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Line 19: − M31F10v2.jpg|{{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 /> − M31F11.jpg|{{figure number|4}}Cored boring through the West Bay fill sequence. Boring is located near the distal end of the bay infill. Diameter of core is 8 cm (3 in.).<ref name=Colemanetal_1981 /> − M31F12.jpg|{{figure number|5}}Cored boring through the West Bay bay fill sequence. Boring is located in central part of the bay-fill sequence. Diameter of core is 8 cm (3 in.).<ref name=Colemanetal_1981 /> Line 37:
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→Bay-fill deposits
M31F8.jpg|{{figure number|1}}High-altitude air photograph of a relatively large bay-fill environment in the lower delta plain of the Mississippi River delta.<ref name=Colemanetal_1981>Coleman, J. M., and D. B. Prior, 1981, Deltaic environments of deposition ''in'' P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: AAPG Memoir 31, p. 139-178.</ref>
M31F8.jpg|{{figure number|1}}High-altitude air photograph of a relatively large bay-fill environment in the lower delta plain of the Mississippi River delta.<ref name=Colemanetal_1981>Coleman, J. M., and D. B. Prior, 1981, Deltaic environments of deposition ''in'' P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: AAPG Memoir 31, p. 139-178.</ref>
M31F9.jpg|{{figure number|2}}Summary diagram illustrating the major characteristics of the bay-fill deposits in the lower delta plain.<ref name=Colemanetal_1981 />
M31F9.jpg|{{figure number|2}}Summary diagram illustrating the major characteristics of the bay-fill deposits in the lower delta plain.<ref name=Colemanetal_1981 />
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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|300px|{{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 />]]
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]]).
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.
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M31F11.jpg|{{figure number|4}}Cored boring through the West Bay fill sequence. Boring is located near the distal end of the bay infill. Diameter of core is 8 cm (3 in.).<ref name=Colemanetal_1981 />
M31F12.jpg|{{figure number|5}}Cored boring through the West Bay bay fill sequence. Boring is located in central part of the bay-fill sequence. Diameter of core is 8 cm (3 in.).<ref name=Colemanetal_1981 />
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[[: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.