ABSTRACT: Lithology and Depositional History of Pre-Black River Rocks in the Illinois Basin

LUNEAU, BARBARA A., Intera Information Technologies, Longmont, CO

Cambrian-Ordovician rocks of the Illinois basin provide a lithologically diverse set of potential reservoirs for deep basin exploration. At the base of the Sauk Sequence, the Upper Cambrian Mount Simon Sandstone unconformably overlies Precambrian basement across much of the basin, and overlies older sedimentary rocks in the Reelfoot rift and Moorman syncline. Deposition of Mount Simon arkoses, subarkoses, and quartzarenites was strongly influenced by basement topography and subsidence over the failed rift complex. Eau Claire Formation deposition records progressive drowning of clastic source areas. Dolomite mudstones and ooid grainstones blanket Lower Eau Claire glauconitic sandstones. Fine-grained sandstones, siltstones, and shales of the Davis Formation overlie this subsequence.

As the Sauk transgression progressed, accumulating sediment buried paleotopography, and homogenous dolomite mudstones of the Knox Group were deposited. Deposition of the Gunter/Jordan and New Richmond sandstones record lowering of relative sea level. Dolomite mudstone and sandstone of the Everton Dolomite were deposited following a minor marine transgression that occurred between the Sauk and Tippecanoe sequences.

Deposition of the Tippecanoe sequence began with the rocks of the Ancell Group. St. Peter Sandstone quartzarenites were deposited in shallow marine environments. Sandstone grades upward into the lithologically heterogeneous Dutchtown Formation and Joaquin Dolomite. Interbedded sandstone, organic-rich shale, dolomite, limestone, and anhydrite record deposition mainly in a restricted peritidal setting. Continued low-energy deposition resulted in the accumulation of the homogenous limestone of the Black River Group. Open marine conditions were not reestablished in the Illinois basin area until deposition of limestones of the Galena Group.

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