Coal reviewed: Depositional controls, modern analogues and ancient climates

Ancient economic coals usually contain more than 90% organic matter and less than 1% sulphur. Sedimentological reconstructions show the interbedded clastics to be fluvial-plain, deltaic, barrier coastline, estuarine or alluvial-fan deposits. However, bases of coal beds unconformably transgress these various facies, and coals only rarely pass laterally into clastic facies. Thus coal-forming peats post-dated the underlying clastic sedimentation and most coals did not form in deltas. Local coal thicknesses can be controlled by compaction of underlying sediment, syndepositional faulting, or basement structure. Coals with more than 1%-2% sulphur are usually overlain within 10 m by marine sediments which provided SO ₄ ^2- by diffusion. However, most coals contain less sulphur and must be freshwater in origin. Lithotype and spore profiles vertically through coals indicate a characteristic change of peat floras. This resulted from equisites-dominated seat earths, followed by diverse tree-sized vegetation which formed bright coal and lastly by low diversity stunted vegetation which formed dull coal. These features compare well with peats formed in modern ombrogenous blanket or raised bogs. The ancient mires formed in rainwaters (low sulphur), excluded clastic sediments (low ash), had a lateral floral zonation, usually post-dated clastic deposition and occurred inland from the shoreline. Modern ombrogenous mires are abundant, especially in humid temperate climates. These peats are forming today during a period of long-term climatic cooling in areas with a frequent reliable rainfall. Ancient coals may also have formed during long-term climatic cooling epochs when volcanic CO ₂ was being removed from the atmosphere. This could be regarded as a Gaian biosphere maintaining Earth temperatures similar to those of the present day.