...
A massive body of rock characterised by its age, lithological structure, conditions of formation (depositional environment) and other features that distinguishes it from adjacent rocks
...
For example, a river bed may have a corse-grain lithofacies at the central deep channel where water is fast and fine-grain lithofacies on the shallow sides where water is slow.
See Also
...
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology