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Evolution of a Fan Channel on the Surface of the Outer Mississippi Fan: Evidence from Side-Looking Sonar

  • Suzanne O’Connell
  • William B. F. Ryan
  • William R. Normark
Part of the Frontiers in Sedimentary Geology book series (SEDIMENTARY)

Abstract

A deep-towed, side-looking sonar survey of an outer fan, distributary channel near DSDP Sites 614 and 615 on the Mississippi Fan defined three distinct morphologic zones: (1) a relatively “straight” channel zone (sinuosity of 1.1), (2) a channel termination zone, and (3) a splay and “finger-shaped distributary” zone. Boundaries between zones are gradual with no direct connection observed between this channel and the leveed channel that was mapped and drilled farther upfan during DSDP Leg 96.

In the “straight” channel zone, a clearly defined, shallow (10 m, 33 feet), incised channel exists within a broader area of surficial erosional features. Locally, the inner walls and floor of the channel area are marked by very low-acoustic backscatter on the side-looking sonar images. In some reaches, the channel path is parallel to the alignment of several distinct north-northeast lineations in the backscatter intensity. Low-relief levees (less than 5 m, 15 feet) are discontinuous along the margins of the straight channel zone and die out before the channel terminates. Detached splay and “finger-shaped distributaries” consist of branching lobate-like features with low relief that extend beyond the termination of the single, relatively straight channel.

The side-looking sonar images record at least five stages of development, each of which may have consisted of more than one event or series of depositional events. In order from oldest to youngest: (1) channeled erosion during which the broad area of finger-shaped distributaries was created, (2) erosion causing lineation or grooving on the fan surface, (3) formation of the “straight” channel and the broader erosional area in which it lies, (4) both development of and destruction of levees on parts of the channel, and (5) blanketing the sea floor with fine-grained sediment that returns low acoustic backscatter. The erosional events that sculpted the fan surface were probably initiated by an unconfined sediment gravity-flow event, whose source was either in the main channel-levee region farther upfan or on the continental slope. These data suggest that the area was not part of the most recently active depositional lobe on the outer Mississippi Fan.

Keywords

Debris Flow Sonar Image Acoustic Backscatter High Backscatter Turbidite System 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer Science+Business Media New York 1991

Authors and Affiliations

  • Suzanne O’Connell
  • William B. F. Ryan
  • William R. Normark

There are no affiliations available

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