This is an old revision of the document!
Table of Contents
Chronostratigraphy - Geological Time Scales
Chronostratigraphy is the process by which geological time is divided up into meaningful “chunks” of named units. This is hierarchical with categories such as Eons, Eras, Periods, Epochs and Ages/Stages… together with various “Sub-” categories as well. Thanks to Mr. Spielberg and a rather confused Tyrannosaurus rex (which didn't even live during the Jurassic!) almost everybody has heard of the Jurassic Period. But the Jurassic is but one of three Periods that make up the Mesozoic Era which itself is but one of three Eras that make up the Phanerozoic Eon.
Going in the other direction hierarchically, in the Middle Jurassic Epoch, the Bajocian Stage is subdivided into Late, Middle and Early Substages but in New Zealand (for example) there is an interval within the Middle Bajocian called the middle Temaikan.
The latest addition to the lexicon is the “Anthropocene”, beloved of many modern geologists and, especially, environmentalists. At present, this term is not formally defined as scientists are still debating just exactly how to recognise it. However, when it becomes officially accepted it will become the latest Epoch of the Quaternary Period and the latest Epoch in Earth History.
Although there are many terms, each with their own typically hard-to-remember name, chronostratigraphic units are convenient “blocks” of geological time to enable geologists to communicate with one another about the rocks they see in their back yards.
In the 21st century after over 200 years of mulling over the issues. it may come as a surprise to many geologists, that many of the chronostratigraphic (e.g. Stage) names in which we place so much emphasis (e.g. Aptian, Visean, Miocene etc.) still lack a formal definition. The reason for this is… historical… in all senses of the word.
Similarly, even some stages which were once thought to be well-known, understood and “stable” in the hierarchy have been redefined and, in many cases, now occupy positions quite different to what they were in the past. For example, relatively recent changes in the position of the Maastrichtian – Campanian boundary (which has jumped 2 ammonite zones and three planktonic foram zones!) have rendered many publications and industry reports misleading due to references to sections of “Early Maastrichtian” age which are now more correctly “Late Campanian”.