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Framboidal Pyrite Associated with Organogenic Dolomite in the Mississippian Michigan Formation, Subsurface of Western Michigan




College of Liberal Arts and Sciences

Date Range



In western Michigan mining exposed six stratigraphic units composed of Mississippian-age gypsum, shale, and dolomite. This study focuses on unit two (second from youngest). Two types of laminae (siliciclastic-rich, SRL; and siliciclastic-poor, SPL) occur in the dolomite, which our studies to date indicate is organogenic in origin. In addition to the abundance of siliciclastics in the SRL, dolomite rhombs and pyrite framboids are larger, framboids are more plentiful, and their shape is more irregular than those in the SPL. These differences between the types of laminae indicate variations in their depositional environments and diagenetic histories. Relatively high available iron concentrations may account for the larger amount of pyrite in the SRL, whereas differences in conditions during formation of the pyrite may explain the size and shape variations (e.g., formation in water column versus sediment). Selective recrystallization due to higher porosity and permeability may explain the larger size of the dolomite rhombs in the SRL. This recrystallization is suggested to have occurred at depth with water originating from the dehydration of gypsum, which may account for preliminary data suggesting relatively heavy ´O18 in dolomite in the SRL versus the SPL.

Conference Name

Michigan Academy of Science, Arts, and Letters

Conference Location

Alma, Michigan

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