Abstract: The Sebree Trough - Did It Really Exist? Upper Ordovician Sequence Stratigraphy Provides an Answer
HOHMAN, JOHN C. and BRIAN D. KEITH
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.
AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky