Lower Cretaceous of Southern Alberta and Adjoining Areas

Abstract

The total stratigraphic interval studied in southern Alberta and northern Montana includes all of the Lower Cretaceous strata between the sub-Cretaceous unconformity and the base of the Fish Scale sandstone, which occurs near the Lower Cretaceous-Upper Cretaceous boundary. Abrupt facies changes in the Lower Cretaceous sediments in western Alberta necessitate a different nomenclature in the Foothills area than in the Plains area. In the southern Alberta Foothills the continental sediments of the Blairmore group can be divided into two distinct lithologic parts. The lower Blairmore consists dominantly of gray shale and gray to buff protoquartzite-type sandstones and contains two members. The Cadomin member at the base of the lower Blairmore is a persistent chert and quartzite pebble conglomerate which can be traced throughout the Alberta Foothills. The Calcareous member, consisting of gray to buff argillaceous limestones, calcareous shale, and buff calcareous quartzose sandstone, occurs at the top of the lower Blairmore. The upper Blairmore sediments show a marked change in color and lithologic character from those below. The sediments are dominantly green, and the sandstones are usually highly feldspathic an chloritic arkosic graywackes and lithic graywackes. The Crowsnest formation overlies the Blairmore group in the southwestern corner of Alberta and is composed of a series of tuffs, agglomerates, breccias, and flows. In the Plains area the Lower Cretaceous is divided into four major lithologic units, in ascending order: Mannville group, Joli Fou formation, Viking formation, and an unnamed unit above the Viking formation and below the base of the Fish Scale sandstone. The Mannville group is dominantly non-marine but contains some marine strata in central Alberta. The lower Mannville usually contains a basal quartzose sandstone member called the Cut Bank and Sunburst sandstones in southern Alberta, and the Ellerslie sandstone in central Alberta. The top of the lower Mannville is placed at the top of the Calcareous member which is correlated lithologically with the Calcareous member in the Blairmore group. The upper Mannville consists of kaolinitic subgraywacke-type sandstones, siltstones, shales, and thin coal beds, and is equivalent to the lower part of the upper Blairmore at the west. In central Alberta a marine Glauconitic sandstone occurs near the base. The Joli Fou formation is a marine dark gray shale body at the base of the Colorado group which loses its lithologic identity in western and southern Alberta. The Viking formation consists of a marine succession of salt-and-pepper protoquartzites interbedded with gray siltstones and shales. In southern Alberta the Viking and Joli Fou formations are not recognizable as two distinct units, and the entire succession is called the Bow Island formation, which is correlated with parts of the upper Blairmore and Crowsnest formations in the Foothills area. An isopach map of the Lower Cretaceous indicates the Alberta Foothills was the site of a marginal basin in Lower Cretaceous time. Petrographic and lithofacies studies suggest the Lower Cretaceous End_Page 590------------------------------ sediments were mainly derived from an orogenic positive area in south-central British Columbia, with minor contributions from a mildly positive area in northern Saskatchewan and Manitoba. The lithologic character of the lower Blairmore suggests a meta-sedimentary source area, whereas the arkosic sediments of the upper Blairmore are indicative of an igneous source. The abrupt decrease in the feldspar content of the sandstones eastward, accompanied by an increase in kaolinitic clay matrix, is attributed to physical and chemical processes, and to dilution of the western derived sediments by clastics from an eastern source.