Print this pageAbstract: Dolomitization Patterns in Paleozoic of Williston Basin
Mechanisms and processes of dolomitization may be suggested by dolomite-distribution patterns in hand specimens and on a regional scale as in examples chosen from the lower Paleozoic and Mississippian of the Williston basin.
Mottled dolomitization patterns in the Ordovician Yeoman Formation (= Lower Red River) reflect preferential dolomitization of burrow fills. Sediment between burrows had been rendered impermeable during very early diagenesis by cementation. Dolomitization occurred prior to complete sediment lithification (early diagenetic) and halite molds suggest that dolomitization occurred in association with refluxing hypersaline brines. The vertical distribution of dolomite in the formation, however, indicates that such brines did not supply the necessary magnesium; this was supplied more probably from the sediment itself (dolomitization by cannibalization). In parts of the overlying Stony Mountain Formation, dolomite is present in an argillaceous matrix between undolomitized limestone nodules. Th nodules formed during very early diagenesis as a result of patchy lithification. Later severe pressure solution has formed microstylolites between nodules, along which later dolomitizing fluids entered the rock. This late diagenetic dolomitization also caused alternation of matrices in the burrow-mottled sediment type of the Yeoman Formation. On a regional scale, limestones in the lower Paleozoic are present only beneath an argillaceous member of the Stony Mountain Formation. This umbrella-like action of the "Stony Mountain shale" suggests dolomitization occurred from descending ground waters. Basin-wide dolomitization of overlying strata (including the Middle Devonian Winnipegosis Formation) suggests that dolomitization was a Devonian event, accomplished during deposition of the Prairi Evaporite or during phases of later salt dissolution.
Mississippian carbonate rocks (Madison Group) are dolomitized and cemented by calcium sulfate to depths of up to 150 ft (46 m) beneath an unconformity but only when overlain by red beds and/or evaporites of the Jurassic-Triassic Watrous Formation. Restriction of dolomitization to the area of Watrous cover and an inverse relation between Watrous red bed thickness and the depth of replacement indicate that dolomitization occurred during the time of Watrous deposition. During Watrous evaporite deposition a vast sabkha complex lay across the Williston basin. Calcium sulfate precipitated there was derived from ground water in the underlying red beds. Ground water was replenished from northward-migrating, compaction-driven formation waters in the Mississippian carbonate rocks. Ion filtratio by the clay-rich Watrous red bed unit caused water in contact with the Mississippian carbonate rocks to become more saline and to exceed gypsum saturation. Consequent gypsum cementation (and minor replacement) of these carbonate rocks generated brines with a high Mg/Ca ratio capable of dolomitizing the remaining limestones.
AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana