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Directly capping the distributary-mouth bar are a variety of overbank splays and shallow-bay deposits. These overbank splays are commonly referred to as natural levees. During annual river floods, the river stage normally tops the distributary channel, and numerous small overbank or crevasse splays result. Each splay will be maintained for one or several years until it builds up or grades the natural levee to flood level, and then it will cease activity. Thus, capping the distributary-mouth bar is commonly a series of small coarsening-upward sequences representative of these splay deposits. Deposits are characterized by a wide variety of sedimentary structures, with climbing current ripples and small-scale cross laminae being the most common in the coarser grained parts. [[:file:M31F18.jpg|Figure 3J]] illustrates some of the ripple-type structures present in this environment.
Directly capping the distributary-mouth bar are a variety of overbank splays and shallow-bay deposits. These overbank splays are commonly referred to as natural levees. During annual river floods, the river stage normally tops the distributary channel, and numerous small overbank or crevasse splays result. Each splay will be maintained for one or several years until it builds up or grades the natural levee to flood level, and then it will cease activity. Thus, capping the distributary-mouth bar is commonly a series of small coarsening-upward sequences representative of these splay deposits. Deposits are characterized by a wide variety of sedimentary structures, with climbing current ripples and small-scale cross laminae being the most common in the coarser grained parts. [[:file:M31F18.jpg|Figure 3J]] illustrates some of the ripple-type structures present in this environment.
[[:file:M31F19.jpg|Figures 4]], [[:file:M31F20.jpg|5]], and [[:file:M31F21.jpg|6]] show parts of a continuously cored boring (approximately 130 m in depth) taken through the distributary-mouth-bar deposits in the Mississippi River delta. [[:file:M31F19.jpg|Figure 19]] illustrates the sedimentary structures in the core representing lower sections of the distal-bar deposits. Even within this 10 m section of core the change from a predominance of clay in the lower part of the distal bar to a predominance of fine, silty and sandy laminations is apparent. Several regions show highly contorted bedding believed to be primarily due to slumping rather than coring processes.
[[:file:M31F19.jpg|Figures 4]], [[:file:M31F20.jpg|5]], and [[:file:M31F21.jpg|6]] show parts of a continuously cored boring (approximately 130 m in depth) taken through the distributary-mouth-bar deposits in the Mississippi River delta. [[:file:M31F19.jpg|Figure 4]] illustrates the sedimentary structures in the core representing lower sections of the distal-bar deposits. Even within this 10 m section of core the change from a predominance of clay in the lower part of the distal bar to a predominance of fine, silty and sandy laminations is apparent. Several regions show highly contorted bedding believed to be primarily due to slumping rather than coring processes.
[[:file:M31F20.jpg|Figure 5]] represents approximately 7 m of core taken in the lower segment of the distributary-mouth-bar deposits. The predominance of sand laminations intercalating with silty clay and silt laminations is apparent. Darker colored zones in lower parts of the core represent transported organic debris. Also shown in this sequence is a section of highly contorted core, once more believed to be the result of slumping rather than core disturbance. The predominance of small-scale ripple laminations is obvious within the sequence. [[:file:M31F21.jpg|Figure 6]] represents 7 m of cored boring taken from the uppermost part of the distributary-mouth-bar deposit. The core consists primarily of sand deposits, with sand-sized particles making up 80 to 90% of the total unit. Dark layers are stratification representing transported organic debris. Note that most of the stratification is nearly flat-lying, and few or no steep dips exist, except for small-scale cross-bedding. One core indicates tremendous disturbance and distortion of lamination and undoubtedly represents a portion of a slump block. Ripple laminations, climbing ripples, and larger scale trough cross-laminations are the most dominant sedimentary structure types.
[[:file:M31F20.jpg|Figure 5]] represents approximately 7 m of core taken in the lower segment of the distributary-mouth-bar deposits. The predominance of sand laminations intercalating with silty clay and silt laminations is apparent. Darker colored zones in lower parts of the core represent transported organic debris. Also shown in this sequence is a section of highly contorted core, once more believed to be the result of slumping rather than core disturbance. The predominance of small-scale ripple laminations is obvious within the sequence. [[:file:M31F21.jpg|Figure 6]] represents 7 m of cored boring taken from the uppermost part of the distributary-mouth-bar deposit. The core consists primarily of sand deposits, with sand-sized particles making up 80 to 90% of the total unit. Dark layers are stratification representing transported organic debris. Note that most of the stratification is nearly flat-lying, and few or no steep dips exist, except for small-scale cross-bedding. One core indicates tremendous disturbance and distortion of lamination and undoubtedly represents a portion of a slump block. Ripple laminations, climbing ripples, and larger scale trough cross-laminations are the most dominant sedimentary structure types.
Although in present world deltas, river-mouth tidal ridges form one of the major sand bodies associated with rivers prograding into basins displaying macro- or high-tidal ranges, literature on these river-mouth tidal ridges is sparse. Many past river deltas undoubtedly existed in similar environmental settings, and they must therefore form prominent sand bodies in many ancient-rock deltaic sequences. Major literature associated with these types of features includes that of Off,<ref name=Off_1963>Off, Theodore, 1963, [http://archives.datapages.com/data/bulletns/1961-64/data/pg/0047/0002/0300/0324.htm Rhythmic linear sand bodies caused by tidal currents]: AAPG Bulletin, v. 47, p. 324-341.</ref> Keller and Richards,<ref name=Kellerandrichards_1967>Keller, G. H., and A. F. Richards, 1967, Sediments of the Malacca Straits, southeast Asia: Jour. Sed. Petrology, v. 37, p. 102-127.</ref> Reineck and Singh,<ref name=Reineckandsingh_1967>Reineck, H. E., and I. B. Singh, 1967, Primary sedimentary structures in the Recent sediments of the Jade, North Sea: Marine Geol., v. 5, p. 227-235.</ref> Houbolt,<ref name=Houboult_1968>Houbolt, J. J. H. C., 1968, Recent sediments in the southern bight of the North Sea: Geol. en Mijnbouw, v. 47, p. 245-273.</ref> Klein,<ref name=Klein_1970>Klein, G. de V., 1970, Depositional and dispersal dynamics of intertidal sand bars: Jour. Sed. Petrology, v. 40, p. 1095-1127.</ref> Ludwick,<ref name=Ludwick_1970>Ludwick, J. C., 1970, Sand waves and tidal channels in the entrance to Chesapeake Bay: Old Dominion Univ., Inst. Oceanography, Tech. Rept. 1, 79 p.</ref> Meckel,<ref name=Meckel_1975>Meckel, L. D., 1975, Holocene sand bodies in the Colorado Delta, Salton Sea, Imperial County, California: in M. L. Broussard, ed., Deltas, models for exploration, 2nd ed: Houston Geol. Soc., p. 239-265.</ref> and Wright and Thom.<ref name=Wrightandthom_1975>Wright, L. D., and B. G. Thom, 1975, Sediment transport and deposition in a macrotidal river channel: Ord River, Western Australia: Estuarine Research, v. II: New York, Academic Press, p. 309-321.</ref>
Although in present world deltas, river-mouth tidal ridges form one of the major sand bodies associated with rivers prograding into basins displaying macro- or high-tidal ranges, literature on these river-mouth tidal ridges is sparse. Many past river deltas undoubtedly existed in similar environmental settings, and they must therefore form prominent sand bodies in many ancient-rock deltaic sequences. Major literature associated with these types of features includes that of Off,<ref name=Off_1963>Off, Theodore, 1963, [http://archives.datapages.com/data/bulletns/1961-64/data/pg/0047/0002/0300/0324.htm Rhythmic linear sand bodies caused by tidal currents]: AAPG Bulletin, v. 47, p. 324-341.</ref> Keller and Richards,<ref name=Kellerandrichards_1967>Keller, G. H., and A. F. Richards, 1967, Sediments of the Malacca Straits, southeast Asia: Jour. Sed. Petrology, v. 37, p. 102-127.</ref> Reineck and Singh,<ref name=Reineckandsingh_1967>Reineck, H. E., and I. B. Singh, 1967, Primary sedimentary structures in the Recent sediments of the Jade, North Sea: Marine Geol., v. 5, p. 227-235.</ref> Houbolt,<ref name=Houboult_1968>Houbolt, J. J. H. C., 1968, Recent sediments in the southern bight of the North Sea: Geol. en Mijnbouw, v. 47, p. 245-273.</ref> Klein,<ref name=Klein_1970>Klein, G. de V., 1970, Depositional and dispersal dynamics of intertidal sand bars: Jour. Sed. Petrology, v. 40, p. 1095-1127.</ref> Ludwick,<ref name=Ludwick_1970>Ludwick, J. C., 1970, Sand waves and tidal channels in the entrance to Chesapeake Bay: Old Dominion Univ., Inst. Oceanography, Tech. Rept. 1, 79 p.</ref> Meckel,<ref name=Meckel_1975>Meckel, L. D., 1975, Holocene sand bodies in the Colorado Delta, Salton Sea, Imperial County, California: in M. L. Broussard, ed., Deltas, models for exploration, 2nd ed: Houston Geol. Soc., p. 239-265.</ref> and Wright and Thom.<ref name=Wrightandthom_1975>Wright, L. D., and B. G. Thom, 1975, Sediment transport and deposition in a macrotidal river channel: Ord River, Western Australia: Estuarine Research, v. II: New York, Academic Press, p. 309-321.</ref>
River distributaries debouching into high-tidal regions commonly display a funnel-shaped configuration, with widths attaining several kilometers. Linear, elongate tidal ridges aligned parallel with each other in the direction of tidal flow are the most prominent channel and river mouth accumulation forms. They appear to be directly related to bidirectional sediment transport patterns, high-tidal amplitudes, and tidal-current symmetry. Tidal ridges described by Coleman<ref name=Coleman_1976 /> and Wright and Thum<ref name=Wrightandthum_1975 /> in the Ord River are typical of the study shoals found in river-dominated deltaic distributaries.
River distributaries debouching into high-tidal regions commonly display a funnel-shaped configuration, with widths attaining several kilometers. Linear, elongate tidal ridges aligned parallel with each other in the direction of tidal flow are the most prominent channel and river mouth accumulation forms. They appear to be directly related to bidirectional sediment transport patterns, high-tidal amplitudes, and tidal-current symmetry. Tidal ridges described by Coleman<ref name=Coleman_1976 /> and Wright and Thom<ref name=Wrightandthom_1975 /> in the Ord River are typical of the study shoals found in river-dominated deltaic distributaries.
Those tidal ranges in the Ord River range in relief from 10 to 22 m and compositely account for over 5 x 10<sup>6</sup> cu m of total sand accumulation. Tidal ridges average roughly 2 km in length and 300 m in width, with crests emergent or near the surface at low tide. A few are permanently emergent and vegetated by mangrove. In some deltas, tidal ridges attain extreme lengths of 10 to 15 km. Deltas displaying similar types of shoals have been described by Off<ref name=Off_1963 /> and Meckel.<ref name=Meckel_1975 /> Meckel<ref name=Meckel_1975 /> referred to tidal ridges at the mouth of the Colorado delta, Gulf of California, as tidal bars. Ridges at the mouth of the Colorado display relief of 7 to 10 m and a crest to crest spacing of several kilometers. In cross-sect on they vary from roughly symmetrical to distinctly asymmetrical, with steep sides commonly facing the downstream direction of tidal propagation.
Those tidal ranges in the Ord River range in relief from 10 to 22 m and compositely account for over 5 x 10<sup>6</sup> cu m of total sand accumulation. Tidal ridges average roughly 2 km in length and 300 m in width, with crests emergent or near the surface at low tide. A few are permanently emergent and vegetated by mangrove. In some deltas, tidal ridges attain extreme lengths of 10 to 15 km. Deltas displaying similar types of shoals have been described by Off<ref name=Off_1963 /> and Meckel.<ref name=Meckel_1975 /> Meckel<ref name=Meckel_1975 /> referred to tidal ridges at the mouth of the Colorado delta, Gulf of California, as tidal bars. Ridges at the mouth of the Colorado display relief of 7 to 10 m and a crest to crest spacing of several kilometers. In cross-sect on they vary from roughly symmetrical to distinctly asymmetrical, with steep sides commonly facing the downstream direction of tidal propagation.
Although little coring has been done on these types of deposits, Figure 22 is an attempt to summarize data presently available. The upper left diagram illustrates the distribution of some of the tidal ridges seaward of the mouth of the Shatt-el-Arab River delta, which flows into the Persian Gulf. Lengths of the tidal ridges at the river mouth range from 5 to 15 km, with some of the larger ridges displaying widths of 2 km. General spacing of the ridges across the distributary-mouth-bar area ranges from a minimum of about 2 km to slightly over 5 km. The upper righthand diagram shows a typical vertical sequence resulting from river-mouth progradation and lateral migration of ridges. In general, the coarsening upward sequence displayed agrees quite well with data presented from the lower Colorado delta<ref name=Meckel_1975 /> and Ord River delta.<ref name=Coleman_1976 />
[[file:M31F22.jpg|thumb|300px|{{figure number|7}}Summary diagram illustrating the major characteristics of river-mouth tidal ridge deposits in the subaqueous delta plain.<ref name=Colemanetal_1981 />]]
Although little coring has been done on these types of deposits, [[:file:M31F22.jpg|Figure 7]] is an attempt to summarize data presently available. The upper left diagram illustrates the distribution of some of the tidal ridges seaward of the mouth of the Shatt-el-Arab River delta, which flows into the Persian Gulf. Lengths of the tidal ridges at the river mouth range from 5 to 15 km, with some of the larger ridges displaying widths of 2 km. General spacing of the ridges across the distributary-mouth-bar area ranges from a minimum of about 2 km to slightly over 5 km. The upper righthand diagram shows a typical vertical sequence resulting from river-mouth progradation and lateral migration of ridges. In general, the coarsening upward sequence displayed agrees quite well with data presented from the lower Colorado delta<ref name=Meckel_1975 /> and Ord River delta.<ref name=Coleman_1976 />
Sand units are generally well sorted and display a variety of small-scale and large-scale cross-stratifications. One of the more common sedimentary structures within sand bodies is the small-scale bidirectional or herring-bone stratification type. Shell debris is generally common, both scattered throughout sand deposits and concentrated into thin lag-type deposits. Parallel sand layers are common throughout the entire sequence of sandy deposits and probably result from deposition during the upper flow regime, especially during low tide, when water depths across the shoals are quite low and velocities are quite high.
Sand units are generally well sorted and display a variety of small-scale and large-scale cross-stratifications. One of the more common sedimentary structures within sand bodies is the small-scale bidirectional or herring-bone stratification type. Shell debris is generally common, both scattered throughout sand deposits and concentrated into thin lag-type deposits. Parallel sand layers are common throughout the entire sequence of sandy deposits and probably result from deposition during the upper flow regime, especially during low tide, when water depths across the shoals are quite low and velocities are quite high.
Thompson<ref name=Thompson_1968>Thompson, R. W., 1968, Tidal flat sedimentation on the Colorado River delta, northwestern Gulf of California: Geol. Soc. America Mem. 107, 133 p.</ref> measured flood and ebb currents of 100 to 135 cm/sec, with maximum velocities of more than 200 cm/sec, in bars at the mouth of the Colorado. Although exposures are generally limited within the tidal ridges, shallow pits and box cores near the tops of many tidal ridges have large-scale trough cross-bedding, with the probabilities that within the uppermost sequences, large-scale cross-bedding could be preserved. Directional properties throughout the sequence generally show a net downstream direction; however, upstream-oriented cross-stratification is not uncommon, and thus current roses would probably show the bidirectional pattern.
Thompson<ref name=Thompson_1968>Thompson, R. W., 1968, Tidal flat sedimentation on the Colorado River delta, northwestern Gulf of California: Geol. Soc. America Mem. 107, 133 p.</ref> measured flood and ebb currents of 100 to 135 cm/sec, with maximum velocities of more than 200 cm/sec, in bars at the mouth of the Colorado. Although exposures are generally limited within the tidal ridges, shallow pits and box cores near the tops of many tidal ridges have large-scale trough cross-bedding, with the probabilities that within the uppermost sequences, large-scale cross-bedding could be preserved. Directional properties throughout the sequence generally show a net downstream direction; however, upstream-oriented cross-stratification is not uncommon, and thus current roses would probably show the bidirectional pattern.
The lower left diagram in [[:file:M31F22.jpg|Figure 7]] depicts the probable sand isopach associated with a river-mouth tidal-ridge environment. This particular isopach is based on limited data and is patterned after the Ord River mouth. Sand thickness throughout the isopached interval would undoubtedly vary and be concentrated into the linear type of ridges seen topographically in modern deltas. Log response (lower right diagram, [[:file:M31F22.jpg|Figure 7]]) displays extreme variation because of sand thickness; the base of the sand deposit displays a gradational contact to a rather abrupt basal scour plane associated with those ridges of prominent scour. In general, the ridges tend to display the coarsest and best sorted sand units and are illustrated by core holes 3, 5 and 7.
The lower left diagram in [[:file:M31F22.jpg|Figure 7]] depicts the probable sand isopach associated with a river-mouth tidal-ridge environment. This particular isopach is based on limited data and is patterned after the Ord River mouth. Sand thickness throughout the isopached interval would undoubtedly vary and be concentrated into the linear type of ridges seen topographically in modern deltas. Log response (lower right diagram, [[:file:M31F22.jpg|Figure 7]]) displays extreme variation because of sand thickness; the base of the sand deposit displays a gradational contact to a rather abrupt basal scour plane associated with those ridges of prominent scour. In general, the ridges tend to display the coarsest and best sorted sand units and are illustrated by core holes 3, 5 and 7.
==Subaqueous slump deposits==
==Subaqueous slump deposits==
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.
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.
[[file:M31F23.jpg|thumb|300px|{{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 />]]
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.
[[file:M31F24.jpg|thumb|300px|{{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 />]]
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.
[[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 />]]
[[: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|thumb|300px|{{figure number|11}}Summary diagram illustrating the major characteristics of slump deposits in the subaqueous delta plain.<ref name=Colemanetal_1981 />]]
[[: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.
[[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 />]]
[[: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]].
<gallery mode=packed heights=200px widths=200px>
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 />
</gallery>
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.
''Literature on subaqueous slumps.'' <ref name=Shepard_1955>Shepard, F. P., 1955, Delta front valleys bordering the Mississippi distributaries: Geol. Soc. America Bull., v. 66, p. 1489-1498.</ref> <ref name=Shepard_1973>Shepard, F. P., 1973, Sea floor off Magdalena delta and Santa Marta area, Colombia: Geol. Soc. America Bull., v. 84, p. 1955-1972.</ref> <ref name=Fisk_1956>Fisk, H. N., 1956, Nearshore sediments of the continental shelf off Louisiana: Proc. 8th Texas Conf. on Soil Mech. and Foundation Eng., p. 1-23.</ref> <ref name=Moore_1961>Moore, D. G., 1961, Submarine slides: Jour. Sed. Petrology, v. 31, p. 343-357.</ref> <ref name=Ocamb_1961>Ocamb, R. D, 1961, Growth faults of south Louisiana: Gulf Coast Assoc. Geol. Soc. Trans., v. 2, p. 139-175.</ref> <ref name=Morganetal_1968>Morgan, J. P., J. M. Coleman, and S. M. Gagliano, 1968, [http://archives.datapages.com/data/specpubs/structu1/data/a153/a153/0001/0100/0145.htm Mudlumps: diapiric structures in Mississippi Delta sediments]: AAPG Memoir 8, p. 145-161.</ref> <ref name=Bea_1971>Bea, R. G., 1971, How sea floor slides affect offshore structures: Oil and Gas Jour., v. 69, no. 48, p. 88-92.</ref> <ref name=Bruce_1972>Bruce, C. H., 1972, Pressured shale and related sediment deformation: a mechanism for development of regional contemporaneous faults: Gulf Coast Assoc. Geol. Soc. Trans., v. 22, p. 23-31.</ref> <ref name=Woodburyetal_1973>Woodbury, H. O., et al, 1973, Pliocene and Pleistocene depocenters, outer continental shelf, Louisiana and Texas: AAPG Bull., v. 57, p. 2428-2439.</ref> <ref name=Woodburyetal_1977>Woodbury, H. O., J. H. Spotts, and W. H. Akers, 1977, Movement of sediment on Gulf of Mexico continental slope and upper continental shelf: 9th Offshore Tech. Conf., Proc., v. 1, p. 59-68.</ref> <ref name=Colemanetal_1974 /> <ref name=Garrison_1974>Garrison, L. E., 1974, The instability of surface sediments on parts of the Mississippi delta front: U.S. Geol. Survey Open File Rept., Corpus Christi, Texas, 18 p.</ref> <ref name=Hedberg_1974 /> <ref name=Busch_1975>Busch, D. A., 1975, [http://archives.datapages.com/data/bulletns/1974-76/data/pg/0059/0002/0200/0217.htm Influence of growth faulting on sedimentation and prospect evaluation]: AAPG Bulletin, v. 59, p. 217-230.</ref> <ref name=Whelanetal_1975>Whelan, Thomas, III, et al, 1975, The geochemistry of Recent Mississippi River delta sediments: gas concentration and sediment stability: Preprints 7th Ann. Offshore Tech. Conf., Houston, Texas, May 5-8, 1975, p. 71-84.</ref> <ref name=Coleman_1976 /> <ref name=Edwards_1976>Edwards, M. B., 1976, Growth faults in Upper Triassic deltaic sediments, Svalbard: AAPG Bull., v. 60, p. 341-355.</ref> <ref name=Robertsetal_1976>Roberts, H. H., D. Cratsley, T. Whelan, III, 1976, Stability of Mississippi delta sediments as evaluated by analysis of structural features of sediment borings: Eighth Ann. Offshore Tech. Conf., Houston, Texas, May 3-5, 1976, p. 9-28.</ref> <ref name=Colemanandgarrison_1977 /> <ref name=Embleyandjacobi_1977>Embley, R. W., and R. Jacobi, 1977, Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins: Marine Geotechnology, v. 2, p. 205-228.</ref> <ref name=Molniaetal_1977>Molnia, B. F., P. R. Carlson, and T. R. Bruns, 1977, Large submarine slide in Kayak Trough, Gulf of Alaska: in Landslides: Geol. Soc. America Rev. Eng. Geol., v. 3, p. 137-148.</ref> <ref name=Carlson_1978>Carlson, P. R., 1978, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0062/0012/2400/2412.htm Holocene slump on continental shelf off Malaspina Glacier, Gulf of Alaska]: AAPG Bulletin, v. 62, no. 12, p. 2412-2426.</ref> <ref name=Priorandcoleman_1978a /> <ref name=Priorandcoleman_1978b /> <ref name=Colemanandprior_1978 /> <ref name=Rider_1978>Rider, M. H., 1978, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0062/0011/2150/2191.htm Growth faults in Carboniferous of Western Ireland]: AAPG Bulletin, v. 62, no. 11, p. 2191-2213.</ref>
==See also==
==See also==