Dolomitization and dolomite neomorphism: Trenton and Black River Limestones (Middle Ordovician) northern Indiana, USA

The Trenton and Black River Limestones are dolomitized extensively along the axis of the Kankakee Arch in Indiana, with the proportion of dolomite decreasing to the south and southeast of the arch. Planar and nonplanar dolomite replacement textures and rhombic (type 1) and saddle (type 2) void-filling dolomite cements are present. Three stages of dolomitization, involving different fluids, are inferred on the basis of petrographic and geochemical characteristics of the dolomites. Nonferroan planar dolomite has relatively high delta(18)O values (-1.8 to -6.1 parts per thousand PDB) and has Sr-87/Sr-86 ratios (0.70833 to 0.70856) that overlap those of Middle Ordovician seawater, Petrography, geochemistry, and the geometry of the dolomitized body suggest that the planar dolomite was formed in Middle and Late Ordovician seawater during the deposition of the overlying Maquoketa Shale, Ferroan planar and nonplanar dolomite occurs in the upper few meters of the Trenton Limestone, confined to areas underlain by planar dolomite, This dolomite contains patches of nonferroan dolomite with cathodoluminescence (CL) characteristics similar to underlying planar dolomite, Ferroan dolomite has relatively low delta(18)O values (-5.1 to -7.3 parts per thousand PDB) and has slightly radiogenic Sr-87/Sr-86 ratios (0.70915 to 0.70969) similar to those obtained for the overlying Maquoketa Shale, These data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying Maquoketa Shale during burial. The absence of ferroan dolomite at the Trenton-Maquoketa contact, in areas where the earlier-formed nonferroan planar dolomite also is absent, indicates that the fluid expelled from the overlying shale did not contain enough Mg2+ to dolomitize limestone. Type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite, suggesting that it also formed from shale-derived burial fluids. CL growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the aids of the Kankakee Arch. Type 2 saddle dolomite cements precipitated late; their low delta(18)O values (-6.0 to -7.0 parts per thousand PDB) are similar to those of the type 1 dolomite cement, However, fluid-inclusion data indicate that the saddle dolomite was precipitated from more saline, basinal fluids and at higher temperatures (94 degrees to 143 degrees C) than the type 1 cements (80 degrees to 104 degrees C). A trend of decreasing fluid-inclusion homogenization temperatures and salinities from the Michigan Basin to the axis of Kankakee Arch suggests that these fluids emerged from the Michigan Basin after precipitation of type 1 cement.