ABSTRACT

Interpretive cross sections based on detailed descriptions of 33 outcrops and cores are used to better understand the relative effects of tectonics, eustasy, tides, and climate on Upper Mississippian (middle Chesterian) stratigraphy in the tectonically active, tide-dominated Illinois basin. The cross sections show that five mixed carbonate-siliciclastic, high-frequency sequences in the Bethel through Glen Dean formations can be correlated around the outcrop belt through areas with very different subsidence histories.

The sequence boundaries are marked by paleosols and incised valleys and can be correlated basin-wide within a framework of distinctive marker beds. Because of its updip position, lowstand systems tracts are not preserved in the Illinois basin. The transgressive systems tracts generally consist of one or two parasequences that are typically composed of tidally influenced quartz sandstone filling incised valleys at the base overlain by open-marine skeletal limestone, shallow-marine shale, and heterolithic siliciclastic tidal-flat deposits. The maximum flooding surface (MFS) for the sequences is picked at the base of the deepest water limestone facies. Highstand systems tracts are composed of 1 to 6 regressive parasequences that consist of basal offshore skeletal limestone capped by laterally extensive shale and heterolithic siliciclastic tidal-flat facies.

The basin-wide extent of the sequence boundaries and maximum flooding surfaces across tectonic highs and lows suggests that the sequences were produced by eustatic sea-level changes rather than local tectonics or autogenic processes. The sequences were likely produced by moderate- to high-amplitude (30-100 m) fourth-order ( 400 ky) glacio-eustatic sea-level changes driven by the transition from the greenhouse conditions of the Early Mississippian to the "icehouse" conditions of the late Paleozoic. The lateral extent and frequency of component parasequences suggests that they were likely produced by fifth-order sea-level changes (10-100 ky). The sequences may be bundled into third-order composite sequences, but the third-order signal is obscured by the magnitude of the fourth-order sea-level changes a feature typical of ice-house stratigraphies.

The sequences can be used as time slices to identify spatial and temporal variations in differential subsidence between the Cincinnati Arch and the more rapidly subsiding Basin Interior. Episodes of high and low differential subsidence occurred every two to three sequences. These subsidence variations had a major impact on lithofacies distribution and onlap and offlap geometries in sequences and parasequences. The occurrence of some widespread seismically disturbed beds suggests that active faulting occurred during deposition. Normal faulting appears to have occurred during periods of high differential subsidence and reverse faulting during periods of low differential subsidence. Differential subsidence and related normal and reverse faulting may have occurred in response to phases of thrust loading and quiescence in the Appalachian orogenic belt to the east. Even in this tectonically active setting, however, it is the eustatic signal that generates basin-wide, mappable stratigraphic sequences.