STABLE ISOTOPIC AND MINOR MINERAL EVIDENCE FOR EARLY FORMATION OF DOLOMITE IN THE MISSISSIPPIAN MICHIGAN FORMATION, SUBSURFACE OF WESTERN MICHIGAN

Extensive exposures of gypsum, shale, and dolomite in the Mississippian Michigan Formation in Kent County in western Michigan have been made accessible through mining. These strata are separated into six units. Each contains a thick bed of gypsum at the base and is generally capped by thin to medium beds of shale and dolomite. Either laterally extensive or discontinuous beds of detrital-rich dolomite are found in some of the gypsum units. This study focuses on dolomite found in the second to the youngest stratigraphic unit. This bed is 3-20 cm thick, contains finely crystalline (averaging about 10µm), planar-s, dolomite rhombs, is rich in detrital grains (silt and angular to sub-rounded sand of sublitharenite composition), and largely lacks benthic fossils. Electron microprobe and x-ray diffraction analyses indicate the dolomite is ferroan, non-stoichiometric (rich in calcium), and poorly ordered. Reflected light and scanning electron microscopy have revealed a significant presence of pyrite associated with some of the dolomite. The pyrite is concentrated in laminae rich in siliciclastic material and is largely framboidal. Stable isotope analyses indicate the dolomite is heavy in δ¹⁸O (up to +5.3‰) and light in δ¹³C (-4.0 to -0.6‰) relative to Mississippian, marine dolomite. The fine crystal size and heavy δ¹⁸O values 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 δ¹³C 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.