Conference Name: Geological Society of America Conference 2011
Gypsum in the Michigan Formation in Kent County has been separated into six units. Each contains a thick bed of gypsum capped by beds of shale and dolomite. This study focuses on dolomite found in the second to the youngest unit. This bed contains finely crystalline dolomite rhombs, is rich in detrital grains, and largely lacks benthic fossils. Electron microprobe and x-ray diffraction analyses indicate the dolomite is ferroan, non-stoichiometric, and poorly ordered. Reflected light and scanning electron microscopy have revealed a significant presence of pyrite. The pyrite is concentrated in laminae rich in siliciclastic material and is largely framboidal. Stable isotopes indicate the dolomite is heavy in ´18O (up to +5.30) and light in ´13C (-4.0 to -0.60) relative to Mississippian, marine dolomite. The fine crystal size and heavy ´18O are consistent with dolomite forming syndepositionally in an evaporitic environment and may indicate crystallization at relatively low temperatures without significant recrystallization at depth. The light ´13C may be due to penecontemporaneous incorporation of light, organic carbon through the metabolic action of sulfate-reducing bacteria. These bacteria are thought to play a key role in the formation of protodolomite. The reducing environment created by these bacteria likely resulted in the early formation of the pyrite, and the lack of oxygen may account for the general scarcity of benthics associated with the dolomite. Thus, both evaporitic conditions and organogenic processes are likely keys to the origin of the dolomite, which is interpreted to have formed during or soon after deposition.