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Outcrop Characterization of the Starved Rock Member, Saint Peter Sandstone (Ordovician) at Starved Rock, Illinois

Wroblewski, Anton F. J.1  and Frank E. Stroik2
1 Department of Earth Science, Northeastern Illinois University, Chicago, IL 
2 Department of Geology, University of Georgia at Athens, Athens, GA

The Saint Peter Sandstone is a well-sorted, almost pure quartz arenite deposited during a major mid-Ordovician lowstand. Clastics prograded basinward across a subaerially exposed carbonate platform via eolian transport, as evidenced by the well-rounded, frosted texture of the quartz grains. The Starved Rock Member of the Saint Peter Sandstone is preserved as a northeast-southwest trending belt of strata that has been interpreted as an offshore bar or a barrier island. Spectacular three-dimensional exposure of the outcrop belt allows detailed characterization of the internal geometry and facies distribution within the Starved Rock Member. Small- and large-scale (up to 60 cm high) trough cross-stratification, low-angle planar bedding, high-angle planar-tabular cross-stratification, and horizontal laminations are preserved in the Starved Rock Member. Paleocurrents taken from trough axes and foreset orientations reveal primarily southeastward sediment transport, with subordinate northwestward currents. The normal orientation of the high-angle planar-tabular cross-stratification with regard to the trough axes suggests ridge migration over runnels in an upper shoreface. At least four 3 meter-thick, coarsening-upward successions are preserved, with flooding surfaces represented by low-relief erosional contact of trough-cross-stratification (runnel) upon low-angle planar bedding (swash zone). Concretions preserved at the tops of upward-coarsening successions represent mineral precipitation in simple, vertical burrow fills.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004