Abstract

Emphasis on the association between relative sea-level lowstand and the formation of sandy deep-water fans has tended to downplay the significance of high sediment supply and its potential to create deep-water fans, even during sea-level highstands. The Lance– Fox Hills–Lewis shelf margin in southern Wyoming suggests that high supply was critical in causing the accretion of this moderately wide Maastrichtian shelf margin, at a minimum rate of 47.8 km/m.y., and the generation of large, sand-rich fans during every shoreline regression across the shelf. It is surprising that fans developed from shelf-margin clinoforms that show systematically rising shelf-edge trajectories (proxy for rising relative sea level) as well as from those that show flat trajectories (stable to falling relative sea level). However, the latter, producing more sediment bypass, resulted in bigger and thicker fans, whereas the former produced somewhat smaller and thinner fans. We term the former highstand fans and suggest caution in using the lowstand model for high-supply systems.

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