ABSTRACT

The Lower Jurassic in the Rocky Mountain House area of central Alberta includes the lower part of the Fernie Formation, which may be subdivided into two distinct lithologic units: the Nordegg Member, and the Poker Chip Shale Member. These units were studied by power X-ray diffraction to determine semiquantitative values of mineralogy. The values were verified by data from atomic-absorption spectrometry and insoluble-residue calculations.

Quartz, calcite, and dolomite comprise the essential minerals of the Nordegg Member, with pyrite, siderite, and bituminous matter as accessories. On the basis of these minerals and the lithologic data, three major facies can be identified: a dolomitic sandstone along the eastern margin, which passes westerly into a bioclastic limestone, and a chert farther to the west. Deposition of the Nordegg Member was on a stable shelf with a shoreline lying to the east, and deeper waters to the west. Two possible ultimate sources for the abundant silica required for biogenic deposition of chert are available--volcanic ash from central British Columbia, or silica carried in solution by streams flowing westerly off the Precambrian Shield area.

Mineralogy of the Poker Chip Shale Member is less variable, with quartz, illite and kaolinite being the essential constituents and pyrite, siderite, and glauconite the accessory minerals. No facies relationships were observed in the Poker Chip Shale Member, but a depositional environment similar to that of the Nordegg Member is suggested.