IDENTIFYING CYCLOTHEMS IN THE PENNSYLVANIAN TRADEWATER FORMATION IN THE SUBSURFACE OF THE ILLINOIS BASIN

Cyclothems were initially described in Pennsylvanian Strata of the Illinois Basin (ILB) in the early twentieth century, initiating substantial research to correlate the strata of these cyclothems, both within the ILB and across the continent. Accurate correlation of cyclic strata allows for a greater understanding of spatially-changing depositional environments, temporal changes in climate, and ultimately the controls on cyclic deposition and their relative importance in different parts of the basin.

Some of the better documented cyclothems in the ILB and North America are found within Middle-Late Pennsylvanian strata, as the low relief depositional setting allowed for laterally-extensive marker horizons. However, cyclothems of Lower-Middle Pennsylvanian strata in the ILB are not as well documented, with the relatively high relief of the pre-Pennsylvanian unconformity resulting in more spatially-restricted deposits with fewer laterally-continuous marker beds. Consequently, Lower-Middle Pennsylvanian cyclothems have historically been difficult to correlate over regional distances.

To attain a more comprehensive understanding of changing environmental conditions during the Pennsylvanian and the relative impact of autocyclic and allocyclic controls on deposition, Lower-Middle Pennsylvanian cyclothems are being examined. Sequence stratigraphic techniques are being applied to core and geophysical data, and cyclic strata will be correlated from the outcrop belt into the subsurface of the ILB. This paper will outline the first results of a study to correlate cyclothemic strata from the eastern margin of the ILB along a transect that runs toward the basin center, using biostratigraphic markers to verify correlations. The correlated strata will be evaluated through sedimentological study of outcrop, core, and three-dimensional subsurface maps derived from geophysical data, in order to determine the depositional setting. Once correlation is complete, facies relationships can be documented and geochemical and mineralogical analyses conducted to evaluate the controls on cyclicity of Lower-Middle Pennsylvanian strata in the ILB.