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Ordovician strata in Manitoba comprise two vertically distinct parts, a lower clastic sequence, and an upper sequence of dolomites and dolomitic limestones. The latter have been divided into three formations having the character of para-time rock-units. These are recognizable in the outcrops of southern Manitoba and east-central Saskatchewan and can be traced into the subsurface. The lowermost, and by far the thickest, of the carbonate formations is the Red River.
The outline and shape of the depositional Williston Basin is reflected in the present structural attitude of the Red River, which also manifests the presence of a number of positive tectonic elements within the basin.
The formation thickens from the basin periphery toward the International Border, its center of maximum deposition being beyond the area of study, in central North Dakota. Both the rate of thickening and the degree of structural dip increase towards the basin center. Thinning of the formation is apparent over local positive areas.
The Red River is herein divided into Lower, Middle, and Upper units based on mass lithologic characters. The Lower and part of the Middle unit are restricted westward, indicating generally transgressive marine conditions during deposition. Minor cyclical fluctuations involving interbedded evaporites and bioclastic material are evident in the Upper Red River.
Three broad lithofacies may be discerned within the formation each of which is believed to reflect deposition within a relatively distinct environment, dependent primarily on the influence of water depth for its salient characteristics. These environmental zones blend into each other both laterally and vertically but within a single para-time unit tend to be related to structural features and to geographic position relative to the basin center. Their distribution implies the existence of a shelf-edge circumscribing the basin.
The development of porosity appears directly influenced by variations in lithology and by the degree of secondary dolomitization to which the rock has been subject.
The salinity of waters contained in the total Ordovician carbonate section is greatest nearest the basin center, fresh water predominating in closer proximity to the peripheral outcrops. Known occurrences of petroleum appear to bear a positive relationship to the distribution of highly saline formation waters.
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