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The AAPG/Datapages Combined Publications Database
Williston Basin Symposium
Abstract
NDGS/SKGS-AAPG
Fourth International Williston Basin Symposium, October 5,
DEPOSITIONAL ENVIRONMENTS OF THE UPPER DEVONIAN BIRDBEAR FORMATION, SASKATCHEWAN
ABSTRACT
The Upper Devonian Birdbear Formation is the uppermost unit of the Saskatchewan Group, and is equivalent to the Birdbear (Nisku) Formation of North Dakota and Montana, and approximately equivalent to the Nisku Formation of the Winterburn Group of the Alberta subsurface. Problems regarding the correlation of the Saskatchewan section with rocks in Alberta may be due to the transitional lithologies found between the deeper water sediments of the Alberta Basin and the shallow water shelf sediments of the Montana and North Dakota portions of the Williston Basin.
Detailed lithofacies mapping and sedimentological studies have revealed the presence of thirteen microfacies within the Birdbear. The dominant microfacies are close and complex associations of laminated to cryptalgal dolomite-lime mudstone, structureless crystalline dolomite and bedded to nodular-bedded anhydrite with interlaminated massive dolomite. These microfacies are dominant in the upper member of the Birdbear Formation. The lower member is dominated by mud-supported (as opposed to grain-supported) carbonates, with typical microfacies being bedded lime mudstone, pelletoidal bioclastic lime wackestone to packstone and nodular lime mudstone to nodular bioclastic lime mudstone-wackestone. There are local occurrences of highly fossiliferous, bioclastic open-marine lime wackestone to packstone, and oolitic lime packstone to grainstone. Dolomitization of the lower member is well developed, apparently controlled by the position of landward restricted lagoons behind erratically distributed shoals.
The pattern and distribution of microfacies point to depositional environments of very low wave and current energy, increased salinity and limited amounts of terrigenous clastic influx. In western Saskatchewan, an argillaceous dolomite to lime mudstone is dominant, indicating the influx of terrigenous elastics from positive structural features marginal to the Williston and Alberta Basins.
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