Mid-Continent Section

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Sequence Stratigraphic Control on Distribution and Porosity Evolution in Cherts in the Mississippian of the Mid-Continent


Chert and silicified carbonate in the Mississippian of the Mid-Continent are highly variable and often contain significant porosity. Silicification of carbonate was initiated by dissolution of biogenic amorphous silica and the subsequent precipitation of opal-CT and quartz. Carbonate replacement was achieved by a force of crystallization controlled replacement, where dissolution of carbonate material was driven by re-precipitation of stable silica phases. Dissolution of carbonate and silica, as well as the volumetric change from amorphous silica to quartz, created pore space in many of the cherts. Porosity abundance in chert is controlled by the relative rate of carbonate sedimentation during deposition, as it controls the ratio of carbonate to spicules. Slow sedimentation resulted in the dissolution and re-precipitation of silica as nearly pure, non-porous chert. Faster sedimentation resulted in adequate ratios for nearly complete replacement of limestone by silica, forming abundant porosity in the rock. Rapid sedimentation minimized the effect of silicification and porosity evolution. In outcrop, gradational changes from pure, to highly porous, to less porous cherts are observed vertically at multiple frequencies due to varying orders of sea level cyclicity. The link between chert variety, relative sea level fluctuation, and the observed sequence stratigraphic framework aids in explaining the controls on porosity distribution at both the 3rd and 4th order scales.