Sequence stratigraphy of the Mississippi Fan (Late Miocene-Pleistocene), northern deep Gulf of Mexico

Summary

The Mississippi Fan (Late Miocene-Pleistocene) is a large submarine fan deposited in the northern deep Gulf of Mexico (Fig. l6.1). An isopach map of the Mississippi Fan indicates a maximum thickness of greater than 4 km (Fig. 16.2). The fan is the deep-water part of a large Neogene depocentre filling the northern Gulf of Mexico. From analysis of 19000 km of multifold seismic data across the fan, Weimer (1989, 1990, 199D defined 17 seismic sequences, each characterized by a series of channel, levee and associated overbank deposits, along with other mass transport deposits (Figs 16.3 and 16.4). Key seismic facies and lateral relationships are illustrated by sequences 11 and 12 (Figs 16.4–16.10). At the base of 11 sequences are a series of seismic facies consisting of mounded, hummocky, chaotic and subparallel reflections, which constitute 10-20% of the sediments in the sequences (Fig. 16.10). These facies are externally-mounded in cross-section and occur in two general regions of the fan (Fig. 16.10). In the upper and middle fan they occur below channels and are elongated in shape, mimicking the channel’s distribution. In the middle to lower fan they have a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows and turbidites, and are informally called ‘mass transport complexes’.

Overlying this basal interval and characteristic of all sequences are well-developed channel-levee systems which constitute 80–90% of the fan’s sediments. Channels consist of high amplitude, subparallel reflections. Levee sediments have subparallel reflections that have moderate to high amplitudes at the base changing upward to low amplitude (Figs 16.5–16.8). The vertical change in amplitude may reflect a decrease in the grain size and bed thickness of the levee sediments. Overbank sediments consist of interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites derived from the channel, as well as slides and debris flows derived from the slope (‘debris aprons’). Laterally continuous ‘doublet’ reflections, which cap each channel-levee system, are interpreted to be condensed sections that form from hemipelagic sediments.

Late Miocene-Pleistocene eustatic cycles are interpreted to have been the major factor controlling the timing and style of sedimentation in the fan. ‘Mass transport complexes’ are interpreted to have formed during a lowering of sea level and reflect sediments derived from retrogressive slumping during the formation of submarine canyons along the upper slope and outer shelf. Channel-levee systems were deposited when sea level was near its lowest position and sediment derived from deltas was transported into the deep basin via submarine canyons. During highstands in sea level a thin layer of hemipelagic sediment was deposited on the fan surface.