Paleoenvironmental Variability across the Cretaceous-Tertiary Boundary in the Alberta Foreland Basin, as Interpreted from Fluvial Deposits and Paleosols, Red Deer River Valley, Alberta, Canada

The integration of sedimentological, paleotectonic and paleopedogenic data across the Cretaceous-Tertiary (KT) boundary of south-central Alberta indicates fluvial aggradation and variability of paleosol morphology in response to foreland orogenesis. The depositional history records an evolution from amalgamated, multi-story, braided sand bodies to accretionary, single-story, overbank-prone meandering deposits. The distribution of paleosols throughout the section is also cyclic. Immature, well-drained paleosols are associated with the braided deposits, whereas mature, poorly-drained paleosols are interbedded with the meandering deposits. Two large-scale aggradational fluvial cycles are observed within the study interval and are interpreted to record variations in sediment supply and tilt of the depositional profile associated with foreland tectonism. Orogenic pulses are reflected in outcrop by amalgamated fluvial deposits interbedded with immature paleosols. Waning orogenesis is characterized by reduced fluvial sedimentation rates and an increase in the number of mature paleosols. Orogenic quiescence is associated with an increase in channel sinuosity, and poorly drained, gleyed, coal-capped paleosols. The KT boundary is located three meters above the tectono-stratigraphic transition from amalgamated, braided fluvial systems with well-drained paleosols to accretionary, meandering fluvial systems with poorly-drained paleosols. A gradual shift towards a more poorly-drained paleosol morphologies and increasingly accretionary fluvial styles suggests that the KT event was not the cause of increasingly cool and wet conditions across the boundary, but that the boundary lies at the inflection point between a well-drained to poorly-drained depositional cycle.