STRATAL ARCHITECTURE WITHIN OSAGEAN (MISSISSIPPIAN) CARBONATE SEQUENCES, MIDCONTINENT SHELF OF IOWA

Osagean carbonate deposition across the central Midcontinent region of Iowa can be subdivided into two general facies tracts: 1) inner-shelf area of shallow subtidal to peritidal carbonate facies, primarily packstones (fine skeletal, peloidal, intraclastic, coated-grain, or oolitic) and mudstones (fenestral to brecciated) of the Gilmore City Formation and proximal “Keokuk” facies; and 2) middle-shelf area of entirely subtidal carbonate facies, primarily crinoidal packstones, dolomitized mudstones, and siliceous lithologies of the Burlington and Keokuk formations. Sediment accumulation was slow across the middle shelf, and the stratigraphic succession is punctuated by condensed units and starved submarine surfaces. Inner-shelf strata of the Gilmore City Fm include three stratigraphic sequences displaying a stacked succession of parasequences. Each parasequence (most 1 to 5 m thick) forms a shallowing-upward package, commonly with a peritidal cap. The lower sequence downlaps a basinward-expanding submarine disconformity developed across the middle shelf area, and upper strata of the downlap succession share relationships with the lower Burlington Fm. Middle and upper Gilmore City strata show an abrupt facies transition at the inner-shelf break, replaced by proximal facies of the Burlington Fm. Succeeding Keokuk deposition is marked by a complex stack of shallowing-upward parasequences, which are capped by peritidal facies in inner-shelf areas.