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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==

Overlying the large-scale cross-bedding is a [[Core_description#Maturity|well-sorted]] sand sequence composed of repeated cyclic sedimentation units of climbing ripples, convolute laminations, and parallel well-sorted sand laminations ([[:file:M31F5.jpg|Figure 4A]]). Each of the cyclic units represents deposition during a single flood and a change from lower flow to upper flow regime. Such units range in thickness from 0.5 to over 1.5 m. In some areas the flood deposit is poorly sorted and consists primarily of silts and sands displaying small-scale ripple laminations ([[:file:M31F5.jpg|Figure 4B]]).

Overlying the large-scale cross-bedding is a [[Core_description#Maturity|well-sorted]] sand sequence composed of repeated cyclic sedimentation units of climbing ripples, convolute laminations, and parallel well-sorted sand laminations ([[:file:M31F5.jpg|Figure 4A]]). Each of the cyclic units represents deposition during a single flood and a change from lower flow to upper flow regime. Such units range in thickness from 0.5 to over 1.5 m. In some areas the flood deposit is poorly sorted and consists primarily of silts and sands displaying small-scale ripple laminations ([[:file:M31F5.jpg|Figure 4B]]).

During times of extremely high flooding, rapid deposition often takes place on the point bar because of river currents cutting across the meander loop, attempting to shorten its course. In those instances, thick sequences of rapidly deposited, well-developed climbing ripples capped by contorted laminations are common ([[:file:M31F5.jpg|Figure 4C]]). Deposition of 1 to 2 m can take place over only a few hours to a day. Most commonly the distorted bedding simply caps the sand unit, but the entirebed can be rendered into a fluid state and massive contortions can be present ([[:file:M31F5.jpg|Figure 4D]]). The zone of maximum contorted bedding often closely approximate the low-water river level and extends upward into the zone covered by flood waters. The uppermost unit of the meander point bar often consists of silt and sand beds (few centimeters thick) displaying ripple laminations alternating with silty-clay and clay beds exhibiting root burrows ([[:file:M31F5.jpg|Figure 4E]]). In these uppermost units iron oxide and calcium carbonate nodules usually are abundant.

During times of extremely high flooding, rapid deposition often takes place on the point bar because of river currents cutting across the meander loop, attempting to shorten its course. In those instances, thick sequences of rapidly deposited, well-developed climbing ripples capped by contorted laminations are common ([[:file:M31F5.jpg|Figure 4C]]). Deposition of 1 to 2 m can take place over only a few hours to a day. Most commonly the distorted bedding simply caps the sand unit, but the entire bed can be rendered into a fluid state and massive contortions can be present ([[:file:M31F5.jpg|Figure 4D]]). The zone of maximum contorted bedding often closely approximate the low-water river level and extends upward into the zone covered by flood waters. The uppermost unit of the meander point bar often consists of silt and sand beds (few centimeters thick) displaying ripple laminations alternating with silty-clay and clay beds exhibiting root burrows ([[:file:M31F5.jpg|Figure 4E]]). In these uppermost units iron oxide and calcium carbonate nodules usually are abundant.

Sand bodies display wide variations in current directional properties, primarily because of the sinuous nature of the channel and its migrational variations. This is especially true in units consisting of large-scale cross-bedding. The upper climbing-ripple part of the deposit shows much less scatter in directional properties. Dip angles are generally low and primarily reflect cross-bedding, except in the upper contorted bedding unit, where erratic dips of 15 to 20° are common.

Sand bodies display wide variations in current directional properties, primarily because of the sinuous nature of the channel and its migrational variations. This is especially true in units consisting of large-scale cross-bedding. The upper climbing-ripple part of the deposit shows much less scatter in directional properties. Dip angles are generally low and primarily reflect cross-bedding, except in the upper contorted bedding unit, where erratic dips of 15 to 20° are common.

With continued but slow accumulation, the deposit eventually fills to a level where the surface of the swamp is exposed to oxidizing conditions for several months during the year. Thus, oxidation begins to remove the organic content from the sediment. Movement of groundwater fluids through the sediments results in formations of numerous small, rounded pellets or iron oxide and calcium carbonate, commonly encrusting the root burrows. The clays within this unit often display a slight red coloration, illustrated in [[:file:M31F7.jpg|Figure 6]].

With continued but slow accumulation, the deposit eventually fills to a level where the surface of the swamp is exposed to oxidizing conditions for several months during the year. Thus, oxidation begins to remove the organic content from the sediment. Movement of groundwater fluids through the sediments results in formations of numerous small, rounded pellets or iron oxide and calcium carbonate, commonly encrusting the root burrows. The clays within this unit often display a slight red coloration, illustrated in [[:file:M31F7.jpg|Figure 6]].

The lower two diagrams in Figure 6 illustrate a typical isopach map of a small lake delta infill and variations in log response through such a deposit. The isopach tends to indicate a wedge of coarse clastic sediments sandwiched between lower freshwater organic lacustrine deposits and overlying swamp and marsh deposits. Most often the logs will document the coarsening-upward sequence of this delta fill. Within the sand body, dip angles are often quite high, reaching 10 to 15°, and result primarily from the foresets of the rapidly prograding lacustrine delta. Often, resistivity kicks are extremely common within such a setting and are responses to lignite, coal, and iron-rich seams which form within these essentially red bed deposits. The sand body itself normally displays a graded base; however, in some areas, generally near breaks in the riverbank, thick, sharp-based sands can often accumulate immediately within the region of the actual crevassing.

The lower two diagrams in Figure 6 illustrate a typical isopach map of a small lake delta infill and variations in log response through such a deposit. The isopach tends to indicate a wedge of coarse clastic sediments sandwiched between lower freshwater organic lacustrine deposits and overlying swamp and marsh deposits. Most often the logs will document the coarsening-upward sequence of this delta fill. Within the sand body, dip angles are often quite high, reaching 10 to 15°, and result primarily from the [[foreset]]s of the rapidly [[prograde|prograding]] lacustrine delta. Often, resistivity kicks are extremely common within such a setting and are responses to [[lignite]], [[coal]], and iron-rich seams which form within these essentially red bed deposits. The sand body itself normally displays a graded base; however, in some areas, generally near breaks in the riverbank, thick, sharp-based sands can often accumulate immediately within the region of the actual crevassing.

==Further reading==

==Further reading==