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Energy resources: Coal
Energy resources: Coal

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Energy resources: Coal

1.3.1 Peat formation in deltas and coastal barrier systems

Since mires require poor drainage, low-lying land close to coastal areas might provide the right conditions for peat to form. Most extensive areas of modern peat formation are indeed situated not far above sea-level, and as Figure 2 shows, they are commonly associated with river deltas and coastal barriers. Such environments would also have been significant areas of peat production in the geological past. However, the flooding of an area alone does not guarantee significant accumulations of peat; high productivity of organic matter is also required.

Figure 2
Figure 2 (a) The location of mires in deltas and coastal barrier systems. When river water spills through a breach or crevasse in a levee, a deposit known as a crevasse splay is formed. (b) A modern-day example of a coal-forming mire from the Okeefenokee Swamp, Florida. (c) Reconstruction of a late Carboniferous low-latitude swamp forest.

Deltas act as conduits bringing sediment via distributary river channels out into an open body of water (often the sea). In between these river channels are areas that receive less sediment, and it is here that high rates of subsidence and organic matter production may promote the development of mires (Figure 3).

Figure 3
Figure 3 Satellite image showing dark-coloured mires on the Mississippi delta south of New Orleans. These mires are forming around small lakes, and both are surrounded by a network of stream channels. The main river channel is approximately 500 m wide, and is flanked by natural levees that have been strengthened to reduce flooding. As a result the mires grow unchecked by annual deposition of sands and silts during floods. (This image was taken in 2001 — well before the major floods in this area caused by Hurricane Katrina in 2005.)

  • What is likely to happen to these areas when adjacent rivers are in flood?

  • Floodwaters will inundate the area and deposit a layer of sand or mud, which will contaminate the peat. This will (temporarily at least) halt the development of peat and, as we shall see later, contribute to the impurity of coal so formed.

As Figure 2a shows, a coastal barrier is a ribbon-like beach that protects a lake (or lagoon) on the landward side of the barrier from the sea. In this protected environment mires can develop along the fringes of the lagoon, or adjacent to tidal channels that cross the area.

  • Considering the proximity to the sea, what process is most likely to contaminate the mire with sediment?

  • Storms will tend to wash sand off the coastal barrier and redeposit it on the mire.

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