Comparison of Outcropping and Buried Reefs in Middle Devonian Winnipegosis Formation, Manitoba and Saskatchewan

D. M. Kent, H. R. McCabe

In 1986, Winnipegosis reefs became prime exploration targets in the Williston basin, following the discovery of producible oil in the Home Tableland 8-22-2-9W2 well in southeast Saskatchewan. An understanding of the facies distribution in these reefs is difficult to acquire since few of them have been penetrated by more than one well. However, the exposed reefs in the Manitoba outcrop belt may be useful to develop facies models applicable to subsurface reefs.

Winnipegosis reefs grew in an elongated northwest-trending basin extending from North Dakota to northwest Alberta. The basin was flanked on the west and southeast by an exposed hinterland to which was accreted a carbonate wedge of shallow shelf sediments. The reefs developed on a platform of normal marine carbonates underlying the entire basin. Differentiation into shelf and basin settings occurred after deposition of the platform sediments.

The shelf sediments are predominantly lime mudstones and skeletal wackestones with local packstone and grainstone accumulations. The components of these lithologies include brachiopods, crinoids, solitary corals, ostracods, bryozoans, amphiporids, green algae, and peloids. Locally, there are stromatoporoid-coral patch reefs.

The basinal deposits include starved-basin sediment consisting of kerogenous lime mudstones with intercalated reef-derived biogenic material, overlain by varvitic couplets of lime mudstone and kerogenous sediment. These pass upward into, and are interbedded with, layered anhydrites that are rich in black organic material either as an admixture to the anhydrite or as thin laminae. These sediments are interrupted by local buildups of lime mudstone, skeletal wackestone, packstone, and grainstone. The buildups range in height from 30 to 100 m above the starved-basin sediments. Those in southeast Saskatchewan are between approximately 30 and 60 m.

Both subsurface and outcropping reefs appear to have been initially colonized by crinoids and codiacean algae in a wackestone lithology. In subsurface, wackestones continue to dominate the reefs vertically, to near the top where coral-stromatoporoid framestones and algal laminites take over. Outcrop reefs present a lateral variation from dipping reef-flank material through a coral-stromatoporoid facies into a reef-interior facies of pelleted grainstones and wackestones. At one outcrop locality, the reef is capped by algal laminites developed on peloidal and skeletal grainstones, all of which appear to have undergone vadose diagenesis.

The outcrop reefs are thought to occupy three paleogeographic positions: upper basin slope, toe-of-slope, and central basin. In addition, they have two morphologies--pinnacle and flat top. The flat-top reefs are thought to have isolated coral-stromatoporoid buildups along their margins. The areal extent of the outcrop reefs varies depending on morphology, but it is limited to between 0.5 and 10 km.

We anticipate that the preceding features may be analogous for subsurface reefs. However, the problem of the relationship between buried reefs and enclosing anhydrite cannot be solved in the outcrops and will require core and detailed seismic analysis.

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