رخساره های زمین شناسی

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 Florida and the Bahamas. 1 km = 0.6 mi.

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 North America and elsewhere.  See also: Paleogeography

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 North America consitute the original cyclothems. Problems have arisen because these and other cycles can originate from several causes, the effects of which may be very similar and may be superimposed. Thus, cyclothems are caused partly by the advance and abandonment of delta lobes and partly by oscillatory changes of sea level. Second, progressive changes in a sedimentary basin, such as the slow seaward progradation of a delta or tidal flat, may be interrupted by rare random events, for example, a hurricane, which can erode much of the previous cyclic sedimentary record and leave its own unique lithofacies instead. Analysis of cyclicity is based on application of Walther's law. This law, first defined in the nineteenth century, stemmed from the recognition that vertical successions of strata are formed largely by lateral migration of sedimentary environments. The law states that only those facies found side by side in nature can occur superimposed on each other in vertical succession. Recognition of widespread cyclic units is the basis of sequence stratigraphy. Sequences record regional or global changes in sea level brought about by tectonics or eustacy.  See also: Cyclothem; Depositional systems and environments

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, Himalayas) and appears to result from tectonic thrusting of one continent over another. Lowpressure hornfels or greenschist-amphibolite facies series develop in response to intrusion of hot magma at shallow levels of the Earth's crust; this is a process referred to as contact metamorphism. As illustrated by these examples, metamorphic facies series can be used as an indicator of the plate tectonic history of a geologic region.  See also: Eclogite; Magma; Plate tectonics

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.