CRETACEOUS LAKE DEPOSITS IN THE EARLY AND LATE FORELAND BASIN DURING NON-MARINE PERIODS IN WEST-CENTRAL ALBERTA, CANADA

During both the early and late development of the s foreland basin in Alberta, non-marine depositional environments in the west-central foredeep are most apparent in the Lower Cretaceous (Albian, Aptian) and Upper Cretaceous (Campanian, Maastrichtian). Based on outcrop and core observations along with subsurface well-log analysis and mapping, we recognize evidence for persistent lake systems contained within the same region of the central Alberta foothills during these two key intervals. Evidence for a lacustrine environment in the older unit (Gladstone Fm.) includes thin cyclical packages (~1-3m thick) containing laminated black mudstone and siltstone, thin coal beds, coarsening up packages with lenticular and wavy-bedded heterolithics, and ripple cross-laminations indicative of unidirectional and oscillatory (wave) currents. At the top of the packages roots, terrestrial bioturbation (e.g. meniscate backfilled burrows), and other pedogenic features indicate exposure and repetitive base-level drop. The presence of a type I kerogen (oil) in nearby reservoirs, which is typically sourced from lacustrine environments, further supports this interpretation. Evidence for the presence of a persistent lake system in the younger unit (Brazeau Fm.) system is similar, in that it contains deposits with black shales, wave-ripple bedforms, roots, bioturbation; the lacustrine units also preserve stromatolites. Subsurface well logs and outcrops indicate coarsening-upwards in thin cycles (~2-4 m thick), which are typically stacked in larger coarsening-upwards packages; these are interpreted as mouth bars and shorefaces that prograde into standing water. Along with the similarities in facies and packaging, the two units are present in the same area, despite their vertical separation being hundreds of meters. We propose that the study area may represent a sub-basin bounded by deep-seated structural features activated during foreland basin development.