Petrographic and Geochemical Study of Gypsum and Dolomite in the Mississippian Michigan Formation, Subsurface of Western Michigan
P. E. Videtich1 and S. A. Tourre2
1Department of Geology, Grand Valley State University, Allendale, MI
2URS Corporation, Sacramento, CA
The Mississippian Michigan Formation in the subsurface of western Michigan is comprised of gypsum-shale-dolomite sequences. The gypsum and dolomite were petrographically and geochemically analyzed to determine their origin. The majority of the gypsum is alabaster with a mosaic texture and some vertically oriented nodules. The gypsum is 1 to 6 m in thickness and has a high gypsum to matrix ratio. The dolomite (generally <0.2 m thick) is unfossiliferous and non-stoichiometric. It is composed of very fine (most <10 lm), subhedral to anhedral crystals. Ripples and mudcracks, formed in the underlying shale, are preserved in the dolomite as are terrestrial plant fossils.
d34S in the gypsum ranges from +15.2 to +17.9 per mil CDT and 87Sr/86S ranges from +0.70792 to +0.70806. All these data fall within the range of estimated isotopic values for other Mississippian marine sulfates. d18O and d13C in the dolomite are negatively correlated and the d18O is as heavy as+5.3‰ PDB and d13C as light as –4.7‰.
The megascopic characteristics of the gypsum suggest subaqueous deposition, perhaps in marginal seas. The similarities between the isotopic values of the gypsum in the Michigan Formation and reported Mississippian values indicate that the gypsum precipitated from water of marine derivation with little or no influence from continental waters. Petrographic observations and isotopic data for the dolomite suggest that the precursor dolomite sediment formed in a shallow, restricted sea that was periodically freshened resulting in mixing of waters. The carbonate sediment was likely dolomitized either penecontemporaneously or soon after burial.
AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan