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