Near-Surface and Burial Diagenesis of Mississippian Burlington and Keokuk Formations: ABSTRACT

Abstract

The Burlington and Keokuk formations (Osagean) in Illinois and Missouri exhibit a complex diagenetic history of multiple episodes of calcite cementation, dolomitization, dedolomitization, chertification, compaction, and minor Mississippi Valley-type mineralization. Geochemical data and plane and cathodoluminescent light petrography define the complete paragenetic sequence. The earliest widespread diagenetic event was replacement of lime mud by luminescently zoned dolomite (dolomite I). Dolomite I rhombs were later partially replaced by unzoned, luminescently red dolomite (dolomite II). A regional calcite-cemented stratigraphy of 5 luminescent and nonluminescent zones postdates dolomites I and II and predates upper Meramecian deposition. Between the third and fourth calcite-cement zones, minor dedolomitization was accompanied by a third dolomite generation (dolomite III) that syntaxially overgrew and/or replaced dolomites I and II. Chertification of lime mud began before replacement by dolomite I. Additionally, chalcedony and megaquartz filled voids and partially replaced the first 5 calcite-cement zones. Two regional compaction events are observed. The first occurred between calcite-cement zones 3 and 4, whereas the second postdates calcite cement zones. The preceding events are postdated by minor Mississippi Valley-type mineralization, which includes dissolution vugs partially filled by baroque dolomite, sphalerite, pyrite, quartz, and ferroan calcite. A mixing zone is proposed for dolomite I based on petrographic timing and complementary stable isotope data. Regional variations in luminescent and nonluminescent calcite-cement zones suggest precipitation in ground-water systems with lateral and vertical Eh gradients. Late diagenesis by hot brines is indicated by minor Mississippi Valley-type mineralization and supported by fluid inclusion data and depleted carbon and oxygen isotope values. End_of_Article - Last_Page 272------------