ALLUVIAL PLAIN ENVIRONMENTS

The classification, description, and distribution of the alluvial plain and other depositional environments, such as deltaic, interdeltaic, marine, etc. of the northern Gulf coastal plains have been discussed in considerable detail by R. J. LeBlanc (1950). The alluvial plains of the Gulf Coast rivers consist of two principal environments: (a) stream meander belt, and (b) floodbasin or backswamp.

A stream meander belt is the zone within which a stream constantly shifts its course by undercutting one bank and depositing sediment on the opposite bank. Most streams develop and abandon several courses or meander belts, which form the highest topographic features of alluvial valleys.

A flood basin is the topographically low area between meander belts or between a meander belt and the valley wall. These areas are overflow basins for slack river water during flood stages. A typical alluvial valley is characterized by several meander belts and intervening flood basins (see Figure 2).

River meander belt deposits comprise the coarser "trains" of alluvial sediments since the river constantly reworks and winnows its own deposits. The coarser materials are repeatedly transported for short distances down stream and are redeposited. Most of the finer-grained sediments are transported further distances downstream and to the adjacent backswamps during high water or flood stages. Flood basins are the main depositional sites of the finer grained river alluvium.

Meander Belt Environments

The meander belt consists of several distinct subenvironments: (a) point bar, (b) natural levee, (c) abandoned channel, and (d) cutoff meander or oxbow lake

Point bar environment. The point bar* environment (Figures 3, 4, and 5) includes the depositional areas encircled by the outer bank of a river meander. It is the site of most active deposition within the alluvial valley

Cross profiles of meander loop channels are asymmetrical. The thalweg (axis of greatest channel depth) occurs near the outer bank, which is steep and subject to much undercutting and slumping. On the cross-channel profile, the river bed rises gently from the thalweg towards the inner bank. The gentle slope of the inner bank is depositional in contrast to the under cut outer bank which is erosional (see Figure 3)

River crossings are channel areas between successive meanders where the main current crosses the channel from the undercut bank of one meander to the undercut bank of the next downstream meander. The river at the crossings is considerably shallower than near the concave bank of a meander. Its cross channel profile is somewhat u-shaped.

Migration of the channel of a meander loop results from scouring and undercutting of the outer bank and deposition on the inner bank, and produces a series of arcuate accretionary bar ridges and swales (sloughs, lakes, and chutes) within the point bar area. These features record stages in development of the point bar. The depositional bank slopes gently from the youngest prominent ridge towards the thalweg, but small ridges may cause minor undulations of this slope. The relief of the arcuate swell and swale features of the inner area of the meander loop varies in elevation from low to flood-stage levels. However, most of this area is flooded only during maximum river stages.

Natural levees Natural levees are very low, asymmetrical ridges which flank the flood-stage channel of a stream. The crest of the natural levee is near the channel, and the levee slopes gently toward the backswamps. The width of the natural levee is from 60 to over 700 times its height. The height varies with the size of the stream; Mississippi natural levees are approximately 15 feet high and Brazos levees are about 5 feet high Natural levees consist of very fine sands9 silts, and clays which are deposited by channel overflow waters. Coarser materials are deposited on the levee crest, and progressively finer materials on the levee backslopes and in flood basins. Although soil development and vegetation destroy much of the bedding' very small ripple lamination is common in natural levee deposits Natural levees within stream meander belts, together with the higher point bar deposits' form alluvial ridges which are the highest parts of alluvial valleys.

Abandoned channels cutoff meanders, and channel plugs. Abandoned channels such as Oyster Creek (Figure 2), formed by major upstream river diversions and also cutoff meanders (Figures 6 and 7) are subsequently filled with finer clastic materials, mainly thin-bedded and laminated clays, silt, and organic matter. These channel fillings are collectively called channel plugs. Cutoff meander fillings, also called clay plugs, may form crescent-shaped impermeable barriers within meander belt sands. Cutoff meanders containing clay plugs are more common in well-developed meander belts; compare the poorly developed modern Brazos and the well-developed Oyster Creek-Brazos meander belts (Figure 2). However, the size and areal distribution of clay plugs and meander belt sands vary with the size of the stream. Nanz (1956) and the New Orleans Area staff have pointed out that fine "rained channel fillings may form impermeable barriers which influence hydrocarbon accumulations in Cretaceous alluvial deposits of the Wisner field, Franklin Parish, Louisiana


*Areas within the channels of river meander loops are called points by Mississippi river boat captains since these are points of land and submerged bars projecting across the general trend of river traffic.