Geochemical and petrologic evidence of the origin and diagenieis of a Late Mississippian, supratidal dolostone

Abstract

We examinedmajorelementcompositions, carbonandoxygenisotopiccompositions, andmacro/microfabricsof a 15-30em thick. bedded dolostone writ in the upper part of the Elwren Formationin southern Indiana for evidence of the dolostone's origin, early alteration, and burial diagenesis. Burial diagenetic alterationof the bedded dolostone and associated nodules is indicatedby the presence of recrystallized dolomite with light oxygen isotopic compositions. However, the Elwren Formationdolostones from different outcrops exhibit similar, systematic, stratigraphic trends of key primary microfabrics, organic matter content, carbon and oxygen isotopic compositions, and major elementcompositions. Therefore, the oxygenisotopicshift causedby burial diagenesisis inferredto have affected the dolostonesin a uniform manner so as to preserverelative,primary, oxygenisotopic differences between differentparts of the dolostone system. This permits our use of the isotopicdata to suggest the processes of formation of the bedded dolostone and the nature of the early alterationof the dolostone.Collectively, our data indicate that the bedded dolostone was deposited as regionally extensive, penecontemporaneous dolomite from hypersaline brines in a shallow,flat bottomed,marginalmarinelakeor lagoonat least 15 kID in radius. The water-bodywas laterfilled with siliciclastic mud that, along with the dolostone,was then pedogenically altered,givingrise to dolostonenodules formed by redistributionof thebeddeddolostonein the soil. Fabricsinsidethe nodules andcompactionalslickensidesaroundthe nodulesshow that the beddeddolostone was induratedbeforeit was subjectto pedogenesis and thatthe nodulesformedpriorto significantburial. Oxygenisotopicandmajorelement data indicate that the soil environmentwas at first dominatedby an isotopically light fluid, perhaps seawater, and later was saturated by evaporativebrine similar to that in which the bedded dolostoneformed.