EXTRAORDINARY STROMATOPOROID-ECHINODERM BUILDUPS IN THE LATE ORDOVICIAN (KATIAN) KIMMSWICK LIMESTONE OF EAST-CENTRAL MISSOURI

In the Kimmswick Formation of east-central Missouri, small reef-like buildups occur within dominant massive grainstones. During the Katian age (~451 MA) this area lay in a transitional shallow water environment between the Ozark Dome and the deep water Sebree Trough. The Kimmswick Formation is relatively homogeneous in faunal composition and lithology throughout the midwest. Here, however, we report on a unique mound paleocommunity of encrusting stromatoporoids, cyathocystid edrioasteroids, camerate and cladid crinoids, paracrinoids, edrioblastoids, bryozoans, tabulate and rugose corals, including many taxa that are rare elsewhere. Stromatoporoids and encrusting echinoderm holdfasts formed a projecting framework boundstone, affording a suite of microenvironments suitable for colonization. Roadcuts show one large and other smaller buildups displaying various stages of growth. The large buildup displays a vertical succession of: 1) initial stabilization assemblages consisted of stromatoporoids (and possible undetermined binding organisms) that bound echinodermal grainstones; 2) main stabilization dominated by thin, flattened stromatoporoid morphologies, which appear to be the chief binders that stabilized the sediment, producing a viable hard substrate upon which larger core reefal masses formed; 3) colonization, diversification and formation of the main mass of the buildup, dominated by various encrusting echinoderms intergrown with stromatoporoids and other undetermined binding organisms. Cyathocystid edrioasteroids, often in inverted life orientation, occur in dense aggregations within buildup cavities. Stalked echinoderms (crinoids, paracrinoids, and edrioblastoids) exploited higher tiers, and occur attached to and intergrown with stromatoporoids by root-like to lobate holdfasts. The buildups grade upward into a stromatoporoid-dominated assemblage eventually buried by grainstones with scattered receptaculitid fragments. This occurrence affords a unique opportunity to investigate in more detail rare paleoenvironments and ecological interactions during the Late Ordovician of Laurentia. Future study will make detailed comparisons with similar-aged stromatoporoid-cyathocystid buildups of Baltica and the eastern United States.