STRATIGRAPHIC FRAMEWORK AND DEPOSITIONAL SYSTEM OF THE MIDDLE DEVONIAN LINGLE FORMATION, ILLINOIS BASIN

The Lingle Formation (upper Middle Devonian) is a major oil-producing interval along the northern and southern flanks of the Sangamon Arch in the northwestern part of the Illinois Basin. This study focuses on stratigraphic framework and depositional setting of the Lingle along the southeast margin of the Sangamon Arch, central Illinois. The upper Middle Devonian succession in southern Illinois has been the subject of several stratigraphic classifications. Following the original description, it was subdivided into Lingle and Alto formations. As these units were not lithologically distinct and could not be distinguished with any confidence, the St. Laurent Formation of Missouri was extended into Illinois. However, subsequent authors have used the names synonymously as St. Laurent (Alto and Lingle) Formation. The name Lingle has persisted and remains current in the Illinois oil industry as well as among some academic researchers. Therefore, the name St. Laurent is abandoned, and the name Lingle Formation is adopted for upper Middle Devonian strata that underlie the Upper Devonian New Albany Shale and overlie the lower Middle Devonian Grand Tower or older deposits.

The Lingle consists of mixed carbonate-siliciclastic deposits. It was deposited in a warm, gently sloping homoclinal ramp platform in which high energy bioclastic packstone to grainstone/quartz sandstone facies graded basinward to low-energy muddy sediments. In the study area, the Lingle represents an unconformity bounded depositional sequence superimposed by small scale shallowing-upward cycles. A thin distinctive high gamma-ray shale marker at the base of the formation in the study area that is coeval with an argillaceous phosphatic limestone in the deeper part of the basin record maximum flooding horizon. The highstand package may encompass laterally discontinuous bioclastic grainstone, dolostone, and sandstone/dolomitic sandstone reservoirs, which commonly display northeast-southwest trend suggesting deposition along a paleoshoreline roughly parallel with the trend of the Sangamon Arch. The rather thin and southward thickening transgressive package records a north-northwest, relatively rapid diachronous transgression over the lower Middle Devonian strata across the Illinois Basin.