In the Monongahela Group, lacustrine limestones are intimately associated with important coal deposits as evidenced by (1) their common biogenic origin within embayments between clastic deltaic lobes, (2) the interbedding of thick limestones and minable coal seams, and (3) their paleogeographic proximity. A preliminary study of the lower Monongahela Group in the northern coal basin of West Virginia shows that the seemingly monotonous limestones are actually variable in terms of both paleontology and petrology. Freshwater fossils in these beds include micro- and macro-invertebrates (darwinulid and cypridid ostradoces, mollusks, and abundant Spirorbis), vertebrates (fragments of amphibians and fish), and megaplants, and calcareous algae. These rocks are classified as finely laminated micrite, pelmicrite, and intramicrite. Sedimentary structures such as mud cracks, desiccation chips, caliche, and bird's-eye all indicate frequent drying. Water-level fluctuations that commonly affected the lakes, as interpreted from limestones, also affected nearby coal swamps in terms of plant types, water chemistry, and degradation rates. These then determined coal lithotype and ash content. During the periods of subaerial exposure, rare gypsum crystals were precipitated interstitially in the carbonate mud, and the concentration of minerals like gypsum may be one source of sulfur for pyrite in adjacent high-sulfur coals. Hence, the understanding of depositional environments in the lacustrine limestones provides further insights into details of coal formation.

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