Seismic Facies, Characteristics, and Variations in Channel Evolution, Mississippi Fan (Plio-Pleistocene), Gulf of Mexico

Abstract

The Mississippi Fan consists of 17 discrete channel-levee systems deposited during the late Pliocene and Pleistocene. Each channel-levee system comprises one to several individual channels that vary considerably in distribution, size, and geometry. Their physical characteristics are best described using four parameters—aggradation, lateral migration, sinuosity, and bifurcation. Values for these parameters differ considerably both between and within individual channel-levee systems. Specifically, maximum aggradational values for channels range from 250 to 850 m, whereas lateral migration within the channel valleys range from 0 to 12 km. Channel sinuosities range from none to a moderate amount. Downfan channel bifurcation varies from 0 to 27. The lateral migration of high-amplitude reflections (coarse-grained deposits) within channel valleys is usually associated with sinuous, possibly meandering channels. However, one channel (#13) has 10–12 km of lateral migration and low sinuosity, indicating that lateral migration of submarine fan channels is not always associated with high sinuosity. Channel width/thickness ratios generally decrease downfan, with upfan lateral migration changing downfan to bifurcating channels that show little lateral migration. In many channel-levee systems, the initial channels are relatively small and the levee sediments are possibly slightly coarser-grained; with growth they evolve into wider, larger channels with finger-grained levee sediments.The depositional variations within the channel valleys are caused by the interplay of several factors. Aggradation reflects the thickness of the coarse-grained channel-fill deposits, and the thickness is partially influenced by the position on the fan surface. Lateral migration was caused probably by deflection of turbidity flows away from previous deposits, as well as downfan avulsion and longevity of channel. Sinuosity is probably controlled by changes in gradients of valley slope and channel slope, and changes in the grain size of the turbidity flows. Bifurcation can be caused by flowstripping, abrupt increases in volume of turbidity flows being delivered to the basin, channel blockage by slides, and levee failure away from the channel. In any channel-levee system, the vertical change in channel deposits and grain size of levees were affected by changes in the grain sizes of flows being delivered to the basin. The initial flows were possibly slightly coarser-grained and became fine-grained during later channel deposition. KeywordsLateral MigrationSeismic FaciesCondensed SectionChannel EvolutionOverbank SedimentThese 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.