The Ironton/Galesville sandstone complex represents a transition from the basal, Franconian lowstand to a carbonate shelf deposited during the ensuing, upper Cambrian sea-level rise. Previous workers thought Ironton/Galesville quartz arenites were `sheet sandstones' because they are thin and widespread on the craton. However, observations of cores and logs from the northern Illinois subsurface suggest otherwise.

The Galesville Sandstone is made up of an amalgamated series of downlapping and seaward-stepping facies elements representing a prograding, strandplain and/or barrier island system. Mean grain size increases upward from very-fine to fine or medium sand, and inferred energy levels of sedimentary structures also increase upward in the Galesville, which was deposited in shoreface beach and coastal (eolian) dune environments.

A widespread ravinement surface separates the Ironton from the underlying Galesville. The Ironton `Sandstone' is comprised of three facies: I1) highly burrowed, nearly massive sandstone; I2) tabular cross-stratified sandstone, largely devoid of biogenic traces, and I3) impermeable, micritic dolomite beds. Cyclic shifts from normal-marine, shallow bays (I1) to tidal flats (I2) and shallower, restricted bays (I3) resulted in complex facies intercalations. Ironton reservoir heterogeneity increases upward as the sandstone beds become thinner and less abundant. The upper Ironton is almost entirely composed of dolomite.

Although the Ironton and Galesville are stratigraphically separate units, they are hydraulically connected. The basal Ironton sandstone (predominantly facies I1) serves as the primary conduit through which gas is injected and withdrawn at Pontiac. The underlying Galesville Sandstone's primary gas-storage role is that of a large, supporting volume.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah