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

Abstract

Identification of bypass at the shelf margin is critical to deepwater exploration. The shelf margin of an early Eocene fourth-order sequence with an attached basin-floor fan in the Central Spitsbergen Basin was examined in this study. Turbidity currents were fed mainly by hyperpycnal flow emerging from shelf-edge deltas. The lifespan 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 meters (33-49 feet) thick and 100-200 meters (328-656 feet) 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 ~1.5 kilometers (0.9 miles) 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 output. The late lowstand was therefore marked by a second pulse of shelf-margin accretion with only limited bypass to the basin floor, and a heterolithic, prograding complex downlapped the early lowstand channels. Transgression ultimately led to abandonment of the shelf-edge delta complex and accumulation of mainly mudstone on the margin. The shelf-margin architecture exhibited in this sequence should serve as a type example of a deepwater feeder system in which hyperpycnal flow is the primary initiator of turbidity currents for sand accumulation on the slope and basin floor