Stratigraphy

Etymologically, the term stratigraphy (from the Latin stratum and the Greek graphia) means “description of strata.” The word stratum (plural, strata) was introduced by Nicolaus Steno in the 17th century to describe a layer of rock or sediment that is delimited by horizontal surfaces with lateral continuity and which represents a unit of time of deposition. The meaning of a stratum deals not only with the geometry of the stratified rocks but also with their genesis, assuming that it is a sedimentary unit deposited under constant physical conditions. Therefore, stratigraphy is not just a simply descriptive science, but it also deals with problems related to the genesis of the strata, the description of rock successions, and their interpretation in terms of a general timescale. Stratigraphers study all attributes of rocks as strata (e.g., form, distribution, lithologie composition, fossil content, geophysical and geochemi-cal properties) and infer their origin and geologic history.

The origin of stratigraphy has its roots in scientists trying to determine the age of the earth. It was first introduced in the geological nomenclature by Alcide d'Orbigny in the 19th century, as a consequence of the need to order processes in time, so as to establish a geological chronology. Since Amadeus W. Grabau published the first treatise on stratigraphy in 1913, the term stratigraphy has been used to refer to a geological science that studies the order and the relative position of the strata, the stratified rocks being its main focus. The sequence of layering of the rocks, together with the evolution of life, is the basis of the relative timescale. In addition, stratigraphy can yield information about the processes affecting the deposition of sediments, which can result in a complex layout of the strata. Thus stratigraphers study and interpret the processes recorded in the sedimentary successions in order to determine the nature and arrangement of the stratified rocks, correlate rocks and events, and order the sequences of rocks and events in a relative timescale.

Since 1917 the improvement of radiometric techniques has allowed scientists to determine the absolute age of rocks and to work out the duration of the intervals of geologic time that had been previously established by means of fossils. The development of the oil industry between 1920 and 1940 led to a significant improvement in the knowledge of lithostratigraphy and the geometry of stratified sedimentary bodies. Until the mid-1900s the French school supported a historical meaning of stratigraphy, whereas the North American school focused on a dynamic concept of this science and on the interpretation of the geometries of sedimentary bodies. It was this second concept that was more broadly accepted within the scientific community. Afterward, the advent of plate tectonics led stratigraphers to concentrate their studies on the mobility of sedimentary basins and their evolution through time. Data from [Page 1202]international research programs on marine geology, such as the Deep Sea Drilling Project, the use of paleomagnetism, sedimentary geochemistry and seismic stratigraphy, together with the improvement of the biostratigraphical methods and the abundant studies on radiometric dating, contributed to the current meaning of stratigraphy as a science and to its subdivision into several branches. The most important of these are lith-ostratigraphy (study of the geometrical bodies of stratified rocks, their geometry and genesis), bio-stratigraphy (study of the temporal distribution of fossils in the stratigraphie record), chronostratig-raphy and geochronology (determining the age of the stratigraphie units and establishing a worldwide stratigraphie scale), magnethostratigraphy (establishing a scale of changes in magnetic polarity through time), chemostratigraphy (analysis of stable isotopes and chemical elements in the stratified rocks), sequence stratigraphy (recognizing major events recorded in the stratigraphie record), and basin analysis (reconstruction of the spatial and temporal distribution of each unit of stratified rock within a sedimentary basin).

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