Clinker bed breccias are among the rarest breccia types.  They occur where coal beds used to be.  Coal is combustible (that’s why coal is used as an energy source in human society).  Coal beds can catch on fire accidentally by human carelessness, leading to environmental disaster areas (for example, Centralia, Pennsylvania).  Coal beds can also be ignited naturally by lightning strikes, grass fires, forest fires, or spontaneous combustion.


Some ancient coal beds in the geologic record have burned away, leaving behind clinker beds.  “Clinker” is the term for scoriaceous, non-combustible materials left behind after coals burn.  The rock shown below is a breccia formed by baking and collapse of rock from above a naturally burned-out coal bed.  This clinker bed breccia consists of a scoriaceous matrix (dark brown) with large to small angular fragments of altered bedrock from above the original coal bed.  Some early explorers of the American Great Plains, not knowing the origin of this material, called natural clinker “pumice” or “scoria”.


Clinker lithologies are intensely altered from previously existing rocks.  Specifically, they form by intense heating from a coal bed fire, sometimes resulting in melting and the formation & subsequent cooling of paralava.  As such, clinker is a metamorphic rock, the result of thermal metamorphism.


Clinker Bed Breccia (15.3 cm across) from the Fort Union Formation (Paleocene).  Collected along Rt. 12, ~11 miles east-northeast of Miles City, northern Custer County, eastern Montana, USA.  Many lignite coals in western North Dakota, eastern Montana, and northeastern Wyoming have burned in the recent geologic past and are now clinker beds.



Locality info. provided by LaVonne Hunze.



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