High-Resolution Sequence Stratigraphy of Late Mississippian (Chesterian) Carbonate and Mixed Carbonate-clastic Reservoir Facies in the Illinois Basin: An Outcrop and Core Study

SMITH, LANGHORNE B., Chevron Production Co., New Orleans, LA, and READ*, J. FRED, Dept. of Geological Sciences, Virginia Tech, Blacksburg, VA

Eight 4th-order (400 k.y.) disconformity-bounded mixed carbonate-siliciclastic sequences were deposited in the tectonically-active, tide-dominated Illinois basin during the Late Mississippian greenhouse to icehouse transition. Detailed, lithologic cross-sections were constructed through the Chesterian Ste. Genevieve through Glen Dean interval which show an upward change in character from carbonate-dominated sequences bounded by caliche and breccia paleosols to mixed-carbonate siliciclastic sequences bounded by red, slickensided mudrock paleosols and incised valleys. The 4th-order sequences are composed of 5th-order parasequences that can be correlated basin-wide. Parasequences in the basal, dominantly carbonate sequences are composed of patchy ooid grainstone tidal ridge reservoir facies which interfinger with skeletal limestone and are capped by laterally extensive muddy carbonate units. Parasequences in the overlying mixed carbonate siliciclastic interval commonly have basal quartz sandstone valley fill and tidal sand ridge reservoir facies overlain by skeletal limestone and shale-dominated siliciclastics.
The sequences can be bundled into sequence pairs and composite sequences. Composite sequences are composed of 4 sequences and are bounded by better developed disconformities that commonly coincide with biostratigraphic zone boundaries. High energy reservoir facies are widespread in transgressive sequence tracts and late highstand sequence tract (where present) and confined to updip areas in the early highstand sequence tracts.
Increasing amplitude 4th-order glacio-eustasy produced the sequences and caused the upward increase in incised valleys and deeper water carbonate deposition. Parasequences were produced by 5th-order glacio-eustatic sea-level fluctuations (20-100 k.y.). Sequence pairs and composite sequences were produced by 3rd-order sea-level fluctuations possibly in combination with local tectonics. Spatial and temporal variations in differential subsidence between the eastern and western shelves and the more rapidly subsiding basin interior caused variations in onlap/offlap geometries of sequences and parasequences. Increasingly wetter wet-dry seasonality caused an upward increase in siliciclastic influx and concurrent decrease in ooid deposition. The increasing-amplitude eustasy and progressively more humid climate were caused by the onset of continental glaciation on Gondwana.

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