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Variations in organic-rich sediment (peat) from the Mississippi River delta plain can be explained by differences in three interrelated parameters: (a) depositional setting, (b) balance between subsidence and detrital influx, and (c) marine inundation. Variations are observed in geometry of the overall deposits, organic matter content, and the mineralogy of peats and related ashes.
Using a vibracorer, samples were collected from two brackish areas (Avery Island/Sale-Cypremort and Barataria basin) and one freshwater area (Gueydan). Geometries of the deposits are different for the two brackish settings; interdistributary peats of Barataria basin tend to be discontinuous and somewhat thicker than blanket peats of Avery Island/Sale-Cypremort. The freshwater peat is an elongate body in a Pleistocene channel cut.
Peats in all areas average 90% moisture content. Dried peat averages 81.3% organic matter with a bulk density of 0.12 g/cm3. Holocene differential compaction has been minimal (< 4%).
Conventional ash values indicate different distributions of organic material for the two brackish areas. For the area containing interdistributary peats, approximately 5% of the subsurface contains more than 70%
organic matter, whereas for blanket peats, 14% of the section has more than 70% organic matter.
Mineralogic data collected on low-temperature ashes indicate that major minerals in brackish peat consist of kaolinite, quartz, and water-soluble salts, whereas minerals from freshwater samples are primarily kaolinite and quartz. X-ray diffraction and electron microscopy show the presence of smectite, illite, abundant siliceous spicules, framboidal pyrite, biotite, pyroxene, and rutile in the insoluble fraction.
Suitability of these deposits as coal precursors has been discussed by several workers. Superficial examination of data generated so far suggests that only thin coal seams with high ash would be produced. However, the presence of decomposing siliceous sponge spicules in the ash and the possibility of ash being leached by fresh ground water, suggest that much ash may be lost during early diagenesis. Hence, this environment may have more potential as a modern coal-forming model than it first appeared.
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