SEQUENCE STRATIGRAPHY, SEDIMENTARY FACIES AND TRILOBITE BIOFACIES OF THE LATE ORDOVICIAN (LATE SANDBIAN-EARLY KATIAN) SUCCESSION OF MISSOURI

The Deicke, Millbrig and House Springs K-bentonites facilitate high-resolution correlation of the late Sandbian—early Katian succession of the mid-continent. They provide the framework for analysis of third order depositional sequences in Missouri. In the St. Louis region, the M4 sequence (Sandbian) is composed of peritidal carbonates of the Plattin Subgroup. The M4-M5 boundary lies just above the base of the Glencoe Member of the Spechts Ferry Formation (Decorah Subgroup). It is marked by an erosion surface that cuts down onto the Millbrig K-bentonite that can be traced at least 140 km to the north. Within the M5 sequence, lower ranked sequences correspond to the Glencoe Member and overlying Kings Lake Formation. A regional break at the base of the Kimmswick Subgroup likely records the base of the M6 sequence. Facies change across the M5-M6 boundary from relatively deep subtidal (Kings Lake Formation) to shallower subtidal bioturbated carbonate sands. Trilobite biofacies also shift from low diversity, Isotelus-Calyptaulax dominated faunas to more diverse, Ceraurus-rich assemblages. Within the Kimmswick, a sharp facies offset marked by the appearance of coarse, cross-bedded bioclastic rudstone and grainstone may mark the base of the C1 sequence. The Missouri succession demonstrates the remarkable extent of warm water, peritidal carbonate deposition across a broad swathe of eastern and central North America during the M4 interval. It also confirms the significance of the M4-M5 boundary in marking a change in style of sedimentation. Although evidence for warm water carbonate deposition persists into M5 and even M6 in Missouri, this boundary marks a regional shift from peritidal to exclusively subtidal environments.