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Lower Paleozoic limestones in the Williston basin are only preserved from complete dolomitization where they are overlain by an argillaceous member of the Ordovician Stony Mountain Formation. This umbrellalike relation was originally interpreted as indicating that dolomitization resulted from the actions of descending brines. Basin-wide dolomitization of overlying strata (including the Middle Devonian Winnipegosis Formation) and an absence of Upper Devonian and higher regional dolomitization suggest that dolomitization was also a Middle Devonian event, which occurred during deposition of the overlying Prairie Evaporite.
Winnipegosis-Prairie Evaporite relations in Saskatchewan, however, indicate that a different genetic link existed between evaporite deposition and the regional dolomitization of subjacent carbonate rocks. It is believed that the Prairie evaporites were precipitated largely from groundwaters that entered the basin by means of carbonate buildups in the Winnipegosis Formation. Groundwater seepage first deposited travertine, "vadose" pisolites, and carbonate muds on mound tops and flanks, then caused massive precipitation and growth of interstitial, sediment-replacing and sediment-displacing gypsum in the mound-flanking beds. Concentration and processing of brines during their downward migration allowed final precipitation of chevron halite in salt flats on the basin floor. Groundwater mo ements also caused regional dolomitization of pre-Prairie Evaporite carbonate rocks which lay along flow paths. Groundwaters beneath the relatively impermeable Stony Mountain shale were stagnant and, consequently, did not cause dolomitization of their carbonate hosts.
Dolomitization of lower Winnipegosis blanket ("platform") limestones occurred concurrently with early compaction. This timing is consistent with the suggested dolomitization model.
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