Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological
Survey and Resource Assessment Division).
Mississippian Period (362 to 320 million year old rocks)
After a brief regression, the Kaskaskia Sea returned to Missouri at the beginning of the Mississippian. The Kaskaskia never receded in the nearby Illinois Basin. Although the inundation did begin with a silica base, this layer is only expressed as a sandstone or shale in the northeast and southwestern parts of the state. Much like the Ordovician, the Mississippian sequence is composed of 4 series: the Kinderhookian, Osagean, Meramecian and Chesterian. Unlike the Ordovician, the rock layers differ greatly, depending on the sector of the state. About a quarter of all the rocks exposed in Missouri are Mississippian in age.
Most Mississippian strata are crystalline, fossiliferous, or micritic (muddy) limestones, interbedded with thin shales. The Mississippian sea began as a clear, warm, shallow sea with profuse carbonate reefs. As the time period progressed, the seas became progressively muddier as a result of the Acadian orogeny to the northeast. The Mississippian sea may have covered much of Missouri, although, as usual, the deposit thinned over the uplift areas, and was subsequently removed.
Perhaps the most typical Mississippian strata is the Burlington Limestone.
This crystalline, extremely fossiliferous limestone covers most of the state and extends into Iowa and Arkansas. Typical characteristics include layers of chert nodules, and a sedimentary structure caused by pressure solution called "stylolites". The spaces in the stylolite may also become filled with chert or quartz.
During the Mississippian, crinoids were king. These echinoderms resemble plants, complete with a calyx (head) and a holdfast resembling roots. The common name for crinoids is "sea-lilies", though they are actually animals. After death, the "stem" breaks apart into tens to hundreds of fragments, which are one of the most common Mississippian fossils.
Above the Burlington, lies the Keokuk limestone, world famous for "geodes". A geode is a hollow round stone, usually filled with crystals. Sedimentary geodes are formed when mineralized water is trapped in lime muds, or when mineralized water later migrates into open spaces in the limestone. A crack to allow fluids in and out is essential. The walls of the space become coated (more or less) usually with calcite or quartz. Once the walls are stabilized, crystallization proceeds, with crystal growth size limited only by the space available and the mineralized fluid. Eventually, the geode runs out of crystallization materials, the geode dries out, or the space becomes entirely filled with mineral. Once exposed at the surface, geodes, which are generally round or ovoid and less susceptible to weathering than the surrounding rock, will loosen from the limestone. A sharp whack with a hammer is all that is required to yield a fistful of crystals.
At the end of the Mississippian, the retreating Kaskaskian sea overwhelmed the carbonate sequences with silica rich muds and sands. As the carbonates became exposed, karstification once again worked its weathering on the newly formed land surface.
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