HOHMAN, JOHN C., Department of Geological Sciences, Indiana University, Bloomington, IN; and KEITH, BRIAN D., Indiana Geological Survey and Department of Geological Sciences, Indiana University, Bloomington, IN
The stratigraphic relationships of the "Sebree Trough" (a narrow shale-filled
basin) with the laterally adjacent Trenton (Galena) and Lexington carbonate
platforms have remained obscure despite several studies of the Upper Ordovician
of the Illinois Basin and Cincinnati Arch areas. A sequence stratigraphic
analysis utilizing outcrops, cores, and wireline logs establishes a framework
of unconformities and marine-flooding surfaces that unravels the true stratigraphic
relationships among the "Sebree Trough" and the carbonate platforms to
either side.
This sequence stratigraphic analysis reveals the evolution from a carbonate-dominated
platform setting for the Trenton Limestone, Galena Group, and lower Lexington
Limestone to a siliciclastic-dominated foredeep setting for the upper Lexington
Limestone and Maquoketa Group. This transition involved three stages of
platform deposition; each successive stage was characterized by increasingly
dominant siliciclastic deposition. The first stage consisted of the deposition
of relatively pure limestones of the Trenton platform (Trenton and Galena
and the basal part of the Lexington). Shaly limestones of the lower Lexington
platform (middle part of the Lexington Limestone) were deposited during
the second stage. The third stage was the deposition of both shaly limestone
of the upper Lexington platform (upper part of the Lexington Limestone)
and widespread, laterally equivalent shales that marked the initial siliciclastic-dominated
deposition associated with the developing Maquoketa foredeep basin. The
shales of the "Sebree Trough" are, therefore, age-equivalent only to the
upper Lexington shaly carbonates, but not to either the lower and middle
Lexington or the Trenton carbonates. Thus, the presence of the "Sebree
Trough" on previously published maps and cross sections is simply an artifact
of correlation of strata that are not time equivalent.
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