What is a "facies"?

What is a "facies"?
A "facies" (pronounced FAY-sheez) is a part of a rock formation that possesses a certain characteristic. The character could be a physical attribute, such as grain size ("conglomerate facies"); it could be a paleontologic attribute ("Hell Creek Faunal Facies", "Bug Creek Faunal Facies"); it could even be an interpretive attribute ("lower shoreface facies"). Therefore, when one talks about the "Hell Creek Faunal Facies", one is referring to a part of the Hell Creek Formation (the lower 99%) that contains a unique assemblage of fossils. The other facies found in the Hell Creek Formation (the Bug Creek Faunal Facies) occurs at the very top of the formation, and it is very limited in geographic extent (it is found only in a one square-mile area). There is some disagreement regarding what, exactly, the Bug Creek Faunal Facies represents. Most researchers believe that the Bug Creek beds are storm channel deposits that were eroded from both the underlying Cretaceous rocks and from the landscape that contained Paleocene animals (for example, see: Lofgren, D.L., C.L. Hotton, and A.D. Runkel. 1990. Reworking of Cretaceous dinosaurs into Paleocene channel deposits, upper Hell Creek Formation, Montana. Geology 18: 874-877). If this is true, then the Bug Creek Faunal Facies represents a mixture of both Cretaceous and Paleocene fossils. Some researchers, such as J.D. Archibald, believe that the Bug Creek Faunal Facies represents a true life community at the very end of the Cretaceous, and that this life community was in a process of gradual replacement of Cretaceous animals with Paleocene animals. Regardless of which interpretation is correct, all researchers agree that the underlying Hell Creek Faunal Facies represents a pristine assemblage composed only of Cretaceous fossils.

Reference:

Lofgren, D. L. 1995. The Bug Creek Problem and the Cretaceous-Tertiary Transition at McGuire Creek, Montana. University Of California Publications In Geological Sciences 140: 1-185.