High-Resolution Sequence Stratigraphy of Late Mississippian Carbonate and Mixed Carbonate-Siliciclastic Reservoirs, Illinois Basin
SMITH, JR., LANGHORNE B., and J. FRED READ
Nine 4th-order (approx. 400 k.y. duration) disconformity- bounded sequences
(5 to 75 meters thick) were deposited in the tectonically-active Illinois basin
during the Late Mississippian Ste. Genevieve through Glen Dean interval.
Subsidence variations through time between the eastern shelf and more rapidly
subsiding central basin influenced parasequence and reservoir distribution in
the sequences. High differential subsidence caused more parasequences to onlap
in the TST, and HST parasequences to be disconformity-bounded updip or to
progradationally off lap. Low differential subsidence caused parasequences to
have a more "layer-cake" distribution.
Carbonate sequences 1 to 4 have type 2 sequence boundaries marked by caliche, breccia and eolianites and are composed of 4 to 9 carbonate and limestone-shale parasequences. The parasequences have patchy eolianites at the base, overlain by laterally interfingering skeletal grain stone and ooid grainstone tidal ridges (reservoir facies), and are capped by laterally extensive muddy carbonate/fossiliferous shale units and updip disconformities.
Mixed carbonate-clastic sequences 5 to 9 commonly have type 1 sequence boundaries marked by red and green paleosols, traceable laterally into incised valleys. Mixed carbonate clastic parasequences are composed of basal marine quartz sandstone valley-fills and sand ridges (reservoir facies) overlain by sheet-like skeletal limestones and capped by fossiliferous shale. The upward increase in incision on sequence boundaries, the increased siliciclastic component and the 400 k.y. duration of sequences suggest that the sequences were produced by increasing-amplitude 4th-order eustasy driven by the onset of Gondwana glaciation.