The Ullin Limestone (Mississippian) in the Illinois Basin is a light to medium gray, fine-grained skeletal mudstone to wackestone with beds of bryozoan and crinoid-rich grainstone and packstone. This lithofacies is widespread on the structurally higher parts of the La Salle anticline in Crawford and Jasper Counties, Illinois and adjacent areas of Indiana. This shallow to deep shelf facies grades laterally down the western slope of the La Salle anticline in Lawrence and Wabash Counties, Illinois, into the Fort Payne Formation through an intermediate facies. The intermediate facies consists of medium to dark gray lime mudstone which is actually a calcisiltite consisting of bryozoan, crinoids, and other fossil fragments that have been broken and abraded to the limit of recognizability.
The Fort Payne Formation in White County, Illinois, is a dark yellowish brown silicious lime mudstone interpreted to have been deposited in a deep-water basin. Bedding in the Fort Payne varies from irregular fine laminae to nodular bedded and burrowed. Silica is present as disseminated clay-sized quartz, chert nodules, and white quartz nodules. A small amount of crinoid debris is present.
Lateral continuity of beds from the shelf facies into the basinal facies can be demonstrated by correlation of electric log marker horizons. Total thickness of the interval is 122 to 153 m in Wabash and White Counties, Illinois.
Pre-existing depositional topography indicates that the Fort Payne was deposited in water approximately 300 meters deep. Most of the carbonate mud that makes up the Fort Payne was probably winnowed from the surrounding shallow shelves and transported to the center of the basin. Little indigineous carbonate was likely to have been produced at that depth. Silica could have been derived from silicious planktonic organisms, sponges, clay minerals, and clay-sized detrital quartz. Small quantities of organic matter and pyrite give the Fort Payne its characteristic dark color. Much of the Fort Payne is burrowed, not bioturbated, indicating limited bottom dwellers due to a dysaerobic depositional environment.
