188
6 DEPOSITIONAL SYSTEMS
In 1865, Lyell was still writing:
The term "formation" expresses in geology any assemblage of rocks which have some character in common, whether of origin, age, or composition. Thus we speak of stratified and unstratified, freshwater and marine, aqueous and volcanic, ancient and modern, metalliferous
and non-metalliferous formations.
In modern usage this definition better fits facies than formation.
Over the years, however, the term "formation" became more and more restricted to
the description of discrete mappable rock units. This concept was finally embalmed in
Article 6 of the Code of Stratigraphic Nomenclature of the American Commission on
Stratigraphic Nomenclature, namely:
The formation is the fundamental unit in rock stratigraphic classification. A formation is a
body of rock characterized by lithologic homogeneity. It is prevailingly, but not necessarily
tabular and is mappable at the earth's surface or traceable in the subsurface.
Similarly, the Geological Society of London states:
The formation is the basic practical division in lithostratigraphical classification. It should possess some degree of internal lithological homogeneity, or distinctive lithological features that
constitute a form of unity in comparison with adjacent strata. (Geol. Soc. London, Recommendations on Stratigraphical Usage, 1969)
Thus Prevost's original term used for describing a rock unit by lithology and paleontology has been restricted to a lithostratigraphic unit. Meanwhile, Gressly's term "facies" became used in ever vaguer ways. In particular, it ceased to be used to define rocks
purely by lithology and paleontology. The paleogeographic and tectonic situation of a
rock also became one of the defining parameters (e.g., Stille, 1924). Thus the terms "geosynclinal," "orogenic," and "shelf facies" came into use. The concept of facies was also
applied to metamorphic rocks (Eskola, 1915). Recognition of the breadth of usage of
facies is shown by Krumbein and Sloss's terms "lithofacies," "biofacies," and "tectofacies" (1959). "The expressions of variation in lithologic aspect are lithofacies and the
expressions of variation in biologic aspect as biofacies" (1959, p. 268). "Tectofacies are
defined as the laterally varying tectonic aspect of a stratigraphic unit" (1959, p. 383).
Further extensive use of the term "facies" came when environmental connotations were
added. As early as 1879, von Mojsisovics wrote: "Following Gressly and Oppel, one now
customarily applies the term facies to deposits formed under different environmental
conditions" (translation in Teichert, 1958b). As the analysis of ancient sedimentary environments progressed this approach to facies became increasingly common until the
same rock formation could be termed "flysch, .... geosynclinal," or "turbidite facies."
Only the first of these three usages is descriptive and consistent with the original definition. The terms "geosynclinal" and "turbidite" give a genetic connotation to the term,
describing its interpreted tectonic situation and depositional process.
More recently still the term "facies" has been applied to describe the character of seismic data. Terms like "mounded, .... even, .... wavy," and "contorted" are applied to seismic character, and used to interpret the depositional environment (e.g., Mitchum et aL,
1977; Neal et aL, 1993). Further reviews of the origin of the facies concept and its sub-
6 DEPOSITIONAL SYSTEMS
In 1865, Lyell was still writing:
The term "formation" expresses in geology any assemblage of rocks which have some character in common, whether of origin, age, or composition. Thus we speak of stratified and unstratified, freshwater and marine, aqueous and volcanic, ancient and modern, metalliferous
and non-metalliferous formations.
In modern usage this definition better fits facies than formation.
Over the years, however, the term "formation" became more and more restricted to
the description of discrete mappable rock units. This concept was finally embalmed in
Article 6 of the Code of Stratigraphic Nomenclature of the American Commission on
Stratigraphic Nomenclature, namely:
The formation is the fundamental unit in rock stratigraphic classification. A formation is a
body of rock characterized by lithologic homogeneity. It is prevailingly, but not necessarily
tabular and is mappable at the earth's surface or traceable in the subsurface.
Similarly, the Geological Society of London states:
The formation is the basic practical division in lithostratigraphical classification. It should possess some degree of internal lithological homogeneity, or distinctive lithological features that
constitute a form of unity in comparison with adjacent strata. (Geol. Soc. London, Recommendations on Stratigraphical Usage, 1969)
Thus Prevost's original term used for describing a rock unit by lithology and paleontology has been restricted to a lithostratigraphic unit. Meanwhile, Gressly's term "facies" became used in ever vaguer ways. In particular, it ceased to be used to define rocks
purely by lithology and paleontology. The paleogeographic and tectonic situation of a
rock also became one of the defining parameters (e.g., Stille, 1924). Thus the terms "geosynclinal," "orogenic," and "shelf facies" came into use. The concept of facies was also
applied to metamorphic rocks (Eskola, 1915). Recognition of the breadth of usage of
facies is shown by Krumbein and Sloss's terms "lithofacies," "biofacies," and "tectofacies" (1959). "The expressions of variation in lithologic aspect are lithofacies and the
expressions of variation in biologic aspect as biofacies" (1959, p. 268). "Tectofacies are
defined as the laterally varying tectonic aspect of a stratigraphic unit" (1959, p. 383).
Further extensive use of the term "facies" came when environmental connotations were
added. As early as 1879, von Mojsisovics wrote: "Following Gressly and Oppel, one now
customarily applies the term facies to deposits formed under different environmental
conditions" (translation in Teichert, 1958b). As the analysis of ancient sedimentary environments progressed this approach to facies became increasingly common until the
same rock formation could be termed "flysch, .... geosynclinal," or "turbidite facies."
Only the first of these three usages is descriptive and consistent with the original definition. The terms "geosynclinal" and "turbidite" give a genetic connotation to the term,
describing its interpreted tectonic situation and depositional process.
More recently still the term "facies" has been applied to describe the character of seismic data. Terms like "mounded, .... even, .... wavy," and "contorted" are applied to seismic character, and used to interpret the depositional environment (e.g., Mitchum et aL,
1977; Neal et aL, 1993). Further reviews of the origin of the facies concept and its sub-