Ancient Coal-Forming Environments
Four compositional groups, based on the independent parameters of ash yield, sulfur forms, palynology, petrography, and low-temperature ash mineralogy are recognized in the Stockton and Fire Clay coal beds (Kanawha Formation, Middle Pennsylvanian) from the central Appalachian Basin. The four compositional groups are (1) an arboreous lycopod-dominant group, defined by high percentages of Lycospora and vitrinite (telocollinite), generally low but variable ash yield and sulfur content, and a mixed illite-kaolinite-quartz low-temperature ash mineralogy; (2) a transitional group defined by a mixture of lycopods (arboreous and “herbaceous” forms) and ferns, increased percentages of inertinite and desmocollinite (degraded vitrinite), low ash yield and sulfur content, and an increasingly dominant kaolinite ash mineralogy; (3) an “herbaceous” lycopod and fern group defined by high percentages of Densosporites (and related crassicingulate taxa) and ferns, high inertinite and desmocollinite contents, very low ash yield and sulfur content, and a kaolinite-dominant mineralogy; and (4) a mixed group defined by a palynoflora co-dominated by arboreous and “herbaceous” lycopods and ferns, with catamite and cordaites occurring in increased percentages, and variable petrography. Typically, this group is high in ash yield, low in sulfur content, and has an illite-dominant ash mineralogy.
The vertical arrangement of these groups define compositional cycles, which are believed to represent different developmental stages of a domed peat swamp. The commonly observed vertical profile of Lycospora/vitrinite-rich coal layers changing upward to fern-, small lycopod-, and inertinite-rich coal layers defines the first half of the cycle. A return, gradual or abrupt, to Lycospora/vitrinite-rich coal layers toward the top of the bed defines the second half of the cycle. The first half of the cycle is interpreted to represent a progressive change from more planar to more domed peat conditions. This change is marked by increased exposure to air and oxygenated rain water and the establishment of floral seres. The second half of the cycle is interpreted to represent a change from more domed to more planar conditions. Compositional “half cycles,” common in both the Fire Clay and Stockton coal beds, are usually the result of cycle truncation by inorganic partings.
Both planar and domed swamp environments contribute to peat formation in modern domed peat systems. Similarly, both of these environments are recognized in the Fire Clay and Stockton coal beds. A majority of these two coal beds are believed to have been derived from planar, to perhaps moderately domed, peat environments. In contrast, a relatively small part of these beds are thought to have been significantly domed.