Synchronicity, Geological

Synchronicity is the state of being synchronous or simultaneous; of occurring, existing, or being formed at the same time. The term synchronicity is applied to the co-occurrence of any geologic features or events in time, which are called synchronous events, or to rock surfaces on which every point has the same geologic age. Rocks formed in the same part of geologic time, which have identical or nearly identical geologic ages, are called synchronogenic. Synchronicity of events in multiple cores across a wide area has been used, for example, to identify turbidites as seismically generated (and therefore present at the same time in different settings) and to distinguish them from other types of turbidites.

A stratigraphie surface in which every point has the same geologic age is called a synchrone: a zone representing equal time. Chronostratigraphy, a brand of stratigraphy that deals with the age of the geologic strata and their time relations, is the use of chronostratigraphic correlation to demonstrate correspondence in age and chronostratigraphic position between geologic units. Chronostratigraphic [Page 1212]correlation uses chronohorizons (chronostrati-graphic horizons). Each chronohorizon corresponds to a stratigraphie surface or interface that is everywhere of the same age, and theoretically it has no thickness. However, it commonly corresponds to a very thin and distinctive interval that is essentially isochronous over its whole geographic extent and thus constitutes an excellent time-reference or time-correlation horizon. Many biohorizons, bentonite beds, and coal beds are valuable horizons for chro-nostratigraphic correlation, although the most geographically widespread horizons are those marked by magnetic reversals or those caused by global events, such as the red-rusty layer with impact evidence that is commonly found at the Cretaceous-Tertiary (K-T) boundary. To explain the origin of this layer, most authors agree that a large meteorite impacted the Yucatan Peninsula in Mexico at the K-T boundary, triggering catastrophic mass extinctions, as well as an anomalous enrichment in the element iridium and other extraterrestrial impact evidence such as shocked quartz grains or nickel-rich spinels in the boundary sediments globally. The meteorite impact and the deposition of the layer with impact evidence are thus synchronous events that occurred in an almost instantaneous geological period of time, and the K-T boundary layer can be used as an excellent chronohorizon, allowing chronostratigraphical correlation globally. Therefore, this layer can be considered as an isochronous (synchronous) surface having everywhere the same age within a body of strata. Magnetic reversals are also recorded globally in the stratigraphie record, as the magnetic field has its origin in the melt core of the planet, and magnetic reversals have occurred throughout earth history.

Other methods of chronostratigraphic correlation are based on biostratigraphy, philogeny, biomagnethostratigraphy, ecobiostratigraphy, quantitative and statistical studies of fossil assemblages, chemostratigraphy, cyclostratigraphy, and event-stratigraphy. Biostratigraphical correlation compares the stratigraphie distribution of taxa in different areas. Integrated biostratigraphy is one of the main tools of bio- and geochronology; it consists of the biostratigraphical study of different fossil groups, which provide different biozonations that can be combined allowing correlation of different biostratigraphical scales, increased resolution of the scales, as well as recognizing the diachronism of some biostratigraphical boundaries.

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