REGIONAL STRATIGRAPHIC RELATIONS OF THE BURLINGTON-KEOKUK FORMATIONS (MISSISSIPPIAN) ACROSS THE CENTRAL U.S., AND THE NATURE OF THE SUB-BURLINGTON DISCONFORMITY

The classic Osagean Burlington-Keokuk (B-K) succession of the Mississippi Valley is characterized by skeletal grainstones and dolomitized mudstones deposited in an offshore storm-influenced middle-shelf setting. Sediment accumulation never filled up accommodation space, and the middle-shelf area remained entirely subtidal for the duration of B-K deposition. The succession is punctuated by several prominent condensed units and/or starved surfaces, marked by sculpted hardgrounds or phosphatic enrichment (bone beds). Northwestward (shoreward) relations of the B-K have long remained unresolved, but latest Kinderhook-Osagean strata across the inner-shelf facies tract of northern Iowa include cyclic successions of subtidal and peritidal carbonate units. These strata encompass the Gilmore City Fm and overlying units ("Keokuk"). New drill cores in northern Iowa provide strong lithostratigraphic comparisons between the middle and upper Gilmore City Fm and nearby proximal facies of the Burlington Fm. Correlative strata of the lower Gilmore City are entirely absent across the central middle shelf (type B-K area), but this interval downlaps a basinward-expanding regional disconformity developed on top of Maynes Creek-Wassonville carbonates in proximal areas of the middle shelf. The Gilmore City Fm is overlain by strata historically allied with the B-K or Keokuk-Warsaw. This interval as seen in northern Iowa cores is subdivided into five cyclic packages, each marked by subtidal marine carbonates at the base and capped by peritidal or brecciated facies. These strata compare closely with the type Keokuk succession which, although lacking peritidal facies, is also subdivided into five units (split at regional bone beds/hardgrounds). The conodont and foram biostratigraphy currently remains equivocal, but available evidence supports the correlation of supra-Gilmore City strata with the Keokuk Fm.