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The term facies was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy (see Cross and Homewood 1997), which replaced the earlier notions of Neptunism.
The simplest definition of facies is provided by Reading (1996) - 'A facies is a body of rock with specified characteristics... A facies should ideally be a distinctive rock that forms under certain conditions of sedimentation, reflecting a particular process or environment.'
Generally, facies are distinguished by what aspect of the rock or sediment is being studied. Thus, facies based on petrological characters such as grain size and mineralogy are called lithofacies, whereas facies based on fossil content are called biofacies.
These facies types are usually further subdivided, for example, you might refer to a "tan, cross-bedded oolitic limestone facies" or a "blueschist facies". The characteristics of the rock unit come from the depositional (or, in the case of 'blue schist', the metamorphic) environment and original composition. Sedimentary facies reflect depositional environment, each facies being a distinct kind of sediment for that area or environment.
Since its inception, the facies concept has been extended to related geological concepts. For example, characteristic associations of organic microfossils, and particulate organic material, in rocks or sediments, are called palynofacies. Discrete seismic units are similarly referred to as seismic facies.
The sequence of minerals that develop during progressive metamorphism (that is, metamorphism at progressivley higher temperatures) define a facies series and depend on the pressure, or range of pressures, at which the progressive metamorphism occurred.
Walther's Law of Facies
Walther's Law of Facies, named after the geologist Johannes Walther , states that the vertical succession of facies reflects lateral changes in environment. A classic example of this law is the vertical stratigraphic succession that typifies marine trangressions and regressions. However, the law is not applicable where the contact between different lithologies is non-conformable (i.e. sedimentation was not continuous), or in instances of rapid enviromental change where non-adjacent environments may replace one another.
Cross, T. A., and Homewood, P. W., (1997), Amanz Gressly's role in founding modern stratigraphy. Geological Society of America Bulletin 109 (12) 1617-1630.
Reading, H. G. (Ed.), (1996), Sedimentary Environments and Facies. Blackwell Scientific Publications. ISBN 0632036273
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