Depositional History of the Coal-Bearing Upper Jurassic-Lower Cretaceous Kootenay Formation, Southern Rocky Mountains, Canada

Abstract

The coal-bearing Kootenay Formation of Late Jurassic-Early Cretaceous age in Alberta and British Columbia is a classic example of sedimentary sequences built by delta progradation. The formation is composed of 3,500 ft of cyclicly alternating sandstone, shale, and coal, with a few conglomerate beds. The sandstone beds are poorly to moderately sorted, fine- to medium-grained lithic graywacke and proto-quarzite. Shale is of illitic composition with minor amounts of kaolinite. These Upper Jurassic-Lower Cretaceous sediments were derived mainly from the sedimentary-rock terrains located in the Purcell Mountains and the Shuswap Complex. Direction of transportation was to the northeast. Deposition of the Kootenay Formation was initiated by epeirogenic uplift of the source area in Late Jurassic time. Terrigenous sediments were initially deposited on a low-lying coastal plain, which advanced seaward by accretion processes into the shallow sea of the eastern Canadian Cordillera miogeosyncline. In the vertical section, the siltstone and mudstone beds of the open marine environment grade upward into transitional prodelta facies of the Fernie Formation composed of sandstones and siltstones containing the trace fossil Thalassinoides. The prodelta deposits in turn grade upward into delta front sheet sand containing the trace fossil Rhizocorallium. The overlying well-sorted and highly burrowed sandstone of a barrier bar and “misch” tidal flats contains the trace fossil Arenicolites, and grades upward into lower delta plain deposits. These are composed of sandstone beds ranging up to 50 ft in thickness and represent channel fill and point bars of meandering distributaries, laterally interfingered into interdistributary sequences. Coal, as much as 30 ft thick formed as peat in coastal marshes and in fresh-water swamps of interdistributary basins of the lower delta plain. Within the upper delta plain, thick sandstone beds are more common, and coal formed in fresh-water swamps of the flood basins. The delta-plain sediments grade upward into alluvial plain deposits characterized by the occurrence of sheet form beds of conglomerate, lithic sandstone, siltstone and silty shale, and decreased thickness of individual cyclothems. Increased frequency in alternation of rock units is explained by the increase of channel instability and tendency of river channels to form a braided channel system. This system provided unfavorable conditions for coal development. Uplifting of the depositional area in Neo-comian time terminated sedimentation and initiated denudation of the previously deposited sediments. During additional strong uplifting of the inland area, the rejuvenated piedmont-plain deposits represented by conglomerate beds of the Cadomin Formation were spread over the eroded surface of the Kootenay Formation. This study demonstrates the use of sedimentological studies as a tool for exploration and evaluation of coal basins.