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Atchafalaya Bay, Louisiana: Regional Subsidence and Contemporary Delta Formation
R. J. Shlemon (1)
Approximately 30 percent of the lower Mississippi River is presently diverted into its Atchafalaya distributary. As a consequence lacustrine deltas are rapidly filling Atchafalaya Basin lakes, increasingly coarse-grained sediments are entering Atchafalaya Bay, and now a new delta--though probably geologically short-lived--is beginning to form. Understanding the geological and hydrological processes giving rise to this newly-born delta might provide a clearer insight into the evolution of deltas as recorded in ancient sediments.
In the initial subaqueous phase of deltaic development (between 1952 and 1962) over 120 km2 of Atchafalaya Bay had been covered by at least 0.5 m of new sediment. Local filling near the delta apex exceeded 2 m. The following phase, an estimated 50-year period of rapid subaerial expansion and shoreline accretion, has just begun. Comparison with modern Mississippi River subdeltas suggests the Atchafalaya Delta will eventually deteriorate owing to subsidence, compaction, and probable abandonment of the lower river course for a more direct, higher gradient route to the sea.
Analysis of tide records from Eugene Island and other Louisiana coastal stations indicates that in the last 30 years the rate of sea level rise in Atchafalaya Bay ranged from 0.80 to 1.32 cm/yr, due almost exclusively to regional subsidence. This exceeds even the rapid glacio-eustatic sea level rise between 10,000 and 6,000 years before present, an estimated 0.07 cm/yr. Despite deposition into this rapidly subsiding trough, the Atchafalaya Delta is still prograding; its ultimate internal form will reflect an interaction of sediment supply, wave energy, and regional tectonism. Unless modified by man, the Atchafalaya Delta will expand across its bay some 14.2 to 16.9 km2/yr until about the year 2020, creating approximately 950 km2 of new coastal land.
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