Sequence Stratigraphy of Cambrian-Ordovician Units in Illinois Basin

E. I. Egbobawaye
University of Alberta, Department of Earth and Atmospheric Sciences, 2-04 Earth Science Building, Edmonton, AB, T6G 2E3, Canada

The depositional sequence and stratigraphic analysis of Cambrian-Ordovician strata in Illinois basin involve interpretation of subsurface data to unravel sedimentation, sea level fluctuation, and lithologic variation during this geologic period. This research integrated well log data, outcrops, core samples, and thin section analysis to help understand the depositional sequence, and sea level history from Cambrian to Early Ordovician in Illinois basin. Correlation of well logs help to establish the thickness of Formations not exposed at the surface, their depth of occurrence, stratigraphic sequence, and lateral continuity in the subsurface.

The results of sieve analysis and petrographic study of grain-size for textural and compositional maturity respectively, reveal a sub-mature to super-mature lithologic characteristic for the Formations investigated. Chronostratigraphically, the Formations are [Mt Simon, Eau Claire, Galesville, Ironton, Franconia, Oneota, and St. Peters]. The generalized depositional sequence shows cycles of Sauk Sea transgression and regression episodes occurred from Cambrian to Ordovician as evidenced in the stratigraphic sequence of the Paleozoic units in Illinois basin.

This study subdivided the Cambrian sequence on the basis of geophysical well logs into parasequence I and parasequence II. The presence of glauconite, and skolithos burrows in the samples analyzed is indicative of shallow marine depositional environment for the Mt. Simon Formation at the base of Cambrian.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas