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

The Trenton and Black River Limestones are dolomitized extensively along th e axis of the Kankakee Arch in Indiana, with the proportion of dolomite dec reasing to the south and southeast of the arch. Planar and nonplanar dolomi te replacement textures and rhombic (type 1) and saddle (type 2) void-filli ng 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, geochemistr y, and the geometry of the dolomitized body suggest that the planar dolomit e was formed in Middle and Late Ordovician seawater during the deposition o f the overlying Maquoketa Shale, Ferroan planar and nonplanar dolomite occu rs in the upper few meters of the Trenton Limestone, confined to areas unde rlain by planar dolomite, This dolomite contains patches of nonferroan dolo mite with cathodoluminescence (CL) characteristics similar to underlying pl anar 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 rat ios (0.70915 to 0.70969) similar to those obtained for the overlying Maquok eta Shale, These data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying Ma quoketa Shale during burial. The absence of ferroan dolomite at the Trenton -Maquoketa contact, in areas where the earlier-formed nonferroan planar dol omite 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 fe rroan dolomite, suggesting that it also formed from shale-derived burial fl uids. CL growth zoning patterns in these cements suggest that diagenetic fl uids moved stratigraphically downward and toward the southeast along the ai ds 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 simil ar to those of the type 1 dolomite cement, However, fluid-inclusion data in dicate that the saddle dolomite was precipitated from more saline, basinal fluids and at higher temperatures (94 degrees to 143 degrees C) than the ty pe 1 cements (80 degrees to 104 degrees C). A trend of decreasing fluid-inc lusion homogenization temperatures and salinities from the Michigan Basin t o the axis of Kankakee Arch suggests that these fluids emerged from the Mic higan Basin after precipitation of type 1 cement.