ABSTRACT

The Middle Devonian Watt Mountain Formation and its Gilwood Sandstone Member are described from cores of twelve wells in the Mitsue area and two in the Nipisi area. The formation consists of interbedded sandstone and mudstone, with a dolomite unit near the base. Sandstone petrography was studied in twenty-eight thin sections, and mudstone mineralogy in ten samples by X-ray diffraction and differential thermal analysis.

The Gilwood Sandstone Member is a poorly to moderately well sorted, mainly feldspathic sandstone in the Mitsue area and an arkose in the Nipisi area. The primary cement is carbonate, which partly replaces the matrix, and which is in turn partly replaced by anhydrite.

The brown to green mudstones of the Watt Mountain Formation consist mainly of quartz, dolomite, feldspar, illite, chlorite, and minor amounts of anhydrite. The colour is not related to iron content but probably is related to the oxidation state of the iron.

The composition of the Gilwood Sandstone indicates that the source area was primarily an igneous-metamorphic terrain, most probably the Peace River high. The environment of deposition of the Watt Mountain Formation in the study-area was near-shore, deltaic or coastal plain, probably ranging from shallow-marine to continental. Differences in the composition of the Gilwood Sandstone between the Mitsue and Nipisi areas are probably due to different distances of transport and to slightly different environments of deposition.