View source for Delta plain, subaqueous

==Subaqueous slump deposits==
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file:M31F23.jpg|{{figure number|8}}Schematic illustrating depicting the major types of submarine landslides, diapirs, and contemporary faults in the Mississippi River delta. <ref name=Colemanetal_1981 />
file:M31F24.jpg|{{figure number|9}}Side-scan sonar mosaic of subaqueous landslide gullies in the Mississippi River delta. The width of the mosaic is 1.5 km, and the superimposed grid is a 25-m square. The slope is from top (approximately 10-m water depth) to bottom (water depth 60 m).<ref name=Colemanetal_1981 />
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Recent detailed marine geologic investigations on subaqueous parts of continental shelves seaward of many river deltas experiencing high depositional rates have revealed contemporary recurrent subaqueous [[gravity]]-induced mass movements as common phenomena worthy of consideration as an integral component of the normal deltaic process and marine sediment transport. Off river deltas such as the Mississippi, Magdalena (Colombia), Orinoco (Venezuela), Surinam (Surinam), Amazon (Brazil), Yukon (Alaska), Niger (Nigeria), Nile (Egypt), and Hwang-Ho (China), subaqueous slumping and downslope mass movement of sediments are common processes. Instabilities and mass movement of sediment in these regions generally display the following characteristics: (a) instability occurs on very low angle slopes (generally less than 2°); (b) large quantities of sediment are transported from shallow water to deeper water offshore along well-defined mudflow gullies (debris flows) and in a variety of translational slumps. Although individual mudflow features vary in size and frequency, they generally possess a source area consisting of subsidence and rotational slumping, an elongate, often sinuous chute or channel (mudflow gully), and a composite depositional area composed of overlapping lobes of remolded debris.

River deltas displaying an abundance of submarine landslides are generally characterized by high rates of sediment accumulation within both fine-grained and coarse-grained fractions. Sediments therefore have an extremely high water content and most commonly display excess pore fluid pressures.

The abundance of fine-grained organic material within fine-grained clays is also subjected to rapid degradation by biochemical processes and produces large accumulations of sedimentary gas (primarily methane and carbon dioxide). The basic conditions for failure exist when stresses exerted on the sediment are sufficient to exceed its strength. This can be due to stress increases, strength reduction, or a combination of the two.

The Mississippi River delta and adjacent shelf region have been sites of active investigation for decades. The past decade has seen substantial advances in the systematic utilization of various techniques for marine geological exploration. Application of side-scan sonar and high-resolution seismic techniques has allowed substantial improvements in documentation and mapping of subaqueous flowslides off the delta. Essentially the entire subaqueous part of the Mississippi delta has been covered with overlapping side-scan sonar imagery and high-resolution seismic lines on a grid spacing of 250 m. Using these techniques, and aided by a large number of off-shore foundation borings, Coleman et al.,<ref name=Colemanetal_1974>Coleman, J. M., et al, 1974, Mass movements of Mississippi River delta sediments: Gulf Coast Assoc. Geol. Soc. Trans., v. 24, p. 49-68.</ref> Coleman,<ref name=Coleman_1976 /> Coleman and Garrison,<ref name=Colemanandgarrison_1977>Coleman, J. M., and L. E. Garrison, 1977, Geological aspects of marine slope instability, northwestern Gulf of Mexico: Marine Geotechnology, v. 2, p. 9-44.</ref> and Prior and Coleman<ref name=Priorandcoleman_1978a>Prior, D. B., amd J. M. Coleman, 1978a, Disintegrating retrogressive landslides on very-low-angle subaqueous slopes, Mississippi Delta: Marine Geotechnology, v. 3, no. 1, p. 37-60.</ref> <ref name=Priorandcoleman_1978b>Prior, D. B., amd J. M. Coleman, 1978b, Submarine landslides on the Mississippi River delta-front slope: Geoscience and Man, v. XIX, p. 41-53: School of Geosciences, Louisiana State Univ., Baton Rouge.</ref> <ref name=Colemanandprior_1978>Coleman, J. M., and D. B. Prior, 1978, Contemporary gravity tectonics--an everyday catastrophe? in Uniformitarianism--a contemporary perspective: AAPG Annual Meeting Abstract, v. 62, p. 505.</ref> have identified a wide variety of slope deformational features. These data form the basis for the following discussion of subaqueous mass-movement deposits.

The main types of slope and sediment instability mapped in 5- to 300-m water depths are illustrated schematically in [[:file:M31F23.jpg|Figure 8]], which shows their distribution around a single distributary. Similar spatial organization can be identified around the entire periphery of the modern river delta. Although a large number of types have been identified, the major types having geological significance include peripheral slumping, elongate retrogressive slides and mudflow gullies (and their associated overlapping depositional lobes), and large shelf-edge arcuate slumps and contemporaneous faults.

Peripheral slumps are associated primarily with immediate distributary-mouth-bar deposits. Bottom slopes in this region range from 0.2 to 0.6°. Peripheral slumps display abrupt stair-step scarps on the seafloor, the heights of the scarps ranging from 3 to 8 m. Tensional cracks are often present upslope from the major scarp, and frequently small mud vents are associated with these scarps.

Sizes of the slump blocks vary considerably, ranging from only a few hundred meters to well over 3 km. Although the features are referred to as rotational types of slumps because of the extremely low-sloping bottom gradients, the major movement is translational downslope. Displacements of the sediments begin as shallow rotational slumps, continuing over basal shears as predominantly translational movement.

Depth of the shear plane, and hence the thickness of the block, varies, but rarely exceeds 35 m. Movement rates are hard to determine, but repeated surveys over a 1-year period display movements ranging from a few hundred meters to nearly 1,000 m. This type of block slumping results essentially in downslope movement of sediment from shallow-water environments to deeper offshore and outer continental shelf water depths. Since the features originate in and near the distributary-mouth bar, they are frequently responsible for carrying coarse distributary-mouth bar sands farther offshore into deeper waters.

Instability of a second major type consists of elongate, retrogressive mudflow gullies and their overlapping depositional lobes. These features extend radially seaward from each major distributary and occur in water depths from 10 to 100 m. Depositional lobes extend farther seaward to water depths as great as 300 m. Each feature possesses a long, narrow chute or channel linking a depressed, hummocky source area on the upslope end to composite overlapping depositional lobes of fans on the seaward end. [[:file:M31F24.jpg|Figure 9]] is a side-scan sonar mosaic of several of these mudflow gullies. Source areas are normally bowl shaped and bounded by distinct scarps ([[:file:M31F24.jpg|Figure 9, A]]), with the interior of the depression normally characterized by extremely hummocky, chaotically arranged blocks of clasts in a matrix of highly fractured, flowed sediments.

Narrow chutes or gullies ([[:file:M31F24.jpg|Figure 9, B]]) extend downslope at approximately right angles to the regional depth contours and achieve lengths exceeding 8 to 10 km. They are rarely straight, and in plan view they display high sinuosity, with alternating narrow constrictions and wide bulbous sections. Widths of the gullies range from 20 to 50 m at the narrow section to 600 to 800 m where gullies are widest. Gully floors are generally depressed from a few meters to 20 m below the adjacent intact bottom. Slopes of side walls range from 1° to highs of 15°, and small rotational side slumps are often apparent.

During failure and movement of sediments, the material is apparently viscous enough to occasionally be ejected out of the narrow channel, forming overbank or natural-levee-type splays ([[:file:M31F24.jpg|Figure 9, C]]). On seaward ends of the elongate chutes, broad overlapping composite depositional lobes composed of debris discharged from the gullies are present. Depositional lobes display extremely irregular bottom topography characterized by crenulated blocky, disturbed debris and often abundant mud vents and volcanoes. Seaward mud nose scarps range in height from a few meters to more than 25 m. In plan view, scarps are curved and adjacent lobes often coalesce, forming an almost continuous complex sinuous frontal scarp possibly extending for distances of 20 to 25 km more or less parallel with the bathymetric contours.

Depositional areas are composed of several overlapping lobes owing to periodic discharge events, and each discharge is associated with its own distinctive nose. Seaward of the edge of the lobes, extensive small-scale pressure ridges are arranged sinuously and parallel. Extensive fields of mud vents and volcanoes emitting gas, water and fluid mud are found associated with the lobes and directly seaward of the noses; these undoubtedly result from rapid loading of underlying sediment as well as consolidation processes within the debris itself. Thicknesses of the lobes are difficult to determine, but each distinct lobe is normally 20 or so meters thick, and because of overlapping, the total thickness of mudflow can often approach 50 to 60 m. In one area of the Mississippi River delta, in water depths of approximately 200 to 250 m, depositional lobes cover approximately 770 sq km with discharged debris volume of 11.2 x 10<sup>6</sup> cu m.

A third major type of sediment instability of significant geological importance is arcuate rotational slump and [[growth fault]]. This type commonly occurs on the outer continental shelf in front of the advancing or prograding deltaic system. Large, arcuate-shaped families of shelf-edge slumps and deep-seated contemporaneous faults tend to be active along the peripheral margins of delta fronts. In most instances, these large-scale features tend to cut the modern sediment surface, often forming localized scarps on the seafloor. These surface scarps provide localized areas for accumulation of downslope mass-moved shallow-water sediment. In many places shelf-edge slumps tend to give a stairstepped appearance to the edge of the continental shelf and are highly reminiscent of rotational peripheral slumps higher on the continental shelf, near the mouths of the modern distributaries. However, these features generally occur on a much larger scale and cut a column of sediment ranging from 50 to 150 m in thickness.

Lateral continuities of individual slump scarps range from a few kilometers to as much as 8 to 10 km, and scarps on the seafloor produced by this slumping process may have heights of 30 m. A similar type of slump is commonly referred to as a contemporaneous or growth fault and is the feature moving continuously along the shear plane with deposition. Hence with time and continued movements, offsets of individual marker beds increase with depth, and thickness of these beds increases abruptly across the fault.

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M31F25.jpg|{{figure number|10}}High-resolution seismic record run across an active growth fault seaward of the mouth of South Pass, Mississippi River delta. A. Seismic line showing active growth fault seaward of a large upslope mudflow. Note the increased thickness of sediment on the downthrown side of the fault. Horizontal scale is 300 m between shot points and vertical scale is 25 milliseconds per time line, or 19 m (62.5 ft). B. Detailed subbottom seismic record run across an active growth fault. Note the presence of a rollover structure and the increased accumulation of sedimentation on the downthrown side of the fault. Horizontal scale is 300 m between shot points and vertical scale is 10 milliseconds per time line, or 7.6 m (25 ft).<ref name=Colemanetal_1981 />
M31F26.jpg|{{figure number|11}}Summary diagram illustrating the major characteristics of slump deposits in the subaqueous delta plain.<ref name=Colemanetal_1981 />
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[[:file:M31F25.jpg|Figure 10]] illustrates active growth faults in the Mississippi River delta seaward of the mouth of South Pass. [[:file:M31F25.jpg|Figure 10A]] shows a large mudflow lobe upslope from an active growth fault. This fault extends from the surface to depths beyond the bottom of the record. Sparker data run simultaneously indicate that the fault extends 700 to 750 m below sea bottom before merging into a bedding-plane fault. Offsets in the uppermost units are generally 5 to 10 m, while at depth (400 m), offsets of marker beds approach 70 to 80 m. Note the increased thickness of the sediment units on the downthrown side of the fault and the small rollover anticline or reverse-drag characteristic of this type of fault.

The amorphous zone upslope represents a surface mudflow that has progressed to and slightly beyond the limits of the faults. As surface mudflow crosses the fault zone, it thickens. It is highly possible that increased thicknesses on the downthrown sides of these faults result from movement of subsurface mudflows across the fault zone. As the fault zone is blanketed by a large mass of rapidly introduced mass-moved sediment, surface scarps on the seafloor are eliminated. Continued movement along the fault, however, will cause a new scarp, and given enough time, another mudflow will then move across the feature, adding increased amounts of sediment on the downthrown side.

This type of interaction between surface mudflow movements and contemporaneous faults may quite possibly play a large role in maintaining the continuing movement along these fault planes. [[:file:M31F25.jpg|Figure 10B]] shows details of the upper levels of a growth fault in the same vicinity.

A feature commonly associated with growth faults and of extreme importance to [[petroleum]] trapping is the association of rollovers or reverse drag with the downthrown side of a growth fault ([[:file:M31F25.jpg|Figure 10A, B]]). These features are common on the contemporaneous faults presently active in the delta. Rollover structures tend to form soon after deposition of sediment on the downthrown side and do not require a considerable amount of overburden and weighting to form. Mass-moved material flowing downslope from higher levels on the delta front (sands, silts, and clays) contains high water and gas contents. It is speculated that, as sediment accumulates slightly more thickly on the downthrown side of the fault, early degassing and dewatering associated with movement along the fault take place. Pore waters and pore gases are permitted to escape upward in the zone of movement associated with the fault, thereby decreasing the volume of sediment and allowing an early change in density to occur nearly contemporaneously with the fault. As greater and greater amounts of sediment are added and overburden pressures become increasingly larger, this feature is then amplified and becomes more pronounced with time and depth.

[[:file:M31F26.jpg|Figure 11]] illustrates through a summary diagram some of the major characteristics associated with subaqueous slump deposits. Although boring control and core control are limited in deeper offshore waters, enough foundation borings have penetrated some of the sequences to give a fairly good indication of the deposit types accumulating offshore on the downthrown sides of some slump fault features. In addition, numerous articles on Gulf Coast Tertiary sequences indicate the type of deposition associated with slump deposits. The upper right-hand diagram illustrates a vertical sequence commonly associated with offshore slump deposits. The first striking characteristic is the extreme variations in [[grain size]]. Sandy deposits generally occur as distinct isolated blocks showing both sharp base and sharp tops. Grain size depends on the source of the slump material, and in such a deltaic setting, sources are commonly distributary-mouth-bar deposits trapped on the downthrown sides of these slump features.

Thus many sedimentary structures are the same as those described for distributary-mouth-bar deposits. Having been mass moved downslope, however, they lie on entirely marine clay deposits and thus normally have a sharp lower bounding surface. The upper surface is also usually extremely sharp and generally is characterized by a high degree of intensive burrowing on the top of the sand body. Because most of the deposits are mass moved, depositional dips increase significantly, and high-angle dips of 10 to 25° are not uncommon in these beds. Fracturing and localized faulting and slump structures are also abundant in most of the sand bodies.

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M31F27.jpg|{{figure number|12}}Core photographs of subaqueous slump deposits. Diameter of cores A, B, and E is 13 cm (5 in.) and of cores C and D 8 cm (3 in.). A. X-ray radiograph of highly distorted clay layers in marine deposits beneath the slump block. B. X-ray radiograph of multiple fracturing in clays in the shear plane zone. C. X-ray radiograph of silt and sand core in the slump block. Note that bedding is preserved with only minor fracturing but is tilted at angles of 20 to 30°. D. X-ray radiograph of disturbed structures in mudflow deposit that caps a slump block. E. X-ray radiograph of core in normally deposited marine clays, which often cap the slump deposits. ote the lack of disturbance in these deposits. <ref name=Colemanetal_1981 />
file:M31F28.jpg|{{figure number|13}}Part of a cored boring through a slump block composed of distributary mouth bars sands. This slump block is off Southwest Pass, Mississippi River delta, and is underlain and capped by marine clays. Diameter of cores is 8 cm (3 in.).<ref name=Colemanetal_1981 />
file:M31F29.jpg|{{figure number|14}}Part of a core taken through a mudflow lobe on the continental shelf off the modern Mississippi River delta.<ref name=Colemanetal_1981 />
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[[:file:M31F27.jpg|Figure 12A-E]] illustrates specific types of stratification often encountered in finer grained sequences separating slumped sand blocks. Both normal bedded marine clay deposits ([[:file:M31F27.jpg|Figure 12E]]) and highly distorted marine clays ([[:file:M31F27.jpg|Figure 12A, B, D]]) are common within the finer grained sequences.

[[:file:M31F28.jpg|Figure 13]] shows part of a cored boring through a slump block composed of distributary-mouth-bar sands. The core was taken in a buried slump block off Southwest Pass, Mississippi River delta. The sand unit is capped above and below by marine clays having faunal content that indicates middle to outer continental shelf depths. The sand body itself contains no fauna except for an occasional shallow-water microfossil. The thickness of the slump block cored is approximately 15 m, of which approximately 10 m is illustrated in [[:file:M31F28.jpg|Figure 13]]. The lowermost part of the sand body is characterized by extremely distorted sedimentary structures, with little or no primary stratification preserved. However, within the same slump block unit, well-preserved small-scale stratification exists as isolated blocks. There is a tendency for excessive dips to be present in the deposits, and fracturing systems such as those shown in the first two cores are also common within the sequences. The lateral extent of the sand body is relatively unknown, though additional borings in the area tend to show a minimum lateral continuity of at least 1 km. It seems highly probable that there is reservoir continuity across this particular sand body.

[[:file:M31F29.jpg|Figure 14]] represents a part of a core taken through a mudflow lobe that moved down from shallower waters in the delta front across the continental shelf and on to outer continental slope water depths. This particular mudflow consists primarily of fine-grained silty clays of the distal bar and prodelta clay deposits. As is evident in the core, a wide variety of distorted laminations are present. Numerous fractures exist throughout the entire deposit; however, this particular depositional lobe has moved downslope from water depths of approximately 20 m and now lies at the edge of the continental shelf, where water depths are about 130 m (downslope movement of approximately 3 to 4 km). Even with this magnitude of movement, primary stratification is still preserved within individual block , as shown in the cored boring in [[:file:M31F29.jpg|Figure 14]].

The lower two diagrams in [[:file:M31F26.jpg|Figure 11]] attempt to illustrate a sand isopach map associated with a growth fault offshore, and accumulation of slump-block material on the downthrown side of a growth fault. This type of isopach is common in many of the published papers discussing Tertiary depocenters. Variations in electric-log response in various parts of the isopach map are shown in the lower right-hand diagram of [[:file:M31F26.jpg|Figure 11]]. One of the most characteristic features of sand bodies deposited by slumping processes is the extremely blocky character of the electric-log response. Sands generally tend to be sharp based, producing rather uniform log response. Correlation of individual kicks on the sand body is extremely tentative simply because of the nature of the slumping process. It is the writers' belief that many of the sand bodies associated with growth fault systems represent shallow-water sand bodies that have slumped downslope into deeper water and become trapped on the seafloor near growth fault systems.

In other instances, normal progradation of deltaic sequences across the growth fault zone results in thicker accumulations of mass-moved sediment on the downthrown side of the fault systems. Mass-moved sediment on the downthrown side is most common near the lower, more marine parts of the delta sequence.