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 ^pmil ^dgr¹³C and -0.3 ^pmil ^dgr¹⁸O 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 ^pmil ^dgr¹⁸O PDB, +2.8 ^pmil ^dgr¹³C PDB, and has a wide range of ^dgr¹⁸O (-0.5 to -6.6 ^pmil PDB) and a narrower range of ^dgr¹³C (+1 to +4 ^pmil). 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 isotop cally heavy (~ +3 to 11 ^pmil ^dgr¹⁸O SMOW). Dolomite II shows a regional geographic trend of decreasing ^dgr¹⁸O and increasing ^dgr¹³C 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.

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