Hyperpycnal flow variability and slope organization on an Eocene shelf margin, Central Basin, Spitsbergen

Identification of bypass at the shelf margin is critical to deep-water exploration. We examine the shelf margin of an early Eocene fourthorder sequence with an attached basin-floor fan in the Spitsbergen Central Basin. Turbidity currents were fed mainly by hyperpycnal flow emerging from shelf-edge deltas. The life span of any turbidity current was determined primarily by the sediment concentration of the flow and the duration of the river flood. High-density hyperpycnal flows created sand-filled slope-channel complexes 10-15 m (33-49 ft) thick and 100-200 m (328-656 ft) wide that served as conduits for bypass to the basin floor. Low-density hyperpycnal flows were unconfined and deposited heterolithic lobes on the slope. Shelf-margin accretion of about 1.5 km (0.9 mi) during the falling stage gave way abruptly to bypass in the early lowstand. Most of the basin-floor fan growth was achieved after shelf-edge incision and before relative sea level rise. Coastal-plain aggradation in the late lowstand sequestered sediment from the shelf-edge distributaries, effectively diminishing high-density hyperpycnal flow output. The late lowstand was therefore marked by a second phase of shelf-margin accretion with only limited bypass to the basin floor, and a heterolithic, prograding complex down-lapped the early lowstand channels. Transgression ultimately led to the abandonment of the shelf-edge delta complex and the accumulation of mainly mudstone on the margin. The shelf-margin architecture exhibited by this sequence should serve as a type example of a deep-water feeder system in which hyperpycnal flow is the primary initiator of turbidity currents for sand accumulation on the slope and basin floor.