Devonian Winnipegosis Reefs of Manitoba Outcrop Belt--Possible Basin Model

Hugh R. McCabe

Devonian Winnipegosis reefs crop out intermittently along a 350-km belt in southwestern Manitoba and represent an almost complete facies sequence ranging from central-basin reefs in the northwest to shelf-edge reefs in the southeast. In addition, structural complexities of the overlying Devonian strata mirror precisely the configuration of the underlying reefs. These outcrop data, supplemented by closely spaced stratigraphic core holes that have been sited specifically with respect to reef-controlled structures, permit development of a tentative reef model.

Reef parameters are (a) size, from less than 0.5 km to a maximum of about 12 km; (b) shape, small pinnacle-type features to broad, irregular, flat-topped, atoll-like complexes, all with relatively steep margins (5°-20°); and (c) height, uniform in any given area, ranging from 40-50 m at shelf edge to 65-90 m in central basin areas.

Internal reef structure shows flat central beds, and flank dips ranging from 20° to 45° or more in the most basinward reefs. Locally, reefs are abundantly fossiliferous, but organic framework is difficult to identify. Black bituminous mudstones in the interreef and reef-flank areas, with or without reef-derived carbonate detritus, are the only time-stratigraphic equivalents of the reefs. (The Ratner-type bituminous carbonate laminites appear to entirely postdate reef development.)

Configuration of the steep-sided reefs and the well-defined shelf edge may have been controlled not only by organic (biohermal?) development, but also by "anoxic containment" resulting from restriction of lateral growth by anoxic bottom waters. Pinnacle reefs could thus have developed, in part, as anoxically contained pinnacle mounds rather than true bioherms.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988