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GEOLOGIC NOTE |
1 Department of Geological Sciences, University of Texas at Austin, 1 University Station, C-1100, Austin, Texas 78712; petter{at}mail.utexas.edu
2 Department of Geological Sciences, University of Texas at Austin, 1 University Station, C-1100, Austin, Texas, 78712
Andrew L. Petter is a Ph.D. aspirant in the Jackson School of Geosciences, University of Texas at Austin. He earned an M.S. degree in geological sciences from the University of Texas at Austin in 2005, where he focused on the topic covered in this article. He is currently studying the Paleogene shelf margin of the Gulf of Mexico and its relationship to transport of reservoir-quality sediment to the deep water.
A University of Texas professor, Ron Steel received his B.Sc. degree in 1967 and his Ph.D. in 1971 from the University of Glasgow. His research is aimed primarily at using clastic sedimentology to address problems in basin analysis and particularly to decipher the signatures of tectonics, sea level change, and sediment supply in stratigraphic successions.
Identification of bypass at the shelf margin is critical to deep-water exploration. We examine the shelf margin of an early Eocene fourth-order 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 downlapped 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.
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