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Abstract

A series of seismic-scale (10 km × 1 km) shelf-sand-stone-tongue outcrops of Barremian and Eocene age in Spitsbergen’s Central Basin have been used to examine the relationship between shelf-edge regime and the accumulation of sands on deepwater slope or basin-floor areas. The setting is a small foreland basin where shelf-slope-basin floor clinoforms indicate water depths of up to 400 meters, slope lengths up to 5 km., slope gradients up to 3 degrees, and basin-floor fans extending away from the slope up to 10 km.

The datasets show that shelf edges dominated by intact deltas have a relatively undisturbed morphologic profile and produce broad shelf-margin progradation, but little or no basin-floor sand accumulation. Basin-floor fans, on the other hand, tend to occur when the time-equivalent outer shelf was dominated by rivers, the shelf edge shows collapse/ block-rotation/ growth faulting, and the slope contains canyons that focus sediment by-pass to the basin floor.

Shelf edges that were dominated by a fluvial sediment supply tend to evolve to a system dominated by shelf-margin deltas, as relative sea level rises on the shelf. This evolution is accompanied by a change in how sandy sediment volumes are partitioned between basin floor and slope. Initial focussed degradation of the shelf edge/slope, accompanied by aggradation of sandy basin-floor fans and late development of channel-levee systems on the lower slope, changes to sandy slope accretion by turbidites that pinch-out before they reach the base of the slope. This cycle is completed by shaly slope aggradation. Most cycles of relative fall and rise of sea level in the Central Basin lack the initial “fluvial/basin-floor fan” phase, and simply start with the “shelf-edge delta/slope accretion” phase. The identification of the rarer cycle (that lacking the basin-floor fans), and the distinction between the two types of cycle is critical for the prediction of deepwater, sandy fans.

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