Lower Delta Plain

The lower delta plain lies between the low-tide mark and the upper limit of the tidal influence. It is typically a brackish to saline environment and is the scene of more active deposition, mostly by crevassing or overbanking from the deltaic distributaries, modified by the input of marine sediments from storms. Crevassing is more common than the occurrence of natural levees because the levees are generally more poorly developed. Vegetation is necessarily salt tolerant and is confined to those plants that can withstand prolonged inundation. In arid settings, the lower delta plain can be a zone of salt flats or active dune fields. (Carter and Woodroffe)

On the lower delta plain, distributaries are more influenced by marine processes, such as tidal fluctuations and salt-water wedge intrusion. Abandoned distributary channels may develop into estuaries and be filled by marginal marine deposits. (Carter and Woodroffe)

Many ancient coal deposits are considered to have formed in deltaic settings. Peats can form throughout the upper and lower delta plain. Brackish and salt marshes on the lower delta plain tend to produce organic-rich muds or high-sulfur peats due to admixture with salt water and frequent clastic influx. Climatic conditions are also important. (Carter and Woodroffe)

The lower delta plain is also known as the delta front.
 
 

Distributaries are small, shifting channels that carry water away from the main river channel and distribute it over the surface of the delta. The distributary channel has sand and silt beds with high mica content. Sometimes very thin beds of shale are present. Thick pockets of transported organics are sometimes present. The beds are abundantly cross-bedded, with plenty of ripple cross-lamination, scour-and-fill structures and discontinuous clay lenses. They may show the effects of both wave and current ripples. The beds generally grade from poorly sorted near the base to well sorted at the top. The sand layer could expand the thickness rapidly by growth faulting. The depth of the channel decreases rapidly as the river mouth bar is approached and water depth over most distributary mouth bars, that occur when the coarsest material is deposited at the mouth of the distributary, rarely exceeds 3 m. There is little migration of distributary channels.
 
  Interdistributary bays are low, marshy mudflats cut by small narrow channels of slow-moving water. They are composed of shales and thin laminations of highly burrowed sand-silt layers. Crossbeds of alternating sandy silts and silty clays are common. A high mica content is also common. Faunal remains are rare, but there may be freshwater pelecypods and gastropods. Ostracods may be present in thin clay zones. Organic debris is located along bedding planes, where thick layers of peat and coal may form. Pyrite is usually abundant and replaces plant debris through pyritization. Scattered shell fragments are often present. Most interdistributary bays are shallow water bodies, possibly with brackish or marine waters. They rarely exceed 7 or 8 m depth and average at approximately 4 m. These bays are commonly elongate is shape with the longest dimension ranging in size from a few hundred meters to about 15 to 20 km. They initially form as a break in the major distributary channel during a flood stage. They gradually increase in flow through successive floods, reach a peak of maximum deposition, wane, and then become inactive. The crevasse system is then inundated by marine waters as a result of this subsidence. It reverts back to a bay environment, thus completing the sedimentary cycle.
 
 

A natural levee is an accumulation of alluvium bordering a stream channel, deposited by the river during times of flood. Giant ripple bedded, small ripple bedded, and laminated and interbedded sands, silts, and clays have been observed. Of these features, the laminated and interbedded sands, silts, and clays are most common. This sequence coarsens upwards and the clastics in the upper levee region are well sorted. The natural levee deposits, which flank the channel, consist of interbedded, laminated, small ripple bedded, and contorted sand, silt, and clay. These may be oxidized and disturbed since they are exposed to weathering during low water stages. At the base, brackish water clays and organic debris are present.
 
 

Crevasse splays are miniature versions of the delta, fanning out from a breach in a levee and forming their own thin sets of channels within the interdistributary bay mudstones. They coarsen upward with sequences of shales to sand (which may be repeated numerous times), but on a thinner, smaller scale and also become finer grained away from the breached levee until they grade laterally into the interdistributary bay-fill muds. The sands are usually poorly sorted. The splay channels are commonly filled with ripple laminations, indicating very rapid sedimentation in the waning flood currents that occur when the water released by the breached levee has nearly drained.

The crevasse splays form during periods of high run off and diverts a significant volume of water and sediment from the main delta stream. The splay forms when sediment is deposited as the water spreads out. They build into shallow bays between or adjacent to major distributaries and extend themselves. In 100 to 150 years, the thickness of a splay can range from 3 to 15 m.
 
 

Although abandonment is a complex process, it is often the result of an accident, such as logjams, loss of gradient advantage, infilling during an environmentally catastrophic event (such as a hurricane), or changes upstream. This will cause the channel to deteriorate and infill. Once the blockage occurs, only local sediments derived from both upstream and downstream will infill the abandoned hole in the ground. The lower regions of the channel are commonly filled with poorly sorted sands and silts containing an abundance of transported organic debris. As the channel is filled, the water becomes more stagnant. The velocities of the currents are maintained at this point. After this, the channel will begin to fill with fine-grained materials. With time, the channel completely fills entirely with fine-grained, poorly sorted sediments. These regions become marshes or swamps. The upper region of the channel is filled with organic debris, logs, and clays with high water content. Occasionally, thin seams of shell debris and silt-infilled animal burrows may be present. Deltaic regions with low tides and high suspended sediment load have no process available to infill the channels with sand or other coarse debris.