ABSTRACT

The Starved Rock Member of the St. Peter Sandstone has been described as a clean, medium-grained sandstone that was deposited in a broad band across northern Illinois. This deposit displays a sequence of sedimentary structures that indicates deposition in shallow water. From the base upward the structures include (1) massive beds; (2) small-scale trough cross-strata; (3) high-angle tabular cross strata; (4) large-scale trough cross strata; (5) alternating beds of horizontal, burrowed laminae and small-scale trough cross strata; and (6) massive and burrowed beds.

Tabular cross strata and small-scale trough cross strata show a consistent northwest orientation, whereas the large-scale trough cross strata show no preferred orientation. Both tabular and large-scale trough cross strata consist of medium-coarse sand, whereas the small-scale trough cross strata and horizontal laminae are of medium sand. Discontinuous horizontal laminae, present at irregular vertical intervals in the section, consist of interlaminated medium to fine sand and silt.

Recently, the sand waves of the North Sea have been proposed as an analog for the St. Peter environment in Wisconsin, because of the presence of such sedimentary structures as very large-scale tabular cross strata. However, the sedimentary structures, directional data, and textures observed in the Starved Rock Member resemble more closely those developed on nonbarred coasts, like that of Oregon. The presence of inferred deeper water sediments within the St. Peter, landward of the Starved Rock Member, indicates that this sand body may have been deposited as an offshore shoal.

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