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The thickness and lithologic character of the Brereton Limestone Member are important factors in determining roof stability in underground mines in the Herrin (No. 6) Coal. Subsurface data from 216 electric logs or drill cores from two areas in the Illinois basin were used to construct isopach maps and fence diagrams of the Brereton Limestone. The Brereton Limestone is absent locally, and ranges from less than 0.1 to 19.5 ft (3 cm to 6 m) in thickness in most of the study area. Petrographic data from 159 thin sections or acetate peels from 48 drill cores permit recognition of 12 rock types in the Brereton: skeletal, foraminiferal, and algal lime mudstones; skeletal, sponge-spicule, and algal wackestones; skeletal and intraclastic packstones; skeletal and intraclastic grai stones; calcareous shale; and calcareous siltstone. All data are from two areas with a sandstone channel dividing them.
Area I shows a general overall thickening of the Brereton toward the sandstone channel but is characterized by mounds of lime mudstone and wackestone, some of which are capped by grainstones, suggesting shoaling. Area II is characterized by a general thickening of the Brereton away from the sandstone channel and generally an inverse relationship between the thickness of the Brereton and the underlying (Energy? and Anna) shales, but has a few carbonate mounds that were shoaling near the end of Brereton deposition. Lime mudstones and wackestones predominate, and some mound tops contain grainstones.
Thickness variations are depositional. Shallow, offshore marine Brereton facies suggest a "bank" depositional model, with irregular and local facies and thickness changes, rather than a "basin" model, with predictable offshore-onshore facies or thickness changes. Detailed petrologic and paleoecological analysis aimed at paleoenvironmental interpretations will probably be necessary to predict local variations in Brereton lithologic character and thickness in many areas.
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