Period (geology)
A system referred to in the earth's history, a time unit of chronostratigraphy. It is similar to the geochronology of the unit period. A system or a period usually includes time intervals of several tens of million years.
Definition
A system is in the hierarchy of ranks of chronostratigraphic units, the average of five units. In geochronology this period the unit period corresponds. The terms system and period are often used in the literature - but not quite correctly - used as synonyms. The boundaries of chronostratigraphic unit system are bounded by isochronous ( simultaneous ) stratigraphic surfaces that are defined by biostratigraphic markers or other events. The systems are relatively defined by the boundary surfaces between two successive systems as well as by their sequence in time, ie the absolute age of the interfaces and the absolute amount of time a system is not included in this definition, since the absolute age of the interfaces can still change or other newer methods of age determination. The determination of these limits are real at a certain location at a certain point in the profile ( GSSP ). From the Ediacaran - border back into the Earth, the system limits are absolute geochronological defined. From this threshold the following including older systems are identical with the respective equivalent geochronological units (cycles ). However, there are efforts to define these limits chronostratigraphic and establish a GSSP. The absolute duration of a system or for a period is usually for historical reasons and varies considerably, from a few million years to several hundred million years ago in the Proterozoic.
Structure
In the hierarchy of chronostratigraphic units, a system is a subordinate unit of a Ärathems, ie multiple systems are combined in a Ärathem. In Geochronology the corresponding periods are grouped into an era. A system can turn itself be divided into several sub- series, a period in several epochs.
Naming
Up the international names of the systems of Chronostratigraphy or their equivalents in geochronology have been all named in the 19th century. The name derives from the historic landscapes (eg Cambrian ), ancient tribes (eg, Silurian ), characteristics of the rock formations (eg chalk), natural resources (carbon ), training of the system in Central Europe (eg Triassic = Trinity ), etc. However, there was about the naming of individual systems often long time no consensus (eg Dyas vs. Perm ). Some older systems have been divided into two systems (eg the old Silurian Ordovician and Silurian in the present ), the tertiary even only a short time ago in the Paleogene and Neogene systems.