ENIGMATIC LAMINAE IN DOLOMITE IN THE MISSISSIPPIAN MICHIGAN FORMATION: SUBSURFACE OF WESTERN MICHIGAN

Siliciclastic-rich laminae (SRL) are common in thin dolomite beds in the Mississippian Michigan Formation. The dolomite is encapsulated in thick gypsum beds and commonly underlain by shale. SRL contain euhedral to subhedral dolomite rhombs (mean ~20 μm) and have a high concentration of framboidal and plate-like pyrite (FeS₂), and celestite. The celestite is irregular and seems to infill pore space. Organics (μm-scale), mostly observed in SRL, are fibrous, branching, or ropey. Siliciclastic-poor laminae (SPL) have smaller dolomite rhombs (mean ~10 μm) and little FeS₂ or celestite. Using EDS the laminae could not be differentiated based on Fe concentrations in the dolomite (mean 3.8 mol % FeCO₃) or based on δ¹⁸O (-1.34 to +5.02 ‰) and δ¹³C (-3.8 to +0.01 ‰).

Heavy δ¹⁸O and light δ¹³C imply evaporitic and anoxic waters. Light δ¹³C indicates incorporation of C¹² via breakdown of organics by sulfate-reducing bacteria (SRB). FeS₂ morphologies support this interpretation. The plate-like texture of some of the FeS₂ suggests replacement of Fe-rich clay minerals due to SRB activity. Framboidal FeS₂, commonly surrounding organics (mm-scale), is known to form in reducing environments dominated by SRB. A large amount of FeS₂ in the SRL is thought to result from a high Fe concentration available for FeS₂ formation at the time of deposition. A minimum amount of Fe was likely required to cause the reaction between H₂S formed by SRB and Fe, and this threshold was exceeded more in the SRL than SPL. This is supported by a uniform background Fe concentration in the SRL and SPL, yet more FeS₂in the SRL. Iron oxide coatings on siliciclastics and Fe-rich clay minerals are likely sources for the increased Fe concentration in the SRL at the time of deposition.

Relatively large dolomite rhombs in the SRL imply recrystallization occurred preferentially in the more permeable (result of clastic abundance) SRL. Because the δ¹⁸O in the SRL vs. SPL is not distinguishable, recrystallization in the SRL likely occurred in a closed system. However, the association of secondary celestite is enigmatic. Celestite commonly is thought to indicate an aragonitic precursor to dolomitization, but this would require interlaminated aragonite (SRL) and dolomite (SPL). Alternatively, the celestite could have formed during recrystallization with gypsum as a source for the Sr.