Mississippian carbonate buildups and development of cool-waterlike carbonate platforms in the Illinois Basin, Midcontinent U.S.A.

Numerous biohermal buildups occur in Mississippian (Lower Carboniferous) strata in the Illinois Basin and adjacent regions. They developed as mud mounds, biodetrital calcisiltite mounds, and bryozoan frame thickets (fenestrate-frame coquina or rudstone) during the Kinderhookian and early Meramecian (Tournaisian and early Visean), and as microbial mud mounds, microbial- serpulidbryozoanboundstones, and solenoporoid (red algal) boundstones during the Chesterian (late Visean and Serpukhovian). True Waulsortian mounds did not develop in the Illinois Basin, but echinoderm (primarily crinoids)-bryozoan carbonate banks and bryozoan frame thickets generally occupied the same niche during the Kinderhookian-early Meramecian. Nutrient availability and the resulting increase in the productivity of echinoderms and bryozoans were apparently detrimental to Waulsortian mound development. Deposition of crinoidal-bryozoan carbonates during the Kinderhookian-Osagean initially occurred on a ramp setting that later evolved into a platform with a relatively steep margin through sediment aggradation and progradation. By mid-Osagean-early Meramecian, two such platforms, namely the Burlington Shelf and the Ullin Platform, developed adjacent to a deep, initially starved basin. Sedimentologic and petrographic characteristics of the Kinderhookian-earliest Meramecian carbonates resemble the modern cool-water Heterozoan Association. This is in contrast with post-earliest Meramecian carbonates, which are typically oolitic and peloidal with common peri tidal facies. The post-earliest Meramecian carbonates, therefore, resemble those of the warm-water Photozoan Association. The prevalence of Heterozoan carbonates in the Illinois Basin correlates with a rapid increase in the rate of subsidence and a major second-order eustatic sea-level rise that resulted in deep-water starved basins at this time. In the starved Illinois Basin, deposition was initially limited to a thin phosphatic shale that was followed later by deposition of up to 200 m of siliceous, spiculitic, and radiolarianbearing limestone. The starved basin was connected to the deep open ocean through a bathymetric depression, which was centered over the failed late Precambrian-Early Cambrian Reelfoot Rift, which extended from the deep-water Ouachita Trough in central Arkansas to southern Illinois, approximately parallel to the trend of the modern Mississippi River. We believe that upwelling of cool, nutrient- and silica-rich deep oceanic water, which entered the basin through this bathymetric depression, resulted in proliferation of pelmatozoans and bryozoans. The subsequent change from cool-water-like carbonates to warm-water-like carbonates appears to be related to decreased subsidence and gradual shallowing of the basin.