STROMATOPOROID-ECHINODERM ECOLOGICAL SUCCESSIONS IN THE LATE ORDOVICIAN (KATIAN): REEF FACIES AND THE ROLE OF KEY BIOTA IN REEF ARCHITECTURE

Abstract

Abstract Ecologically complex buildups within the Kimmswick Limestone of the Galena Group (Upper Ordovician, Katian) near St. Louis, Missouri, display unique communities of stromatoporoids, encrusting cyathocystid and edrioblastoid edrioasteroids, camerate and other crinoids, paracrinoids, bryozoans, tabulate, and rugose corals. Substrate stabilization and vertical ecological successions were influenced by labechiid stromatoporoids that transitioned from laminar to domal/pillar morphologies from the base of the reef to its terminus. Cyathocystid edrioasteroids occurred in dense aggregations within cryptic cavities, often inverted in life orientation. Surrounding facies consisted of bryozoan and chert-rich wackestones-packstones, cross-bedded abraded echinoderm grainstones, gastropod-bivalve grainstones, and echinoderm-bryozoan grainstone/rudstones, while reefal facies comprised stromatoporoid-echinoderm boundstones, and stromatoporoid-cyathocystid framestones. Reef geometry and facies distribution reflected both allogenic and autogenic controls fundamental to the initialization and stabilization of the Shady Valley reefs. Four distinct successional series, from initial stabilization of important binders to colonization and diversification of stemmed echinoderm groups and subsequent domination of labechiid stromatoporoid framebuilders, formed the vertical profile of the reefs. Laminar, domal and irregular frame-building stromatoporoids acted as sediment stabilizers and formed ideal substrates for encrusting hardground fauna, supporting the development of cryptic habitats exploited by diverse echinoderms at the acme of reef diversification. Similar Katian hardground ecological successions occur in coeval reefs elsewhere in North America, Europe, Baltica, and South China. However, the scale of the reef architecture, development of complete ecological successions, and the diversity and multiple ecological roles of labechiid stromatoporoids and echinoderms in the Kimmswick Limestone sets it apart.