ABSTRACT

Cumulative patterns of dolomitization in a subsurface Upper Devonian carbonate platform in the Peace River Arch area, Alberta, are related to progressive burial (< 1 km), then fracture-controlled processes. The paragenetic sequence and dolomite types are defined using plain-light, cathodoluminescence, and fluorescence microscopy, with further definition provided by isotope (¹⁸O, ¹³C, ⁸⁷Sr/⁸⁶Sr) and trace-element (Mn, Fe) data. Onset of fracture-related dolomitization, between depths of 700 and 1000 m, is defined by depth profiles of vitrinite reflectance.

Dolomitization began with near-surface growth of Ca-rich (54-56 mol % CaCO₃), Fe-poor isolated rhombohedra. Their continued growth and coalescence coincided with stylolite formation. This progression was followed by formation of patchily distributed stoichiometric, Fe-poor (< 0.5% Fe) matrix dolomite that formed after onset of chemical compaction and prior to fracture-related dolomitization. A narrow range in ¹⁸O (-4.2 to -5.9 PDB; x = - 5.3) values is defined for all these dolomites; they are more negative than expected for Frasnian marine dolomites. A broad scatter in their ¹³C (-2 to +4 PDB) suggests that they precipitated in zones of ongoing anaerobic organic diagenesis. Sr isotope values (x = 0.7098) of matrix dolomite are radiogenic compared to Frasnian seawater. Progressive burial dolomitization was paralleled by changing Mg sources: (1) diffusion from seawater; (2) chemical compaction of limestone; and (3), as previously modeled for other Leduc platforms, platform-directed flow related to basin sediment compaction.

Deep burial (> 1 km) fracturing focused basin-derived fluids along subvertical structural conduits and into adjacent permeable strata. In the Early Mississippian (< 1 km burial), mixing of intraplatform marine-derived waters and basin-derived brines caused dissolution and replacement of limestone and matrix dolomite by Fe-poor microcrystalline dolomite; additional void-fill dolomite followed. At greater depths (< 2 km), during the Mississippian through the Jurassic, microfractures formed conduits for Mg that was locally derived from chemical compaction of existing dolomites. This fracturing stage is defined by Fe-poor fracture-fill dolomite, as well as sparry and saddle dolomites that formed within interconnected secondary macroporosity. The final stage of dolomitization mark the first appearance of ferroan (> 3 wt % Fe) dolomite in the platform, heralding increased Fe mobilization during burial diagenesis. A negative shift in ¹⁸O values, -6.5 to -15, is associated with successive fracture-related dolomites, and is interpreted to reflect an increase in fluid temperatures. Fracture-related flow was probably rapid; petrographic and isotopic signatures of fracture-related dolomitization can be traced 300 m upsection from the Leduc Formation.