THE MIDDLE DEVONIAN WINNIPEGOSIS FORMATION OF WEST-CENTRAL SASKATCHEWAN

Abstract

The Winnipegosis Formation of west-central Saskatchewan consists of three members which are, in ascending order, the Lower Winnipegosis Member (Lower Member), the upper Winnipegosis Member (Upper Member) and the Ratner Member. The Lower Member is regionally developed, 20 to 50 feet. in thickness, and consists of oncolitic packstone, with less common interlaminated argillaceous-carbonaceous mudstones, dolomite laminites,and intraformational conglomerates at its top. These sediments were deposited in a shallow-water, marine environment, with energy levels ranging from high to low. Low energy conditions prevailed late in lower Winnipegosis time, although scattered shoals had developed and were eroded. Frame-building organisms, such as corals· and stromatoporoids comprise less than 2 percent by volume of core from the Upper Member. They are not in growth position,are heavily abraded, and are less common than sediment-baffling and -binding organisms, such as crinoids, algae, and dendroid stromatoporoids, especially Amphipora sp. The upper Member consists of extensive but discrete carbonate banks, up to 345 feet thick, which are bounded by low-angle average marginal slopes. Both lateral and vertical variations in lithology and faunal content occur within the banks, however, a lack of time control makes definition of facies relationships unreliable. The dominant rock types are laminated, unfossiliferous mudstones and pelleted, biofragmental grainstones, with a fringing cap unit formed during subaerial exposure of some portions of the banks. Laminated mudstones overlie pelleted grainstones in the centers of banks, and also grade vertically into the Shell Lake Member of the lower Prairie Evaporite Formation, indicating that there was increasing restriction in the centers of banks as they developed. The gradational relation­ship at the top of Upper Winnipegosis banks is evidence to suggest that not all portions of the banks were subaerially exposed,as has been postulated by other workers. The off-bank Ratner Member consists of up to 7 T feet of inter­laminated carbonate mudstone and enterolithic anhydrite. These sediments probably were deposited in a quiet-water marine environment after most of the Winnipegosis banks had formed.