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The upper delta plain lies above the level of effective saltwater intrusion and is unaffected by marine processes. Most of the sediments comprising this part of the delta plain originate from the migratory tendency of [[distributary channel]]s, [[overbank]] flooding during annual highwater periods, and periodic breaks in the river banks, in which "crevassing" into adjacent lake basins occurs. The major environments of deposition include braided channels, meandering channels (point bars and meander-belt deposits), lacustrine delta fill, backswamps, and flood plains (swamps, marshes, and freshwater lakes).

The upper delta plain lies above the level of effective saltwater intrusion and is unaffected by marine processes. Most of the sediments comprising this part of the delta plain originate from the migratory tendency of [[distributary channel]]s, [[overbank]] flooding during annual highwater periods, and periodic breaks in the river banks, in which "crevassing" into adjacent lake basins occurs. The major environments of deposition include braided channels, meandering channels (point bars and meander-belt deposits), lacustrine delta fill, backswamps, and flood plains (swamps, marshes, and freshwater lakes).

==Braided-channel deposits==

==Braided-channel deposits==

Braided channels are marked by successive divisions and rejoinings of the flow around [[alluvial]] islands. Most braided river channels are characterized by a dominant bedload transport of sediment, high variations in water discharge, high downstream gradients, and large width-depth ratio of channels. Most braided rivers display rapid and continuous shifting of sediment and position of channels. These channels are found in all climate zones, but, because of their dependence on erratic discharge and high bedload, they are most common in arid and arctic settings. Braiding characteristics of the channel often extend all the way into the delta plain. However, one of the largest braided channels in the world, the Brahmaputra River, has formed in a humid climatic setting. Lateral migration of c annels can be dramatic, as in the Brahmaputra River, where lateral migration rates of several thousand meters during a single flood are not uncommon.<ref name=Coleman_1969>Coleman, J. M., 1969, Brahmaputra River: channel processes and sedimentation: Sedimentary Geology, v. 3, p. 129-239.</ref> In the Rosi River (a tributary of the Ganges River), lateral migration of the channel over the past two centuries has been about 170 km; in a single year the channel may shift over 30 km laterally.<ref name=Reineckandsingh_1973>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> Because of high lateral migration rates and shallow depths of scour, most braided-channel deposits display high lateral continuity but are rather thin (rarely over 30 m thick).

Braided channels are marked by successive divisions and rejoinings of the flow around [[alluvial]] islands. Most braided river channels are characterized by a dominant bedload transport of sediment, high variations in water discharge, high downstream gradients, and large width-depth ratio of channels. Most braided rivers display rapid and continuous shifting of sediment and position of channels. These channels are found in all climate zones, but, because of their dependence on erratic discharge and high bedload, they are most common in arid and arctic settings. Braiding characteristics of the channel often extend all the way into the delta plain. However, one of the largest braided channels in the world, the Brahmaputra River, has formed in a humid climatic setting. [[Lateral]] migration of channels can be dramatic, as in the Brahmaputra River, where lateral migration rates of several thousand meters during a single flood are not uncommon.<ref name=Coleman_1969>Coleman, J. M., 1969, Brahmaputra River: channel processes and sedimentation: Sedimentary Geology, v. 3, p. 129-239.</ref> In the Rosi River (a tributary of the Ganges River), lateral migration of the channel over the past two centuries has been about 170 km; in a single year the channel may shift over 30 km laterally.<ref name=Reineckandsingh_1973>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> Because of high lateral migration rates and shallow depths of scour, most braided-channel deposits display high lateral continuity but are rather thin (rarely over 30 m thick).

[[file:M31F2.jpg|thumb|200px|{{figure number|1}}Photographs of bedding in a braided channel deposit. A. Large-scale cross-bedding in the lower part of a fining-upward cycle on a braided channel. B. Trough-shaped cross-bedding in lenticular sets that form the overlying zone in a fining-upward cycle of a braided channel. C. Ripple drift bedding separated by parallel sand laminations.<ref name=Colemanandprior_1981>Coleman, J. M., and D. B. Prior, 1981, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0100/0139.htm Deltaic environments of deposition], in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], p. 139-178.</ref>]]

[[file:M31F2.jpg|thumb|200px|{{figure number|1}}Photographs of bedding in a braided channel deposit. A. Large-scale [[cross-bedding]] in the lower part of a fining-upward cycle on a braided channel. B. Trough-shaped cross-bedding in lenticular sets that form the overlying zone in a fining-upward cycle of a braided channel. C. Ripple drift bedding separated by parallel sand laminations.<ref name=Colemanandprior_1981>Coleman, J. M., and D. B. Prior, 1981, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0100/0139.htm Deltaic environments of deposition], in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], p. 139-178.</ref>]]

Individual channels split around numerous mid-channel islands or braid bars. During floods erosion occurs on the upstream ends and lateral sides of bars and eroded material is added to the downstream side of the bar. Because each channel has a different depth, lateral migration of the channel results in scour to differing depths. Multiple fining-upward cycles (which are commonly truncated) occur within the resulting sand body. Each fining-upward cycle (owing to deposition of a laterally migrating channel) is characterized by a scoured base and overlying sets of large-scale cross-bedding in which individual sets are up to 1 m thick ([[:file:M31F2.jpg|Figure1A]]). This lower sequence of large-scale cross-bedding can attain thicknesses of up to 7 m. Small-scale ripple bedding, scour and fill structures, organic trash, and clay layers are occasionally found.

Individual channels split around numerous mid-channel islands or braid bars. During floods erosion occurs on the upstream ends and lateral sides of bars and eroded material is added to the downstream side of the bar. Because each channel has a different depth, lateral migration of the channel results in scour to differing depths. Multiple fining-upward cycles (which are commonly truncated) occur within the resulting sand body. Each fining-upward cycle (owing to deposition of a laterally migrating channel) is characterized by a scoured base and overlying sets of large-scale cross-bedding in which individual sets are up to 1 m thick ([[:file:M31F2.jpg|Figure1A]]). This lower sequence of large-scale cross-bedding can attain thicknesses of up to 7 m. Small-scale ripple bedding, scour and fill structures, organic trash, and clay layers are occasionally found.

Overlying this unit is a zone displaying finer grain size and composed of lenticular-shaped units of large-scale cross-bedding (trough type) intercalated with zones of climbing ripple laminations, horizontal laminations, and ripple cross-bedding ([[:file:M31F2.jpg|Figure 1B]]). In some places small-scale laminations are present within certain zones. The uppermost unit of the fining-upward cycle displays horizontal laminations separating well-defined, near-horizontal sets of steeply dipping ripple-drift bedding ([[:file:M31F2.jpg|Figure 1C]]). Small-scale convolute laminations and burrowed sand and silt layers are common in the uppermost part of the unit. However, scouring by later migration of the channel often removes this upper burrowed section.

Overlying this unit is a zone displaying finer [[grain size]] and composed of lenticular-shaped units of large-scale cross-bedding (trough type) intercalated with zones of climbing ripple laminations, horizontal laminations, and ripple cross-bedding ([[:file:M31F2.jpg|Figure 1B]]). In some places small-scale laminations are present within certain zones. The uppermost unit of the fining-upward cycle displays horizontal laminations separating well-defined, near-horizontal sets of steeply dipping ripple-drift bedding ([[:file:M31F2.jpg|Figure 1C]]). Small-scale convolute laminations and burrowed sand and silt layers are common in the uppermost part of the unit. However, scouring by later migration of the channel often removes this upper burrowed section.

[[file:M31F3.jpg|thumb|300px|{{figure number|2}}Summary diagrams illustrating the major characteristics of braided channel deposits (letters on the vertical section refer to core or outcrop photographs).<ref name=Colemanandprior_1981 />]]

[[file:M31F3.jpg|thumb|300px|{{figure number|2}}Summary diagrams illustrating the major characteristics of braided channel deposits (letters on the vertical section refer to core or outcrop photographs).<ref name=Colemanandprior_1981 />]]

[[:file:M31F3.jpg|Figure 2]] summarizes the major characteristics of braided-channel deposits, including lateral relationships (block diagram in upper left); typical vertical sequence (upper right), including grain size, directional properties, dip angles, relative porosity, and sedimentary structures; sand body isopach map (lower left); and representative electric logs (lower right) at selected sites to show variation in log shape. As this diagram illustrates, the typical vertical sequence is characterized by multiple stacked fining-upward cycles of deposition (each representing deposition by a migratory channel). Directional properties within each cycle often display narrow directional spread and are fairly representative of the long-axis orientation or downstream direction of the channel. High dip angles are most often associated with large-scale cross-bedding and distorted layers.

[[:file:M31F3.jpg|Figure 2]] summarizes the major characteristics of braided-channel deposits, including lateral relationships (block diagram in upper left); typical vertical sequence (upper right), including grain size, directional properties, [[dip]] angles, relative porosity, and sedimentary structures; sand body isopach map (lower left); and representative electric logs (lower right) at selected sites to show variation in log shape. As this diagram illustrates, the typical vertical sequence is characterized by multiple stacked fining-upward cycles of deposition (each representing deposition by a migratory channel). Directional properties within each cycle often display narrow directional spread and are fairly representative of the long-axis orientation or downstream direction of the channel. High dip angles are most often associated with large-scale cross-bedding and distorted layers.

The isopach map shows a laterally continuous sand body, often extending 20-50 km laterally in a direction perpendicular to the downslope channel direction. Most braided channels display rather uniform thicknesses across the entire sand body (averaging 15 to 25 m thick) and localized deeper sand-filled scoured pods. Log response often shows an overall blocky shape, with numerous sharp "kickouts" representing local coarse-sand-filled scours. Locally numerous fining-upward cycles can be well defined on the logs. Although distinctive cycles can often be discerned from log data, it is very likely that individual units cannot be carried laterally any great distance, and presence of the numerous thin silt and clay layers discourages thinking that reservoir continuity may extend for great dis ances. Exposures in some of the tar sands in Canada show a lack of reservoir continuity as tars are concentrated along distinct layers within the overall sand body. Potential for porosity traps is great in the braided-channel environment.

The isopach map shows a laterally continuous sand body, often extending 20-50 km laterally in a direction perpendicular to the downslope channel direction. Most braided channels display rather uniform thicknesses across the entire sand body (averaging 15 to 25 m thick) and localized deeper sand-filled scoured pods. Log response often shows an overall blocky shape, with numerous sharp "kickouts" representing local coarse-sand-filled scours. Locally numerous fining-upward cycles can be well defined on the logs. Although distinctive cycles can often be discerned from log data, it is very likely that individual units cannot be carried laterally any great distance, and presence of the numerous thin silt and clay layers discourages thinking that reservoir continuity may extend for great dis ances. Exposures in some of the tar sands in Canada show a lack of reservoir continuity as tars are concentrated along distinct layers within the overall sand body. Potential for porosity traps is great in the braided-channel environment.

A meandering river is one with a channel pattern that displays high sinuosity in plan view (sinuosity greater than 1.5). Meanders are most commonly associated with rivers displaying nonerratic flooding characteristics, high suspended-sediment load (generally fine-grained bedload), and low downslope gradient. Most meandering rivers occur in tropical and temperate climates, where suspended loads and high annual flooding are common, but they will also occur in arctic and arid climates.

A meandering river is one with a channel pattern that displays high sinuosity in plan view (sinuosity greater than 1.5). Meanders are most commonly associated with rivers displaying nonerratic flooding characteristics, high suspended-sediment load (generally fine-grained bedload), and low downslope gradient. Most meandering rivers occur in tropical and temperate climates, where suspended loads and high annual flooding are common, but they will also occur in arctic and arid climates.

Meandering rivers are most common in alluvial valleys of river systems, but in several modern river deltas meandering sections of the distributaries are present, especially in rivers where distributaries are influenced by high tidal ranges or in rivers carrying extremely large coarse bedload sediment.

Meandering rivers are most common in [[alluvial]] valleys of river systems, but in several modern river deltas meandering sections of the distributaries are present, especially in rivers where distributaries are influenced by high tidal ranges or in rivers carrying extremely large coarse bedload sediment.

The channel profile normally displays an asymmetric V shape, the steep cutting bank is referred to as the cut bank; the shallow-sloping depositing side is termed the point bar. Maximum flow velocities are found near the steep concave bank, while lower velocities are present along the point-bar side of the channel. Undercutting of the cut-bank side of the channel during a flood causes oversteepening and slumping of the channel wall. Locally, where the channel scours into sandy deposits, high flood levels (excessive hydraulic head) force river water into the buried sand body. A sudden drop in the river level results in excess pore-water pressure in the buried sand, and water and sand flow back into the river, causing slumping in the overlying bank deposits. In either case, the slumping results in an increase of the channel's cross-sectional area, thereby reducing velocity and causing deposition on the point-bar side of the channel.

The channel profile normally displays an asymmetric V shape, the steep cutting bank is referred to as the cut bank; the shallow-sloping depositing side is termed the point bar. Maximum flow velocities are found near the steep concave bank, while lower velocities are present along the point-bar side of the channel. Undercutting of the cut-bank side of the channel during a flood causes oversteepening and slumping of the channel wall. Locally, where the channel scours into sandy deposits, high flood levels (excessive hydraulic head) force river water into the buried sand body. A sudden drop in the river level results in excess pore-water pressure in the buried sand, and water and sand flow back into the river, causing slumping in the overlying bank deposits. In either case, the slumping results in an increase of the channel's cross-sectional area, thereby reducing velocity and causing deposition on the point-bar side of the channel.

The channel abandoned when a river switches its course to another site is most commonly filled with silty clays, organic clays, and organic trash. The clay-filled channel, therefore, results in numerous nearly isolated sand bodies in the overall meander belt providing innumerable stratigraphic trapping possibilities. This is illustrated in the upper left diagram of Figure 4. The vertical sequence in the meander point-bar sand body is illustrated in the upper right diagram of [[:file:M31F4.jpg|Figure 3]].

The channel abandoned when a river switches its course to another site is most commonly filled with silty clays, organic clays, and organic trash. The clay-filled channel, therefore, results in numerous nearly isolated sand bodies in the overall meander belt providing innumerable stratigraphic trapping possibilities. This is illustrated in the upper left diagram of Figure 4. The vertical sequence in the meander point-bar sand body is illustrated in the upper right diagram of [[:file:M31F4.jpg|Figure 3]].

Most commonly, the vertical sequence shows a fining-upward grain size relationship; a few coarser layers are found near the upper one-third of the sand body. The sand body has a scoured base, and often coarse, organic trash (logs, limbs and clay clasts) is found intercalated with the sandy units. Thin clay and silt layers often separate coarse sandy units. Above this basal unit is normally a massive, thick sand unit displaying large-scale cross-bedding with occasional contorted layers and thin laminations of organic debris. The large-scale bedforms migrate primarily during periods of high flood.

Most commonly, the vertical sequence shows a fining-upward grain size relationship; a few coarser layers are found near the upper one-third of the sand body. The sand body has a scoured base, and often coarse, organic trash (logs, limbs and clay clasts) is found intercalated with the sandy units. Thin clay and silt layers often separate coarse sandy units. Above this basal unit is normally a massive, thick sand unit displaying large-scale [[cross-bedding]] with occasional contorted layers and thin laminations of organic debris. The large-scale bedforms migrate primarily during periods of high flood.

[[file:M31F5.jpg|thumb|300px|{{figure number|4}}Photographs of bedding in a meander point bar. A. Cyclic flood deposits in a point bar. B. Small-scale cross-stratification and organic debris. C. Climbing ripple sequence capped by convolute laminations. D. Highly contorted bedding in point-bar deposits. E. Soil zones alternating with ripple laminations in upper part of point-bar deposits.<ref name=Colemanandprior_1981 />]]

[[file:M31F5.jpg|thumb|300px|{{figure number|4}}Photographs of bedding in a meander point bar. A. Cyclic flood deposits in a point bar. B. Small-scale cross-stratification and organic debris. C. Climbing ripple sequence capped by convolute laminations. D. Highly contorted bedding in point-bar deposits. E. Soil zones alternating with ripple laminations in upper part of point-bar deposits.<ref name=Colemanandprior_1981 />]]

Quiet-water deposition, reducing conditions, abundance of burrowing organisms (especially soft-bodied organisms such as polychaete worms), and occasional wave and current action are characteristic of this environment. Deposits within the lake bottoms consist of dark-gray to black organic-rich clays containing scattered silt lenses. In some lakes organic debris, such as large accumulations of freshwater shell, is present where overturning of the lake waters is a common process.

Quiet-water deposition, reducing conditions, abundance of burrowing organisms (especially soft-bodied organisms such as polychaete worms), and occasional wave and current action are characteristic of this environment. Deposits within the lake bottoms consist of dark-gray to black organic-rich clays containing scattered silt lenses. In some lakes organic debris, such as large accumulations of freshwater shell, is present where overturning of the lake waters is a common process.

Elsewhere near-anoxic conditions exist and deposits consist of extremely organic-rich, fine-grained clay. The most common types of stratification include parallel and lenticular laminations, intense bioturbation, and occasionally distorted primary structures.

Elsewhere near-anoxic conditions exist and deposits consist of extremely organic-rich, fine-grained clay. The most common types of stratification include parallel and lenticular laminations, intense [[bioturbation]], and occasionally distorted primary structures.

Although some of the parallel laminations result from alterations in textural properties, most are the product of alternating flocculated and nonflocculated layers. Within the flocculated layers there is commonly an abundance of small cracks and fractures. Most cracks are oriented perpendicular to bedding, but differing orientations are occasionally encountered. These are probably what has been termed syneresis cracks, having developed from expulsion of fluids when internal forces of attraction between particles are greater than internal forces of repulsion between solid phase particles. Microfaunal remains are usually abundant within lacustrine facies but consist of only a small number of ostracod species. Charophytes are encountered in some samples. Early diagenetic inclusions include vivianite, which is normally associated with drifted plant remains or burrow fills; and pyrite, which is extremely common and generally occurs as small cubes or isolated drusy masses.

Although some of the parallel laminations result from alterations in textural properties, most are the product of alternating flocculated and nonflocculated layers. Within the flocculated layers there is commonly an abundance of small cracks and fractures. Most cracks are oriented perpendicular to bedding, but differing orientations are occasionally encountered. These are probably what has been termed syneresis cracks, having developed from expulsion of fluids when internal forces of attraction between particles are greater than internal forces of repulsion between solid phase particles. Microfaunal remains are usually abundant within lacustrine facies but consist of only a small number of ostracod species. Charophytes are encountered in some samples. Early diagenetic inclusions include vivianite, which is normally associated with drifted plant remains or burrow fills; and pyrite, which is extremely common and generally occurs as small cubes or isolated drusy masses.

[[file:M31F7.jpg|thumb|300px|{{figure number|6}}Core photographs of bedding in lacustrine delta fill. core diameter is 13 cm (5 in.). A. Shell debris in lower portion of lacustrine deposits. B. Highly burrowed organic clays of lacustrine deposits. C. X-ray radiograph of core from the lower portions of the lacustrine delta fill showing alternating parallel lamination of silt and clay with abundant burrowing. D. Laminated silty clays in lower portion of lacustrine delta fill. Note the elliptical Fe2CO3 nodule. E. X-ray radiograph of clays in lower portion of lacustrine delta fill in which high sedimentation rates preclude burrowing organisms. F. Well-stratified silty and sandy deposits of the coarser sediments forming the bulk of the lacustrine delta fill. G. Parallel and lenticular laminations common in the upper portion of the delta fill. Quite often clay-filled burrows are common in the capping sequence over lacustrine delta fill. Note extremely large root burrow. H. High organic backswamp clays that accumulate in a poorly drained reducing swamp environment. Organic stringers and peat deposits are common in this environment. I. X-ray radiograph of core taken in backswamp deposit. Note the stringers of organic debris (dark layer) and the early formation of siderite nodules (Fe2CO3). Pyrite is abundant in this setting. J. Silty clays with abundant iron oxide and calcium carbonate nodules that form in well-drained oxidizing swamp environment.<ref name=Colemanandprior_1981 />]]

[[file:M31F7.jpg|thumb|300px|{{figure number|6}}Core photographs of bedding in lacustrine delta fill. core diameter is 13 cm (5 in.). A. Shell debris in lower portion of lacustrine deposits. B. Highly burrowed organic clays of lacustrine deposits. C. X-ray radiograph of core from the lower portions of the lacustrine delta fill showing alternating parallel lamination of silt and clay with abundant burrowing. D. Laminated silty clays in lower portion of lacustrine delta fill. Note the elliptical Fe2CO3 nodule. E. X-ray radiograph of clays in lower portion of lacustrine delta fill in which high sedimentation rates preclude burrowing organisms. F. Well-stratified silty and sandy deposits of the coarser sediments forming the bulk of the lacustrine delta fill. G. Parallel and lenticular laminations common in the upper portion of the delta fill. Quite often clay-filled burrows are common in the capping sequence over lacustrine delta fill. Note extremely large root burrow. H. High organic backswamp clays that accumulate in a poorly drained reducing swamp environment. Organic stringers and peat deposits are common in this environment. I. X-ray radiograph of core taken in backswamp deposit. Note the stringers of organic debris (dark layer) and the early formation of siderite nodules (Fe2CO3). Pyrite is abundant in this setting. J. Silty clays with abundant iron oxide and calcium carbonate nodules that form in well-drained oxidizing swamp environment.<ref name=Colemanandprior_1981 />]]

[[:file:M31F6.jpg|Figure 5]] illustrates the major characteristics of the lacustrine delta-fill sequence. The upper left diagram is a schematic representation of a small distributary that has been diverted into a shallow freshwater lake. The schematic shows lateral relationships of the various facies, indicating that the bulk of the delta-fill forms a wedge of coarse clastics within an overall deposit consisting of fine-grained organic-rich clays from lacustrine and backswamp deposits. The upper right diagram of Figure 6 illustrates a typical vertical sequence, which normally displays a coarsening-upward trend. The lowermost units consist of lacustrine deposits. Quite often these lowermost units of the lacustrine delta consist of large accumulations or biomasses of shell ([[:file:M31F7.jpg|Figure 6A]]). Normally the shell deb is is encased in a matrix of fine-grained organic clays containing abundant pyrite inclusions. As the sedimentation rate within the delta increases, there is generally a decrease in the amount of coarse faunal remains. Organic activity, however, does not normally cease, and burrowing and abundant bioturbation of soft-bodied organisms can totally obliterate the primary structures ([[:file:M31F7.jpg|Figure 6B]]).

[[:file:M31F6.jpg|Figure 5]] illustrates the major characteristics of the lacustrine delta-fill sequence. The upper left diagram is a schematic representation of a small distributary that has been diverted into a shallow freshwater lake. The schematic shows lateral relationships of the various facies, indicating that the bulk of the delta-fill forms a wedge of coarse clastics within an overall deposit consisting of fine-grained organic-rich clays from lacustrine and backswamp deposits. The upper right diagram of Figure 6 illustrates a typical vertical sequence, which normally displays a coarsening-upward trend. The lowermost units consist of lacustrine deposits. Quite often these lowermost units of the lacustrine delta consist of large accumulations or biomasses of shell ([[:file:M31F7.jpg|Figure 6A]]). Normally the shell deb is is encased in a matrix of fine-grained organic clays containing abundant pyrite inclusions. As the sedimentation rate within the delta increases, there is generally a decrease in the amount of coarse faunal remains. Organic activity, however, does not normally cease, and burrowing and abundant [[bioturbation]] of soft-bodied organisms can totally obliterate the primary structures ([[:file:M31F7.jpg|Figure 6B]]).

In most fill deposits, the burrows are sand filled due to the physiological processes of polychaete worms, which burrow through the muds, concentrating sands within their bodies and leaving behind essentially a sand- or silt-filled burrow. As the sedimentation rate continues to increase, laminations of thicker silt and sandy silt layers alternate with thinly laminated organic clays ([[:file:M31F7.jpg|Figure 6C]]). Burrowing persists upward within the deposits, but not to the point of masking primary stratification. Quite often the parallel-laminated clay deposits consist of extremely organic-rich clays, and leaf remains are common along the bedding planes. Color laminations and inclusions of iron carbonate nodules are often common within this part of the delta fill ([[:file:M31F7.jpg|Figure 6D]]).

In most fill deposits, the burrows are sand filled due to the physiological processes of polychaete worms, which burrow through the muds, concentrating sands within their bodies and leaving behind essentially a sand- or silt-filled burrow. As the sedimentation rate continues to increase, laminations of thicker silt and sandy silt layers alternate with thinly laminated organic clays ([[:file:M31F7.jpg|Figure 6C]]). Burrowing persists upward within the deposits, but not to the point of masking primary stratification. Quite often the parallel-laminated clay deposits consist of extremely organic-rich clays, and leaf remains are common along the bedding planes. Color laminations and inclusions of iron carbonate nodules are often common within this part of the delta fill ([[:file:M31F7.jpg|Figure 6D]]).