Pioneer reef communities within a Middle Triassic (Anisian) to Upper Triassic (Carnian) mixed carbonate-siliciclastic ramp system from the Star Peak Group, South Canyon, central Nevada

Abstract

Reef patterns change drastically between the late Paleozoic and early Mesozoic due to the effects of the end-Permian mass extinction. Reef communities follow an ecological transition from the stabilization stage in the Anisian, to the colonization stage in the Carnian through Norian, to the final diversification and domination stages in the Norian through the Rhaetian. Most eastern Panthalassa reef studies focused on documenting framework-supported, reef structures elevated from the seafloor during the Late Triassic diversification stage. However, to fully understand reef development, we also need to record the precursor stages. Here, we examine field-based and microfacies data using a sequence stratigraphic lens to develop a quantitative assessment of non-skeletal and skeletal carbonate grains. Our results indicate that a reef mound and biostromes developed, during an overall transgressive sequence, due to substrate cementation caused by flooding events. Although taxonomic differences between biostromes and the reef mound change through time, we conclude that these taxonomic changes do not record true first occurrence data. Instead, these fossil accumulations probably occurred due to random chance of larval recruitment. Even though the reef types found here preserve similar taxa that have been shown to promote high-reef growth (e.g.: calcified demosponges, calcareous algae, and bivalves), low-reef growth structures are found at our site. Thus, reefs containing large amounts of micrite built by low-growing binding and encrusting organisms typify this Middle and Upper Triassic section, confirming that this section records the colonization stage of reef recovery.