Changes

The distributary-mouth bar is the area of shoaling associated with the seaward terminus of a distributary-mouth channel. Shoaling is a direct consequence of a decrease in velocity and a reduction in carrying power of a stream as it leaves the confines of its channel. Accumulation rates are extremely high, probably higher than in any other environment associated with the delta. In some places depositional rates of coarse sediments at the mouth of the Mississippi reach 1 to 3 m per year.

The distributary-mouth bar is the area of shoaling associated with the seaward terminus of a distributary-mouth channel. Shoaling is a direct consequence of a decrease in velocity and a reduction in carrying power of a stream as it leaves the confines of its channel. Accumulation rates are extremely high, probably higher than in any other environment associated with the delta. In some places depositional rates of coarse sediments at the mouth of the Mississippi reach 1 to 3 m per year.

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 [[hydrocarbon migration|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 [[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.

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.