CAMBRO-ORDOVICIAN TRANSITION IN ILLINOIS: NATURE OF THE C-O BOUNDARY AND CRITERIA FOR ITS RECOGNITION IN THE DEEP PART OF THE ILLINOIS BASIN

The Cambro-Ordovician (C-O) boundary presents a stratigraphic problem in the deep area of the Illinois Basin where the Gunter Sandstone, used to mark and correlate the base of the Ordovician System in northern and central parts of Illinois, is absent. To define and evaluate the C-O boundary, lithofacies and vertical facies trends of the Cambro-Ordovician transition in Illinois (the uppermost Cambrian Eminence Formation and the lowermost Ordovician Gunter Sandstone and/or Oneota Dolomite) have been investigated. An easily recognizable high Gamma ray marker, probably a K-bentonite bed, constantly occurs at about the same depth below the C-O boundary, the Eminence-Gunter/Oneota contact. The gamma ray deflection and the overlaying associated strata below the boundary are very persistent laterally and constitute a uniform succession that is easily recognizable in the subsurface. The C-O boundary coincides with the most regressive surface separating the Eminence highstand systems tract and the transgressive systems tract of the overlying Oneota Dolomite. The boundary is planar and subaerial exposure features such as karst, caliche, ferruginous surface, and paleochannels typically are absent.

During the Late Cambrian short term sea-level drawdown, carbonate deposition continued in the southeastern portion of the Illinois Basin because subsidence was equal to or in excess of base-level fall. However, the coeval quartz arenite facies of the Gunter Sandstone was deposited on the exposed central, western, and northern parts of Illinois and the neighboring states, possibly in an aeolian setting. Exceptional textural and compositional maturity, frosted bimodal grains, excellent rounding even in finer grain size, thin lamination, large scale cross-bedding, and paucity of shale suggest aeolian deposition followed by varying amount of reworking during post Cambrian sea-level rise. This study provides a starting point for more detailed sequence stratigraphic analysis of the entire Cambro-Ordovician succession in the Illinois Basin, thus, offering a unique framework in which facies distribution through time can be examined to define potential reservoirs and seals for petroleum exploration and carbon sequestration.