Geochemistry of Regionally Extensive Dolomites, Burlington and Keokuk Formations (Mississippian), Iowa and Illinois: ABSTRACT

The Burlington and Keokuk Formations (Mississippian) contain 2 major generations of dolomite throughout the several ten-thousand square kilometers of study area. Dolomite I (oldest) is luminescently zoned, Ca-rich, Fe- and Mn-poor, and has Mississippian ⁸⁷Sr/⁸⁶Sr; dolomite II is unzoned, nearly stoichiometric, Fe- and Mn-rich, and has radiogenic ⁸⁷Sr/⁸⁶Sr; dolomite III, a minor zone, is nonluminescent and very Fe-rich. Dolomites I and II formed before nonmarine calcite cements, and all 3 dolomites formed before Late Mississippian under shallow burial conditions.

Stable isotopes of dolomite I average +2.3 ‰ δ¹³C and −0.3 ‰ δ¹⁸O PDB. Dolomite I is slightly depleted in ¹³C and ¹⁸O with respect to estimated Mississippian marine dolomite. Thus, dolomite I may have formed in waters only slightly depleted in ¹⁸O and ¹³C compared with seawater at low temperatures (~ 25°C). Dolomite II averages −4.1 ‰ δ¹⁸O PDB, +2.8 ‰ δ¹³C PDB, and has a wide range of δ¹⁸O (−0.5 to −6.6 ‰ PDB) and a narrower range of δ¹³C (+1 to +4 ‰). These data suggest that temperatures of precipitation were less than the 80°C−110°C ranges implied by the contained 2-phase fluid inclusions, unless the waters were isotopically heavy (~ + 3 to 11‰ δO SMOW). Dolomite 11 shows a regional geographic trend of decreasing δ¹⁸O and increasing δ¹³C from north to south. This trend can be accounted for by either a northward decrease in temperature or in ¹⁸O content of diagenetic waters.

We suggest a model in which dolomite I precipitated in marine-dominated mixed waters at low temperatures, the bulk of the dissolved constituents being derived intraformationally. Dolomite II formed as a replacement of dolomite I in a system dominated by nonmarine waters and/or slightly elevated temperatures, deriving much of their constituents from sub-Burlington strata.