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The role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models. A two-dimensional, finite-difference code (Bethke 1985) was used to simulate single-phase flow of variable-salinity fluids driven by compaction, convection, and topography in carbonate platforms and adjacent basinal sediments. A simple hydrostratigraphic model consisting of a platform-reef complex surrounded by basin-filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence. Most pore fluids in the shale migrate to the surface at velocities less than 0.1 mm/yr and basin-derived fluids are focused into the reef from only a limited distance of 10 km. Relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform-reef complex. Magnesium mass-balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization. Two-layer hydrostratigraphic models involving variable-salinity fluids show that evaporation of seawater results in downward and seaward fluid migration; the magnitude of flow is a function of fluid density. Even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments, albeit at rates of less than 1 cm/yr. Paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface-derived waters evaporated past gypsum saturation is a viable dolomitization mechanism. Calculations utilizing rates of Mg mass flux show that dolomitization of large carbonate platforms (>10 km3) by fluids below halite saturation requires millions of years of reflux-driven flow. /P> Thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow. The thermal "pump" produces a convection cell in which lateral and vertical flow achieve rates of 75.0 and 13.0 mm/yr. respectively. The potential for dolomitization is high because a large reservoir of Mg (seawater) is tapped, elevated temperatures drive fluids further into the stability field for dolomite, and kinetic inhibition of dolomite nucleation and growth is decreased. Simulations of high-frequency glacio-eustatic sea-level oscillations suggest that a dynamic hydrologic regime may be established. A sea-level fall of 2 m lasting for 10,000 yr can drive lateral flow at rates up to 0.15 m/yr. Dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea-level fluctuations.

Saddle dolomite, anhydrite and coarsely crystalline, late mosaic calcite cements in the Devonian Swan Hills Formation at the Rosevear Field in Alberta, Canada are interpreted from petrographic and geochemical evidence as the products of burial diagenesis. These inferred burial cements are commonly found as a partial to complete filling of moldic and vuggy porosity in massive replacement dolostones. Fluid inclusions (Th = 125-159°C; Tm = -18 to -22°C), 87Sr/86Sr ratios (0.7101-0.7115), enrichment in Ba and Zn, and light 18O values (- 6 to - 11.2, PDB) suggest that saddle dolomite precipitated from hot, radiogenic brine with salinities six times greater than seawater. Ions for saddle dolomite precipitation may have been sourced from off-reef shales, the Precambrian basement, Middle Devonian halite, Mesozoic clastics and Swan Hills replacement dolomite undergoing pressure solution. In the presence of a "normal" paleogeothermal gradient (30°C/km), saddle dolomite would have formed in the Late Cretaceous-Paleocene, at depths of 3-5 km. This time corresponds to the emplacement of gas at the Rosevear Field. Dolomite-hosted anhydrite cement with nonradiogenic 87Sr/86Sr (0.7083) and low 34S compositions (15 to 23 CDT) has been quantitatively modeled using a two-component mixing equation to constrain fluid and ion sources. The results indicate that ions for cementation were derived from fluids that passed through and dissolved Middle Devonian anhydrite. This produced a Sr-rich brine with nonradiogenic 87Sr86Sr and low 34S. These fluids mixed with radiogenic formation water already in the reservoir to precipitate anhydrite cement with the observed isotopic composition Fluid inclusions (Th = 90 to 170°C; Tm = -6 to -17°C) and elevated 87Sr/86Sr compositions (0.7105-0.7173) in coarsely crystalline, late mosaic calcite support precipitation during the hydrological transformation of the Western Canada Basin into a topographically-controlled flow regime. The reduced precipitation temperatures of some fluid inclusions in late mosaic calcite compared to saddle dolomite may be attributed to the decrease in western basin paleogeothermal gradients following the Laramide Orogeny. The presence of calcitized anhydrite, coarsely crystalline calcite with light 13C values (-5.4 to -13.2 PDB), S-enriched pyrite, and 8% H2S in reservoir gases provide evidence for thermochemical sulfate reduction during burial diagenesis at Rosevear.

Iron content and cathodoluminescence define six regionally extensive calcite cement zones over 100,000 sq. km of study area in the Burlington-Keokuk Formation of Illinois and Missouri. The regional cement stratigraphy is composed of mappable zones, analogous to lithostratigraphic units, which represent diagenetic episodes during which calcite with distinct cathodoluminescent properties precipitated in a given time interval relative to other diagenetic events such as cement fracturing and calcite dissolution. Individual cement zones may not be strictly isochronous throughout the study area but are not strongly diachronous. Two different, but related, cathodoluminescent sequences have been identified within pre-Pennsylvanian cements in the Burlington-Keokuk Formation: 1) within lower strata, early, nonferroan cement consists of both nonluminescent and luminescent calcite (zones II-IV); 2) within upper strata, early, nonferroan cement consists of entirely luminescent calcite (zone II). Petrographic relationships indicate that zone II is time equivalent with zones II-IV in lower Burlington-Keokuk strata. The occurrence of cement zones II-IV encased in Pennsylvanian shale and quartz sand within pre-Pennsylvanian karst cavities constrains the age of early, nonferroan calcite as pre-Pennsylvanian. In ll cases, pre-Pennsylvanian cements are followed by ferroan, dull to moderately luminescing calcite (zone V) which, in turn, is overgrown by nonferroan, moderate to brightly luminescing calcite (zone VI). Cross-cutting relations of Burlington-Keokuk syntaxial calcite with Mississippi Valley-type mineralization phases constrains the minimum age of cement zones V and VI as Early Permian. Pre-Pennsylvanian cements formed in a shallow, meteoric aquifer characterized by vertically segregated chemical facies. Pre-Pennsylvanian cements within lower strata precipitated from waters of varying Eh, while those in upper strata formed from reduced waters. Post-Mississippian/pre-Early Permian cements precipitated under less than 1 km of overburden from reducing meteoric waters or subsurface brines.