Stratigraphy, Age, and Interregional Correlation of the Exshaw Formation, Alberta Rocky Mountains

Abstract

ABSTRACT The Exshaw Formation of the Alberta Rocky Mountains and foothills consists of a lower, black shale unit and an upper, siltstone unit, which are separated by a gradational contact. At the type section on Jura Creek, near Exshaw, Alberta, the upper contact of the Exshaw is redefined so that the formation includes all, rather than merely the lower part, of an unbroken sequence of dark-yellowish orange-weathering calcareous siltstone. At the type section, the black shale unit is 33 ft thick and the siltstone unit 127 ft. X-ray diffraction data from the type section indicate that the shale is composed mostly of quartz and contains only minor amounts of illite, feldspar, and carbonates. The siltstone has considerably more calcite and dolomite than most of the shale. Throughout its area of outcrop, the Exshaw unconformably overlies limestones of the Devonian Palliser Formation, and is sharply (disconformably?) overlain by shales at the base of the Mississippian Banff Formation. The silts and clays, and some of the carbonaceous material in the black shale unit, were probably derived from soils and deposited in shallow-marine, euxinic lagoons. The siltstone unit was deposited in a regressive, marginal-marine environment of widespread, well-oxygenated tidal flats. Physical continuity is demonstrated between the Exshaw Formation and a) the lower two members of the Bakken Formation in the subsurface of southeastern Alberta and Saskatchewan, b) the Sappington Member of the Three Forks Formation in Montana. Conodont evidence from the Exshaw Formation and circumstantial conodont and spore evidence from the Sappington Member and Bakken Formation show the age of the Exshaw to be Devonian and Mississippian. The systemic boundary is probably positioned near the middle of the black shale unit. Evidence previously cited for the age of the Exshaw at Crowsnest Pass is shown to be more pertinent to the age of the overlying Banff Formation. End_Page 32-------------------------