Changes

==Bay-fill deposits==

==Bay-fill deposits==

One of the major facies associated with many deltas is the large areal extent of bay fills or crevasses that break off of main distributaries and infill the numerous interdistributary bays within the lower delta plain. These sequences form the major land areas in the lower delta plain. Crevasse deposits build into shallow bays between or adjacent to major distributaries and extend themselves seaward through a system of radial bifurcating channels similar in plan to the veins of a leaf.

One of the major facies associated with many deltas is the large areal extent of bay fills or crevasses that break off of main distributaries and infill the numerous interdistributary bays within the lower delta plain. These sequences form the major land areas in the lower delta plain. Crevasse deposits build into shallow bays between or adjacent to major distributaries and extend themselves seaward through a system of radial bifurcating channels similar in plan to the veins of a leaf.

The interdistributary bays into which the crevasses prograde are normally open bodies of water, often completely surrounded by marsh or distributary channels. More often, however, they partially open to the sea or connect to it by small tidal channels. Most of the bays are shallow-water bodies, rarely exceeding 7 to 8 m in depth and averaging approximately 4 m, containing brackish to marine waters. The bays are commonly elongate, with their longest dimension ranging in size from a few hundred meters to approximately 15 to 20 km. The process of crevassing or infilling has been recorded historically in many cases.<ref name=Colemanandgagliano_1964>Coleman, J. M., and S. M. Gagliano, 1964, Cyclic sedimentation in the Mississippi River deltaic plain: Gulf Coast Assoc. Geol. Socs. Trans., v. 14, p. 67-80.</ref> Each bay fill forms initially as a break in the major distributary channel during flood stage, gradually increases in flow through successive floods, reaches a peak of maximum deposition, wanes, and becomes inactive. As a result of subsidence, the crevasse system is inundated by marine waters, reverting to a bay environment, and thus completing a sedimentary cycle.

The interdistributary bays into which the crevasses prograde are normally open bodies of water, often completely surrounded by marsh or distributary channels. More often, however, they partially open to the sea or connect to it by small tidal channels. Most of the bays are shallow-water bodies, rarely exceeding 7 to 8 m in depth and averaging approximately 4 m, containing brackish to marine waters. The bays are commonly elongate, with their longest dimension ranging in size from a few hundred meters to approximately 15 to 20 km. The process of crevassing or infilling has been recorded historically in many cases.<ref name=Colemanandgagliano_1964>Coleman, J. M., and S. M. Gagliano, 1964, Cyclic sedimentation in the Mississippi River deltaic plain: Gulf Coast Assoc. Geol. Socs. Trans., v. 14, p. 67-80.</ref> Each bay fill forms initially as a break in the major distributary channel during flood stage, gradually increases in flow through successive floods, reaches a peak of maximum deposition, wanes, and becomes inactive. As a result of subsidence, the crevasse system is inundated by marine waters, reverting to a bay environment, and thus completing a sedimentary cycle.

[[:file:M31F8.jpg|Figure 1]] shows a high-altitude photograph of a relatively large bay-fill environment in the lower delta plain of the Mississippi River delta. This particular crevasse, Cubits Gap Crevasse, occurred in 1862, when a break in the levee diverted some 6 to 8% of the Mississippi River water through the crevasse opening.<ref name=Welder_1959>Welder, F. A., 1959, Processes of deltaic sedimentation in the lower Mississippi River: Tech. Rept. 12, Coastal Studies Inst., Louisiana State Univ., Baton Rouge, 90 p.</ref> By 1870 only a small number of low shoals were visible on the maps. By 1903 essentially the entire bay area had been infilled with crevasse-splay or bay-fill sediments. Since that time, subsidence and deterioration have resulted because of a lack of clastic sedimentation, and presently only the major distributary patterns are obvious on high-altitude photographs.

[[:file:M31F8.jpg|Figure 1]] shows a high-altitude photograph of a relatively large bay-fill environment in the lower delta plain of the Mississippi River delta. This particular crevasse, Cubits Gap Crevasse, occurred in 1862, when a break in the levee diverted some 6 to 8% of the Mississippi River water through the crevasse opening.<ref name=Welder_1959>Welder, F. A., 1959, Processes of deltaic sedimentation in the lower Mississippi River: Tech. Rept. 12, Coastal Studies Inst., Louisiana State Univ., Baton Rouge, 90 p.</ref> By 1870 only a small number of low shoals were visible on the maps. By 1903 essentially the entire bay area had been infilled with crevasse-splay or bay-fill sediments. Since that time, subsidence and deterioration have resulted because of a lack of clastic sedimentation, and presently only the major distributary patterns are obvious on high-altitude photographs.

Most of the former land surface has now reverted to extremely shallow-water bays, and in approximately 30 to 40 years the entire landmass will have subsided below sea level and the environment will have reverted to an interdistributary bay. The upper left diagram of Figure 9 illustrates, by a series of historic maps, development of the West Bay fill. The break in the major Mississippi channel occurred in 1839, and maps from that time show water depths of 7 to 10 m in the adjacent bay. After the initial levee break, coarse sediment was dumped subaqueously in the vicinity of the break, and no new sub-aerial land developed. However, with continued deposition and a general shoaling in the bay near the break, a bifurcating channel pattern and infilling of the bay sequence developed rapidly This stage of development is illustrated by the map in 1875; most of the channels were still actively prograding, and nearly the entire bay had been filled with sediments or a regressive sequence of delta deposits.

Most of the former land surface has now reverted to extremely shallow-water bays, and in approximately 30 to 40 years the entire landmass will have subsided below sea level and the environment will have reverted to an interdistributary bay. The upper left diagram of Figure 9 illustrates, by a series of historic maps, development of the West Bay fill. The break in the major Mississippi channel occurred in 1839, and maps from that time show water depths of 7 to 10 m in the adjacent bay. After the initial levee break, coarse sediment was dumped subaqueously in the vicinity of the break, and no new sub-aerial land developed. However, with continued deposition and a general shoaling in the bay near the break, a bifurcating channel pattern and infilling of the bay sequence developed rapidly This stage of development is illustrated by the map in 1875; most of the channels were still actively prograding, and nearly the entire bay had been filled with sediments or a regressive sequence of delta deposits.

The map from 1922 shows that many of the channels had been abandoned. They had prograded far enough seaward to lose their gradient advantage, and only a few of the major channels continued to deliver sediments to the bay. Much of the newly exposed land had been converted into luxuriant marsh growth, and organic-rich clays were capping the top of the regressive sequence. With time, plant growth could no longer maintain its productivity because of encroaching marine waters, and slowly the marsh began to break into numerous small lakes and bays. Wind-generated waves in the shallow bays, coupled with subsidence, began to destroy the marsh surface, and by 1958 much of the original land buildout had subsided below sea level, the area reverting to a bay.

The map from 1922 shows that many of the channels had been abandoned. They had prograded far enough seaward to lose their gradient advantage, and only a few of the major channels continued to deliver sediments to the bay. Much of the newly exposed land had been converted into luxuriant marsh growth, and organic-rich clays were capping the top of the regressive sequence. With time, plant growth could no longer maintain its productivity because of encroaching marine waters, and slowly the marsh began to break into numerous small lakes and bays. Wind-generated waves in the shallow bays, coupled with subsidence, began to destroy the marsh surface, and by 1958 much of the original land buildout had subsided below sea level, the area reverting to a bay.

Once the distributary-mouth bar has prograded across the site of the vertical section, it is capped by small over-bank crevasse splays, which often display alternating silt, sand, and clay units, the silts and sands including well-developed small-scale [[ripple lamination]]s ([[:file:M31F10v2.jpg|Figure 3I]]). In some instances, scour into the underlying deposits is apparent; however, the scour planes are of extremely low angle. The uppermost unit capping the bay-fill sequence is essentially an interdistributary bay or a marsh deposit. Interdistributary bays normally consist of highly burrowed silts and silty clays, whereas marsh deposits generally display a sequence of highly burrowed organic clays ([[:file:M31F10v2.jpg|Figure 3J]]). In some deposits iron carbonate or siderite nodules are common, and in most cases pyrite is abundant, replacing plant debris.

Once the distributary-mouth bar has prograded across the site of the vertical section, it is capped by small over-bank crevasse splays, which often display alternating silt, sand, and clay units, the silts and sands including well-developed small-scale [[ripple lamination]]s ([[:file:M31F10v2.jpg|Figure 3I]]). In some instances, scour into the underlying deposits is apparent; however, the scour planes are of extremely low angle. The uppermost unit capping the bay-fill sequence is essentially an interdistributary bay or a marsh deposit. Interdistributary bays normally consist of highly burrowed silts and silty clays, whereas marsh deposits generally display a sequence of highly burrowed organic clays ([[:file:M31F10v2.jpg|Figure 3J]]). In some deposits iron carbonate or siderite nodules are common, and in most cases pyrite is abundant, replacing plant debris.

The lower two diagrams ([[:file:M31F9.jpg|Figure 2]]) illustrate an [[isopach map]] of a bay-fill sequence and variations in log response that can occur within such a sand body. The isopached sand body generally displays a fan-shaped wedge, with the thickest sands generally being found near the initial break in the distributary channel. Often, sands in this vicinity display a sharp base scoured into the underlying interdistributary bay and marsh deposits. Away from the initial break, however, the typical coarsening-upward sequence (or inverted bell-shaped logs) becomes the most common type of log response. Within the overall sand body there are areas where sands have not accumulated to any great thickness, and therefore zones (bore hole 2) in which virtually no sand can be found and the entire sequence consists of interdistributary-bay silts and clays, grading upward to marsh deposits.

The lower two diagrams ([[:file:M31F9.jpg|Figure 2]]) illustrate an [[isopach map]] of a bay-fill sequence and variations in log response that can occur within such a sand body. The isopached sand body generally displays a fan-shaped wedge, with the thickest sands generally being found near the initial break in the distributary channel. Often, sands in this vicinity display a sharp base scoured into the underlying interdistributary bay and marsh deposits. Away from the initial break, however, the typical coarsening-upward sequence (or inverted bell-shaped logs) becomes the most common type of log response. Within the overall sand body there are areas where sands have not accumulated to any great thickness, and therefore zones (bore hole 2) in which virtually no sand can be found and the entire sequence consists of interdistributary-bay silts and clays, grading upward to marsh deposits.

[[:file:M31F11.jpg|Figure 4]] and [[:file:M31F12.jpg|Figure 5]] illustrate a continuous core through the West Bay complex. Location of the boring shown in [[:file:M31F11.jpg|Figure 4]] is toward the outer fringes of the sand body, and the distributary-mouth-bar sands become much thicker and are more apparent in the cored borings. Comparison of these continuous cored borings with the vertical profile in [[:file:M31F9.jpg|Figure 2]] indicates that stratification from the lower part of the bay fill to the upper part changes from parallel laminated and burrowed clays at the base to a higher and higher percentage of those structures associated with current flow near the top of the infilled sequence. Because of position within the overall bay fill, however, thickness of the sand body can vary considerably.

[[:file:M31F11.jpg|Figure 4]] and [[:file:M31F12.jpg|Figure 5]] illustrate a continuous core through the West Bay complex. Location of the boring shown in [[:file:M31F11.jpg|Figure 4]] is toward the outer fringes of the sand body, and the distributary-mouth-bar sands become much thicker and are more apparent in the cored borings. Comparison of these continuous cored borings with the vertical profile in [[:file:M31F9.jpg|Figure 2]] indicates that stratification from the lower part of the bay fill to the upper part changes from parallel laminated and burrowed clays at the base to a higher and higher percentage of those structures associated with current flow near the top of the infilled sequence. Because of position within the overall bay fill, however, thickness of the sand body can vary considerably.