Abstract

The Nisku Formation of the Winterburn Group of Upper Devonian age represents carbonate sedimentation which occurred on a slowly subsiding tectonic shelf rimming the Canadian shield and developed in response to the late Frasnian regression. The carbonate ramp and basin configuration of the Nisku Formation in the Western Canadian Sedimentary Basin is primarily controlled by relative rises in sea level with downwarping producing two major centers of basinal sedimentation; one in West Pembina, the second in northeast British Columbia. Facies belts include: (i) slope-basin carbonates and shales, (ii) deep ramp carbonates and shales, (iii) shallow ramp carbonates and argillaceous carbonates, (iv) isolated coral-algal stromatoporoid biostromes and bioherms, and (v) inner ramp-peritidal carbonates and evaporites.

In the West Pembina area, at the distal end of the ramp-basin transition, isolated bioherms and biostromes developed on shallow carbonate ramps (Lobstick and Bigoray Members). Deep ramp carbonates (Cynthia Member) overlie the shallow ramp carbonates suggesting incipient drowning of these earlier ramps. Drowning produced a major landward shift in the shallow carbonate facies. During this deepening event the buildups on the shallow ramp kept pace with the rapid relative sea level rise. Buildup development was eventually terminated by progradation of tidal flat sediments (Wolf Lake Member) into the area during a regressive event.

In ramp interiors, situated in southern Alberta, the Nisku carbonates are composed of restricted marine sequences together with carbonate-evaporite cycles representing salina-type sedimentation within paleotopographic lows which may or may not be subjected to a sabkha over-print. Carbonate sedimentation within this area is mainly by aggradation.