ABSTRACT

Late Jurassic and early Cretaceous sediments in the southern Rocky Mountain region consist of three lithologic divisions; in ascending order these are: 1) the Fernie Formation, consisting mainly of shale of Jurassic age, 2) the Kootenay Formation, consisting of shale and coal-bearing beds with subordinate sandstone, at least partly of Jurassic age, and 3) the Blairmore Group consisting of a basal conglomeratic member of unknown age overlain by sandstones with subordinate shales or limestones of early Cretaceous age. The precise position of the Jurassic-Cretaceous contact remains controversial.

To the west, sections appear to be most complete, while a hiatus, or unconformity, increases in magnitude towards the east. Thus in the westerly sections coal-bearing Kootenay beds grade into the coarse-grained Blairmore clastics by increase in frequency, thickness and grain size of the sandstone and conglomeratic units. In the easterly sections there is no such gradation and conglomeratic beds of the Blairmore cut into and overstep siltstones and coal-bearing beds of the Kootenay Formation. In the Flathead Ridge section the ridge-forming conglomerate is composed of a series of interdigitating lenses of conglomerate with sandstone, and shale with sandstone.

The majority of rock types analyzed are lithic or rock fragment sandstones and conglomerates, the components of which consist of clastic mineral and rock fragments with clays, and authigenic minerals.

Depositional features such as festoon- and cross-bedding and poorly sorted, bimodal sediments, indicate that the clastics were laid down in a continental or alluvial environment. This is confirmed by evidence of diagenetic features, since acidity and oxidising conditions, characteristic of such an environment, are necessary for the precipitation of the significant authigenic mineral iron oxide, and less significant kaolinite. The precipitation of iron-rich dolomite and hydrocarbons was influenced by more alkaline and saline conditions associated with the occurrence of salt marshes and restricted lakes left on the flood plain by periodic inundations of the sea.

Late Jurassic uplift in the west resulted in increased erosion and rate of transport, which culminated in the deposition of the Elk and Blairmore conglomerates, followed by the quieter estuarine and floodplain conditions of Middle Blairmore time. The courses of the various rivers and their tributaries shifted laterally, so that the gross effect achieved is that of one, homogenous conglomeratic unit stretching from northern Alberta and British Columbia to the United States.

The Kootenay/Blairmore clastics are thus transitional between one type of sedimentary environment in the Jurassic and another in the Cretaceous, and are indicative of one of a series of orogenies in the western hinterland.

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