--> Abstract: Models of Evaporitic Drawdown for a Carbonate Source Rock: Middle Devonian, Saskatchewan, by C. D. Latimer; #91012 (1992).

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ABSTRACT: Models of Evaporitic Drawdown for a Carbonate Source Rock: Middle Devonian, Saskatchewan

LATIMER, CAMERON D., University of East Anglia, Norwich, Norfolk, England

The Ratner Member occurs between the Winnipegosis (Keg River) and Prairie Evaporite formations, in basin settings and between older carbonate buildups. It has been variously interpreted as deep- and shallow-water deposits. It is a cyclic unit of carbonate, calcium sulfate, and organic laminae, and cycles terminate with displacive sulfates. The Ratner transitionally overlies a deep-water (>90 m) organic-rich starved basin deposit that is considered the source rock for Winnipegosis oils. Oil-staining associated with Ratner laminites indicates that it was also a hydrocarbon source. Laminae and cycles are traceable for tens of kilometers. Ratner cycles are found at elevations up to 20 m on flanks of carbonate buildups, indicating that deposition must have occurred in part from comparab e water depths. Mound-flank locations also have greater amounts of displacive sulfates, which are interpreted to have formed under shallow to emergent conditions. This suggests that cycles in basin settings must shallow upward. The main unsolved problem is to determine depositional depths at the beginning of Ratner cycles. If early laminae are deep-water deposits, then cycles reflect salinity increases, decreasing depths, and major fluctuations in evaporative drawdown. If they are shallow, this indicates early substantial drawdown, and cycles can reflect only salinity increases in generally shallow-water settings. A new and detailed microstratigraphy of Ratner laminites is being used to distinguish between these environmental models.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)