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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 [[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.

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==

[[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.

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 [[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.

[[: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.