Facies
Under facies ( Latin facies , face ',' face ') in the broadest sense all the characteristics of are rock body or a rock association understood that from its geological resulting history. They can be purely descriptive features (color, structure (e.g. stratification ), mineral composition , fossils ) or those that are typical for the formation (magmatism, sedimentation) or subsequent changes ( metamorphosis , metasomatosis , weathering ) of a rock. A facies includes all rocks of a certain locality or region that were formed or transformed under the same physical and chemical conditions.
Basics and usage of language
The name originally goes back to the Swiss Amanz Gressly (1838), who defined it as the “sum of all primary organic and inorganic characteristics of a deposit in one place”. (This historical definition refers to sedimentary facies and does not yet represent the much broader concept of facies that is encountered today. Over the course of almost two centuries, the term facies has undergone changes and generalizations.)
The term facies is applied to rock associations with various rock properties. A distinction must be made between purely descriptive facies terms such as "green, marly facies" and facies terms describing the educational conditions such as "highly saline lagoon facies". To characterize the facies, these are provided with descriptive attributes, while structural-chronological criteria are referred to as lithostratigraphic layers (strata) .
In addition to the formation of the rocks (lithofacies), the fossils (biofacies) preserved in the rock can provide information about the climate, habitat and other environmental conditions that prevailed at the time of the deposition. Fossils that (in contrast to index fossils ) are restricted to very specific facies spaces and are typical for these, are called facies fossils .
Based on the three basic types of rock: sediments, igneous rocks and metamorphic rocks , a distinction can be made between sedimentary, igneous and metamorphic facies.
Terms such as microfaces or electrofacies emerge on the basis of examination methods for the more precise definition or delimitation of different facies.
Examples
Some examples of different facies are:
- Neritic facies (to distinguish deep sea facies)
- Fluviatile facies (river sedimentary rocks)
- Ostracod facies (contains shells of small crustaceans)
- Metamorphic facies (rocks changed by pressure and temperature at depth)
Facies areas
Large facies areas (supra-regional facies) can be divided into increasingly smaller facies areas.
In the case of sedimentary rocks, for example, a marine facies could be roughly divided into a deep sea facies (pelagic or abyssal facies) and a shallow water facies (neritic facies). The shallow water facies could in turn be divided into a reef and a lagoon facies. This enables the paleogeography of the sediment deposit area to be reconstructed. For example, the distribution of different carbonate facies in the area of today's Altmühltal results in an exact model of the sea basin, which was the deposit area of the carbonates in the Jurassic Age .
In igneous rocks , facies of different geochemical compositions and thus different mineral stocks and different eruption forms and cooling histories ( plutonites and volcanic rocks ) can provide information about the cause, location and formation of the magmatism.
In metamorphic facies, the rocks are differentiated on the basis of characteristic mineral compositions that are formed under certain pressure and temperature conditions. A distinction is made between sub- facies such as green schist facies or albite - epidote - almandine facies, which correspond to defined metamorphic areas. Areas of different metamorphic facies can be used to reconstruct the metamorphic conditions. For example, the distribution of metamorphic facies of former subduction zones differs characteristically from that of a metamorphosis when subsidence within a sedimentary basin .
Facies developments, facies rule according to Walther
Facies can not only differ spatially in terms of palaeogeography; they can also develop over time. For example, due to an advancement of the sea (transgression), littoral facies (coastal facies) can lie above beach facies. The rule of facies according to Walther (1894) states that with undisturbed stratification, only facies can lie on top of each other, which can also occur next to each other at the same time.
Microfaces
The term microfaces is to be delimited. This does not mean a facies of its own, but the summary of all facies features that can only be seen under the microscope. Examples of this are the type of cementation of sandstones or the occurrence of certain hard parts of microorganisms in limestone . In this way an extremely detailed determination of the sedimentation space and its development is possible from a microfacial point of view, especially with carbonates.
Electrofacies
The term electrofacies must also be distinguished. This does not mean a facies of its own, but the summary of various rock features , which are to be determined by borehole measurements . The term comes from petrophysics . Data from borehole measurements such as electrical conductivity , neutron density or natural gamma radiation of the rock drilled through are plotted against each other in diagrams. Different rocks are now grouped in different areas, which defines different electrofacies.
Individual evidence
- ^ A. Gressly: Observations géologiques sur le Jura Soleurois. In: Nouvelles Mémoires Société Helvétique des Sciences Naturelles 2, pp. 1–349, (1838) (French)