Stratigraphy, sedimentology and depositional environments of the coal-bearing Jurassic-Cretaceous Kootenay Group, Alberta and British Columbia
Authors: Gibson D W 
Published: November 2010

Strata of the Jurassic-Cretaceous Kootenay Group have long been of economic interest as a readily accessible source of thermal and metallurgical coal. This report attempts to synthesize and describe the geology of the Kootenay Group on a regional basis, by
providing detailed information and data on lithofacies, lithofacies relationships, correlation of major rock units, the distribution and continuity of coal seams, and major environments of deposition.

The Kootenay Group, up to 1112 m thick, comprises three formations which, in ascending order, are: the Morrissey, which ranges in measured thickness from 20 to 80 m and comprises a massive cliff-forming succession of fine- to medium-grained sandstone; the Mist Mountain, ranging in measured
thickness from 25 to 665 m and comprising an interbedded succession of predominantly nonmarine sandstone, siltstone, mudstone, rare conglomerate, and economically important thin to thick seams of coal; and the Elk, ranging in measured thickness from a zero erosional edge to 590 m and comprising an
interbedded sequence of nonmarine sandstone, siltstone, mudstone, shale and, locally, chert-pebble conglomerate and thin seams of coal. The Morrissey Formation can be further subdivided throughout most of the area into the Weary Ridge (lower) and Moose Mountain members.

Economically, the Mist Mountain Formation is the most important formation of the Kootenay Group, containing coal seams up to 18 m thick, which range in rank from medium to high volatile bituminous in the south, to low volatile bituminous to semianthracite in the north. Seams are thicker and more
numerous in areas of the Fernie Basin and upper Elk River valley. Coal seams in the Elk Formation are thin, less abundant, regionally discontinuous, and generally lower in rank than those in the Mist Mountain Formation. Coal in the Morrissey Formation is rare and of no economic value.

Petrographic analyses of many thin sections from the Kootenay Group indicate two main detrital components, quartz and lithic rock fragments, the latter including several varieties of chert; grains and pebbles of sedimentary quartzite or quartz arenite; grains of dolostone and limestone, commonly
sandy to silty; grains and pebbles of silicified mudstone-argillite; and grains of collophane. The cement consists predominantly of quartz and chert, and less commonly dolomite, calcite and rare clay minerals. Composition and textural relationships indicate that most, if not all, detrital components
were derived from pre-existing sedimentary rocks located east of the Rocky Mountain Trench.

The analysis of sedimentary facies, facies relationships, petrographic data, the presence or absence of characteristic megafossil, microfossil and microfloral assemblages, suggest that strata of the Kootenay Group formed part of a major, prograding, clastic wedge, deposited within three major
depositional environments. The Morrissey Formation is diagnosed as a beach, beach ridge, and coastal dune depositional environment. The overlying Mist Mountain Formation is interpreted as a fluvial-deltaic-interdeltaic clastic succession, deposited in an environment largely unaffected by marine or
brackish water inundations. The Elk Formation represents deposition within a major, coastal, fluvial-alluvial plain depositional environment. In the Front Ranges of the southern Rocky Mountains, the Elk contains strata characteristic of a distal alluvial fan or braidplain depositional
environment.