ABSTRACT

The Mississippi Fan, in the Gulf of Mexico, is a broad, arcuate Pleistocene accumulation of displaced shallow water sediments. The fan covers an area of about 290,000 km² and it has a maximum thickness of 4000 m. The sediment volume is in excess of 300,000 km³. Seismic reflection profiles show eight acoustical horizons of areas extent, which can be used to produce structure contour and isopach maps. These maps reveal that the fan was constructed by a minimum of seven elongate sediment bodies, called fanlobes. A fanlobe is described as a channel-overbank complex deposited over a relatively short geologic time period; it changes its seismic characteristics from upfan to downfan.

Any of these elongated fanlobes can be divided into four regions, each having certain typical acoustical characteristics that can be recognized on seismic reflection profiles. These four regions are: 1) an upslope erosional canyon, 2) an upper fan, which terminates near the base of the slope, 3) an aggradational middle fan, with a sinuous and migratory axial channel, and 4) an aggradational lower fan, with many channels, of which only one is active at any given time.

The Mississippi Canyon, which is the sediment source for the youngest fanlobe, is located on the outer shelf and upper slope, approximately 60 km west of the present Mississippi Delta. The canyon commences at a water depth of 80 m, has an average width of 12 km, and gradually merges with the upper fan at an approximate water depth of 1200 m. This canyon formed in a relatively short period of time during a period of failing or low sea level, probably as a result of sediment instability on the upper slope and outer continental shelf. Seismically, the base of the canyon fill is characterized by chaotic reflectors and is overlain by discontinuous, slightly irregular reflectors.

The upper fan is characterized by one large, nearly filled channel with poorly developed levees. The channel acted as a conduit to transport the source material downslope. Seismically, the fill of this erosional channel is similar to that of the canyon, but the width-to-depth ratio is larger, and a smaller cut and fill structure normally occupies the upper part of the larger channel. The hyperbolic acoustical patterns at the base of the larger channel may represent slump material or homogeneous sandy deposits, such as from debris flows.

The middle fan, commencing in about 2000 m of water, is the main depositional region of the fanlobe. This part can be as wide as 150 km and as thick as 400 m. The fanlobe is convex upward in cross section, and an aggradational channel is located at its apex. This 2 to 4 km wide channel is migratory in nature, and side-scan sonar images show it as a leveed, sinuous channel. Seismically, the channel occurs as an asymmetrical cut and fill structure with an acoustical high amplitude zone commencing at the base of the cut and fill and then migrating upward stratigraphically to the base of the present day morphological channel. Irregular reflectors emanate from the channel complex and offlap onto the adjacent underlying reflectors (the overbank deposits).

Downslope, in approximately 3100 m of water, the lower fan is characterized by a smaller and less sinuous channel and the presence of abandoned channels trending nearly parallel to the active channel. Seismic reflectors become more or less continuous and are separated by slightly acoustically semi-transparent to chaotic flat lenses and small channel structures. Farther downfan, the reflectors become more parallel and regular; the channels become too small to be identified on seismic profiles.

The seven fanlobes mapped from the regional seismic data indicate that the lobes are not stacked vertically, but shift position, each younger one occupying the low between adjacent and underlying older lobes. The active construction

Figure 1. Site 621, middle fan channel. Generalized lithological column, stratigraphic zonation, uncorrected gamma ray trace, and depth (m, ft) subsea and subbottom. Lithological legend: vertical lines: calcareous ooze; horizontal dashed lines: mud and silly mud; small dots: sandy mud with silt and silt laminae; large dots: sand; circles: gravel.

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of each lobe is probably related to the lowering of sea level and outbuilding of the shelf edge, whereas the thin capping parallel seismic reflectors represent pelagic and hemipelagic slow sedimentation during high sea level stands. Throughout the Pleistocene, each fanlobe shifted generally eastward and basinward; and, although it is difficult to connect a fanlobe with a canyon on the shelf, several filled and buried canyons exist to the west of the modern Mississippi Canyon.

Description of DSDP Drill Sites

Nine sites were drilled in the Mississippi Fan deposits. Four of these sites were located on the middle fan (Sites 621, 622, 617, and 620), one in a slump deposit on the margin of the youngest fanlobe (Site 616), and four on the lower fan (Sites 623, 624, 615, and 614). Paleontological zonation of the sediments from the drill sites allowed computation of average accumulation rates during the late Wisconsin and Holocene. In the middle fan sites, both channel and overbank sediments accumulated at rates ranging from 11 to 12 m/1000 years, whereas lower fan accumulation rates were 7 m/1000 years for the channel sites and 6 m/1000 years for the distal channel mouth deposits. Accumulation rates for the high sea level stand of the Holocene are much lower, ranging from 3 to 30 cm/1000 years, indicating the decreasing sediment input associated with high sea level stands.

In the middle fan, Sites 621 and 622 were drilled in the channel; 621 on the convex thalweg position, and 622 on the concave "point-bar" position. Both sites (figs. 1 and 2) bottomed at about 200 m in gravel overlain by a pebbly mud. This depth coincides with the top of the high amplitude seismic reflector zone, suggestive of a coarse grained channel lag. The nearest source of gravel rich deposits is on the shelf near the head of the Mississippi Canyon, indicating a minimum transport distance to the site of over 220 km. The total channel fill at both sites show a single fining-upward sequence from gravel to sands to interbedded sands, silt, and mud, to more or less homogeneous muds. The sand section at Site 622, on the "point- bar," is about 30 m thicker than at the thalweg site. This occurrence of thick sand, along with the geometry of the high amplitude seismic reflectors and side-scan sonar mapping, strongly suggests that the channel is migratory and aggradational in nature and that it is somewhat analogous to a fluvial meandering river. The gamma logs display sands with sharp bases and rather sharp tops, and the incorporated faunas are primarily displaced shallow water benthic foraminifers. Based on seismic mapping of the high amplitude reflectors, the width of the coarser channel deposits is approximately 7 to 10 km.

Sites 617 and 620 were drilled in the overbank deposits of the middle fan, Site 617 in a swale deposit immediately adjacent to the channel (fig. 3), and Site 620, some 18 km from the channel (fig. 4). Both locations show that the overbank deposits are much finer grained than the channel deposits and consist primarily of fine grained sediments arranged in alternating coarsening-upward trends. These deposits are interpreted to represent mud or fine grained silty turbidites that have been shed out of the adjacent channel. Small-scale distorted layers are extremely common in these deposits. The silt units, displaying abundant small-scale cross laminations, vary in thickness from a few centimeters to more than 5 m. Based on well log response, the overbank sediments are composed of 2.8 percent sand, 24.5 percent silt, and 72.7 percent clay. Foraminifers are relatively more abundant than in the channel site and include some bathyal benthic species. This occurrence of foraminifers suggests that the channel

Figure 2. Site 622, middle fan channel. Generalized lithological column, stratigraphic zonation, uncorrected gamma ray trace, and depth (m, ft) subsea and subbottom. For lithological legend, see figure 1 .

Figure 3. Site 617, middle fan swale/levee. Generalized lithological column, stratigraphic zonation, and depth (m, ft) subsea and subbottom. Lithological legend: horizontal dashed lines: mud and silty mud; dashed lines with dots: silty-sandy mud with thin silt and sand layers (inclined: disturbed zone).

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serves primarily as a conduit for transporting the coarser sediment downfan and that only suspended sediments are delivered overbank to build up the marginal areas of the fan.

Site 616 (fig. 5) was drilled on the eastern margin of the modern fan lobe in an area that has been mapped as a massive "slump deposit" that masks the youngest fanlobe. The upper 100 m of the cored section shows little internal deformation, but alternating zones of very steep dips (up to 65 degrees) separated by highly sheared zones suggest mass movement transport of sediments during the late Pleistocene. Beneath the "slump deposits," two older fanlobes were penetrated, both showing a coarsening-upward trend. The older fanlobe, not completely penetrated, contained 7 percent net sand, while the younger fanlobe (88 m thick) had 33.8 percent net sand.

Four sites were drilled in the lower fan, two adjacent to the channel (Sites 623 and 624) and two near the channel terminus (Sites 615 and 614). In the lower fan, the channel narrows to less than 500 m, decreases in relief (5 to 10 m), and becomes less sinuous. Abandoned channels, mapped from side-scan sonar and roughly paralleling the active channel, suggest that channel switching is a common process. In addition, the lack of significant levees bordering the channel appears to allow sediment to readily spread out of the confines of the channel Sites 623 and 624 were drilled on the margins of the lower-fan channel, Site 624 being located some 6 km west of the active channel. Both sites consist of alternating channel fill and overbank deposits (figs. 6 and 7). The channel fills (primarily sand with thin clay stringers) are generally thin, usually less than 15 m thick. Each channel unit commences with a sharp basal sand grading upward into laminated silty muds and occasionally muds. The overbank intervals are poorly defined coarsening-upward sequences of interbedded thin sands, silts, and silty muds. All the data suggest that, during the construction of a fanlobe, the lower-fan channels occupy different positions for short periods of time, then switch to another nearby position rather than maintain a stable position or migrate laterally.

Sites 615 (fig. 8) and 614 (fig. 9) were drilled near the channel terminus and generally showed coarsening-upward turbidite sequences that constructed channel mouth depositional lobes. Site 615 was the deepest cored boring, penetrating to a depth of 523 m below the seafloor. Two fanlobes were cored; the youngest lobe is 202 m thick and the underlying older lobe is 274 m thick. Both lobes are Wisconsin in age. These are underlain by a late Wisconsin interstadial (Ericson Zone X) consisting of a fining-upward carbonate sequence that probably originated from the West Florida platform. This unit was deposited by a single debris flow or a series of debris flows that occurred over a relatively short period of time. Above the carbonate sequence, the older lobe contains a net sand thickness of 183.0 m (65 percent), while the younger lobe has a net sand thickness of 81.7 m (41 percent). Both lobes display a coarsening-upward trend, with the sands displaying blocky, 2- to 10m-thick sharp bases and tops. In the lower part of the fanlobe, the thin sands display abundant cross laminations and concentrations of organic debris (coffee grounds) and mica. Accumulation rates at these sites, some 600 km from the shelf edge, ranged from 2 m/1000 years (Site 614) to 6.5 m/1000 years for Site 615. As a result of the high accumulation rates, planktonic and benthic fauna are rare and only reworked shallow water species are present.

Conclusions

The drilling in the fan, as well as the analysis of the seismic

Figure 4. Site 620, middle fan overbank deposits. Generalized lithological column, stratigraphic zonation, gamma ray and induction log traces, depth (m, ft) subsea and subbottom. Lithological legend, see figures 1 and 3.

Figure 5. Site 616, eastern fan slump area. Generalized lithological column, stratigraphic zonation, uncorrected gamma ray trace, and depth (m, ft) subsea and subbottom. Lithological legend, see figures 1 and 3; blank: no data.

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data, produces some very important information, which is summarized below (Bouma et al., 1985):

1. The Mississippi Fan consists of several fanlobes, each of which is connected to a submarine canyon that is incised into the outer continental shelf and continental slope. During the Pleistocene, a general migration from west to east and toward deeper water took place.
2. A fanlobe is an elongated body of sediment that is basically a channel-overbank complex, which can be divided into four regions: a) a canyon, which is formed primarily by massive slope failures during falling sea level; b) an upper fan, which terminates near the base of slope and acts primarily as a conduit for transporting sediment downfan; c) a middle fan, which is an aggradational unit with a convex upper surface and a large, sinuous, migratory channel positioned at its apex; and d) a lower fan, which is also aggradation in nature but which has several more or less parallel channels, of which only one is active at any given time. Near the terminus of a channel, small bifurcations may occur, the sediment is no longer confined and spreads out as a series of sheet sands.
3. The single sinuous channel on the middle fan is migratory and aggradational in nature and shows a general similarity to migratory fluvial channels. The channel fill is a fining-upward sequence, commencing with gravels and ending with homogeneous muds. The basal graveliferous and sandy sediments represent a channel lag and are characterized acoustically by high amplitude reflectors that shift laterally and climb stratigraphically.
4. The coarser grained sediments are confined to the channel, and only the finer grained sands, silts, and clays escape to form the overbank deposits. These alternating fine sand, silt, and clay layers are laid down rapidly, as the accumulation rates of the overbank sediment are essentially the same as those of the channel complex. Accumulation rates for both sequences range from 10 to 12 m/1000 years. All sites on the middle fan generally have low faunal content, and in the channel most of the species represent reworked shallow water benthic faunas.
5. In contrast to the single channel on the middle fan, the lower fan contains a number of smaller channels, only one of which seems to be active at any given time. The process of channel switching produces a vertical sequence of alternating fining-upward channel-fill and overbank deposits.
6. Significant amounts of sand, derived from a source area with a low sand:clay ratio, were moved down the fan via a large channel to its distal areas, a transport distance in excess of 600 km. The fanlobes in the lower fan have high net sand percentages; the two lobes cored showed 65 and 41 percent sand.
7. During the Wisconsin Interstadial, a 29 m-thick carbonate debris flow was deposited over older fanlobes. The source of this unit was probably the Florida platform to the east. Such debris flows can deposit significant quantities of sediment over large areas of the basin floor.
8. Sedimentation was extremely high on the Mississippi Fan during the Pleistocene. During late Wisconsin times, sedimentation rates ranged from 11 to 12 m/1000 years for the middle-fan channel and overbank sediments to 5 to 6 m/ 1000 years for the outer-fan areas. Transport distances are large, being in excess of 600 km for the sands deposited in the outer-fan lobes. High rates of sedimentation appear to correlate to periods of low sea level and the initial rise of sea level, while low sedimentation rates are associated with high sea-level stands.