Skeletal abundance of Upper Ordovician coral reefs, Lourdes Formation, western Newfoundland

Abstract

Reefs experience profound evolutionary and ecological changes through the Early Paleozoic, with the rise and fall of the lower Cambrian archaeocyathan reef and the later advent of other sponge and coral reefs. The Upper Ordovician Lourdes Formation of the Long Point Group, western Newfoundland, preserves small coral bioherms in a series of fault blocks along Long Point on the Port au Port Peninsula. These bioherms, preserved in the Black Duck Member 3, contain tabulate corals, bryozoans, and stromatoporoids adding to their framework. The overlying Beach Point Member preserves deeper water, storm-dominated deposition, and bioherms are absent from this setting. An analysis of skeletal abundance through the point counting of thin sections from samples within and outside the reefs reveal a mosaic of skeletal abundance across these settings. The bioherms and associated channel deposits preserve the highest skeletal abundance whereas the settings just outside of the reefs and in facies overlying the reefs preserve notably lower skeletal abundance. This suggests that reef settings harbored a higher diversity and abundance of skeletonizing organisms, more so than environments adjacent to but outside of the reefs. Though variations in depositional processes can affect the abundance of skeletal material through physical processes, the storm-dominated facies of the Beach Point Member generally contain more abundant oncoids, peloids, and intraclasts than skeletal material. Even when examining broadly similar facies within and outside of reefs, this difference in animal abundance remains. Examining Upper Ordovician coral reefs of the Lourdes Formation reveals that the Great Ordovician Biodiversification Event resulted in an increase in skeletal abundance from their earlier reef counterparts, but that the abundance of skeletal animals is driven by particular environmental conditions in reef settings, such as supersaturation of CaCO3, wave activity fostering local increases in oxygen, and warm temperatures. In this way, reefs provided some of the most suitable places for skeletal organisms to thrive in the Early Paleozoic.