Changes

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 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 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.

The most common sedimentary structure consists of a variety of small-scale cross laminae and current ripple drift types ([[:file:M31F18.jpg|Figure 3G-I]]). Quite often mass-movement processes such as small localized slumps result in distorted laminations. This is particularly true near lower sections of the distributary-mouth-bar sequence, where over-pressured sediments are much more common. [[:file:M31F18.jpg|Figure 3G]] illustrates one type of slump structure commonly seen. Near the top of the distributary-mouth bar large accumulations of river-transported organic debris are often present. Water-saturated logs and other organic debris are transported down the rivers in times of flood and discharged into the nearshore zone, where wave action grinds down the coarser wood particles into large concentrations of organic debris. [[:file:M31F18.jpg|Figure 3I]] is a core showing some of the organic laminations present within the upper part of the sand body.

The most common sedimentary structure consists of a variety of small-scale cross laminae and current ripple drift types ([[:file:M31F18.jpg|Figure 3G-I]]). Quite often mass-movement processes such as small localized slumps result in distorted laminations. This is particularly true near lower sections of the distributary-mouth-bar sequence, where over-pressured sediments are much more common. [[:file:M31F18.jpg|Figure 3G]] illustrates one type of slump structure commonly seen. Near the top of the distributary-mouth bar large accumulations of river-transported organic debris are often present. Water-saturated logs and other organic debris are transported down the rivers in times of flood and discharged into the nearshore zone, where wave action grinds down the coarser wood particles into large concentrations of organic debris. [[:file:M31F18.jpg|Figure 3I]] is a core showing some of the organic laminations present within the upper part of the sand body.

Although little coring has been done on these types of deposits, [[:file:M31F22.jpg|Figure 7]] is an attempt to summarize data presently available. The upper left diagram illustrates the distribution of some of the tidal ridges seaward of the mouth of the Shatt-el-Arab River delta, which flows into the Persian Gulf. Lengths of the tidal ridges at the river mouth range from 5 to 15 km, with some of the larger ridges displaying widths of 2 km. General spacing of the ridges across the distributary-mouth-bar area ranges from a minimum of about 2 km to slightly over 5 km. The upper righthand diagram shows a typical vertical sequence resulting from river-mouth progradation and lateral migration of ridges. In general, the coarsening upward sequence displayed agrees quite well with data presented from the lower Colorado delta<ref name=Meckel_1975 /> and Ord River delta.<ref name=Coleman_1976 />

Although little coring has been done on these types of deposits, [[:file:M31F22.jpg|Figure 7]] is an attempt to summarize data presently available. The upper left diagram illustrates the distribution of some of the tidal ridges seaward of the mouth of the Shatt-el-Arab River delta, which flows into the Persian Gulf. Lengths of the tidal ridges at the river mouth range from 5 to 15 km, with some of the larger ridges displaying widths of 2 km. General spacing of the ridges across the distributary-mouth-bar area ranges from a minimum of about 2 km to slightly over 5 km. The upper righthand diagram shows a typical vertical sequence resulting from river-mouth progradation and lateral migration of ridges. In general, the coarsening upward sequence displayed agrees quite well with data presented from the lower Colorado delta<ref name=Meckel_1975 /> and Ord River delta.<ref name=Coleman_1976 />

Sand units are generally well sorted and display a variety of small-scale and large-scale cross-stratifications. One of the more common sedimentary structures within sand bodies is the small-scale bidirectional or herring-bone stratification type. Shell debris is generally common, both scattered throughout sand deposits and concentrated into thin lag-type deposits. Parallel sand layers are common throughout the entire sequence of sandy deposits and probably result from deposition during the upper flow regime, especially during low tide, when water depths across the shoals are quite low and velocities are quite high.

Sand units are generally [[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.

In an upstream direction, depicted by core hole 8, it is highly probable that the base of the sand represents a scoured surface, and thus the sand body displays an extremely sharp base. In drop cores and samples from interridge-ridge areas, sediments tend to be much more poorly sorted. Clay clasts, organic trash, and shell lags are common. It is probable that electric-log response would show extremely erratic and ragged types of patterns as indicated in bore holes 4 and 6.

In an upstream direction, depicted by core hole 8, it is highly probable that the base of the sand represents a scoured surface, and thus the sand body displays an extremely sharp base. In drop cores and samples from interridge-ridge areas, sediments tend to be much more poorly [[Core_description#Maturity|sorted]]. Clay clasts, organic trash, and shell lags are common. It is probable that electric-log response would show extremely erratic and ragged types of patterns as indicated in bore holes 4 and 6.

Although data are sparse from this type of environmental setting, the writers believe such sand bodies to be indeed common in ancient rock sequences, and until a larger number of cored borings and cores on and through these river-mouth tidal ridges are obtained, data presented in [[:file:M31F22.jpg|Figure 7]] remains somewhat speculative. However, observations from literature cited above indicate that the general nature of the deposits is as illustrated in the vertical sequence of [[:file:M31F22.jpg|Figure 7]].

Although data are sparse from this type of environmental setting, the writers believe such sand bodies to be indeed common in ancient rock sequences, and until a larger number of cored borings and cores on and through these river-mouth tidal ridges are obtained, data presented in [[:file:M31F22.jpg|Figure 7]] remains somewhat speculative. However, observations from literature cited above indicate that the general nature of the deposits is as illustrated in the vertical sequence of [[:file:M31F22.jpg|Figure 7]].