International standards and resources
The International Commission on Stratigraphy(Opens in a new tab/window) (ICS) sets the global standards for the scale for expressing the history of Earth, including the International Chronostratigraphic Chart(Opens in a new tab/window) (ICC).
Australian conventions and resources
Geoscience Australia manages the Australian Stratigraphy Commission(Opens in a new tab/window), which develops stratigraphic principles and classification for Australia. Geoscience Australia also maintains the Australian Stratigraphic Units Database(Opens in a new tab/window).
Chronostratigraphic and geochronologic terms
The ICS defines global units (e.g. systems, series and stages) for when rock bodies were formed; these are called chronostratigraphic units and are shown on the ICC. The ICC is, in turn, the basis for the geochronologic units (eon, era, period, epoch and age) of the International Geologic Time Scale.
A chronostratigraphic unit has a binomial name – a proper noun plus a term word. Use initial capitals for both words:
Pleistocene Series Upper Pleistocene Stage
Geochronologic units define the geological time intervals of the ICC. A geochronologic unit name uses the same proper noun as the equivalent chonostratigraphic unit, with the corresponding geochronologic term (eon, era, period, epoch or age):
Pleistocene Series [chronostratigraphic] Pleistocene Epoch [geochronologic] Mesozoic Era Late Jurassic Cretaceous Period
Upper and Lower in formal chronostratigraphic names are replaced by Late and Early, respectively, in the geochronologic timescale:
Upper Triassic Series [chronostratigraphic] Late Triassic Epoch [geochronologic]
See the table below for a comparison of chronostratigraphic and geochronologic unit names.
Main hierarchical levels of formal chronostratigraphic and geochronologic terms
| Chronostratigraphic | Geochronologic |
|---|---|
| Eonothem | Eon |
| Erathem | Era |
| System | Period |
| Series | Epoch |
| Stage | Age |
| Miocene Series | Miocene Epoch |
| Upper/Lower Jurassic [formal] | Early/Late Jurassic [formal] |
| upper/lower Permian [informal] | early/late Permian [informal] |
The term words are not needed if there is no ambiguity:
... from the Cretaceous to the … ... until the end of the Permian
Adjectival words are capitalised only if they are part of the formal name:
in the Middle Jurassic from the Upper to the Middle Pleistocene
Use lower case for informal terms (e.g. early, late) that are used to modify formal names:
from the early Mesozoic to the late Paleozoic
Expressing geological time
Absolute dates can be expressed as follows:
by the Middle to Late Devonian (ca 385 Ma), fish had ...
between 55 ka and 21 ka
formed by 3,500 Ma
older than 4,000 Ga
where ka = kiloannum (a thousand years ago), Ma = mega-annum (a million years ago) and Ga = giga-annum (a billion years ago).
Note that, in expressing a range of time, the largest (oldest) number occurs first.
Duration of time can also expressed in kiloyears (kyr), megayears (Myr) and gigayears (Gyr):
at rates of 20–30 m per Myr
for the past 300 Myr
The Jurassic Period lasted approximately 50 Myr [duration] from ca 201 Ma to ca 152 Ma [absolute dates].
The before present (BP) timescale is often used in geology; see Date and time systems.