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Facies
(geology)
Any observable attribute of rocks,
such as overall appearance, composition, or conditions of formation, and changes
that may occur in these attributes over a geographic area. The term “facies” is
widely used in connection with sedimentary rock bodies, but is not restricted to
them. In general, facies are not defined for sedimentary rocks by features
produced during weathering, metamorphism, or structural disturbance. In
metamorphic rocks specifically, however, facies may be identified by the
presence of minerals that denote degrees of metamorphic change.
Sedimentary
Facies
The term “sedimentary facies” is
applied to bodies of sedimentary rock on the basis of descriptive or
interpretive characteristics. Descriptive facies are based on lithologic
features such as composition, grain size, bedding characteristics, and
sedimentary structures (lithofacies); on biological (fossil) components
(biofacies); or on both. Individual lithofacies or biofacies may be single beds
a few millimeters thick or a succession of beds tens to hundreds of meters
thick. For example, a river deposit may consist of decimeter-thick beds of a
conglomerate lithofacies interbedded with a cross-bedded sandstone lithofacies.
The fill of certain major Paleozoic basins may be divided into units hundreds of
meters thick comprising a shelly facies, containing such fossils as brachiopods
and trilobites, and graptolitic facies. The scale of an individual lithofacies
or biofacies unit depends on the level of detail incorporated in its definition,
and is determined only by criteria of convenience, or availability of outcrop
and subsurface data, depending on whether a geologist wishes to describe a
single outcrop or an entire sedimentary basin.
Facies analysis
The term “facies” can be used in an
interpretive sense for groups of rocks that are thought to have been formed
under similar conditions. This usage may emphasize specific depositional
processes such as a turbidite facies, or a particular depositional environment
such as a shelf carbonate facies, encompassing a range of depositional
processes. See also: Turbidite
Facies analysis is an important
component of stratigraphic and sedimentary geology because it provides a
methodology of systematic description that is fundamental to orderly
stratigraphic documentation and to interpretations of depositional environment
and paleogeography. Most sedimentological basin studies should now begin with
such an analysis, in which the succession is divided into its component
lithofacies and their characteristics described carefully. Lithofacies may be
identified by name or by a code system devised by the researcher, the latter
method being convenient for abbreviated description and notetaking and for
computer processing (Fig. 1).
Fig. 1 Stratigraphic section through a fluvial
deposit illustrating its subdivision into lithofacies and the use of a
lithofacies code scheme. 1 m = 3.3 ft. (After A. D. Miall, A review of the
braided river depositional environment, Earth Sci. Rev., 13:1–62,
1977)
Increasing use is being made of
three-dimensional geometric (architectural) information regarding the extent of
facies units, because such information commonly is environmentally diagnostic.
Mappable stratigraphic units
commonly consist of a single lithofacies or a small assemblage of lithofacies.
This provides the basis for a hierarchical subdivision of stratigraphic
sequences ranging from the individual bed or lithofacies unit through member or
lithosome and the higher-ranking subdivisions such as stratigraphic sequence,
formation, and group. See also:
Facies (geology); Stratigraphy
Facies
assemblages
Groups of facies (usually
lithofacies) that are commonly found together in the sedimentary record are
known as facies assemblages or facies associations. These groupings provide the
basis for defining broader, interpretive facies for the purpose of
paleogeographic reconstruction. For example, a delta plain facies may be
defined, consisting of distributary channel sandstones, levee sandstones and
siltstones, and backswamp mudstones and coal. This may pass seaward into a delta
front facies consisting of distributary mouth bar sandstones and
interdistributary bay mudstones and shell beds. Still further seaward this may
pass into a fine-grained prodelta facies, and landward there may be a fluvial
facies and a lacustrine facies. An example of a carbonate facies assemblage is
illustrated in Fig. 2.
Fig. 2 Hypothetical carbonate shelf showing
relationship of lithofacies to environmental facies; based on the modern shelves
of
The shelly facies and graptolitic
facies described earlier are biofacies terms, but they correspond to distinct
lithofacies assemblages dominated by shelf carbonate sediments and by deep-water
mudstones and siltstones, respectively. Use of the terms provides a convenient
way to describe the broad paleogeography of some of the major Paleozoic
sedimentary basins around the margins of
Many facies assemblages are cyclic
in the sense that the individual lithofacies components tend to follow each
other in a similar order in a stratigraphic succession, reflecting the repeated
occurrence of the same depositional processes. For example, coal beds commonly
(but not invariably) follow a seat earth or root bed. Such assemblages are
termed cyclothems or, simply, cycles. Much sedimentological research has been
devoted to analysis and interpretation of cycles, which occur in most
depositional settings. For example, the Carboniferous coal-bearing deposits of
Europe and of central and eastern
Facies
models
These have been defined for most
sedimentary environments. Each model is intended as a summary of the
depositional processes and products occurring in a specific setting, such as a
tidal flat or a submarine fan. Such models are useful guides when carrying out
preliminary interpretations of an unknown sedimentary succession, because they
provide a range of criteria relating to lithofacies, sedimentary structures,
biofacies, and cyclic characteristics that the geologist can use to confirm or
negate a particular interpretation.
Facies
maps
These are constructed to illustrate
lithologic variations across a sedimentary basin, and they usually reveal
patterns or trends that can readily be interpreted in terms of depositional
environments and paleogeography. Various quantitative techniques have been
devised for emphasizing relevant features. Ratio maps are particularly useful,
for example, ones showing the ratio of total thickness of sandstone beds to
total shale or total carbonate thickness. In elastic coastal environments such a
map may reveal the location and shape of major sandstone bodies, which can then,
from their size, shape, and orientation, be interpreted in terms of an
appropriate sedimentary model (delta, barrier island system, submarine fan, and
so on) in conjunction with diagnostic sedimentological criteria. Location and
trend may be of considerable economic importance if, for example, porous
sandstone bodies are known to contain pools of oil or gas. See also: Barrier islands; Petroleum
geology
Another technique is to plot the
percentage of sandstone or aggregate sandstone thickness in a section. Figure 3
illustrates a map of this type, and reveals a hitherto unsuspected pattern of
lobate sandstone bodies containing a readily identifiable deltaic shape. Thicker
sandstone beds lie closer to the sediment source, and so in this example
transport was clearly from northwest to southeast.
Fig. 3 Plot of sandstone thickness, in meters,
in a Cretaceous sandstone-shale unit, derived from subsurface log analysis. Note
the narrow sandstone-filled channel representing a delta distributary channel
entering the area from the northwest, and the broad lobe of sandstone formed at
the mouth of this channel where the delta splayed out into the sea. (After J.
Bhattacharya, Regional to subregional facies architecture of river-dominated
deltas in the Alberta subsurface, Upper Cretaceous Dunvegan Formation, in
Society for Sedimentary Geology, Concepts in Sedimentology and Paleontology,
vol. 3, 1991)
Similar techniques can be applied to
many successions of carbonate rocks. Thus carbonates are typically classified in
terms of the ratio of biogenic or detrital fragments to carbonate and matrix.
This classification reflects depositional energy (wave and current strength)
because the mud matrix tends to be winnowed out in high-energy environments.
Therefore, a plot of the ratio can be a useful paleogeographic indicator.
However, carbonates are particularly susceptible to diagenetic alteration, which
can obscure primary depositional characteristics and hinder detailed
paleogeographic reconstruction. See
also: Diagenesis; Sedimentary rocks; Sedimentology; Stream transport and
deposition
Andrew D. Miall
Metamorphic
Facies
A metamorphic facies is a collection
of rocks containing characteristic mineral assemblages developed in response to
burial and heating to similar depths and temperatures. It can represent either
the diagnostic mineral assemblages that indicate the physical conditions of
metamorphism or the pressure–temperature conditions that produce a particular
assemblage in a rock of a specific composition.
Changes in pressure and temperature,
due to tectonic activity or intrusion of magma, cause minerals in rocks to react
with one another to produce new mineral assemblages and textures. This is the
process that transforms igneous and sedimentary rocks into metamorphic rocks.
The mineral assemblage that is preserved in a metamorphic rock reflects the
composition of the original igneous or sedimentary parent and the
pressure–temperature history experienced by the rock during metamorphism. Hence,
rocks of the same initial composition that are subjected to the same
pressure–temperature conditions will contain identical mineral assemblages.
Facies names and
boundaries
The metamorphic facies to which a
rock belongs can be identified from the mineral assemblage present in the rock;
the pressure and temperature conditions represented by each facies are broadly
known from experimental laboratory work on mineral stabilities. Several
metamorphic facies names are commonly accepted, and these are shown, along with
their approximate pressure–temperature fields, in Fig. 4a. These facies names
are based on the mineral assemblages that develop during metamorphism of a rock
with the composition of a basalt, which is a volcanic rock rich in iron and
magnesium and with relatively little silica. For example, the dominant mineral
of the blueschist facies (in a rock of basaltic composition) is a sodium- and
magnesium-bearing silicate called glaucophane, which is dark blue in outcrop and
blue or violet when viewed under the microscope. Characteristic minerals of the
greenschist facies include chlorite and actinolite, both of which are green in
outcrop and under the microscope. Basaltic rocks metamorphosed in the
amphibolite facies are largely composed of an amphibole called hornblende. The
granulite facies takes its name from a texture rather than a specific mineral:
the pyroxenes and plagioclase that are common minerals in granulite facies rocks
typically form rounded crystals of similar size that give the rock a granular
fabric. See also: Amphibole;
Amphibolite; Basalt; Blueschist; Glaucophane; Granulite; Pyroxene
Fig. 4 Pressure–temperature fields. (a) Diagram
showing the relationship between the different metamorphic facies and the
physical conditions of metamorphism. Boundaries (tinted areas) between facies
are regions where isograd reactions occur. (b) Diagram showing the relationships
between metamorphic facies, individual rock pressure–temperature paths, and two
typical facies series. Note that the facies series connects maximum temperature
points on pressure–temperature paths followed by different rocks. 1 kilobar =
108 pascals. 1 km = 0.6 mi.
Rocks of different composition show
different mineral assemblages in each of the facies. A shale metamorphosed to
greenschist facies conditions would contain chlorite, biotite, and perhaps
garnet as its key indicator minerals, in contrast to the chlorite + actinolite
assemblage that would be found in a rock of basaltic composition. In the
amphibolite facies, the indicators in a metamorphosed shale would be garnet,
staurolite, or kyanite rather than amphibole. See also: Shale
The boundaries separating individual
facies from one another are chemical reactions called isograds that transform
one key mineral assemblage into another. For example, a typical greenschist is
transformed into an amphibolite by the hornblende isograd reaction chlorite +
actinolite + plagioclase = hornblende, and an amphibolite can be transformed
into a granulite by the reaction hornblende = clinopyroxene + orthopyroxene +
plagioclase. In general, these reactions occur over a range of
pressure–temperature conditions that depend upon the specific compositions of
the minerals involved. In most cases, facies are separated by several reactions
that occur within a restricted interval of pressure–temperature space. Thus the
boundaries between facies are diffuse and are represented by tinted areas rather
than individual lines in Fig. 4.
See also: Hornblende; Phase equilibrium
Facies
series
Different geological terranes show
different combinations of metamorphic facies. For example, one area may show a
progression from blueschist to eclogite to greenschist facies over a distance of
a few kilometers (1 km = 0.6 mi), whereas another area may be characterized by a
sequence from greenschist to amphibolite to granulite facies. Because the
specific facies are related to the pressure and temperature conditions of
metamorphism, these different facies series indicate differences in the
pressure–temperature histories of the two areas. The
blueschist-eclogite-greenschist facies series develops in areas that have
experienced high-pressure metamorphism at relatively low temperatures. This type
of pressure–temperature history is produced in subduction zones where two
lithospheric plates collide with one another. In contrast, the
greenschist-amphibolite-granulite series forms in regions that have undergone
heating at middle to lower crustal depths. This facies series is exposed in many
of the mountain belts of the world (Appalachians, Alps,
Pressure–temperature paths
Because burial and uplift rates may differ from rates of heat transfer, individual rocks may follow paths through pressure– temperature space that are quite distinct from the facies series recorded by the host terrane. Mineral reaction rates are strongly dependent upon temperature and proceed faster during heating than during cooling; pressure has little effect on these rates. Hence, metamorphic facies and facies series generally reflect equilibration at the thermal maximum of metamorphism, regardless of the complexity of the actual pressure–temperature path followed by the rock in response to tectonic movements. As shown in Fig. 4b, individual pressure–temperature paths in many cases intersect the facies series path at a high angle. Disequilibrium mineral assemblages and chemical zoning in some minerals can sometimes be used to reconstruct portions of these pressure–temperature paths and hence gain information on how the depth of burial and the temperature of an individual rock varied through time. This information can be used to refine tectonic interpretations of an area based on the metamorphic facies series.
- A. D. Miall, Principles of Sedimentary Basin Analysis, 3d ed., Springer-Verlag, 1999
- A. Miyashiro, Metamorphic Petrology, 1994
- G. Nichols, Sedimentology and Stratigraphy, Blackwell, 1999
- F. J. Turner, Meatamorphic Petrology: Mineralogical, Field, and Tectonic Aspects, 2d ed., 1981
- R. G. Walker and N. P. James (eds.), Facies Models: Response to Sea Level Change, Geological Association of Canada, 1992
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