Abstract

Research in the modern delta of the Mississippi River has revealed short-term changes and processes that are of significant magnitude. Deltaic lobes, each lobe covering an area of 30,000 sq km and having an average thickness of 35 km, switch sites of deposition on an average of every 1,500 yr. Through short periods of geologic time, this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas. Within a single delta lobe, and operating on an even higher frequency, are bay fills and overbank splays. Bay fills, having areas of 250 sq km and thickness of 15 m, require only 150 yr to accumulate. Four major events have taken place in the modern Balize delta since 1838. Overbank splays are much smaller, covering areas of less than 2 sq km and having thicknesses of 3 m, but are associated with high floods on the river. At the river mouth, continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long, 8 km wide, and a thickness of 80 m in a period of only 200 yr. Differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr. On the subaqueous delta platform, sediment instabilities operate nearly continuously, mass-moving large quantities of shallow-water deposits to deeper-water environments via arcuate rotational slides and mudflow gullies and depositional lobes. All of these changes and processes operate at differing spatial and temporal scales, but all result in deposition of large volumes of sediment over extremely short periods of time.

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