Continuing studies of Illinois basin Pennsylvanian stratigraphy have revealed a fluvial sandstone body of Desmoinesian age in the central and southern part of the Illinois basin. The sand body is a series of multistory or "stacked" sandstones that aggregate more than 300 ft (100 m) thick, as much as 2 mi (3.2 km) wide, and more than 100 mi (160 km) long, making part of a series of interrelated sandstones that constitute the Highland fluvial complex. The sand complex includes as many as

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four cycles of sedimentation, producing a multistory fluvial sandstone sequence similar to two- and three-story sand bodies described from the middle Carboniferous of the Donets basin (USSR).

As noted by D. Swann and P. Potter the Highland and other fluvial complexes represent the Michigan River system that flowed across the basin from the northeast for as much as 250 × 10⁶ years. The long-continued geographic stability of successive, often reestablished, channel courses is evidence of the subsidence history of the Illinois basin, its lack of southern closure, and its relation to the Mississippi Valley embayment.

Certain Pennsylvanian channel sandstones are productive in the Illinois basin, but those of the Highland fluvial complex are not, presumably because of high, uninterrupted permeability, shallow burial, and lack of associated source beds. The multistory sand bodies, however, are accompanied by low-permeability over-bank silts and crevasse splays which appear to seal adjacent coals from contact with marine waters and consequent sulfate contamination.

Multistory sands are possible exploration targets in other cratonic basins. They are identifiable by detailed facies analysis, core drilling, and high-resolution seismic surveys.

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