Upper Delta Plain

The upper most portion of the delta sequence is the delta plain, which is a combination of distributary channels and interchannel environments. Examples of interchannel environments include bays, lagoons, estuaries, flood plains, lakes, tidal flats, and creeks. The delta plain is further divided into two areas: the upper delta plain and the lower delta plain. Although these two areas are comprised of different components, both are dominated by fluvial processes. The upper delta plain is located above the high-tide sea level and therefore, has no marine interaction.

In the upper delta plain deposition comes from both within channels and between channels. There are four main sources of sediment deposition: braided channels, meandering channels, lacustrine delta-fill, and floodplain deposits. These sources are described in detail below. Input from any of these sources is dependent upon the rate of discharge and capacity of the sediment load. As shown below, the shape of a delta is determined by the amount of sediment being carried by the river and the activity of both the waves and tides of the sea.

Braided channel patterns form when any flowing body of water has insufficient discharge to carry its load or has easily erodible banks. Braided fluvial systems are found in the upper reaches of alluvial plains, close to the sediment source. They tend to have steep gradients, lots of coarse sediment (mostly sand and gravel), and rapid discharge fluctuations. At times of small sediment discharge, braided channels are wide and shallow. However, at times of large sediment discharge, braided channels experience sediment overload. Braided channels tend to migrate and interweave with one another, thus creating a network of channels. Between the channels, sediment that is not eroded builds up and forms longitudinal bars, which run basically parallel to the flow direction.

The geometry of braided channels is elongate and fairly straight lengthwise. There is a fining-upward sequence of channel lag gravels, sands (with abundant crossbedding), and vertical accreted and laminated sand and mud. Contacts within the sequence package are gradational. Since braided channels migrate, sequence packages can overlap. This overlapping causes a sharp contact between each package. Fossils found in braided channels are usually only root casts and burrows on vegetated sand flats. Possibly but rarely found are pelecypod and gastropod shells, and ostracodes.


 
 

Meandering channels form in the lower reaches of the upper delta plain. They tend to have a gradient less steep than braided channels. Due to the relatively flat topography, coarser material and sediment is not transported. Meandering channels usually begin from braided channels that are basically straight but over time and distance from sediment source, become more sinuous until finally it is a meandering channel. Meandering channels are snake-like in shape with many bends. They tend to migrate continuously. It is common for the channels to spill over or cut new paths during floods. Resulting features include oxbow lakes and crevasse splays.

There is a fining upward sequence in a meandering channel. The basal unit is channel lag gravel, then the sandy point bar sequence of plane beds, then trough cross-beds, and finally ripple drifts. Similar to braided channels, meandering channels have gradational contacts within the sequence package and sharp contacts between different packages. Characteristic sedimentary structures found in the point bar sands are plane beds, trough cross-beds, and ripple cross-lamination. In terms of fossils, organic matter and fossil wood are common, especially in the floodplain.
 
 

A lake is a landlocked body of standing, nonmarine water. These lacustrine deposits make up an important environment in upper delta plains. They form when there is a depression between the distributary channels and water collects in these depressions. Compaction occurs and the depression subsides over time on the upper delta plain. This combination of compaction and subsidence creates a lake. The size of a lacustrine deposit depends on distributary input, internal drainage, and outflow. Depths usually do not exceed 15 to 20 meters. They are circular or elongate in geometry. Their cross section is lenticular.

There is a coarsening upward sequence of laminated shales, marls, and limestones, then rippled and cross-bedded sandstones, then finally conglomerates. This sequence produces a gradational contact within itself. Lamination of the sequence denotes the alternation of stagnation and overturning of the lake. Sediments of the lacustrine deposit include finely laminated mudstones, marls, and limestones. Along the edges of the deposit, fluvial muds and sands are found displaying cross-beds, wave ripples, and climbing ripple drift. In the depositional basin, turbidite sequences and black shales form. Fossil remains distinguish these lacustrine deposits from marine deposits. Commonly found in these lakes are nonmarine ostracods, diatoms, molluscs, and freshwater fish and insects. Marine invertebrates are not found in this environment.
 
 

A floodplain is the low-lying, generally flat area adjacent to a channel. The size of the floodplain is dependent upon the size of its channel. Small channels have narrow floodplains, whereas large channels have broad floodplains. Floodplains may be laterally or horizontally accreted. Lateral accretion occurs when the deposits build out laterally as a result of inner bank sediment deposition. Vertical accretion occurs as a result of channel overflow and the deposition of sediment along the outside of the channel. Floodwaters breaching the channel may carry silt- and clay-sized sediment and mud onto the floodplain. These floodplain muds are uniformly laminated as they settle out of suspension. Floodplains are typically marshy and can build up humus, soil horizons, and other vegetation.