The Middle Devonian Winnipegosis Formation of Saskatchewan is divisible into upper and lower members

End_Page 619------------------------------

on the basis of a regionally developed, argillaceous interval which forms the uppermost part of the lower member.

The lower member consists of regionally dolomitized marine carbonate rock of relatively consistent thickness and lithology, and has a maximum observed thickness of 54 feet. The upper member is a varied marine carbonate sequence, with three major facies. In the southwest, a wedge of lithologically relatively consistent carbonate rocks has a maximum thickness of about 130 feet along a northwestward-trending axis extending through the Weyburn district in the southeast and the Elbow district in the northwest. This wedge thins somewhat irregularly, though gradually, to a depositional edge in the extreme southwestern part of the area. North and east of the basin, finely laminated carbonate rocks are regionally developed, together with numerous interspersed biofragmental-pelletoidal carbonat banks with biohermal zones. The former have a maximum observed thickness of about 70 feet; the latter may be as thick as 345 feet.

Data indicate that the lower member was deposited as a whole in a broad epicontinental sea. The relatively shallow, open-marine conditions culminated at two different times in basin-wide, reducing, lagoonal conditions, as evidenced by the upper and medial bituminous, argillaceous intervals containing impoverished faunas.

The upper member appears to have been deposited in a shallow sea which deepened toward the northeast. Using a regionally developed, vertically restricted Amphipora zone as a datum, three pre-Amphipora tectonic provinces are discernible. In the southwest, the Elbow-Weyburn basin subsided relatively rapidly as thick shallow-water carbonate sediments accumulated. In the north and east, the comparatively stable Saskatoon shelf was the site of deposition of thin, laminated carbonate sediments and basal bank-carbonate sediments. Flanking the shelf on the north, the Meadow Lake-Sayese basin complex was a depositional site for similar sediments, except that bank sedimentation was further advanced, in response to more rapid, or more prolonged, subsidence.

In post-Amphipora time, subsidence continued in the north and was accelerated in the shelf area which received thick bank accumulations, whereas, in the southwestern basin, carbonate deposition was almost complete.

The consistently developed carbonate wedge occupying the Elbow-Weyburn basin would seem to offer the better prospects for large hydrocarbon accumulations. The abundance of localized carbonate build-ups north and east of this basin presents opportunities for multiple, if comparatively small-scale, accumulations of hydrocarbons.

End_of_Article - Last_Page 620------------