Abstract

Analysis of two Maastrichtian depositional cycles from the Lewis–Fox Hills shelf margin (S. Wyoming, U.S.A.) demonstrates that rivers played the driving role in the bypass of sand to deep water. Waves and tides were of less importance, but they had a significant influence on the relative storage and architecture of sand along the shelf edge, which in turn appear to have been driven by relative sea level and sediment supply.

Clinothem 9 shows main sandstone accumulations in an extensive deltaic sandstone belt along the shelf edge and two deep-water fans. Along the deltaic sandstone belt, river channels fed sand down to deep-water areas, whereas shelf-edge segments with storm-wave domination and tidal influence did not produce channels, but passed basinwards to muddy, turbidite-starved slopes. The wave and tidal regimes, however, likely produced an eastward along-shore drift that fed sediment along the shelf edge and possibly occasionally to deep water via canyon heads. Aggradation (50 m) of the storm-wave deltas indicates a rising relative sea level, which likely favored strong wave influence by allowing open-ocean swells to shape the aggrading, shelf-edge sediment masses. Aggradation also suggests a high sediment supply for deltas to have reached the shelf edge, to have aggraded there, and to have bypassed large volumes of sand to deep water.

Clinothem 10 shows main sandstone accumulations to occur in an incised shelf-edge area with a linked, large deep-water fan. The former exhibits characteristic flat-laminated sandstones (hyperpycnal flows?) and tidally influenced facies in the delta front that are cut by a prominent fluvial incision. Fluvial and overlying tidally influenced estuarine deposits infill the incision, indicating that it is a valley, probably cut during relative-sea-level fall. Outside the valley the shelf edge lacks a continuous and thick sandstone belt, suggesting that wave influence along this shelf edge was relatively weak. A falling relative sea level and wide river incision would have interrupted the otherwise common littoral drift and would have diminished wave influence. At the same time the incised segments of the shelf edge would have provided confined areas for tidal-current amplification and for focused, efficient downslope delivery of sand.