FACIES ANALYSIS AND SEQUENCE STRATIGRAPHY OF THE UPPER CAMBRIAN EMINENCE FORMATION: A MIXED CARBONATE-SILICICLASTIC SUCCESSION IN THE ILLINOIS BASIN

The uppermost Cambrian Eminence Formation (up to 2000 feet) constitutes a southeastward-thickening mixed carbonate-siliciclastic succession in the Illinois Basin and grades laterally into the Jordan Sandstone in the extreme northwest part of Illinois. In central and southern Illinois, the Eminence Formation has long been regarded as a part of the undifferentiated Knox Group because its boundaries with the underlying and overlying formations are generally obscure due to similar lithologies. This study focuses on facies and sequence stratigraphy of the strata that filled the Illinois Basin during late Late Cambrian time for detailed intrabasinal correlation and distinction of the Eminence Formation from the adjacent intervals.

The Eminence Formation includes grainstone facies (preserved in chert nodules) and relics of bioclasts, ooids, and peloids, recording deposition in a shallow marine setting. It comprises three depositional sequences that are correlated with global Late Cambrian 3rd-order sea level cycles. The sequences consist of transgressive and highstand systems tracts separated by maximum flooding horizons (thin dark gray phosphatic shale or argillaceous dolomite) and are superimposed by fourth-fifth-order shallowing-upward cycles. In a typical cycle, gamma ray log response decreases upward and finely crystalline dolomite grades to medium or coarsely crystalline dolomite that may contain relics of shallow marine allochems. The cycle may be capped by a sandy dolomite or thin quartz sandstone layer and reflects the shallowing-upward motif of the original precursor limestone. Although the thickness of the coeval depositional sequences varies across the basin due to differential subsidence, rate of deposition and magnitude of sea level fluctuation, the overall geometry and forms of each sequence are generally similar. Neither cores nor biostratigraphic studies are available for these Cambrian deposits. Thus, sequence stratigraphy, using geophysical logs and well samples, provides a strong tool for intrabasinal correlation of the strata in shallow and deep basin areas of the Illinois Basin, and aids in recognition and correlation of potential reservoirs and seals for petroleum exploration or carbon sequestration.