MASS-BALANCE AND FLUID-FLOW CONSTRAINTS ON REGIONAL-SCALE DOLOMITIZATION, LATE DEVONIAN, WESTERN CANADA SEDIMENTARY BASIN

The source of Mg and the hydrogeologic mechanism by which it is circul ated are critical issues for understanding replacement dolomitization because of the extremely large amount of mass exchanged (primarily Mg for Ca) and the volume of parent water fluxed in the process. Mass bal ance arguments based on dolomite stoichiometry, equilibrium chemistry and basin-scale volumetric data provide the foundation for quantitativ e analyses of specific dolomite occurrences. When applied to the Weste rn Canada Sedimentary Basin (WCSB), these principles suggest that regi onal-scale seepage reflux: dolomitization probably accounts for the ma jority of the dolostones. Moreover, the analyses demonstrate why close d-system dolomitization models, such as burial compaction of shale wat ers, or cannibalization of Mg-bearing strata, such as older dolostones or shales, cannot account for regional dolomite occurrences. The occu rrence of ''hydrothermal dolomite'' (cement and/or replacive phases), which is a localized phenomenon compared to the regional-scale dolosto nes discussed here, could have cannibalized older reflux-related dolos tones in a burial environment, although their origin and age remains t o be demonstrated. Upper Devonian dolostones constitute approximately 50% of the volume (1.24 x 10(5) km(3)) of contemporaneous shallow-mari ne carbonates in the WCSB requiring that approximately 4 x 10(16) kg o f Mg be incorporated in the process. Basin-scale mass balance estimate s indicate that at least 3 to 9 x 10(16) kg of Mg were contained in th e residual evaporitic seawater brines associated with Upper Devonian e vaporite deposits, whereas only about 1 x 10(14) kg was associated wit h connate seawater in all the Paleozoic shales in basin. These results warrant a quantitative analysis of a regionalscale reflux flow system for the Late Devonian as well as abandonment of the shale compaction theory as model for regional-scale dolomitization. A volumetric flow m odel, i.e., a dimensional analysis of Darcy's Law, for the Late Devoni an to early Mississippian indicates that regional-scale, shallow-buria l (probably less than 500 m) reflux is a viable mechanism for regional -scale dolomitization. The model is consistent with the regional distr ibution of dolostones and primary evaporites as well as the early diag enetic, burial and thermal histories of the host strata. It also expla ins the chemical composition of modern connate brines in these rocks.