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Deltaic peats accumulate on abandoned deltaic platforms, in interdistributary basins, in abandoned channels, and as detrital bay deposits.
Within these depositional settings, several parameters determine the potential for extensive peat formation: (1) botanical parent material, (2) climate, (3) frequency of delta-lobe switching, (4) subsidence, (5) detrital influx, and (6) marine inundation. Each factor controls the variability in ash content and mineralogical composition of the peats.
Five areas were cored. Analysis of more than 2,000 samples shows that the average ash content of true peats (less than 25% ash) in the Mississippi delta is 18.3%, and that 5-15% of all sediment in the upper 4.5 m (15 ft) consists of true peat. It appears then that parameters 1 and 3 inhibit extensive peat accumulation, parameter 2 is favorable, whereas 4, 5 and 6 vary depending on the depositional setting.
When studying peats as precursors for coal, one should consider quality as well as quantity of the ash. Examination of ashes indicates that major components are water-soluble salts and silica. Leaching ashes with distilled water result in loss of salts and reduction of ash weights up to 77%. Scanning electron microscopy shows that abundant silica is biogenic and in a state of dissolution. Decrease of the amount of ash by natural dissolution of salts and silica during early diagenesis may result in better quality peats and more true peat overall.
Organic-rich deposits in the Mississippi delta may provide examples of modern coal-forming environments if conditions are properly considered. An understanding of the processes in peats will help in deciphering the rock record with respect to deltaic lignites and coals.
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