DEPOSITIONAL ENVIRONMENTS OF THE PENNSYLVANIAN PARTLOW SANDSTONE, CLARK COUNTY, ILLINOIS

During the Pennsylvanian, the Illinois Basin (ILB) was characterized by a tropical, low-lying coastal plain and adjacent shallow-shelf, leaving behind deposits representative of a series of deltaic, estuarine, and fluvial systems. Cyclothems record the effects of cycles of glacioeustatic change on the depositional systems. Sedimentological correlation of cyclothems promotes a greater understanding of the extent and spatial variation of depositional environments, and provides insight into climate change and the controls on cyclic deposition within the ILB during this period in time. To interpret paleoenvironments, eight cores that penetrate the lower Pennsylvanian Partlow sandstone within the south Johnson Oil Field in Clark County, Illinois, were examined. Core descriptions, facies analysis, and stratigraphic correlation using geophysical data were used to make interpretations.

The Partlow sandstone is situated in the Tradewater Formation and represents a parasequence-scale record of tidally influenced fluvio-deltaic deposition in the ILB. The typical succession in all cores includes a basal shale sharply overlain by sandstone that grades into heterolithic strata. Basal shales are representative of delta-front facies, containing pyrite, no carbonaceous detritus, occasional silt, and frequent horizontal and vertical burrows. The sharp contact between shaly delta front facies and overlying sandstones represents a disconformity. Above this disconformity is an abundance of unidirectional planar to cross-bedded sandstone, indicating a sharp transition to a fluvially-dominated environment. Channel lags and scours, rip-up clasts, and rare carbonaceous debris suggest strong currents and a depositional environment indicative of avulsing distributary channels within a delta plain. Sandstones grade vertically into heterolithic strata that contain bidirectional current indicators, occasional pyrite, horizontal burrows, and rare carbonaceous debris. A transition to tidal influence is recorded where heterolithic strata becomes prominent, suggesting a switch to a tidally-influenced interdistributary bay. Geophysical data supports this transition to a lower energy regime, which is likely the result of allocyclic transgression combined with autocyclic channel migration.