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Line 93: − − 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. − + + − − [[file:M31F25.jpg|thumb|300px|{{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 />]] Line 120:
Line 117: − − [[file:M31F26.jpg|thumb|300px|{{figure number|11}}Summary diagram illustrating the major characteristics of slump deposits in the subaqueous delta plain.<ref name=Colemanetal_1981 />]] − − [[file:M31F27.jpg|thumb|300px||{{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 />]] Line 132:
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→Subaqueous slump deposits
==Subaqueous slump deposits==
==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: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 />
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 (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.
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 10<sup>6</sup> 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 10<sup>6</sup> 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.
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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 />
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 />
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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.
[[: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.