SEDIMENTARY FACIES IN THE JURA-CRETACEOUS KOOTENAY FORMATION, CROWSNEST PASS AREA, SOUTHWESTERN ALBERTA AND SOUTHEASTERN BRITISH COLUMBIA

Abstract

ABSTRACT The Kootenay Formation in the Crowsnest Pass area occupies part of an eastward-thinning, narrow, linear band of Jura-Cretaceous strata comprising part of the Rocky Mountains Foothills and Front Ranges of Alberta and British Columbia. The formation is divided into three main stratigraphic units -- a lower 'Basal Sandstone' member, a middle 'Coal-Bearing' member, and an upper 'Elk' member. The Basal Sandstone member, consisting of massive cliff-forming sandstone, is subdivided into two units, a newly recognized lower 'Unit B', formerly considered to be part of the Fernie Formation, and an upper 'Unit A'. The Coal-Bearing member comprises an interstratified sequence of siltstone, silty shale, mudstone, sandstone and conglomerate, with thin to thick seams of economically important low- to high-volatile bituminious coal. Fourteen major seams have been mined in the Fernie area of southeastern British Columbia, the amount varying with each seam. Coal seams generally become thinner and less numerous toward the east in the Alberta Foothills and Front Ranges. The Elk member consists of a thick, cliff-forming sequence of sandstone, siltstone, mudstone to shale, conglomerate and sparse, thin seams of coal. The conglomerate forms a thick and unusual lithofacies only in parts of the Fernie basin. The Kootenay Formation gradationally overlies strata of the 'Passage Beds' of the Fernie Formation, and is overlain disconformably, at most localities, by the Cadomin Formation of the Blairmore Group. It is postulated that Kootenay strata were deposited as part of a north-to-northeasterly prograding clastic wedge, in the epicontinental Jurassic-Cretaceous sea. Sandstone of the Basal Sandstone member formed part of an elongate interdeltaic beach-barrier island and delta-front sheet sand system. Strata of the Coal-Bearing member are characterized by sedimentary relationships typical of deltaic, interdeltaic and alluvial-plain depositional environments. The Elk member represents deposition mainly in an alluvial plain, with the conglomerate and associated strata of the Fernie area interpreted as part of an alluvial fan.