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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

NDGS/SKGS-AAPG

Fourth International Williston Basin Symposium, October 5, 1982 (SP6)

Pages 75 - 88

GEOLOGY OF THE MIDDLE DEVONIAN DAWSON BAY FORMATION IN THE NORTHERN PART OF THE WILLISTON BASIN

COLIN E. DUNN, Saskatchewan Geological Survey, 201 Dewdney Ave. East, Regina, Saskatchewan

ABSTRACT

The Dawson Bay Formation is generally 30 to 40 m thick. It thins to the west of Saskatchewan, and thickens to about 52 m in Manitoba. Over part of eastern Saskatchewan an increased thickness is attributed to the local development of a halite bed at the top of the formation.

Three (locally four) members are defined from type wells in the Saskatoon area. They are, in ascending order, the Second Red Bed, Burr, and Neely Members, which are capped locally by the Hubbard Evaporite.

Dolomitic mudstone of the Second Red Bed Member is overlain disconformably by carbonates of the Burr Member. These carbonates are dominated by limestone with a normal marine biota of crinoids, corals and brachiopods, and they are characterized by discontinuity surfaces or "hardgrounds". The Neely Member lies disconformably upon a hardground, and consists of an argillaceous limestone (which grades southward into a non-argillaceous sucrosic dolomite) overlain by sucrosic and microcrystalline bituminous limestones containing reefoid accumulations of stromatoporoids, corals and stringocephalid brachiopods. Overlying carbonates are dolomitized algal mats and stromatolites, which are succeeded by intermixed cryptocrystalline dolomite and felted anhydrite.

These sediments were deposited in shallow shelf seas, and the hardgrounds bear witness to several periods when sedimentation ceased. Quiet conditions during deposition of the Neely Member sediments permitted the development of reefoid mounds up to 5 m thick. Sabkha-like conditions prevailed during the final stages of Neely deposition.

Post-depositional events include partial dolomitization: notably in the lower few metres of the Burr Member, the lower 5 m of the Neely Member (in southern Saskatchewan), and the uppermost part of the Neely Member. Bituminous material entered the reefoid part of the Neely Member prior to extensive plugging of porosity by halite. Later events include local subsidence and fracturing of Dawson Bay sediments in areas where dissolution of the Prairie Evaporite has taken place.

Recent discoveries of hydrocarbons in the Dawson Bay Formation in North Dakota renew optimism for making similar discoveries in the northern part of the Williston Basin. Since depositional conditions and diagenesis were similar throughout the basin the porous zones of the Neely Member in Saskatchewan may yet yield oil from structural traps.

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