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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 (Columbia), 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.

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 (Columbia), 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.

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 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 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.

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.

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.

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.

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.

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.

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 10⁶ cu m.

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 10⁶ cu m.

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.

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.

[[: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.

[[: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.

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.

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.

[[: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.

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.

[[: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: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.