ABSTRACT

The outcrops of the Bluff Reef are the most accessible Middle Devonian reef exposures in western Manitoba. The Bluff is a near shore island on the northwest side of Dawson Bay. Unlike other island exposures of the Winnipegosis that are accessible only by boat or float-equipped aircraft, the Bluff is connected to the mainland by a tombolo of Holocene and Pleistocene deposits. The reef outcrops are on the north and northeast margin of the island, but recent corehole drilling at the south end indicates that the shape of the island closely imitates the outline of the Winnipegosis reef. However, the southern half is overlain by 26 to 38 m of Dawson Bay Formation, Second Red Beds and a collapse breccia. Thus, at the north end the reef is about 70 m thick, but in the south half it varies from 43 to 51 m. The reef has a northeast axial trend and a surface length of about 700 m along that axis.

A seven-corehole drilling program indicates that the Bluff Reef rests conformably on the same carbonate rock unit that underlies all Winnipegosis reefs, commonly called the platform. The platform consists of mottled to nodular dolomitized lime mudstone with scattered crinoid columnals and fragmented brachiopod valves. The platform in this area has a variable thickness ranging from about 8 m at both ends of the Bluff to 14 m beneath the central portion.

At the south end of the reef, growth was initiated through a proto-reef community, similar to that found forming the foundations of the Swan Hills reefs of Alberta. This community was dominated by small hemispherical stromatoporoids and thamnoporid corals. They occur in a dolomitized micritic rock with a large proportion of kerogenous material. At the northern end of the reef, the inaugural community appears to have been dominated by crinoids. With the exception of a second proto-reef community a few metres above the founding proto-reef, the remainder of the vertical sequence at the Bluff is dominated by dolomitized micrite containing mainly metozoan skeletal remains as well as peloids and calcareous algal debris. Coreholes and outcrops show that the upper 15 to 30 m at the northern end consist of tabular hemispherical and bulbous forms of stromatoporoids, as well as thamnoporid, fasciculate and chaetitid corals and stringocephalid brachiopods.

Three conclusions can be drawn about Winnipegosis reefs from this study of the Bluff reef: 1) Winnipegosis reef morphology can be extremely changeable; 2) although most of the lithologic variations occur in a vertical sense, there are some subtle and not so subtle lateral changes; and 3) growth of the reef appears to have been initiated about some irregular form on the Middle Devonian sea floor.