USE OF SUBSURFACE DATA FOR MIDDLE AND UPPER PENNSYLVANIAN CYCLOSTRATIGRAPHY IN THE ILLINOIS BASIN

Upper Desmoinesian and lower Missourian (Middle to Upper Pennsylvanian) cyclic successions in the Illinois basin are poorly known due to their geographic variability and scarcity of good exposures. Subsurface data from multiple locations are required to establish correlations within the basin and with the well-known Midcontinent Pennsylvanian succession. About a dozen cores at the Illinois State Geological Survey supply lithologic data, as well as conodonts and palynomorphs for biostratigraphic control. Narrowly-constrained paleontologic correlations are scarce, however, and the cores are too widely spaced for reliable physical correlations.

Modern geophysical log suites, including gamma ray, density, resistivity, and caliper curves, provide thousands of additional data points. Log signatures for limestone, black shale, coal, and sandstone are distinctive. In addition, mature paleosols can be identified in the logs. As blocky, highly fractured claystones, mature paleosols tend to cave in (visible on the caliper curve) and have very low resistivities (generally less than 5 ohm-meters). Paleosols are critical for identifying emergent episodes in the absence of coals, fluvial sandstones, and erosion surfaces.

Subsurface data show that the interval from the Danville Coal to the paleosol beneath the Chapel Coal in the Illinois basin includes the same number of transgressive-regressive depositional cycles (eight) as time-equivalent strata in the Midcontinent region where they are designated Farlington, Altamont, Norfleet, Idenbro, Lost Branch, Checkerboard, Exline, and Critzer cycles. A composite section of these eight cycles in the southwestern part of the basin includes at least 32 successive depositional units and paleosols, but no single location has them all. The succession of depositional cycles becomes obscure toward the east and northeast in the basin as the section becomes dominated by fine to medium-grained progradational siliciclastics.