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In the absence of high tidal range and extremely strong marine energy, the distributary channel pattern is often one of seaward bifurcation ([[:file:M31F16.jpg|Figure 1]]). Because of this bifurcating channel pattern, the distributary-mouth bars at each of the river mouths often merge and form a near-continuous sand strip around the entire periphery of the delta. Shallow offshore slopes and low wave and tide action favor this type of distributary pattern, and turbulent diffusion within the water mass becomes restricted to the horizontal. Bottom friction plays a major role in causing effluent deceleration and expansion. Initially a broad, arcuate radial bar will form at the mouth. However, as deposition on the bar continues, natural subaqueous levees will develop beneath the lateral boundaries of the expanding effluent, where velocity gradients are generally steepest. Development of subaqueous levees tends to inhibit further increases in effluent expansion, so that with continuing bar accretion continuity can no longer be maintained simply by increasing effluent width. As the central part of the bar grows upward, channelization develops along the threads of maximum turbulence, which tend to follow the subaqueous levee. This process results in formation of a bifurcating channel, which has a triangular middle ground shoal separating diverging channel arms. This type of channel pattern is well displayed in the high-altitude photograph shown as [[:file:M31F16.jpg|Figure 1]].

In the absence of high tidal range and extremely strong marine energy, the [[distributary channel]] pattern is often one of seaward [[bifurcation]] ([[:file:M31F16.jpg|Figure 1]]). Because of this bifurcating channel pattern, the distributary-mouth bars at each of the river mouths often merge and form a near-continuous sand strip around the entire periphery of the delta. Shallow offshore slopes and low wave and tide action favor this type of distributary pattern, and turbulent diffusion within the water mass becomes restricted to the horizontal. Bottom friction plays a major role in causing effluent deceleration and expansion. Initially a broad, arcuate radial bar will form at the mouth. However, as deposition on the bar continues, natural subaqueous levees will develop beneath the [[lateral]] boundaries of the expanding effluent, where velocity gradients are generally steepest. Development of subaqueous levees tends to inhibit further increases in effluent expansion, so that with continuing bar accretion continuity can no longer be maintained simply by increasing effluent width. As the central part of the bar grows upward, channelization develops along the threads of maximum turbulence, which tend to follow the subaqueous levee. This process results in formation of a bifurcating channel, which has a triangular middle ground shoal separating diverging channel arms. This type of channel pattern is well displayed in the high-altitude photograph shown as [[:file:M31F16.jpg|Figure 1]].

[[file:M31F18.jpg|thumb|500px|{{figure number|3}}Cores of distributary-mouth bar sequence. Diameter of cores is 13 cm (5 in.). A. Smooth, gray, partially laminated clays of the prodelta deposits. B. Steeply dipping sand-silt laminations characteristic of block slumping often found in the prodelta environment. C. Small lenticular laminations and graded parallel silt laminations common near the top of the prodelta environment. D. Alternating sand, silt, and silty clay laminations in the lower part of the distal bar environment. E. Well-developed parallel silt and sand laminations showing graded bedding and small-scale ripple laminations common in the distal bar deposits. F. Lenticular sand laminations representing "starved current ripples" and small-scale ripple laminations common in the transition zone between the distal bar and distributary-mouth bar. G. Slump structure common near the shear plane in a distributary-mouth bar sequence that has mass-moved seaward. H. Cross laminations common in the distributary-mouth bar sands. The dark material is transported organic debris. I. Large-scale cross laminations common near the top part of the distributary-mouth bar deposits. The dark material is transported organic debris. J. Alternating silty sand and clay layers common to the small overbank splays that cap the distributary-mouth bar deposits.<ref name=Colemanetal_1981 />]]

[[file:M31F18.jpg|thumb|500px|{{figure number|3}}Cores of distributary-mouth bar sequence. Diameter of cores is 13 cm (5 in.). A. Smooth, gray, partially laminated clays of the prodelta deposits. B. Steeply dipping sand-silt laminations characteristic of block slumping often found in the prodelta environment. C. Small lenticular laminations and graded parallel silt laminations common near the top of the prodelta environment. D. Alternating sand, silt, and silty clay laminations in the lower part of the distal bar environment. E. Well-developed parallel silt and sand laminations showing graded bedding and small-scale ripple laminations common in the distal bar deposits. F. Lenticular sand laminations representing "starved current ripples" and small-scale ripple laminations common in the transition zone between the distal bar and distributary-mouth bar. G. Slump structure common near the shear plane in a distributary-mouth bar sequence that has mass-moved seaward. H. Cross laminations common in the distributary-mouth bar sands. The dark material is transported organic debris. I. Large-scale cross laminations common near the top part of the distributary-mouth bar deposits. The dark material is transported organic debris. J. Alternating silty sand and clay layers common to the small overbank splays that cap the distributary-mouth bar deposits.<ref name=Colemanetal_1981 />]]

The upper left diagram of [[:file:M31F17.jpg|Figure 2]] illustrates schematically the process of a bifurcated channel. In the cross sections on this panel, the three major units illustrated include lower prodelta clays, overlain by alternating silt and sand units of the distal bar, and uppermost, distributary-mouth-bar sands. As the diagram shows, distributary-mouth-bar sands form a nearly continuous sand body extending laterally large distances.

The upper left diagram of [[:file:M31F17.jpg|Figure 2]] illustrates schematically the process of a bifurcated channel. In the cross sections on this panel, the three major units illustrated include lower prodelta clays, overlain by alternating silt and sand units of the distal bar, and uppermost, distributary-mouth-bar sands. As the diagram shows, distributary-mouth-bar sands form a nearly continuous sand body extending laterally large distances.

The upper right-hand diagram shows the most common vertical sequence within the distributary-mouth-bar deposits and some of its characteristics. The unit generally displays a coarsening-upward sequence in which depositional dips are extremely low, rarely exceeding 1° except in areas where slump deposits result in high angles within the mass-moved sediment. [[:file:M31F18.jpg|Figure 3A and B]] illustrates some of the characteristic structures found within the lowermost units of prodelta clays. Parallel, colored clay laminations, thin graded silt and silty clay parallel laminations, bioturbation (generally confined to the clay laminations), and slump structures are common within the prodelta deposits. Microfaunal remains generally indicate marine deposition, and diversity of species is generally quite high, indicating an open, inner to outer shelf depositional environment. These deposits display most of the characteristics of normal marine shelf deposits and are differentiated only by their rate of accumulation.

The upper right-hand diagram shows the most common vertical sequence within the distributary-mouth-bar deposits and some of its characteristics. The unit generally displays a coarsening-upward sequence in which depositional dips are extremely low, rarely exceeding 1° except in areas where slump deposits result in high angles within the mass-moved sediment. [[:file:M31F18.jpg|Figure 3A and B]] illustrates some of the characteristic structures found within the lowermost units of prodelta clays. Parallel, colored clay laminations, thin graded silt and silty clay parallel laminations, [[bioturbation]] (generally confined to the clay laminations), and slump structures are common within the prodelta deposits. Microfaunal remains generally indicate marine deposition, and diversity of species is generally quite high, indicating an open, inner to outer shelf depositional environment. These deposits display most of the characteristics of normal marine shelf deposits and are differentiated only by their rate of accumulation.

For a given length of time, prodelta deposits are thicker (because of high sedimentation) than an equivalent section of normal marine shelf deposits. Rapid deposition often results in slightly lower amounts of bioturbation and deposits displaying excess pore fluid pressures. Graded bedding zones are sometimes present in the prodelta deposits, but are rarely found in normal marine shelf deposits. X-ray radiography of the cores reveals that many of the parallel laminations are defined either by inclusions of diagenetic origin or extremely slight differences in textural characteristics. Because of the high rates of deposition normally associated with prodelta deposits, intense bioturbation is usually confined only to the lowermost parts of the deposit, where it grades downward into norma open-marine shelf environments, that often show intense bioturbation.

For a given length of time, prodelta deposits are thicker (because of high sedimentation) than an equivalent section of normal marine shelf deposits. Rapid deposition often results in slightly lower amounts of bioturbation and deposits displaying excess pore fluid pressures. Graded bedding zones are sometimes present in the prodelta deposits, but are rarely found in normal marine shelf deposits. X-ray radiography of the cores reveals that many of the parallel laminations are defined either by inclusions of diagenetic origin or extremely slight differences in textural characteristics. Because of the high rates of deposition normally associated with prodelta deposits, intense bioturbation is usually confined only to the lowermost parts of the deposit, where it grades downward into norma open-marine shelf environments, that often show intense bioturbation.

The distributary-mouth bar is the area of shoaling associated with the seaward terminus of a distributary-mouth channel. Shoaling is a direct consequence of a decrease in velocity and a reduction in carrying power of a stream as it leaves the confines of its channel. Accumulation rates are extremely high, probably higher than in any other environment associated with the delta. In some places depositional rates of coarse sediments at the mouth of the Mississippi reach 1 to 3 m per year.

The distributary-mouth bar is the area of shoaling associated with the seaward terminus of a distributary-mouth channel. Shoaling is a direct consequence of a decrease in velocity and a reduction in carrying power of a stream as it leaves the confines of its channel. Accumulation rates are extremely high, probably higher than in any other environment associated with the delta. In some places depositional rates of coarse sediments at the mouth of the Mississippi reach 1 to 3 m per year.

The sediments are constantly subjected to reworking, not only by stream currents but by waves generated in the open-marine waters beyond the channel mouth. A general understanding of the processes and mode of formation of the distributary-mouth bar is critical to understanding the evolution and vertical relationships illustrated in [[:file:M31F17.jpg|Figure 2]]. As the low-density, turbid, fresh river water flows out of the distributary mouth over denser saline marine waters, the lighter effluent waters expand and lose velocity. Coarser sediments (the sands) settle rapidly, both from suspension and bedload migration, and almost all of the sand is deposited within the vicinity of the distributary mouth.

The sediments are constantly subjected to reworking, not only by stream currents but by waves generated in the open-marine waters beyond the channel mouth. A general understanding of the processes and mode of formation of the distributary-mouth bar is critical to understanding the evolution and vertical relationships illustrated in [[:file:M31F17.jpg|Figure 2]]. As the low-density, turbid, fresh river water flows out of the distributary mouth over denser saline marine waters, the lighter effluent waters expand and lose velocity. Coarser sediments (the sands) settle rapidly, both from suspension and bedload [[hydrocarbon migration|migration]], and almost all of the sand is deposited within the vicinity of the distributary mouth.

Because of variations in turbulence at the river mouth and different process intensities between low river stage and high river stage, silts and clays will occasionally be deposited with sands in this environment. However, reworking by marine and riverine processes results in cleaning and [[Core_description#Maturity|sorting]] of the sediments. As a result, the distributary-mouth bar commonly consists of clean, well-sorted sand and thus is obviously a potential reservoir rock for hydrocarbons. The remaining finer grained suspended load carried by the river is distributed widely by the expanding river effluent and forms distal bar and prodelta environments.

Because of variations in turbulence at the river mouth and different process intensities between low river stage and high river stage, silts and clays will occasionally be deposited with sands in this environment. However, reworking by marine and riverine processes results in cleaning and [[Core_description#Maturity|sorting]] of the sediments. As a result, the distributary-mouth bar commonly consists of clean, well-sorted sand and thus is obviously a potential reservoir rock for hydrocarbons. The remaining finer grained suspended load carried by the river is distributed widely by the expanding river effluent and forms distal bar and prodelta environments.

[[: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: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.

The lower two diagrams on [[:file:M31F17.jpg|Figure 2]] represent a sand isopach map of a distributary-mouth-bar system in which individual distributary-mouth bars have merged, forming a delta-front sand body type. The distributary pattern is shown as solid dark lines. A boring through the distributary channel itself would show very erratic sand distribution in the distributary channel. Electric-log responses and their variations are shown in the lower right-hand diagram of [[:file:M31F17.jpg|Figure 2]]. Most borings show a coarsening-upward sequence, with the sand body varying in thickness, depending on the location of the core with reference to the distributary channels themselves. In general, the nearer the boring to the axis of the distributary, the sharper the base of the sand body, and gradational contacts become less well defined. Distally (away from the distributary-channel axis), the sequence displays a much greater tendency toward a large transition from distal bar to distributary-mouth bar.

The lower two diagrams on [[:file:M31F17.jpg|Figure 2]] represent a sand isopach map of a distributary-mouth-bar system in which individual distributary-mouth bars have merged, forming a delta-front sand body type. The distributary pattern is shown as solid dark lines. A boring through the distributary channel itself would show very erratic sand distribution in the distributary channel. Electric-log responses and their variations are shown in the lower right-hand diagram of [[:file:M31F17.jpg|Figure 2]]. Most borings show a coarsening-upward sequence, with the sand body varying in thickness, depending on the location of the core with reference to the distributary channels themselves. In general, the nearer the boring to the axis of the distributary, the sharper the base of the sand body, and gradational contacts become less well defined. Distally (away from the distributary-channel axis), the sequence displays a much greater tendency toward a large transition from distal bar to distributary-mouth bar.

Sand units are generally [[Core_description#Maturity|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 [[Core_description#Maturity|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.

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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 (Colombia), Orinoco (Venezuela), Surinam (Surinam), Amazon (Brazil), Yukon (Alaska), Niger (Nigeria), Nile (Egypt), and Hwang-Ho (China), subaqueous slumping and downslope mass movement of sediments are common processes. Instabilities and mass movement of sediment in these regions generally display the following characteristics: (a) instability occurs on very low angle slopes (generally less than 2°); (b) large quantities of sediment are transported from shallow water to deeper water offshore along well-defined mudflow gullies (debris flows) and in a variety of translational slumps. Although individual mudflow features vary in size and frequency, they generally possess a source area consisting of subsidence and rotational slumping, an elongate, often sinuous chute or channel (mudflow gully), and a composite depositional area composed of overlapping lobes of remolded debris.

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

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.

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.

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.

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.

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.

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.

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.

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.