Bulletin 021

Author(s) Date 1966-12-31

The Lower Cretaceous Blairmore Group of the Alberta Foothills is a thick wedge of non-marine detrital strata divisible into three formations that correlate with the Mannville Group and overlying Joli Fou-Viking (Bow Island) succession of the Plains. The lower unit comprises a widespread basal conglomerate overlain by thin-bedded siliceous sandstone and dark silty shale, correlative with quartzose sandstone and shale in the lower part of the Mannville Group (McMurray Formation) in the Plains. The middle unit consists of thick, lensing, green, feldspathic sandstone, siltstone, and varicoloured shale, with thin tuff and bentonite interbeds. These beds grade eastward into grey, feldspathic sandstone, dark silty shale, and thin coal beds that form the upper part of the Mannville Group in the Plains. The upper unit consists of lensing, grey, siliceous sandstone, siltstone, and varicoloured shale, grading up in the southernmost Foothills into a thick succession of bedded pyroclastic detritus (Crowsnest Member ).These beds grade laterally in the southwestern Plains into marine sandstone and shale of the Bow Island Formation but are absent to the north in the central Foothills owing to nondeposition or erosion.

The lower and middle Blairmore successions contain the same non-dicotyledonous flora which in the north-central Foothills and in the upper Mannville beds of the Plains is associated with foraminifera of middle Albian age. The upper Blairmore unit contains a dominantly dicotyledonous flora, correlating via the Bow Island Formation with the middle to late Albian Joli Fou-Viking succession of the central Plains. Together with the change in sandstone composition at the middle-upper Blairmore boundary, the marked difference in composition of the two Blairmore floras indicates a prominent break in deposition in the Alberta Foothills at the end of middle Blairmore time.

Petrographic analyses of both Foothills and Plains sandstones show that major changes in the detrital composition of the Blairmore-Mannville rocks coincide with formation boundaries. Sandstones from the lower and upper parts of the Blairmore Group contain abundant siliceous sedimentary or metasedimentary detritus, whereas those from middle Blairmore and upper Mannville strata contain abundant volcanic detritus. The volcanic material apparently originated from vents along the western margin of the depositional basin in southeastern British Columbia, spreading to the north and east across the Alberta Plains into western Saskatchewan.

Authigenic constituents, present as locally derived clastic material and as intergranular cements, also show marked differences instratigraphic and geographic distribution. The distribution of silicate cements can be related partly to differences it the composition of the associated detritus kaolinite and quartz being typical of the siliceous sandstones in the lower and upper parts of the Blairmore Group, and chlorite, illite, and laumontite of the volcanic sandstones in the middle part. However, the absence of chlorite cement from the coal-bearing middle Blairmore beds of the central Foothills and from the upper Mannville sandstones of the Plains, together with other evidence, shows that silicate cement composition is related to physical-chemical factors associated with deposition and later burial, as well as detrital composition. The distribution of authigenic dolomite and siderite also can be related to depositional factors, whereas calcite is erratically distributed as a sandstone cement in all the formations examined.

Variation in gross lithology, sandstone composition, and fossil content of the Blairmore and Mannville Groups can be used to divide the rocks into several laterally interfingering facies that are discordant with and in places cut across formation and time boundaries. Lower and middle Blairmore and correlative Mannville strata together form a major transgressive-regressive cycle of deposition associated with the invasion of the boreal Clearwater Sea, non-marine, fluviatile sediments being present in the south and predominantly marine and shoreline sediments in the north. Upper Blairmore and correlative Mains strata form a separate cycle of sedimentation related to the northward transgression of the Gulfian Colorado Sea, exhibiting facies aspects similar to those of the underlying succession, but with a different paleogeographic distribution.

Mellon, G.B. (1967): Stratigraphy and petrology of the Lower Cretaceous Blairmore and Mannville Groups, Alberta Foothills and Plains; Research Council of Alberta, RCA/AGS Bulletin 21, 296 p.