The geological column is a composite diagram that shows in a single column the vertical or chronologic arrangement of the subdivisions of geologic time, or the sequence of rock units of a given region. Geologic time includes the part of the earth’s history that is represented by and recorded in the successions of rocks, or the time extending from the formation of the earth as a separate planetary body to the beginning of written history. Earth scientists use a common language to talk about geologic time. That common language is standard, and it is ruled by the Geologic Timescale (GTS). The modern GTS consists of two different scales: the relative timescale, which is made of chronostratigraphic units, and the chronometrical or absolute timescale, which consists of geochronologic units. Due to the complexity and duration of geologic history, both scales are divided into hierarchical levels that are used by historical geology to analyze the history of our planet and of life on Earth.
Principles and Development
The standard geological column represents an ideal succession containing rocks from all ages, the earliest rocks on Earth at the bottom of the column and the youngest ones at the top. The construction of the geological column is based on the underlying principles of stratigraphy, first proposed by Nicolaus Steno around 1669. According to his principle of superposition, the oldest stratigraphic units are located at the bottom of the column and the youngest at the top, with dips adjusted to the horizontal. The resulting geological column indicates the relations between the stratigraphic units and the subdivisions of geologic time, and their relative positions to each other. The principle of superposition is the basis for establishing the relative ages of all strata and the fossils that they contain.
The geological column was developed largely during the early 19th century, and its origin probably begins with the story of the first geological map of England, published by William Smith in 1815. Smith was the first to realize that fossils were arranged in order and regularly in strata, always in the same order from the bottom to the top of a section, each stratum being characterized by particular types of fossils. These observations led him to propose the principle of faunal succession. Furthermore, the relative order of the formations was proved to be the same even in distant locations of Great Britain. The application of these two simple principles (superposition and faunal succession) led to the construction of the first geological column. In addition, the geological column was based on the uniformi- tarian principles (the present is the key to the past, i.e., processes operating in the past were constrained by the same laws of physics that operate today) first proposed by James Hutton in the mid-18th century and further developed by Charles Lyell.
The standard geological column aims to establish a classification system to organize systematically the rocks of the earth’s crust into formal units corresponding to intervals of geologic time. Such units must be of global extent to allow correlation. Among the formal units for stratigraphic classification, chronostratigraphic units—units based on the time of formation of the rock bod- ies—offer the greatest potential for worldwide application because they are based on their time of formation, and are therefore the most accepted units to mark positions in the stratigraphic column. Other units such as lithostratigraphic, bio- stratigraphic, and unconformity-bounded units are all of limited areal extent and thus unsatisfactory for global synthesis. The biostratigraphic units are nevertheless unique in the sense that the fossils they contain show evolutionary changes through geologic time that are not repeated in the stratigraphic record. Due to the irreversibility of evolutionary change, biostratigraphic units are indicative of geologic age. However, owing to the imperfection or incompleteness of the fossil record, and the dependence of the fossil-producing organisms on biogeography and depositional facies, the boundaries of the biostratigraphical units commonly lie at different stratigraphic horizons and, similar to unconformity-bounded units, they may be diachronous and represent all or parts of one or several chronostratigraphic units. Magnetostratigraphic polarity units approach synchronous horizons because their boundaries record the rapid reversals of the earth’s magnetic field. Although magnetostratigraphic polarity units may be useful guides for chronostratigraphic position and have a potentially worldwide extent, they have relatively little individuality because one reversal looks like another, and they can usually be identified only by supporting age evidence. Therefore, magnetostratigraphic polarity units require extrinsic data such as biostratigraphic data or stable isotope analyses for their recognition and dating. All these stratigraphic units are based on one property each, and they will not necessarily coincide with those based on another.
For convenience, a chronostratigraphic scale has been created to divide the rock record into chro- nostratigraphic units, which are relative time units. Chronostratigraphic units are divisions of rock bodies based on geologic time. They are studied in Chronostratigraphy, the branch of Stratigraphy that deals with the relative time relations and ages of rock bodies. The purpose of the chronostratigraphic classification is to organize systematically the rocks of the earth’s crust into chronostratigraphic units corresponding to intervals of geologic time. These intervals of geologic time are called geochronologic units, and they actually measure time in years before the present. The geochronologic scale helps to calibrate the chronostratigraphic scale to linear time. Chronostratigraphy aims to provide a basis for time correlation and to create a reference system to record events of geologic history; in order to achieve these goals, the scale is standardized by the International Commission on Stratigraphy (ICS).
Chronostratigraphic units are tangible stratigraphic units because they encompass all the rocks, layer upon layer, formed within a certain time span of the earth’s history regardless of their compositions or properties. By definition, they are worldwide in extent, and their boundaries, which are called chronostratigraphic horizons or chronohorizons, are synchronous, everywhere the same age. Whereas other kinds of stratigraphic units are identified on the basis of observable physical features, chronostratigraphic units are distinguished and established on the basis of their time of formation as interpreted from these observable properties. Several hierarchical levels may be distinguished among chronostratigraphic units, namely eonothem, erathem, system, series, and stage (from the most to the least comprehensive levels). Their rank and relative magnitude are a function of the time interval represented by their rocks. The oldest eonothem is the Archean, and it is followed by the Proterozoic, and by the most recent Phanerozoic. The names of the different erathems are related to the ideas of evolution, representing major changes of the development of life. The oldest erathem of the Phanerozoic is thus called Paleozoic, which means “ancient life,” and it is followed by the Mesozoic, meaning “middle life,” and by the Cenozoic, which means “recent life.” The names of most formal stratigraphic units more commonly consist of an appropriate geographic name (usually, the geographical regions where they were first found and studied) combined with an appropriate term indicating the kind and rank of the unit. Position within a chro- nostratigraphic unit is expressed by adjectives indicative of position, such as basal, lower, middle, upper, and so on. Stages can be subdivided into substages or grouped into superstages. A stage is defined by its boundary stratotypes, that is, sections that contain a designated point in a stratigraphic sequence of almost continuous deposition, chosen for its correlation potential. The lower and upper boundary stratotypes represent specific moments in geologic time, and the geologic time between them is the time span of a stage (generally between 2 and 10 million years). Special attention is paid to the selection of the lower boundaries of chronostratigraphic units, since the upper boundary of a given chronostratigraphic unit corresponds to the lower boundary of the succeeding unit. Therefore, each chronostrati- graphic unit is defined in the rock record by a boundary stratotype that is formally known as a Global Stratotype Section and Point (GSSP), which provides an unequivocal definition of the chronostratigraphic units in the stratigraphic record. If possible, boundary stratotypes must be identified in marine, fossiliferous, and continuous sections that are well exposed and easily accessible, and they should contain synchronous marker horizons that allow long-distance correlation. An example of a geologically synchronous boundary stratotype is the Cretaceous/Tertiary boundary stratotype, whose GSSP is located at the El Kef section in Tunisia. This boundary stratotype contains multiple markers, including evidence of the mass extinction of marine and terrestrial groups such as calcareous plankton or dinosaurs, the restructuring of other faunal groups such as benthic foraminifera, the deposition of a rusty-red layer with an anomalous concentration of iridium, microtektites, shocked quartz grains, and Ni-rich spinels, isotope anomalies (negative shift in C-13), and so forth. These palaeontological, mineralogical, geochemical markers are related to a global event, the impact of an asteroid on Earth, and they allow worldwide correlation of the Cretaceous/ Tertiary boundary. Boundary stratotypes are important because, apart from defining stages, they also define series (whose time spans range from 13 to 35 million years) and systems (normally from 30 to 80 million years each).
A chronozone is a chronostratigraphic unit of unspecified rank, and it includes all rocks formed everywhere during the time span of some designated stratigraphic unit or geologic feature. Although chronozones are formal chronostratigraphic units, they are not part of the hierarchical chronostrati- graphic classification.
The time during which a chronostratigraphic unit was formed corresponds to a geochronologic unit, which corresponds to a unit of time, a subdivision of geologic time. The geologic timescale is based on geochronologic units, and it is the timeintangible-equivalent to the physical geological column, which is based on chronostratigraphic units. Time cannot be found in a rock body, but we can assign a certain age to rock bodies through the analysis of tangible features. Therefore, the geological column can also be divided into geochronologic units. The geologic timescale is usually presented in the form of a chart showing the names of the various stratigraphic units, including chronostratigraphic units and geochronologic units. Unlike the chronostratigraphic scale, which is based on relative time units, the geochronologic or chronometric scale measures time in years before the present. The geologic timescale results from joining the chronostratigraphic and the geochronologic scales.
As in the chronostratigraphic units, several hierarchical levels can be distinguished among geochronologic units, namely eon, era, period, epoch, and age, with eons being the most comprehensive levels and ages the smallest levels. Eras are divided into periods, the periods are further divided into epochs, and the latter into ages. These units are equivalent to chronostratigraphic units: for example, an age represents the time during which a stage was formed and it takes the same name as the corresponding stage, and an epoch is the geochronologic equivalent of a series. Eras and eons take the same name as their corresponding erathems and eonothems. Position within a geochronologic unit is expressed by adjectives indicative of time, such as early, late, latest, and so on. The time span during which a chronozone was deposited corresponds to a chron. Although the International Stratigraphic Guide states that “a chronozone includes all rocks formed everywhere during the time span of some designated stratigraphic unit or geologic feature,” most chronozones and their corresponding chrons are derived from previously established biozones or biostrati- graphical units (bodies of stratified rocks that are characterized by their fossil content).
Standard Global Chronostratigraphic (Geochronologic) Scale
All units of the standard chronostratigraphic and geochronologic hierarchies are theoretically worldwide in extent. They provide a standard scale of reference, what is known as the Standard Global Chronostratigraphic (Geochronologic) Scale, that aims to date all the rocks everywhere and to relate all rocks everywhere to the earth’s geologic history. The standard geological column and its equivalent geologic age system have been built up by superposition of local columns from many different localities. A local geologic column is called a “geologic section,” and it consists of any sequence of rock units found in a given region either at the surface or below it. Although the geologic column is not found complete at any place on Earth and the representative sediments common to all the major divisions cannot be found all together in a single section, the relative order of the formations still remains the same; in addition, such relative order also fits the geologic column.
The quantitative (numerical) measurement of geologic time is dealt with by geochronometry, a branch of geochronology. The improvement of radiometric techniques since 1917 has allowed scientists to determine the absolute ages of rocks and to work out the duration of the intervals of geologic time, which had been previously established by means of fossils. More recently, enhanced methods of extracting linear time from the rock record have enabled high-precision age assignments. Apart from high-resolution radiometric dating, some of these calibration methods include the use of geochemical variations, Milankovitch climatecycles, andmagneticreversals. Radioactivity allowed scientists to date chronostratigraphical units, contributing to the development of the modern GTS. Technologic advances in measuring magnetic properties of rocks, together with the intense drilling of oceanic sediments and their biostratigraphical calibration by means of microfossils, have led to an improved chronology (magnetobiochronology). Moreover, the rapid development of cyclostratigraphy during the past decades has led to the construction of an astronomical timescale for dating events in the geologic record, based mainly on the relation between sedimentological, geochemical, or palaeontological cycles and variations in the earth’s orbital parameters.
Recent Developments and Future Directions
The construction of the geological column has been under way for the last 2 centuries, and it has overcome several obstacles such as the precision and accuracy of correlation and dating tools, the limits of the stratigraphic database, or problems of nomenclature. In March 2005 the current available stratigraphic and geochronologic information was compiled by the project “Geologic Time Scale 2004” and published by a team designated by the International Commission on Stratigraphy. The results of this project summarized the history and status of boundary definitions of all geologic stages, compiled integrated stratigraphy (biologic, chemical, sealevel, magnetic, etc.) for each period, and assembled a numerical age scale from an array of astronomical tuning and radiometric ages. A combination of zones, polarity chrons, stages, and ages was carried out in order to calculate the best possible timescale. Earth scientists thus keep concentrating their efforts on the construction and improvement of the GTS. Research on Ocean Drilling Program (ODP) cores, and on Integrated Ocean Drilling Program (IODP) cores in the near future, will improve the calibration of various biostratigraphical scales, and together with the development of new tuning strategies will probably extend the astronomical timescale downward. Even so, there are still some stratigraphic and geochronologic issues to be resolved in the next updated version of the Geologic Time Scale, the GTS2008, which is expected to include a consensus on all stage boundary stratotypes, which is one of the main challenges for the future.
See also Chronostratigraphy; Dating Techniques; Geologic Timescale; Geology; Hutton, James; Lyell, Charles; Smith, William; Steno, Nicolaus; Time, Planetary
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