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Buried peat zones represent former marsh and swamp surfaces formed by cyclic sedimentation processes. They originate approximately at mean sea level and provide modern analogs for coal-forming environments.
To better understand variations in stratigraphy and organic and mineralogical properties between peats in different deltaic settings, two basins were chosen for study: Vermilion Bay and Barataria basin. The first area represents a blanket-peat-forming environment; the second exemplifies interdistributary peat accumulation.
The stratigraphy of the upper 4 m (13 ft) of the western Vermilion Bay area is that of a freshwater swamp (> 35% organic matter and abundant cypress wood fragments). Due to marine inundation, the present surface is covered with a saline marsh. The swamp deposit at 1 m (3 ft) depth possibly correlates with a marsh deposit at the same depth in eastern Vermilion Bay, where three depositional cycles, each containing blanket peats, can be recognized in the upper 9.5 m (31 ft) of the subsurface. The Barataria interdistributary basin displays a great horizontal and vertical organic matter variability. Hence, stratigraphy cannot be determined, but organic-rich pockets up to 4 m (13 ft) thick are known to exist. In addition, the Pleistocene surface deepens from -2 m ( -6.6 ft) in western Ve milion Bay to -50 m ( -165 ft) in Barataria basin as a result of differential subsidence and surface irregularities.
Characteristically, the peats average 90% moisture, 80% organic matter (20% ash), and a bulk density of 0.12 g/cm3, the latter two numbers based on dry weight. When these data are related to depth, it appears that compaction during the first few thousand years after deposition is minimal. The frequency distribution of organic matter percentage ranges, for all sediments, shows that in eastern Vermilion Bay 15% of the material is peat; by comparison, in Barataria basin, 5% of all material is peat.
Preliminary results from mineralogical and elemental analyses of fresh and brackish peats indicate the presence of clay minerals, quartz, pyrite, gypsum, siliceous spicules, and smaller amounts of the trace mineral rutile. Minerals appear to vary with the type of peat. Elemental inorganic compositions also vary with depositional setting and post-depositional salt-water encroachment.
Peat deposits in the deltaic plain show a great variability in stratigraphy and characteristics due to four conditions: (1) difference in depositional setting; (2) depth to the Pleistocene; (3) intermittent interruption of marsh growth by influx of detrital clastics; (4) marine inundation of freshwater peats.
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