ABSTRACT: Early-Middle Ordovician Evolution of the Illinois Basin: Paleontologic and Sedimentologic Constraints

SHAW, T. H., Queens College, City University of New York, Flushing, NY

This study documents the age and depositional relationships of the early-Middle Ordovician Ancell Group, and its regional equivalents, of the Illinois basin by providing a basis for defining the early geometry of the basin and for qualitatively evaluating the relationship of basin subsidence to the Taconic orogeny.

The study contains two major elements: development of a conodont-based biozonation for age control and lithologic and geophysical-log characterization of the constituent lithofacies to provide a basis for determining the temporal and spatial relationships of depositional environments.

The conodont biozonation is based on conodont samples collected from cored subsurface sections and the ranges of species present graphically correlated to the Middle Ordovician composite standard. Lithofacies have been identified on the basis of detailed description and petrographic sampling of cores and outcrops. Where available, geophysical-logs have been compared to the lithic succession in cores to characterize the geophysical response of the differing rock types.

Based on these data, three genetic units can be identified within the Ancell sequence: (1) a retrogradational transgressive unit with a relative sea level maxima within the Dutchtown Formation; (2) a progradational unit that has a relative sea level minima represented by evaporite-bearing, upper Joachim Dolomite; and (3) a subtidal aggradational unit including the uppermost Joachim, Starved Rock Sandstone and Glenwood Formation.

The Tonti Member of the St. Peter Sandstone is a time-transgressive sandstone lithotype that has a facies relationship with the Dutchtown Formation, Joachim Dolomite and Glenwood Formation.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)