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ABSTRACT: Diagenetic Overprinting of the Manistique Group (Middle Silurian) in the Michigan Basin

Karen S. Mater, William B. Harrison III

Diagenetic overprinting is extensive in the Manistique Group, altering most primary depositional fabric and sedimentary components. Dolomitization was the first diagenetic alteration to occur. Micritic sediment was replaced with nonplanar dolomite, and fossils composed of aragonite or high-Mg calcite were replaced with planar-e dolomite. Biogenic silica dissolved, then reprecipitated as chert, replacing the nonplanar dolomite by force of crystallization. Small patches of dolomite, dolomite crystals with corroded edges, and fossils preserved in chert suggest that the chert is diagenetic and occurred after the formation of dolomite. Fossil preservation is good in the chert, with skeletal microstructures sometimes visible. The fossil fragments composed of low-Mg calcite were initially unaltered, but later dissolved. Physical and chemical compaction was next in the sequence of events. Stylolites developed in the dolomite, especially at the contact between the dolomite and the chert, owing to increased pressures resulting from increasing depths of burial. A second stage of pervasive dolomitization occurred next. This event recrystallized the dolomite leaving little evidence of the fossils, except where preserved in the chert. Hydrothermal alteration was next in the paragenetic sequence. Some of the chert was dissolved and silica then reprecipitated as megaquartz and chalcedony in fractures and pores. Precipitation of pyrite is followed by anhydrite. Sphalerite also occurs in small quantities. Saddle dolomite is common, lining and sometimes filling vugs and fra tures. The final diagenetic event to occur was the pore filling precipitation of halite.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990