CLINKER BED BRECCIA
Clinker bed breccias are among the rarest breccia
types. They occur where coal beds used to be. Coal is
combustible (thatÕs 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. ÒClinkerÓ 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 a
natural coal fire in the geologic past. 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 ÒpumiceÓ.
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 geologic (or recent) past and are now clinker beds.
Locality info. provided by LaVonne Hunze.