Stratigraphy (Latin ‘stratum’ + Greek ‘graphia’), is the interdisciplinary science field that describes all rock bodies that form the Earth's crust and the manner in which they are organised into distinctive units that are then mapped. These stratigraphic maps are based on the inherent properties or attributes of rock bodies in order to establish their distribution and relationship in space and their succession in time. The distinctions and the maps are used to interpret geologic history.
Definitions and organisation
The primary term, stratum (plural is strata) refers to a layer of rock with specific properties which distinguish it from adjacent layers. A layer of ancient mud which has hardened over time into shale may be covered with a layer of deposits from an ancient inland sea which formed after the mud was deposited. These two types of strata will show very different charateristics. The younger or upper layer of limestone, for example, deposited at the bottom of ancient body of water will have a different colour and different chemicals composing it (the remains of animal shells for example).
Stratigraphic classification is derived from the organisation of the Earth's rock bodies found in their original relationships. Classification into distinct units is based on properties that are used in stratigraphic research. In the case of a layer of shale from an ancient mud slide or a period of deposits from rain which then carried soil to the bottom of a new lake or the mouth of a river can then be covered by later deposits of standing bodies of water which contain fish and shell fish. Later seismic events might bend and twist these two layers in some places making their relative ages difficult to determine. Research would then be necessary to ascertain which layer came first, in their original relationship so that they could then be categorised. The important thing is to determine the order in which the layers were deposited
Stratigraphic units are bodies of rock for which distinct properties allow for classification of the Earth's rocks. The units are based on properties or attributes or combinations of both that rocks have. Shale has characteristics different from limestone, basalt, an extruded volcanic rock, different characteristics from metamorphic layers, rocks that were changed by heat. These differences mean that stratigraphic units based on one property will not necessarily coincide with those based on another unit.
Stratigraphic terminology encompasses the total of terms for units used in stratigraphic classification. These may be either formal or informal.
- Formal stratigraphic terminology uses unit-terms defined and named according to guidelines established by international conventions.
- Informal stratigraphic terminology uses unit-terms as ordinary nouns in a descriptive sense, not as a part of a specific scheme of stratigraphic classification. Informal terminology has the effect of creating confusion and are not encouraged.
Stratigraphic nomenclature is a system of proper names established through international convention and are given to specific stratigraphic units.
Minor bodies of rock in the various established categories of stratigraphic classification are designated as zones. Zones are indicated by a prefix, e.g., lithozone (types of rock, chemical composition, subsequent changes to the rock layers, etc.), biozone (fossilised life forms that differ from zone to zone or are continuous), chronozone (specific period of time in which the layers of rock or strata were deposited).
An interface (point of physical contact) between two different layers is called an horizon and is indicative of specific positions in a stratigraphic sequence of rock layers. Horizon types are indicated by a prefix, e.g., lithohorizon, biohorizon, chronohorizon indicating the differences in adjacent strata with reference to type of rock, fossils and time respectively.
Demonstrated correspondence in character and/or stratigraphic position are called a correlation. Correlation types are indicated by a prefix, e.g., lithocorrelation, biocorrelation, chronocorrelation.
Dating and determining the time sequence of the events in the Earth’s history is the science of geochronology.
Subdivisions of geologic time are separated into geochronologic units.
Geochronometry is a branch of geochronology that deals with the quantitative (i.e. numerical) measurement of time. Designated abbreviations for period of years are
- ka = thousand
- Ma = million
- Ga = billion (milliard of thousand million)
The term facies was originally restricted to mean to a lateral variation in lithologic aspects of stratigraphic units. The meaning of facies has been broadened to mean a wide range of concepts including but not exclusive of environment of deposition, lithologic composition, geographic, climatic or tectonic association.
Biostratigraphy involves the identification of fossils and their position relative to their occurrences in space and time. Fossil groups are confined to specific sedimentary layers which reflect changes in the earth’s environment. Fossils only occur in the lithosphere having been formed in the terrestrial (land), freshwater and marine (sea) environments.
Chronostratigraphy deals with rock bodies. This is in contradistinction to geochronology which deals with the dating and subdivision of geologic time.
There is a historical reason for this distinction. Before radioactive decay of natural elements was discovered, stratigraphers had no means to determine the true age of rocks. However, correlation of stratigraphic units allowed the establishment of the order geologic events in time based on biostratigraphy and the superposition principle. A chronostratigraphic scale was established, wherein geologic time was subdivided into a hierarchy of chronostratigraphic units of unknown duration.
With the discovery of radioactive decay, radiometric dating techniques have collectively become an important means to determine the ages of boundaries between chronostratigraphic units. A chronostratigraphic scale that integrates absolute ages is called a geologic time scale.
Lithostratigraphy is the study of lithology and the description and nomenclature of the rocks of the Earth based on their lithology and their stratigraphic relations.
Lithology is the analysis of the composition of rocks and their texture.
A lithostratigraphic unit may consist of sedimentary, or igneous, or metamorphic rocks. Lithostratigraphic units are defined and recognized by observable physical features. Lithostratigraphic units are not defined by age, the time span they represent, geologic history, or the manner in which they were formed. Lithostratigraphic units are defined entirely by their continuity and the extent of their lithologic features.
Magnetostratigraphy is a field within stratigraphy that studies the magnetic characteristics of rock bodies.
If the magnetic properties of rocks have measurable differences stratigraphically, that is, from one strata to the next, those differences can be used to identify their relationships and identify varying stratigraphic units. Stratigraphic units are known collectively as "magnetostratigraphic units" ("magnetozones").
The most useful magnetic property for magnetostratigraphy results from a change in the direction of the magnetization of the rocks. Crystals in rocks are magnetically aligned with the Earth’s magnetic field. The Earth’s magnetic fields has changed over the eons and those magnetic alignments are ‘recorded’ in the crystals of rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. The change in the earth’s magnetic fields is caused by reversals in the polarity of the Earth's magnetic field, the Earth’s magnetic poles literally change locations. These reversals of Earth’s polarity have taken place many times during geologic history. The direction of the remnant magnetic polarity recorded in a stratigraphic sequence can then be used as the basis for a division of each sequence into units with the same magnetic polarity. These units are called "magnetostratigraphic polarity units".
- Age (geology)
- Cambrian (geology)
- Earth science
- Geologic ages of earth history
- Geologic time scale
- Sedimentary geology
- Stage (geology)
References and notes
- Definitions and Procedures International Commission on Stratigraphy
- Biostratigraphy Els Gervais and Hubert Jansen, J & G Consultants
- Salvador A.E., 1994 - International Stratigraphic Guide - A guide to stratigraphic classification, Terminology and procedure. Geological Society of America. ISBN 0-8137-7401-2
- confer section on History in Citizendium Geological Ages of Earth History
- Lithostratigraphic Units International Stratigraphic Guide]
- Magnetostratigraphic polarity units International Commission on Stratigraphy