Chronostratigraphy is the branch ofstratigraphy that studies the ages of rockstrata in relation totime.
The ultimate aim of chronostratigraphy is to arrange the sequence ofdeposition and the time of deposition of all rocks within ageological region, and eventually, the entire geologic record of theEarth.
The standard stratigraphicnomenclature is a chronostratigraphic system based onpalaeontological intervals of time defined by recognisedfossil assemblages (biostratigraphy). The aim of chronostratigraphy is to give a meaningfulage date to these fossil assemblage intervals and interfaces.[1]
Chronostratigraphy relies heavily uponisotope geology andgeochronology to derive hard dating of known and well defined rock units which contain the specific fossil assemblages defined by the stratigraphic system. In practice, as it is very difficult toisotopically date most fossils andsedimentary rocks directly, inferences must be made in order to arrive at an age date which reflects the beginning of the interval.
Themethodology used is derived from thelaw of superposition and the principles ofcross-cutting relationships.
Becauseigneous rocks occur at specific intervals in time and are essentially instantaneous on a geologic time scale, and because they containmineral assemblages which may be dated more accurately and precisely by isotopic methods, the construction of a chronostratigraphic column relies heavily uponintrusive andextrusive igneous rocks.
Metamorphism, often associated withfaulting, may also be used to bracket depositional intervals in a chronostratigraphic column. Metamorphic rocks can occasionally be dated, and this may give some limitations to theage in which a bed could have been laid down. For example, if a bed containinggraptolites overliescrystallinebasement at some point, dating the crystalline basement will give a maximum age of that fossil assemblage.
This process requires a considerable degree of effort and checking of field relationships and age dates. For instance, there may be many millions of years between a bed being laid down and an intrusive rock cutting it; the estimate of age must necessarily be between the oldest cross-cutting intrusive rock in the fossil assemblage and the youngest rock upon which the fossil assemblage rests.
Chronostratigraphic units, with examples:[2]
It is important not to confuse geochronologic and chronostratigraphic units.[3][4] Chronostratigraphic units are geological material, so it is correct to say that fossils of the speciesTyrannosaurus rex have been found in the Upper Cretaceous Series.[5] Geochronological units are periods of time and take the same name as standard stratigraphic units but replacing the terms upper/lower with late/early. Thus it is also correct to say thatTyrannosaurus rex lived during theLate Cretaceous Epoch.[6]
Chronostratigraphy is an important branch of stratigraphy because the age correlations derived are crucial in drawing accurate cross sections of the spatial organization of rocks and in preparing accurate paleogeographic reconstructions.
