Body of rock strata possessing unifying characteristics usually indicative of the environment in which the rocks were formed. The term is also used to describe the environment of formation itself or unifying features of the rocks that comprise the facies.
Features that define a facies can include collections of fossils, sequences of rock layers, or the occurrence of specific minerals. Sedimentary rocks deposited at the same time, but representing different facies belong to a single chronostratigraphic unit (see stratigraphy). But these same rocks may belong to different lithostratigraphic units. For example, beach sand is deposited at the same time that mud is deposited further offshore. The beach sand eventually turns to sandstone while the mud turns to shale. The resulting sandstone and shale strata comprise two different facies, one representing the beach environment and the other the offshore environment, formed at the same time; the sandstone and shale belong to the same chronostratigraphic unit but distinct lithostratigraphic units.
The set of characteristics that distinguish one facies from another in a given chronostratigraphic unit is used to interpret local variations in environments that existed at the same time. Changes in environment can be deduced from a variety of features. One facies in a body of rock might consist of porous limestone containing fossil reef-building organisms in their living positions. This facies might laterally pass into a reef-flank facies of steeply dipping deposits of rubble from the reef, which in turn might grade into an interreef basin composed of fine, clayey limestone.
Ancient floods and migrations of the seashore up or down can also be traced by changes in facies.
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