The St. Peter—Platteville transition consists of three formations representing an upward change in deposition from quartz arenites to shallow-water carbonate rocks. The St. Peter Sandstone is the lower unit and is composed of conglomerate and shale overlain by quartz arenites; it is divided into the Kress, Tonti, and Starved Rock Members. The Glenwood Formation is the middle unit and consists of argillaceous sandstone, arenaceous mudstone, fissile shale, and arenaceous dolosiltite. The Pecatonica Formation is the upper unit and consists of laminated and unlaminated carbonate rocks, sandstone with carbonate cement, and brecciated calcisiltite.

The Kress and Tonti Members were deposited as basal units during transgression over an erosional surface, and the Tonti Member accumulated as a sheet sand in progressively shallower water northward. The transgression was followed by recession due to localized aggradation. The Starved Rock Member was deposited as a barrier-island complex, while the Glenwood Formation accumulated in a lagoon north of the barrier island. Transgression resumed, and the Pecatonica Formation was deposited in a shallow subtidal environment over most of the area. Deeper water carbonate rocks, however, formed where the former lagoon was deepest, and tidal flat carbonate rocks formed over the submerged barrier island.

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