Faculty Scholarly Dissemination Grants

Enigmatic Laminae in Dolomite in the Mississippian Michigan Formation: Subsurface of Western Michigan

Department

Geology Department

College

College of Liberal Arts and Sciences

Date Range

2012-2013

Disciplines

Social and Behavioral Sciences

Abstract

Siliciclastic-rich laminae (SRL) in dolomite in the Michigan Formation contain euhedral to subhedral dolomite rhombs and have a high concentration of framboidal pyrite (FeS2), and celestite. Organics, mostly observed in SRL, are fibrous or ropey. Siliciclastic-poor laminae (SPL) have smaller dolomite rhombs and little FeS2 or celestite. The laminae could not be differentiated based on Fe concentrations or oxygen and carbon. Heavy oxygen and light carbon imply evaporitic, anoxic waters. Light carbon 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 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 time of deposition. A minimum amount of Fe was likely required to cause reaction between H2S formed by SRB and Fe, and this threshold was exceeded more in SRL than SPL. This is supported by a uniform background Fe concentration in the SRL and SPL, yet more FeS2 in the SRL. Iron oxide coatings on siliciclastics and Fe-rich clay minerals are likely sources for increased Fe concentration in the SRL at time of deposition.

Conference Name

Geological Society of America

Conference Location

Charlotte, NC

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