AN INVESTIGATION OF REEF BUILDUP STAGES IN THE LATE ORDOVICIAN (KATIAN) KIMMSWICK LIMESTONE, EAST-CENTRAL MISSOURI: THE ROLE OF TAXA, BIOVOLUME AND STRATIGRAPHY

The Kimmswick Limestone of east-central Missouri is dominated by fossiliferous grainstones of Late Ordovician (Katian) age that formed in a shallow-water environment between the present-day Ozark Dome and the deep-water Sebree Trough. The Kimmswick Limestone has been characterized as relatively homogeneous in faunal composition and lithology throughout the midwest. Here we report on a unique and distinctive reef paleocommunity of encrusting stromatoporoids, cyathocystid edrioasteroids, crinoids, paracrinoids, bryozoans, and corals, including many taxa that are rare elsewhere.

The reef paleocommunity has never been documented before in the Kimmswick, therefore we provide an in-depth investigation of the buildup diversity and reef community structure. In this study stratigraphic analysis, taxon point counts and biovolume estimates of bulk samples at each buildup stage revealed changes in the density of key biota and their structural role. Stratigraphic observations and multivariate analysis determined the presence of all four reef buildup stages (stabilization, colonization, diversification and domination), concluding that it is in fact a true reef and not a carbonate mound, which complete only two buildup stages and are more commonly found in Ordovician strata. Initial and main stabilization reef stages of the buildup consisted primarily of crinoid rudstones with increasing stromatoporoid mounds and bryozoans. Colonization of the buildup shows dominance by laminated stromatoporoids with common bryozoans and echinoderms. Diversification stage consists primarily of encrusting echinoderms with stromatoporoids abundant throughout. Lateral variation into stromatoporoid boundstones is also observed within diversification stage. A terminal stage caps the buildup, showing evidence of reworking and drowning of the buildup with crinoidal debris dominant and oncoids, bryozoans and broken receptaculitids common.

Establishing a detailed narrative of the community dynamics of the Kimmswick reef provides a unique opportunity to assess the paleoenvironment and ecological interactions during the Late Ordovician as a result of the Great Ordovician Biodiversity Event as well as adding to our knowledge of atypical reef systems.