--> Fan Growth by Offset Stacking of Elongate Lobes: Examples from Modern and Ancient Submarine Fans, by Peter J. Fischer and Victor B. Cherven; #91024 (1989)

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Fan Growth by Offset Stacking of Elongate Lobes: Examples from Modern and Ancient Submarine Fans

Peter J. Fischer, Victor B. Cherven

Bathymetric maps across many modern submarine fans reveal a "classic" fan shape and a system of branching distributary channels radiating from a larger upper fan channel or submarine canyon. Most fan models based on this geometry imply, tacitly or explicitly, that the fan shape is the result of radial sediment distribution over much of the fan surface during most turbidity current events. Sediment deposited from a single turbidity current thus would form a fairly widespread blanket that thins downfan and laterally away from channels.

In contrast, high-resolution seismic data across several modern fans and detailed well-log cross sections and maps across ancient subsurface fans show numerous elongate sediment bodies (lobes) with limited lateral extent. The geometry of these lobes implies longitudinal rather than radial transport. These bodies have sometimes been termed "channel-levee complexes," but they extend across the upper, middle, and lower fan and include nonchannel and nonlevee facies as well. They are far larger than the "depositional lobes" postulated to be present at the mouths of distributary channels on the middle or lower fan.

Radial fan growth is by channel avulsion near the fan apex, causing lobe abandonment and the growth of a new lobe in the topographic low bordering the abandoned lobe. Through time, the cycle (1) lobe growth, (2) abandonment, (3) compaction, and (4) subsidence results in offset stacking of multiple lobes of slightly different ages.

Examples of modern and ancient fans from our work in California and published studies document that fan growth happens by offset stacking of elongate lobes. At least four lobes (up to 30 m thick and 10-15 km long) are present in the upper Pleistocene Conception fan, offshore southern California. Over 15 lobes, which are up to 150 m thick and are identifiable downfan for more than 5 km, are present in the Maestrichtian Winters fan of the Sacramento basin.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.