The influence of depositional environments on coal petrographic composition of the Lower Kittaning seam, western Pennsylvania

Abstract

Lateral variations in maceral composition of the Lower Kittanning seam can be explained in terms of differential preservation of plant materials, which is influenced by the pH/Eh of water within the peat swamp and by the rate of subsidence. Within the central part of the basin, high telocollinite contents are observed; this type of vitrinite represents large, well-preserved pieces of woody tissue whose preservation was favored by rapid burial. Towards the margins of the basin, high desmocollinite contents are seen; slower burial, combined with the influx of neutral, oxygenated fresh water permitted decomposition of the precursor materials, producing this highly degraded type of vitrinite. Vertical variations in telocollinite and desmocollinite content can also be related to burial rates and pH/Eh conditions within the swamp. Throughout the basin, high telocollinate contents observed in the lower part of the seam resulted from rapid burial under relatively low pH conditions. Continued high rates of subsidence led to an influx of neutral pH, oxidizing waters associated with the encroaching marine environment. Under these new conditions, more degradation of plant tissues occurred, producing the high desmocollinite contents observed in the upper part of the seam. This effect was most pronounced in the center of the basin. An alternative explanation would be a change in vegetation type in response to the changing conditions; the high desmocollinite content would then reflect a shift to a community dominated by smaller, possibly herbaceous, plants. In addition to a change in vitrinite type towards the top of the seam, a decrease in total vitrinite content, and increases in total liptinite and inertinite contents are observed. The increase in liptinite content is due to either (1) the higher resistance to decomposition of liptinite precursor materials (when compared to vitrinite precursors) in the more neutral conditions, or (2) a change from a dominantly arborescent vegetation to a more herbaceous one. The increase in inertinite results from an increase in transported material (inertodetrinite) and/or an increase in the formation of inertinite macerals, such as degrado-fusinite, possibly in response to the encroaching marine conditions. An additional control on maceral composition is the level of metamorphism of the coal. Total liptinite content decreases towards the southeast; in this area, the coal has attained a sufficiently high rank that the reflectance of liptinite macerals approaches that of vitrinite, thus making their identification difficult.