ABSTRACT: Subdivision of the Late Wisconsin Mississippi Fanlobes

WAGNER, J. B., Mobil Exploration & Producing Services, Dallas, TX

The two youngest fanlobe sequences of the Mississippi Fan can be subdivided into four major regions based on surface characteristics and sedimentary processes: (1) an upslope erosional canyon; (2) upper fanlobe at base of slope with a nearly filled large channel; (3) middle fanlobe, aggradational in character and convex in cross section with a sinuous and migratory axial channel; and (4) aggradational lower fanlobe with one recently active channel and several abandoned ones.

Seismically, the erosional canyon is observed truncating shelfal sediments and acts as a conduit for transporting large amounts of continentally derived sediments into deeper water environments. Also evident from seismic are large slump deposits suggesting retrogressive slumping and the presence of cut and fill structures within the canyon indicating reactivation of this topographic low. The upper fanlobe constitutes the transition from the erosive nature of the canyon to the more aggradational character of the middle fan. Within the middle fan area, morphologies are observed that closely resemble those characteristic of fluvial systems. The region is a convex-shaped aggradational body with a meandering channel system located along its apex. Moving from middle to lower fan is accompan ed by a reduction in sinuosity and both the channel and overbank sequences become very thin with channels eventually unable to confine their sediment load. As a result, sheet sand deposition takes place and has been suggested as being equivalent to depositional lobes. However, no mounded lobe geometries are recognized on multifold seismic data.

Seismic data as well as side-scan sonar data illustrate that channel/overbank deposits are primary components of fanlobe development. Mass-movement deposits are also common in fanlobe construction and further complicate an already complex distribution of seismic facies.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)