Paper No. 3
Presentation Time: 9:00 AM-6:00 PM

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


DAVIS, Adam J., Geology Department, Baylor University, One Bear Place #97354, Waco, TX 76798 and VIDETICH, Patricia E., Geology Department, Grand Valley State University, 1 Campus Drive, Allendale, MI 49401, davisaj205@gmail.com

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.