supporting a benthic flora and fauna of all major trophic groups occupying most niches. These reefs occur in the transition period from algal-dominated bioherms to metazoan-dominated bioherms of the later Phanerozoic. Cryptalgal microstructures can be separated into a number of recurring types and a classification is proposed that can be amended to include types not recognized in the St. George. 'Calcareous' algae such as Girvanella and Renalcis are interpreted to be diagenetic taxa resulting from the calcification of blue-green algae. -- The calcareous sediments of the St. George underwent diagenetic modification beginning on the sea floor and continuing until the Upper Mississippian. Processes included synsedimentary and burial calcite cementation, neomorphism, silicate authigenesis, silicification, burial and epigenetic dolomitization. Galena and sphalerite mineralization occurred during epigenetic dolomitization and colloform vein formation. The paragenetic sequence seems typical of Lower Paleozoic peritidal shelf and epeiric carbonate sediments