Figure 13. The relationship of glacial 

 advance and retreat to continental shelf 

 exposure and sedimentation during the Late 

 Quarternary (after Fisk 1956). 



the relationship of sea level changes to 

 delta and river valley response. 



In addition to causing cutting and 

 valley filling, changes in sea level 

 resulted in migration of the site of 

 sediment deposition. During falling sea 

 level, deposition shifted seaward, 

 depositing deltaic sediments at or near 

 the edge of the continental shelf. The 

 progradation of the deltas seaward over 

 thick sequences of shelf clays resulted in 

 major sedimentary loading of the 

 underlying clays, causing rap-id downbowing 

 and subsidence. As sea level began to 

 rise, the delta site shifted landward. 



expanses of coastal wetlands, some 50 - 60 

 percent larger than present-day wetlands, 

 existed along the Louisiana coast. 

 Borings along the present-day coastline 

 and offshore often hit these buried 

 freshwater marsh and swamp deposits. 



Warming of the Late Pleistocene 

 climate returned polar meltwaters to the 

 ocean basins, raised sea level, and 

 progressively decreased the stream 

 gradients and carrying capacities of the 

 rivers. As a result, the channels filled 

 and large expanses of coastal wetlands 

 were buried beneath the present 

 continental shelf. Sedi.nentation could 

 not keep pace with the rising sea level 

 and the rapid subsidence, and a series of 

 deltas were left stranded on the present 

 continental shelf. 



Seismic data and offshore foundation 

 borings have been used to reconstruct the 

 major deltaic lobes at various times 

 during the last major rise of sea level. 

 The positions of these lobes, shown in 

 Figure 15 a through d, illustrate that at 

 different times in the past the area of 

 the coastal wetlands was governed by the 

 locus of deposition of the major deltaic 

 lobe. The presence of numerous delta 

 lobes, now buried beneath the continental 

 shelf deposits, points out the role that 

 submergence plays in controlling the total 

 area of coastal marshes. If submergence 

 did not occur along the Louisiana coast, 

 many of these older deltaic lobes would 

 still be present, and the present-day 

 coastal marshes would be much more 

 extensive. 



The most recent cycle of sea- level 

 lowering and subsequent rise to its 

 present level began about 80,000 years ago 

 (Fisk and McFarlan 1955). This Late 

 Quaternary cycle began in response to 

 cooling Pleistocene climates. Sea level 

 was lowered approximately 150 - 170 m 

 below its present level by withdrawal of 

 water into the expanding Wisconsin-stage 

 glaciers. Streams along the gulf coast 

 and Mississippi River eroded extensive 

 valleys across the shelf and dumped their 

 sediment at or near the present-day shelf 

 edge. The generalized locations of these 

 river channels, now buried beneath the 

 younger deltaic sediments, are shown in 

 Figure 14. During this period large 



The latest phase of the Quaternary 

 cycle, characterized by relative stability 

 of climates and relatively small changes 

 in sea level, began approximately 5,000 - 

 5,000 years ago. This sequence involves 

 the modern delta cycles described by Fisk 

 and McFarlan (1955) and Frazier (1967). 

 Figure 16 illustrates the major 

 Mississippi River delta lobes that have 

 developed during this period. Although 

 numerous, slightly differing terminologies 

 have evolved to describe the individual 

 delta systems and their ages, most 

 authorities agree on at least seven delta 

 lobes. The result of the building and 

 subsequent abandonment of the Late Recent 

 delta lobes was construction of a modern 



15 



