--> Abstract: Chemostratigraphic and Sequence-Stratigraphic Correlation of the Upper Ordovician Maquoketa Group of the Illinois Basin, by J. M. Guthrie, J. C. Hohman, and L. M. Pratt; #90987 (1993).

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GUTHRIE, JOHN M., JOHN C. HOHMAN, and LISA M. PRATT, Department of Geological Sciences, Indiana University, Bloomington, IN

ABSTRACT: Chemostratigraphic and Sequence-Stratigraphic Correlation of the Upper Ordovician Maquoketa Group of the Illinois Basin

The Upper Ordovician Maquoketa Group of the Illinois basin is a thick, clastic wedge of sediment that prograded westward across Indiana, Illinois, and Iowa from the eastern Taconic highlands of the Appalachian area. Sequence stratigraphy defines two dark, laminated, phosphatic shale horizons within the Maquoketa Group that represent: 1) an initial flooding horizon on a sequence boundary (flooding unconformity) at the base of the Maquoketa Group, and 2) a maximum flooding horizon stratigraphically higher within the Maquoketa Group.

Geochemical data obtained from three cores can be used to identify a unique signature for each horizon. These data include total organic carbon contents, total sulfur contents, hydrogen index values, and the carbon isotopic composition of the organic matter.

Utilizing these geochemical signatures, the two horizons have been traced regionally across the Illinois basin from Indiana to Iowa. These horizons define the upper and lower boundaries of a westward thinning wedge of calcareous shale and thinly bedded limestone which onlaps onto carbonates underlying the Maquoketa Group. The sedimentary wedge laps out on the western margin of the basin with the two phosphatic horizons merging to form a single horizon. In contrast, there is an eastward truncation of the wedge due to erosion associated with a sequence boundary (type I unconformity) at the top of the Maquoketa Group, removing the upper phosphatic shale horizon but preserving the lower horizon. This complex stratigraphy illustrates the value of integrating sequence stratigraphic concepts and geochemical data in correlating fine grained siliciclastic rocks such as the Maquoketa Group.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.