In comparison with other causes of subsidence, 

 crustal downwarping has a minor effect on the 

 Chenier Plain region. The Pleistocene surface lies 

 only about 10 m (33 ft) below tlie Recent surface 

 at the present shoreline (plate 2). Below the 

 Mississippi River delta the depth of the Pleisto- 

 cene surface is over 300 m (984 ft) (Fisk and McFarlan 

 1955, Gould 1970). Land subsidence caused from de- 

 watering processes is usually less dramatic in the 

 Chenier Plain than farther east because of the rela- 

 tively thin section of Recent deposits that overlie 

 the Pleistocene surface. Nevertheless, the overall 

 net rate of subsidence (or sea level rise) is signifi- 

 cant and averages about 1.75 cm (0.69 in) per year 

 on the Chenier Plain. 



2.2.3 RECENT SEDIMENTARY ENVIRONMENTS 



During the geologic formation of the Chenier 

 Plain, the Mississippi River occasionally constructed 

 deltas close to its eastern flank, just as the Atchafa- 

 laya River, located between the Mississippi River 

 and the Chenier Plain, is presently doing. The west- 

 ward movement of reworked former delta sediments 

 combined with sediments from adjacent active 

 Mississippi River distributaries are thought to be the 

 main source of sediments for the Chenier Plain. 

 It is also evident that the rivers within the region 

 contributed sediments to the coast (Howe et al. 

 1935, Van Lopik and Mclntire 1957). 



Deposits of marine origin are represented in the 

 lower part of the Recent sedimentary wedge. Although 

 not deUneated in every core examined, they exist in 

 theory based on an understanding of processes that 

 must have been operating during sea level rise. Thus, 

 they may only be distinguishable from overlying 

 nearshore, marsh, bay, or beach deposits by their 

 relationship to the erosional Pleistocene surface 

 (Byrne et al. 1959, Gould and McFadan 1959). Gulf 

 bottom, marsh, lake, and bay deposits cap the marine 

 deposits and comprise sequences of sand, silt, clay, 

 and organic deposits representing open Gulf, bay, 

 lake, .and marsh or swamp habitats (Byrne et al. 

 1959, Kane 1959, Coleman 1966). 



The open Gulf marine deposits are highly variable 

 depending on their proximity to the sediment source 

 and to the offshore energy conditions. They inter- 

 finger with marsh, bay, or lake deposits close to the 

 shoreUne where erosion or accretion has occurred. 

 The open Gulf deposits are distinguished by the 

 marine fauna, distinctive sedimentary structures, 

 and absence of accumulations of organic detritus. 



Bay, lake, and marsh deposits are closely con- 

 nected both vertically and laterally. As a result of 

 small changes in rates of sea level rise and subsi- 

 dence, and in current patterns, what was a coastal 

 marsh became a lake or bay within a relatively short 

 time. Types of marsh habitat also changed in this 

 dynamic setting. Marsh deposits fomied organic 

 layers that can be dated by their radiocarbon con- 

 tent to reconstruct the depositional history of the 

 area (Byrne et al. 1959, Gould and McFadan 1959, 



and Coleman 1966). Swamp deposits are confined to 

 river valleys and do not represent a major depositional 

 element in the Chenier Plain. 



Bay and lake deposits differ from each other 

 chiefly in their exposure to varying degrees of river 

 and tidal influence. They can be recognized in the 

 subsurface by their hthologic, faunal, and sedimentary 

 properties. Virtually every water body in the Chenier 

 Plain is subject to some tidal influence except where 

 engineering projects disrupt the natural process. The 

 inland water bodies resulted from the drowning of 

 reUct Pleistocene entrenched valleys, as was the case 

 for East Bay, Sabine Lake, and Calcasieu Lake along 

 the coast, and for White Lake and Grand Lake, 

 located inland from major Gulf connections (Fisk 

 1944). Many small lakes originated as marsh ponds 

 diat enlarged when sahnity changes or other stresses 

 interrupted the marsh building processes. Many 

 irregularly shaped lakes represent old river or tidal 

 stream courses that were abandoned. 



2.2.4 CHENIER RIDGES 



The Chenier Plain is characterized by sand and 

 sheU fragment ridges that parallel the shoreline 

 (fig. 2-2a and b). These ridges are of three basic 

 origins: barrier islands, river mouth accretions, 

 and recessional beach ridges. The cross sections of 

 sediment facies in figure 2-3 were constructed from 

 unpublished data in the Louisiana State University 

 Coastal Studies Institute files. 



Barrier islands or spits are progradational features 

 produced by longshore transport of sand-size or larger 

 particles. Barrier islands represent accumulation of 

 sediments that develop seaward of embayments that 

 are usually connected with the Gulf by inlets. They 

 are usually connected with the Gulf by inlets. 

 Barrier islands are nourished by sediments from 

 physiograpliic structure while undergoing erosion 

 and retreat. Growth usually occurs downdrift and 

 landward by spit and accretion ridge fomiation. 

 Bolivar Peninsula (fig. 2-3, cross sections A and B) 

 is the single example within the study area. How- 

 ever, barrier islands probably existed along other 

 coastal sections of tlie Chenier Plain during sea level 

 rise. 



River mouth accretion ridges are another feature 

 of the Chenier Plain created by progradation. These 

 multiple bars fonn concavely seaward where the 

 excess of sand-size particles deposited at river mouths 

 are reworked by waves and currents into complex 

 accretionary patterns (fig. 2-3, outlet of Sabine Lake). 

 Multiple rivemiouth ridges converge to fonn a single 

 recessional ridge extending between the river inlets. 

 The fanning pattern at river mouths differ from 

 barrier spits that fomi broad single ridges or multiple 

 ridges with less seaward concavity. River-mouth 

 ridges are well-developed at tlie mouth of the Sabine 

 River, with less extensive development occurring at 

 the mouth of the Calcasieu River (fig. 2-2a). A series 

 of tliese accretion ridges, representing older shore- 

 lines occur as far north as Little Pecan Island and 



10 



