Chemostratigraphic Applications to Low-Accommodation Fluvial Incised-Valley Settings: An Example from the Lower Mannville Formation of Alberta, Canada

Abstract

Abstract Incised valleys are a ubiquitous feature of the Lower Cretaceous Mannville Group of the Western Canada Sedimentary Basin. The Basal Quartz, a member of the Lower Mannville Formation, is present in two north–south-oriented subparallel valley-form systems in southern Alberta, the western valley form termed the Taber–Cutbank Valley and the eastern valley form termed the Whitlash Valley. This paper details the application of chemostratigraphy to discriminate between the three informal lithostratigraphic units of the Basal Quartz, namely the Horsefly, the Bantry–Alderson–Taber (BAT), and the Ellerslie units in these valley forms. In the Taber–Cutbank Valley, the Horsefly, BAT, and Ellerslie units each have unique chemostratigraphic signatures that enable them to be readily differentiated using inorganic geochemical data. The changes in elemental chemistry that allow this differentiation are inferred to reflect changes in sediment provenance, paleoclimate, and surface residence time. The whole-rock geochemistry of the Horsefly unit, the basal unit of the fill, is demonstrated to remain relatively constant longitudinally in the Taber–Cutbank Valley, therefore demonstrating, when compared to the vertical changes in geochemistry, that temporal or stratigraphic changes are of a greater magnitude than longitudinal changes within in a single valley form. The whole-rock geochemistry of the Horsefly unit in the two valley forms, which in previous studies has been demonstrated to be homotaxial by conventional stratigraphic techniques, is markedly different between the two valley forms. The geochemical differences suggest that the Horsefly unit was subjected to more prolonged and/or intense weathering in the eastern Whitlash Valley than in the western Taber–Cutbank Valley, and that the provenance of the Horsefly unit is fundamentally different between the two valley systems. The application of chemostratigraphy to the Basal Quartz of the Lower Mannville Formation demonstrates that the technique can be utilized as a viable correlation tool in low-accommodation incised-valley settings; however, it is most effective when the whole-rock geochemical data are placed in a regional context by integration with a detailed stratigraphic framework. Once chemostratigraphy is used in conjunction with these other data streams, the differences in geochemistry of the Horsefly unit between the Whitlash and Taber–Cutbank Valleys can be used to infer that the Horsefly unit deposited in the two valleys are two homotaxial, not coeval subunits.