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The lower delta plain lies within the realm of river-marine interaction and extends landward from the shoreline to the limit of tidal influence. Large areal extent of the lower delta plain is common where tidal range is large and seaward gradients of the river channel and delta are low. Most commonly in this environment, channels become more numerous and often show a [[bifurcating]] or [[anastomosing]] type of plan view pattern. Environments between [[distributary channels]] comprise the largest percentage of the lower delta plain and consist of actively migrating [[tidal channel]]s, [[overbank splay]]s ([[natural levees]]), [[interdistributary bay]]s, [[bay fill]]s ([[crevasse splay]]s), [[marsh]]es, and [[swamp]]s. Deltas having an extremely high tidal range and an arid climate are often characterized by interdistributary evaporite or barren [[salt flat]]s, where intricate networks of [[tidal creek]]s occur and are separated by broad evaporative sequences. From the standpoint of sand-body formation, the major environmental sequence consists of bay-fill deposits, which often form thin clastic wedges stacked one on top of another and separated by interdistributary bay and marsh deposits. The major environmental sequences described consist of bay-fill deposits (interdistributary bay, crevasse splay-natural levee, and marsh) and abandoned distributary-fill deposits.

The lower delta plain lies within the realm of river-marine interaction and extends landward from the shoreline to the limit of tidal influence. Large areal extent of the lower delta plain is common where tidal range is large and seaward gradients of the river channel and delta are low. Most commonly in this environment, channels become more numerous and often show a [[bifurcation|bifurcating]] or [[anastomosing]] type of plan view pattern. Environments between [[distributary channels]] comprise the largest percentage of the lower delta plain and consist of actively migrating [[tidal channel]]s, [[overbank splay]]s ([[natural levees]]), [[interdistributary bay]]s, [[bay fill]]s ([[crevasse splay]]s), [[marsh]]es, and [[swamp]]s. Deltas having an extremely high tidal range and an arid climate are often characterized by interdistributary [[evaporite]] or barren [[salt flat]]s, where intricate networks of [[tidal creek]]s occur and are separated by broad evaporative sequences. From the standpoint of sand-body formation, the major environmental sequence consists of bay-fill deposits, which often form thin clastic wedges stacked one on top of another and separated by interdistributary bay and marsh deposits. The major environmental sequences described consist of bay-fill deposits (interdistributary bay, crevasse splay-natural levee, and marsh) and abandoned distributary-fill deposits.

==Bay-fill deposits==

==Bay-fill deposits==

By 1978 the entire region had been inundated by marine waters, and the West Bay complex had reverted to a shallow-marine interdistributary environment. Given time, another crevasse will eventually form on the bank of the Mississippi and another period of progradation will ensue, again filling the interdistributary bay with detrital sediments. It is this process of repeated filling, alternating with periods of marsh destruction, that forms the bulk of cyclic deposits in the lower delta plain.

By 1978 the entire region had been inundated by marine waters, and the West Bay complex had reverted to a shallow-marine interdistributary environment. Given time, another crevasse will eventually form on the bank of the Mississippi and another period of progradation will ensue, again filling the interdistributary bay with detrital sediments. It is this process of repeated filling, alternating with periods of marsh destruction, that forms the bulk of cyclic deposits in the lower delta plain.

[[file:M31F10v2.jpg|thumb|550px|{{figure number|3}}Core photographs showing stratification in bay-fill deposits. Cores are 13 cm (5 in.) in diameter. A. Burrowed and laminated silts and clays of the interdistributary bay environment. B. Ripple laminations and burrowed zones in silts and silty clays in the lower part of the bay fill sequence. C. Well-laminated silts and silty sands of the crevasse infilling. The sandy layers often display small climbing ripple sequences. D. Cross-laminated silts and sands of the crevasse infilling sequence. E. Well-sorted and cross-laminated sand layers alternating with silts and silty clays associated with the lower part of the distributary-mouth bar of the prograding distributaries. F. Cross-stratified sands and silty sands in the lower part of the distributary-mouth bar (bay fill sequence). G. Small-scale cross-stratified sands of the distributary-mouth bar (bay fill sequence). H. Well-sorted cross-stratified sands of the upper part of the distributary-mouth bar (bay fill sequence). I. Alternating silts and silty clays of the overbank splays that cap the bay fill sequence. J. High organic clays that form in the marsh environments and cap the bay fill sequence.<ref name=Colemanetal_1981 />]]

[[file:M31F10v2.jpg|thumb|550px|{{figure number|3}}Core photographs showing stratification in bay-fill deposits. Cores are 13 cm (5 in.) in diameter. A. Burrowed and laminated silts and clays of the interdistributary bay environment. B. Ripple laminations and burrowed zones in silts and silty clays in the lower part of the bay fill sequence. C. Well-laminated silts and silty sands of the crevasse infilling. The sandy layers often display small climbing ripple sequences. D. Cross-laminated silts and sands of the crevasse infilling sequence. E. [[Core_description#Maturity|Well-sorted]] and cross-laminated sand layers alternating with silts and silty clays associated with the lower part of the distributary-mouth bar of the prograding distributaries. F. Cross-stratified sands and silty sands in the lower part of the distributary-mouth bar (bay fill sequence). G. Small-scale cross-stratified sands of the distributary-mouth bar (bay fill sequence). H. Well-sorted cross-stratified sands of the upper part of the distributary-mouth bar (bay fill sequence). I. Alternating silts and silty clays of the overbank splays that cap the bay fill sequence. J. High organic clays that form in the marsh environments and cap the bay fill sequence.<ref name=Colemanetal_1981 />]]

The upper right-hand diagram in [[:file:M31F9.jpg|Figure 2]] illustrates the typical vertical sequence resulting from bay infilling. As can be seen, it is a coarsening-upward sequence, with shallow brackish water clays and organic debris forming the lower part and well-sorted clastics forming the upper sand body. The upper unit is essentially distributary-mouth bar deposits associated with the prograding distributary. The lowermost part of the bay fill generally consists of alternating silts and silty clays, with the clays often showing silt- and sand-infilled burrows ([[:file:M31F10v2.jpg|Figure 3A]]).

The upper right-hand diagram in [[:file:M31F9.jpg|Figure 2]] illustrates the typical vertical sequence resulting from bay infilling. As can be seen, it is a coarsening-upward sequence, with shallow brackish water clays and organic debris forming the lower part and well-sorted clastics forming the upper sand body. The upper unit is essentially distributary-mouth bar deposits associated with the prograding distributary. The lowermost part of the bay fill generally consists of alternating silts and silty clays, with the clays often showing silt- and sand-infilled burrows ([[:file:M31F10v2.jpg|Figure 3A]]).

With continued and increasing sedimentation in the interdistributary bay, coarser particles and more rapid deposition occur. Then silty and sandy stringers begin to intercalate with silty clays. Burrowing is generally reduced ([[:file:M31F10v2.jpg|Figure 3B]]). As the distributary system advances farther into the bay, delivery of coarser grained clastics begins at the site of the vertical section, and often sands and silty sands alternate with thin silty clay laminations. Graded bedding and some small-scale climbing ripple structures are the most common types of lamination ([[:file:M31F10v2.jpg|Figure 3C]]). As the distributary mouth progrades closer to the site of the vertical section, small-scale cross-bedding and occasionally organic trash within the sandy deposits become the most common types of stratification ([[:file:M31F10v2.jpg|Figure 3D]]).

With continued and increasing sedimentation in the interdistributary bay, coarser particles and more rapid deposition occur. Then silty and sandy stringers begin to intercalate with silty clays. Burrowing is generally reduced ([[:file:M31F10v2.jpg|Figure 3B]]). As the distributary system advances farther into the bay, delivery of coarser grained clastics begins at the site of the vertical section, and often sands and silty sands alternate with thin silty clay laminations. Graded bedding and some small-scale climbing ripple structures are the most common types of lamination ([[:file:M31F10v2.jpg|Figure 3C]]). As the distributary mouth progrades closer to the site of the vertical section, small-scale [[cross-bedding]] and occasionally organic trash within the sandy deposits become the most common types of stratification ([[:file:M31F10v2.jpg|Figure 3D]]).

The lower part of the distributary-mouth bar itself is often characterized by cross-bedded sands alternating with sandy silts and silty clays. In general little or no scouring is evident in this part of the bay-fill sequence ([[:file:M31F10v2.jpg|Figure 3E, F]]). The bulk of the sand deposit associated with the advancing distributary-mouth bar is composed of [[cross-stratified sand]]s and sandy silts, displaying a wide variety of climbing ripples and small-scale festoon-type cross-bedding ([[:file:M31F10v2.jpg|Figure 3G, H]]). A high [[mica]] content and transported organic debris along bedding planes are common in this part of the vertical sequence.

The lower part of the distributary-mouth bar itself is often characterized by cross-bedded sands alternating with sandy silts and silty clays. In general little or no scouring is evident in this part of the bay-fill sequence ([[:file:M31F10v2.jpg|Figure 3E, F]]). The bulk of the sand deposit associated with the advancing distributary-mouth bar is composed of [[cross-stratified sand]]s and sandy silts, displaying a wide variety of climbing ripples and small-scale festoon-type cross-bedding ([[:file:M31F10v2.jpg|Figure 3G, H]]). A high [[mica]] content and transported organic debris along bedding planes are common in this part of the vertical sequence.

[[file:M31F13.jpg|thumb|300px|{{figure number|6}}Summary diagram illustrating the major characteristics of the abandoned distributary deposits in the lower delta plain.<ref name=Colemanetal_1981 />]]

[[file:M31F13.jpg|thumb|300px|{{figure number|6}}Summary diagram illustrating the major characteristics of the abandoned distributary deposits in the lower delta plain.<ref name=Colemanetal_1981 />]]

The distributary channel is the natural flume which accommodates and directs a part of the water and sediment discharged from the parent river system to the receiving basin. In most deltas, the distributary channels are rather stable and do not display a tendency toward lateral migration, thereby preventing the formation of [[point-bar deposit|point-bar]] or [[meander-belt deposit]]s. In some deltas, for example, with high bedload streams or in those environmental settings where tidal range is high, migration of the distributary channel can take place, resulting in formation of deposits similar to channel deposits described in the [[delta plain, upper|upper delta plain]].

The [[distributary channel]] is the natural flume which accommodates and directs a part of the water and sediment discharged from the parent river system to the receiving basin. In most deltas, the distributary channels are rather stable and do not display a tendency toward [[lateral]] migration, thereby preventing the formation of [[point-bar deposit|point-bar]] or [[meander-belt deposit]]s. In some deltas, for example, with high bedload streams or in those environmental settings where tidal range is high, migration of the distributary channel can take place, resulting in formation of deposits similar to channel deposits described in the [[delta plain, upper|upper delta plain]].

Although little research has been conducted, the lack of channel migration in the lower delta plain is undoubtedly due to the fact that most river channels scour down through their distributary-mouth-bar deposits into underlying [[marine clay]]s. This scouring provides an entrenchment of the distributary channel with minimal tendencies for lateral migration. Active distributary channels vary considerably in size, some only a few meters wide and 1 to 2 m deep, and others of a large major river delta system with channels reaching 1 km in width and 30 m in depth. Depth within the channel decreases rapidly as the river-mouth bar is approached, and water depths over most distributary-mouth bars rarely exceed 3 m.

Although little research has been conducted, the lack of channel migration in the lower delta plain is undoubtedly due to the fact that most river channels scour down through their distributary-mouth-bar deposits into underlying [[marine clay]]s. This scouring provides an entrenchment of the distributary channel with minimal tendencies for lateral migration. Active distributary channels vary considerably in size, some only a few meters wide and 1 to 2 m deep, and others of a large major river delta system with channels reaching 1 km in width and 30 m in depth. Depth within the channel decreases rapidly as the river-mouth bar is approached, and water depths over most distributary-mouth bars rarely exceed 3 m.

Abandonment of a distributary channel is an extremely complex process, and in many areas is simply an accident. Log jams, loss of gradient advantage, infilling during a catastrophic period (such as a hurricane), or changes in the upstream part of the river will cause the channel to deteriorate and infill. Deprived of an active influx of sediment and water, the channel will then undergo an infilling process in which only local sediments derived from both upstream and downstream will infill the abandoned hole in the ground. The lower parts of the channel are commonly filled with poorly sorted sands and silts containing an abundance of transported organic debris. As a channel shoals, the water becomes more stagnant, and lower current velocities are maintained; soon, fine-grained material begin to infill the channel proper. With time and continued subsidence the channel often fills entirely with fine-grained, poorly sorted sediments. Organic debris, logs, and clays with extremely high water content often form the upper part of the channel fill. Thus in many deltaic regions of low tides and high suspended sediment load, no process is available to infill the channels with sand or other coarse debris.

Abandonment of a distributary channel is an extremely complex process, and in many areas is simply an accident. Log jams, loss of gradient advantage, infilling during a catastrophic period (such as a hurricane), or changes in the upstream part of the river will cause the channel to deteriorate and infill. Deprived of an active influx of sediment and water, the channel will then undergo an infilling process in which only local sediments derived from both upstream and downstream will infill the abandoned hole in the ground. The lower parts of the channel are commonly filled with [[Core_description#Maturity|poorly sorted]] sands and silts containing an abundance of transported organic debris. As a channel shoals, the water becomes more stagnant, and lower current velocities are maintained; soon, fine-grained material begin to infill the channel proper. With time and continued subsidence the channel often fills entirely with fine-grained, poorly sorted sediments. Organic debris, logs, and clays with extremely high water content often form the upper part of the channel fill. Thus in many deltaic regions of low tides and high suspended sediment load, no process is available to infill the channels with sand or other coarse debris.

[[file:M31F14v2.jpg|thumb|300px|{{figure number|7}}Core photographs showing stratification in an abandoned distributary fill deposit. Diameter of cores is 13 cm (5 in.). A. Clay clasts in a sand matrix found near the base of the channel. B. Highly contorted and disrupted bedding in sandy matrix in channel-fill deposits. C. Irregular bedding and burrowed structures in silty clay layers of a channel-fill deposit. D. Contorted bedding and flow rolls in sands and sandy silt deposits of the channel fill. E. High organic clays forming the uppermost part of the channel-fill deposits. <ref name=Colemanetal_1981 />]]

[[file:M31F14v2.jpg|thumb|300px|{{figure number|7}}Core photographs showing stratification in an abandoned distributary fill deposit. Diameter of cores is 13 cm (5 in.). A. Clay clasts in a sand matrix found near the base of the channel. B. Highly contorted and disrupted bedding in sandy matrix in channel-fill deposits. C. Irregular bedding and burrowed structures in silty clay layers of a channel-fill deposit. D. Contorted bedding and flow rolls in sands and sandy silt deposits of the channel fill. E. High organic clays forming the uppermost part of the channel-fill deposits. <ref name=Colemanetal_1981 />]]

Near the base of the infilled channel, often erratic and contorted clay layers are concentrated as clast within the sand body ([[:file:M31F14v2.jpg|Figure 7A]]). Within the deposited sands themselves, contorted structures are very common. Slump-type structures, distorted bedding, clay infills, and occasionally fairly persistent layers of organic trash sandwiched between sand layers are also common. [[:file:M31F14v2.jpg|Figure 7B]] illustrates some of the contorted bedding probably associated with localized slumps within a channel-fill deposit. Silts and silty clays that are deposited in the central part of the channel fill often display thin silty and sandy layers that intercalate with highly burrowed clays. Most commonly the sandy laminations display extremely sharp upper and lower surfaces ([[:file:M31F14v2.jpg|Figure 7C]]). Convolute laminations and other types of distorted bedding such as flow rolls, ball and pillow structures, etc. ([[:file:M31F14v2.jpg|Figure 7D]]), are common within the sand bodies. The uppermost part of the fill consists primarily of organic-rich clays ([[:file:M31F14v2.jpg|Figure 7E]]) that generally show intense root burrowing. Occasionally, thin seams of shell debris and silt-infilled animal burrows can be detected.

Near the base of the infilled channel, often erratic and contorted clay layers are concentrated as clast within the sand body ([[:file:M31F14v2.jpg|Figure 7A]]). Within the deposited sands themselves, contorted structures are very common. Slump-type structures, distorted bedding, clay infills, and occasionally fairly persistent layers of organic trash sandwiched between sand layers are also common. [[:file:M31F14v2.jpg|Figure 7B]] illustrates some of the contorted bedding probably associated with localized slumps within a channel-fill deposit. Silts and silty clays that are deposited in the central part of the channel fill often display thin silty and sandy layers that intercalate with highly burrowed clays. Most commonly the sandy laminations display extremely sharp upper and lower surfaces ([[:file:M31F14v2.jpg|Figure 7C]]). Convolute laminations and other types of distorted bedding such as flow rolls, ball and pillow structures, etc. ([[:file:M31F14v2.jpg|Figure 7D]]), are common within the sand bodies. The uppermost part of the fill consists primarily of organic-rich clays ([[:file:M31F14v2.jpg|Figure 7E]]) that generally show intense root burrowing. Occasionally, thin seams of shell debris and silt-infilled animal burrows can be detected.

Although grain size has a general tendency to show a fining-upward sequence within any one sandy unit, in some deposits there is virtually no change in grain size. Dip angles can also be extremely erratic, resulting primarily from the large number of disturbed and distorted structures found within the channel-fill deposits. Undoubtedly, the rapidity of the infilling process and extremely high porewater content of the clays and silts are responsible for the large amount of distorted bedding.

Although [[grain size]] has a general tendency to show a fining-upward sequence within any one sandy unit, in some deposits there is virtually no change in grain size. [[Dip]] angles can also be extremely erratic, resulting primarily from the large number of disturbed and distorted structures found within the channel-fill deposits. Undoubtedly, the rapidity of the infilling process and extremely high porewater content of the clays and silts are responsible for the large amount of distorted bedding.

[[:file:M31F15.jpg|Figure 8]] is a continuously cored boring through an old distributary channel that existed off Southwest Pass, Mississippi River delta, in the early 1800s. The channel on old maps was approximately 9 to 10 m deep and about 100 to 110 m wide. The base of the channel in the cored boring is identified by the letter A and occurs at a depth below mean sea level of 10.5 m. The clays directly below this scour plane, where burrowing is present, contain marine microfaunal remains. Just above the scour surface is a sand approximately 1 m thick. It contains fairly well sorted fine- to medium-grained sands displaying small-scale current structures. Organic trash and other debris are common along the bedding planes.

[[:file:M31F15.jpg|Figure 8]] is a continuously cored boring through an old distributary channel that existed off Southwest Pass, Mississippi River delta, in the early 1800s. The channel on old maps was approximately 9 to 10 m deep and about 100 to 110 m wide. The base of the channel in the cored boring is identified by the letter A and occurs at a depth below mean sea level of 10.5 m. The clays directly below this scour plane, where burrowing is present, contain marine microfaunal remains. Just above the scour surface is a sand approximately 1 m thick. It contains fairly [[Core_description#Maturity|well-sorted]] fine- to medium-grained sands displaying small-scale current structures. Organic trash and other debris are common along the bedding planes.

Above this unit are a series of alternating sands, silty clays, and clays displaying a large amount of contorted bedding, load casting, and small-scale slumping ([[:file:M31F15.jpg|Figure 8]]). The central part of the fill consists of silty clays alternating with thin silt laminations. Although not especially well displayed in this cored boring, one of the more characteristic features of the laminations within abandoned channels is the extremely sharp nature of both the upper and lower bounding planes of silt and sand laminations within the overall fine-grained channel fill. Graded bedding is rarely observed. The uppermost section of the fill in this distributary channel consists of fine-grained sirs and silty clays containing large amounts of organic debris along the bedding planes. Distorted laminations within the clays and silty clays are apparent (uppermost cores, [[:file:M31F15.jpg|Figure 8]]). The uppermost meter in the abandoned channel in [[:file:M31F15.jpg|Figure 8]] consisted primarily of organic-rich clays and peats, which, because of their extremely high water content, were unable to be preserved and adequately cored and thus are not illustrated in this boring.

Above this unit are a series of alternating sands, silty clays, and clays displaying a large amount of contorted bedding, load casting, and small-scale slumping ([[:file:M31F15.jpg|Figure 8]]). The central part of the fill consists of silty clays alternating with thin silt laminations. Although not especially well displayed in this cored boring, one of the more characteristic features of the laminations within abandoned channels is the extremely sharp nature of both the upper and lower bounding planes of silt and sand laminations within the overall fine-grained channel fill. Graded bedding is rarely observed. The uppermost section of the fill in this distributary channel consists of fine-grained sirs and silty clays containing large amounts of organic debris along the bedding planes. Distorted laminations within the clays and silty clays are apparent (uppermost cores, [[:file:M31F15.jpg|Figure 8]]). The uppermost meter in the abandoned channel in [[:file:M31F15.jpg|Figure 8]] consisted primarily of organic-rich clays and peats, which, because of their extremely high water content, were unable to be preserved and adequately cored and thus are not illustrated in this boring.