Burial Dolomitization of the Middle Ordovician Glenwood Formation by Evaporitic Brines, Michigan Basin
- Bruce Purser,
- Maurice Tucker and
- Donald Zenger
Published Online: 14 APR 2009
Copyright © 1994 The International Association of Sedimentologists
Dolomites: A Volume in Honour of Dolomieu
How to Cite
Simo, J. A., Johnson, C. M., Vandrey, M. R., Brown, P. E., Castrogiovanni, E., Drzewiecki, P. E., Valley, J. W. and Boyer, J. (2009) Burial Dolomitization of the Middle Ordovician Glenwood Formation by Evaporitic Brines, Michigan Basin, in Dolomites: A Volume in Honour of Dolomieu (eds B. Purser, M. Tucker and D. Zenger), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304077.ch11
- Published Online: 14 APR 2009
- Published Print: 25 MAY 1994
Print ISBN: 9780632037872
Online ISBN: 9781444304077
- burial dolomitization models;
- burial dolomitization of middle Ordovician Glenwood formation by evaporitic brines, Michigan Basin;
- stratigraphic north-south cross-section of Glenwood formation;
- lithofacies associations and rock types;
- paragenetic sequence of carbonate lithofacies of Glenwood formation;
- plane-light photomicrograph of mimetic fine-crystalline replacive dolomite;
- carbonate mineral phases of Glenwood formation - four groups;
- fluid-inclusion homogenization temperatures for fracture-filling dolomite cements;
- Sr concentration — Sr isotope variations;
- sources of dolomitizing brine
The Middle Ordovician Glenwood Formation in the Michigan Basin represents a transitional stratigraphic unit from siliciclastic- to carbonate-dominated sedimentation. Deposition occurred in a distal shallow-marine mixed carbonate–siliciclastic setting relative to the shallow beach-to-bar shoreface. deposits of the underlying St Peter Sandstone and the open-marine limestones of the overlying Black River Formation. The Glenwood dolomites form the upper seal of a gas compartment in the deep Michigan Basin.
Diagenesis of the Glenwood Formation carbonate lithologies can be divided into three stages, each occurring in progressively deeper burial diagenetic environments. Early diagenesis occurred in a near-surface, probably marine, setting. Middle diagenesis occurred during shallow burial and is characterized by mouldic porosity, aggrading neomorphism and by precipitation of ferroan and non-ferroan equant calcite cement. Late diagenesis is characterized by deep-burial dolomitization. Dolomite replaced most of the precursor carbonate sediment and occluded fracture, shelter and intergranular porosity. Late postdolomite diagenesis included clay cementation and localized anhydrite formation.
Variations of δ18O and δ13C in carbonate phases define four distinct groups.
1 Middle diagenetic stage neomorphic calcites (n = 3) have δ18O values that vary from 17.4 to 23.6‰ SMOW, and relatively constant δ13C values (–3.5 to –1.3‰ PDB).
2 Most late-stage fine-crystalline replacive dolomites have a narrow range of δ18O values (19.4 ± 0.5‰ SMOW), but δ13C values range from –4.5 to –1.1‰.
3 Late-stage coarse-crystalline replacive dolomites (n = 6) have distinctly lower δ18O values (14.8 to 17.6‰ SMOW), compared to fine-crystalline replacive dolomite, and mainly constant δ13C (–1.5 ± 0.3‰ PDB).
4 Late-stage dolomite cements (n = 17) have δ13C values of –4.4 to –1.3‰ PDB, and all but two δ18O values have a narrow range (18.3 ± 0.7‰ SMOW).
Fluid-inclusion homogenization temperatures of 175 ± 25°C support a late-stage origin of the dolomite cements. At 175°C, the range of δ18O values for dolomite cements suggests precipitation from a water with a constant δ18O value of 6 ± 1‰ vs. SMOW. These data suggest dolomitization from an evaporitic fluid.
Replacive dolomites and dolomite cements have low Sr concentrations (typically <100 ppm). Replacive dolomites have high Rb concentrations (up to 12ppm) and 87Sr/86Sr ratios, which reflect contamination by radiogenic clay. In contrast, dolomite cements have low Rb contents, indicating no clay contamination, and lower 87Sr/86Sr ratios of 0.7084–0.7086, similar to the Sr isotopic composition of Ordovician and Silurian seawater. The homogeneous Sr isotope composition for the dolomite-forming fluids does not allow basinwide circulation of diagenetic fluids, which would probably have interacted with radiogenic sediments, and indicates that the fluids maintained their original chemical signatures for long periods of time, probably in a closed system.
The proposed dolomitizing fluid is middle Palaeozoic evaporitic brines of seawater origin that interacted little with radiogenic sediments and maintained the original 87Sr/86Sr ratios until dolomitization. The timing of dolomitization is uncertain, but the high homogenization temperatures and age of the clay postdating dolomite cements suggests an early Carboniferous age, when the Glenwood Formation reached maximum burial.