Sykes and Pratt reported in 2004 that all coal seams contain internal successions of lithologically distinct layers, although the layering in many seams may not be obvious in exposed faces or cores. The layers represent seam facies, and their detailed analysis can provide useful insight into the autogenic and allogenic controls on peat and coal formation.
Nevertheless, the facies concept has not been extensively applied to coal seams, and hence the development of coal seam facies models has been slow. A major reason for this has been the lack of established methods for portraying the macropetrographic composition and texture of coal.
Successful facies analysis of coal seams begins at the macroscopic scale with the identification of lithofacies, because these provide the only visible and intact record of the original peat facies, including their texture and in-seam distributions. All coals consist of one to three main macropetrographic components, xylite/vitlrain (humified plant parts: xylite in lignites, vitrain in black coals), fusain (fusinitized plant parts, or "charcoal"), and attrital matrix (Schopf 1960).
Seam lithofacies are defined by assemblages of these components, with lithofacies boundaries marked by changes in component proportions, in the size distributions of vitrain/xylite or fusain, and/or in the nature of the matrix. Accordingly, for the identification and analysis of lithofacies, graphical methods of logging coal seam profiles should portray the vitrain/xylite and fusain components, and enable quantification of their size and content. No existing methods satisfy these requirements because they use symbols to portray coal types (see Bustin et al. 1985, p. 49.-61. for selected examples), so we have devised a new method for use with lignites, which has proved effective in identifying and characterizing seam lithofacies.
The method has been applied to lignite seams in the Ravenscrag Formation (Paleogene) of southern Saskatchewan, Canada. A newly exposed face of one seam and fresh cores of 18 other seams showed only indistinct layering, but dry sawing of the lignites revealed much greater proportions of xylite and fusain and the layering became obvious. The clarity of detail revealed in the sawn faces enabled the xylite, fusain, and coal/elastic contacts to be readily traced, providing true-scale graphic representations of the macropetrography. The representations of individual samples are referred to as lithgraptrs whereas those of seam profiles are termed lithologs.
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