ABSTRACT

The Watt Mountain Formation of Middle Devonian age thins basinward away from the Peace River arch. Subsurface cores in an area of about 20,000 km² were studied, with particular attention being given to facies relationships within the Gilwood Member. The Gilwood Member, which is dominantly sandstone, is about 60 m thick near the arch, and pinches out in a distance of 130 km.

Sandstones of the Gilwood Member decrease in both grain size and feldspar content away from the arch while increasing in both textural and mineralogical maturity. Quartz and microcline are the major clastic components. The sandstones contain little clay matrix except near the arch. Throughout the rest of the area the sandstones are cemented by quartz, anhydrite and carbonate, both calcite and dolomite. Both quartz and microcline may be partially replaced by anhydrite and/or carbonate.

Five depositional environments were interpreted from the available cores. Sandstones nearest the granitic rocks of the arch are interpreted as braided-stream deposits. Farther basinward, the sediments grade laterally into meandering-stream, shoreline and marine deposits. Limestones associated with the meandering-stream environment are interpreted as freshwater lake deposits.

Braided-stream and meandering-stream environments were differentiated on the basis of the following features: thickness of individual cycles, sorting, grain size, frequency of shale breaks and intraclasts, nature of overbank deposits, and sedimentary structures.

Thin sandstones interpreted as shoreline deposits were distinguished by mineralogical and textural maturity, sedimentary structures, and evidence of bioturbation. Basinward from the shoreline sandstones, green and red anhydritic to dolomitic shales were interpreted as shallow-water restricted-marine deposits.

Although a large number of tests of the Gilwood have met with very limited success, the updip pinchout of the sandstone still provides a potential target for petroleum exploration. Traps in this area are likely to be small but well control is sparse enough that significant discoveries could still be made.