interbed

An interbed is a stratigraphic layer of rock that lies between two other beds within a sedimentary sequence. The term describes a relatively distinct interval that separates adjacent beds and can differ in lithology, color, mineral composition, or grain size from the surrounding layers. Interbeds form when environmental conditions or sediment supply change during deposition, or when a brief hiatus or condensed interval is preserved between major depositional events.

Interbeds can be thin or thick and may consist of shale, mudstone, siltstone, sandstone, coal, volcanic ash, or other materials. Because they record short-term changes within a formation, interbeds aid in correlating stratigraphic units over distance and in interpreting past environments. Their presence can also affect reservoir quality in hydrocarbon geology, acting as barriers, baffles, or permeability modifiers between more continuous lithologies.

In practice, interbeds are described in terms of interbedded lithologies—for example, interbedded sandstone and shale within a formation. In coal-bearing sequences, shale interbeds between coal seams are a common example and can influence mining conditions and gas content. Interbeds thus provide important clues about depositional history and the internal architecture of sedimentary successions.