Controls on regional distribution patterns of heterozoan and photozoan carbonate facies in tropical transitional carbonate systems: Lower Mississippian, Continental US and southern Canada

Lower Mississippian (Osagean) carbonate systems of the continental United States and southern Canada developed within tropical paleolatitudes but display complex distributions of photozoan and heterozoan facies characteristic of tropical transitional carbonate systems. This study synthesizes published literature to document regional patterns of photozoan and heterozoan facies in shallow-water carbonates and evaluate environmental controls on their distribution. An overall compositional difference occurs across the Transcontinental Arch (TA). North and west of the TA, shallow-water carbonate ramps commonly contain mixed photozoan-heterozoan assemblages, including ooids, peloids, calcareous algae, stromatolites, and benthic foraminifers associated with crinoids, bryozoans, and brachiopods. Photozoans are locally absent nearer western basin margins. In contrast, south and east of the TA, shallow-water systems are dominated by heterozoan assemblages, with photozoans only locally present in areas removed from basin margins. Previous studies have interpreted sea level, climate, and upwelling as dominant controls on deposition. Facies composition patterns correspond closely with proposed areas of upwelling, consistent with an interpretation that upwelled, nutrient-rich, cooler waters adversely affected photozoan development. Sea-level fluctuations affected facies composition by shifting facies belts and potentially by decreasing upwelling intensity during sea-level falls and increasing it during rises. The presence of photozoans near land suggests that land runoff was not a significant factor, consistent with the inferred arid climate. Biosiliceous facies and chert occur in many areas, with silica interpreted to have been sourced through aeolian and upwelling processes. This regional synthesis provides first-level information on factors controlling heterozoan-photozoan facies distributions and establishes a framework for further targeted studies.