Mass balance and fluid flow constraints on regional-scale dolomitization, Late Devonian, Western Canada Sedimentary Basin

摘要

Shields and Brady (1995) provided a thought-provoking discussion that attempts to solve one of the problems that we have worked on for many years, i.e., to place tighter constraints on the flow mechanism(s) and sources of Mg that resulted in regionally pervasive, replacive dolomitization of the Upper Devonian carbonates in the Western Canada Sedimentary Basin (WCSB). The major conclusion of Shields and Brady's (1995) study is that "regional-scale reflux dolomitization accounts for the majority of the dolostones in the WCSB (p. 371)." We agree with the notion that seepage reflux of evaporite brines accounts for some of the Upper Devonian dolomites in the basin, such as the Grosmont Platform in the northeastern part of Alberta (e.g., Machel, 1990; Luo and Machel, 1995) and a few other, much smaller examples (see summary in Mountjoy and Amthor, 1994; Whittaker and Mountjoy, in press). However, a number of Shields and Brady's (1995) assumptions are questionable, and some of their generalizations are not justified. That renders their major conclusion tenuous, at best, and incorrect at least for most of the Alberta part of the WCSB.