ABSTRACT: Seismic Stratigraphy of the Modern Mississippi Fan Turbidite Systems

John B. Wagner, James M. Coleman

The last two late Wisconsin Mississippi fan lobes are depositional units subdivided into four regions: upslope erosional canyon, upper fan, middle fan, and lower fan, based on surface characteristics and sedimentary processes. Individual fan lobe sequences were identified by reflection terminations (including erosional truncation, toplap, downlap, and onlap) and define seismic sequence boundaries on multifold seismic data. External geometries of the fan lobes exhibit a mounded form in cross section becoming more elongate to lens shaped in the dip direction. The two seismic sequence boundaries are horizon 20 (40,000-55,000 years ago) and horizon 30 (approximately 84 Ka), both of which are interpreted to be condensed sections representing either a basin-wide transgression o periods of low sediment accumulation rates associated with highstands of sea level. The Mississippi fan lobes represent a series of onlapping deposits bounded by condensed sections formed as a consequence of variations in sediment supply. Seismically, the sequence boundaries are generally high-amplitude, continuous, doublet reflections, which can be correlated nearly throughout the entire fan. The two fan lobe sequences have a maximum thickness of 350 and 500 m in the middle-fan region.

The mapping of these boundaries illustrates similar geometries with the modern bathymetric surface, suggesting the cyclic nature of fan lobe development may be in response to changing sea level. Three-dimensional modeling of the fan lobe sequences shows that preexisting topography controlled fan lobe geometry. Mass movement and channel/overbank are the primary processes in modern fan lobe construction and are interpreted to be the main components of individual fan lobe sequences.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990