Allochthonous coal

According to the autochthonous, in situ theory of coal formation, peat beds and subsequently coal were formed from the accumulation of plants and plant debris in place. According to the allochthonous theory, the coal-producing peat bogs or swamps were formed from plant debris that had been transported, usually by streams or coastal currents, to the observed burial sites. 

The first theory suggests that coal strata accumulated from plants which had been rapidly transported and deposited under flood conditions. This second theory which claims transportation of vegetable debris is called the allochthonous theory. 

The debate over the origin of coal seams appears to have been decided in favor of in situ (or autochthonous) formation from peats formed slowly in swamps of various descriptions (Diessel, 1980 Stach et al., 1982). One of the key factors in this ascendancy of the peat swamp model over the various allochthonous (or transported) models was the recognition of so-called fossil forests—tree stumps with roots and logs in apparent growth positions on top of coal seams. The peat swamp model has not only become the basis of virtually all studies on coal seam formation but is now also the basis of studies on the coalification of the plant constituents to produce the various coal macerals. For this reason, considerable effort has been directed toward the study of modern peat-forming environments. 

The ascendancy of the peat swamp model has led to some neglect of the evidence for the allochthonous, and catastrophic, deposition of coal seams. Even with abundant evidence for contemporaneous volcanism resulting in volcanically derived inter-seam sediments, such coals are still viewed as having formed in peat swamps that were periodically buried by volcanic debris. 

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