Abstract

The Raven River Member of the Cardium Formation (Turonian, Upper Cretaceous) in the Carrot Creek area of Alberta contains two coarsening-upward sequences of marine mudstones and sandstones. The lower sequence coarsens from black mudstones into bioturbated, fine-grained sandstones, and is capped by a gritty siderite horizon which probably indicates a pause in deposition. The sandstones of the upper sequence contain hummocky cross stratification, suggesting open-marine deposition below fairweather wave base. The two sequences are scoured to variable depths by a major erosion surface which has a relative relief of up to 20 m. A structure map and three-dimensional mesh diagrams of this surface suggest four topographic elements--a relatively high terrace, a bevel at which underlying beds are truncated, and a series of bumps and hollows . These are interpreted to represent a remnant erosional topography, which gradually fades away into the final topographic element, the smooth basin plain . The erosion surface is covered with conglomerate, assigned to the Carrot Creek Member of the Cardium. The conglomerate thickness reaches about 20 m against the bevel and in some of the hollows, but usually thins to a veneer on the terrace, on top of the bumps, and on the distal basin floor. The conglomerate bodies are relatively long and narrow and trend northwest-southeast. They are overlain by marine mudstones of the Dismal Rat Member. The elongate conglomerate bodies appear at first sight to be another case of Western Interior Seaway "offshore bars" encased in marine mudstones many tens of kilometers seaward of time-equivalent shoreline deposits. However, the erosion surface indicates that the conglomerates are not genetically part of the underlying coarsening-upward sequence. We propose a rapid relative lowering of sea level following deposition of the hummocky cross-stratified sandstones. The shoreline advanced many kilometers northeastward into the basin, and during stillstand, wave erosion cut a new shoreface profile (the bevel). During this lowstand, incised rivers supplied gravel to the shoreface. Waves worked the gravel alongshore against the bevel, and into the erosional hollows. Subsequent transgression spread some gravel southwestward back across the terrace. Storms transported gravel stringers into the transgressive muds accumulating in the hollows. Other erosional surfaces of this type exist in Alberta at the Cardium, Viking, and Bad Heart horizons, and in New Mexico at the Gallup-Tocito horizon, suggesting that sea-level changes and shoreface incision may be an alternative explanation for other long narrow "offshore bars" encased in marine shales.

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