SEQUENCE STRATIGRAPHY OF THE UPPERMOST CAMBRIAN-LOWERMOST ORDOVICIAN TRANSITION INTERVAL IN THE ILLINOIS BASIN: IMPLICATIONS FOR DEFINING THE POSITION OF THE C-O BOUNDARY

The position of the Cambro-Ordovician (C-O) boundary in the Knox Group of the Illinois Basin has not been well defined due to the lack of biostratigraphic control. Sequence stratigraphic analysis, however, has provided a strong tool for regional correlation and defining the position of the C-O boundary and the associated units. The C-O transition interval in northern and central Illinois includes the upper part of the Upper Cambrian Eminence Formation and the basal Ordovician Gunter Sandstone. The upper part of the Eminence Formation (up to 152 m) consists of dense dolomite layers with relics of bioclasts, ooids, and peloids intercalated with thin gray shale and quartzose sandstone layers. This interval comprises a depositional sequence that consists of transgressive and highstand systems tracts separated by a maximum flooding horizon (thin dark gray phosphatic shale). The Gunter Sandstone (up to 7.5 m) consists of fine- to medium-grained, mature quartzose sandstone, and overlies, with a sharp contact, the Upper Cambrian Eminence Formation. It grades upward into the Oneota Dolomite and is interpreted as the basal part of the Lower Ordovician transgressive package. The C-O boundary coincides with the most regressive surface separating the upper Eminence highstand package and the transgressive package of the overlying Gunter Sandstone/Oneota Dolomite.

In the deep part of the Illinois Basin, the C-O boundary presents a stratigraphic problem because the Gunter Sandstone is absent, and the Eminence boundary with the overlying Oneota Dolomite is generally obscure because of similar lithologies. Here, a mixed carbonate-clastic unit is present near the C-O boundary. This unit has been classified as the basal Ordovician Gunter Sandstone in western Kentucky. It is over 122 m thick and, in contrast to the Gunter Sandstone, its upper contact with the overlying Oneota/Beekmantown Dolomite is sharp. The unit, here referred to as the Rose Run Sandstone, is interpreted as a highstand deposit within the upper sequence of the Upper Cambrian Eminence Formation (Copper Ridge Dolomite equivalent). The C-O boundary coincides with the most regressive surface separating the upper Eminence/Rose Run highstand package and the transgressive package of the overlying Lower Ordovician Oneota/Beekmantown Dolomite.