facies The term ‘facies’ was first used in 1838 by the Swiss geologist Amanz Gressly (1814–65) in reference to Jurassic rocks in the Jura Mountains. He found that the individual strata there change their characteristics laterally: they are not uniform throughout their extent and undergo facies change. Emile Haug (1907) gave the definition ‘a facies is the sum of lithologic and palaeontologic characteristics of a deposit in a given place’. This useful concept immediately caught on and was used in a great many different ways. Today the word is usually qualified by a descriptive term (for example, ‘sandy facies’, ‘graptolitic facies’), or by a genetic term (for example, ‘deltaic facies’, ‘pelagic facies’, etc.). The usage is also extended to cover associations and packets of rock units rather than single strata.
Several meanings have been attached to the word ‘facies’. It may refer to the petrology of a sediment:
petrofacies e.g. quartz sandstone petrofacies; or it may refer to the lithology of the deposit:
lithofacies, e.g. cross-bedded sandstone facies; to the inferred mode of deposition: e.g.
aeolian facies; to the included fauna:
biofacies e.g. algal-limestone facies; or to a deposit formed with a particular tectonic background:
tectofacies e.g. stable-shelf sandstone. Alternatively, it can be used to describe the inferred environment if deposition: e.g.
estuarine facies. Particular facies may together form a characteristic
facies association or may succeed one another to form a characteristic
facies sequence.
Facies analysis is an important tool in the reconstruction of ancient environments of deposition and hence of palaeogeography. It has also proved invaluable in the prediction and exploitation of economic deposits.
The boundaries between facies, either in the vertical or the horizontal direction, may be sharp or indistinct, depending to some extent upon how the facies are defined. Many writers have used definitions so vague as to bring the concept into disrepute; others have used them as synonymous with lithological units (rock bodies).
Sedimentary facies commonly occur in associations typical of specific depositional environments, and the environments occur next to one another in sequence. This led Johannes Walther in 1894 to declare that facies that occur in conformable vertical successions also occur in laterally adjacent environments. This has been referred to as Walther's ‘law of correlation of facies’.
Since the studies by the Finnish geologist P. Eskola (1915), the term
facies has also been used to denote metamorphic rocks containing similar assemblages of minerals (for example, ‘greenschist facies’, ‘amphibolite facies’) that are interpreted as having formed under comparable conditions of temperature and pressure at depth in the Earth's crust.
D. L. Dineley and and G. Evans
Bibliography
Reading, H. G. (ed.) (1996) Sedimentary environments: processes, facies, and stratigraphy (3rd edn). Blackwell Scientific Publications, Oxford.
Schoch, R. M. (1989) Stratigraphy: principles and methods. Van Nostrand Reinhold, New York.
