Tertiary Lignites of Kemper and Northeast Lauderdale Counties, Mississippi
Shiar Rahaim, Dean A. Dunn
Samples of lignite from the Wilcox Group (lower Eocene) of Kemper County, Mississippi, were subjected to proximate, mineralogic, and grain-size analyses. The results of these analyses indicate that these poor-quality lignites were deposited in small depressions and channels in a deltaic area dominated by herbaceous plants and heavily influenced by fluvial processes.
The dominant ash content of the lignite is composed primarily of sand-sized quartz. The moisture content is lower than expected, because the noncombustible components of the ash retain little or no moisture. Ash content is negatively correlated to ulminite, exinite, and corpohuminite, which appears to indicate that fixed carbon and volatile matter values are low in these lignites owing to the lack of macerals that supply hydrogen and carbon. Moisture has a positive relationship with exinite, corpo-huminite, volatile matter, and fixed carbon, and a negative correlation with ash. This interrelationship among these sets of variables indicates a causal effect as a direct result of a high-energy environment in which large amounts of ash were deposited. This environment would be unfavorable for the deposition and coalification of plant material, and would result in lower production of macerals, thus lowering fixed carbon and volatile matter values. The inorganic sand-sized fraction of the lignite is quite large, and reinforces the conclusion that the lignites were deposited in a high-energy environment.
The dominance of humodetrinite and the small amount of ulminite and textinite indicate the plant community consisted primarily of herbaceous plants (grasslike plants, reeds, and wood from angiosperms), and lacked a significant number of conifers. Gelinite, which is associated with sandy sediments and well-oxygenated waters, is also abundant in the lignite, and appears to indicate a high energy environment with elevated aerobic processes. These aerobic processes might also be responsible for the moderately high amount of fusinite, since the fusinite in this area is not considered to be a result of peat fires. The aerobic activity and biochemical processes possibly caused the fusinization of texinite and ulminite, leading to the negative correlation fusinite has with these two cellular acerals.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.