Carbonate Sequence Stratigraphy of the Cambro-Ordovician Transition and Criteria for Recognition of the C-O Boundary in the Deep Part of the Illinois Basin

The Cambro-Ordovician boundary presents a stratigraphic problem in the deep part of the Illinois Basin due to continuous carbonate deposition and the apparent lithofacies similarities across the boundary. In this part of the basin, the basal Ordovician Gunter Sandstone is absent and the Upper Cambrian through Lower Ordovician carbonate succession (over 6000 feet thick) has long been regarded as the undifferentiated Knox Group. To define the C-O boundary, sequence stratigraphy of the uppermost Cambrian Eminence Formation and the lowermost Ordovician Gunter Sandstone and/or basal Oneota Dolomite in Illinois has been investigated along N-S and SW-NE transects using subsurface data. The Uppermost Cambrian Eminence Dolomite generally is more radioactive as opposed to the overlying and underlying strata and comprises three depositional sequences, which are correlated with the global Late Cambrian (Trempealeauan) 3rd-order sea level cycles. In central and north-central part of the Illinois Basin, the Ordovician Gunter Sandstone, where present, sharply overlies the highstand systems tract of the uppermost Eminence sequence. The Gunter grades upward to generally pure Oneota Dolomite in the transgressive systems tract of the basal Ordovician depositional sequence. In the southern deep area of the basin, the transgressive package of the basal Ordovician Oneota Dolomite overlies the highstand tract of the upper Eminence Dolomite. Therefore, the C-O boundary coincides with the sequence boundary separating the upper Eminence highstand systems tract and the overlying Oneota transgressive systems tract. Neither cores nor biostratigraphic studies are available for the Cambro-Ordovician transition, thus, sequence stratigraphy, using geophysical logs and well samples provides a strong tool for proper recognition of the C-O boundary. Recognition of the Cambro-Ordovician boundary facilitates the subdivision of the Knox Group into lower and upper Knox successions, each consisting of several depositional sequences. This study provides a starting point for more detailed sequence stratigraphic analysis of the entire Cambro-Ordovician succession in the Illinois Basin; it offers a unique framework in which facies distribution through time can be examined to define potential reservoirs and seals for petroleum exploration and carbon sequestration.