Limnic Coal Formation

a coal formation that originated in the interior of a continent in landlocked bodies of water that were not connected with the open sea.

The characteristic features of a limnic coal formation are a small number of coal seams, usually of a complex structure and often having a great but nonuniform thickness, and coal-bearing series that lie unconformably on the underlying rocks. The coal-bearing series consists predominantly of sands and sandstones interbedded with clay deposits. There is a gradual transition from coarse-grained sediments in the lower part of the coal-bearing series to finer-grained ones in the upper part. A particular case of limnic coal formation is the potamic (potamos, “river”) coal formation associated with the formation of flood-plain peat bogs. Limnic coal basins in the USSR include the Cheliabinsk, Turgai, Kansk-Achinsk, and Irkutsk; abroad there are the Kladno and Plzeň basins in Czechoslovakia and others. Depending on their tectonic setting, basins with limnic coal formation contain brown or slightly metamorphosed hard coals. Limnic coal formation is opposed to paralic coal formation.