Geological time scale
Geological time scale refers to the geological evolution of the Earth over time in term of hundreds of millions of years. The Earth has been in existence for approximately 4.6 billion (4.6 x 109) years. During this time, numerous geological events and processes have taken place on the Earth's surface, leading to the formation of a series of rock layers.
Rock dating techniques
The age of these layers can be measured using absolute or relative dating techniques.
Absolute ages of rocks can be determined using radioactive isotopes (e.g. isotopes of U, Th, Rb, Sr). Assuming the half-life of a particular isotope is accurately known, the formation age can be calculated from the present-day parent/daughter isotope ratio. However, sedimentary rocks contain little or no radioactive material. The formation age of these rocks is therefore measured using relative dating techniques.
These techniques are based on some simple concepts.
Nicolauz Steno (1638-1687) proposed the law of superposition, which states that the older rocks in a normal sequence lie underneath younger ones. He also realized that most strata are deposited slowly and in a near-horizontal position, although later they may be folded or even overturned.
Georges Cuvier (1769-1832) noticed that rock sequences often contain linear features or horizons which mark a change in orientation of the layers. These horizons, known as unconformities, represent breaks in geological time during which the underlying beds were tilted and eroded before deposition of the next layer.
These methods for determining the ages of rocks allow geologists to determine a stratigraphic column, or sequence of rocks arranged in chronological order, for a particular locality. However, problems arise when it is necessary to correlate between localities that are separated by large distances or regions where no rocks are exposed. For these purposes, fossils are used to date rocks.
Fossil evidence suggests that organisms first appeared on Earth c. 600 million years ago. Since then, evolutionary change has produced a large diversity of life forms. It is assumed that the evolution of a group of organisms proceeds from simpler to more complex organisms. Therefore, the fossil remains found in a series of rocks can be used to determine their relative ages. Shelly marine fossils are most commonly used as correlative tools, as the original organisms occurred in large numbers over a wide geographical region, and underwent rapid evolutionary change over short time intervals. For example most ammonite species lasted for ~ 2 million years before becoming extinct, which is very short compared to the entire geological timescale. By combining both absolute and relative dating methods, a geological timescale can be constructed.
When reading geological accounts of reservoirs or oilfields, you should be aware that three different sets of terminology may be used to describe the stratigraphy of a region; lithostratigraphy, biostratigraphy and chronostratigraphy.
- Lithostratigraphy (rock stratigraphy) describes the sequence of rock units in terms of their mineralogy, petrography, and internal structure.
- Biostratigraphy describes the rock succession in terms of fossil content and the relative ages of the units.
- Chronostratigraphy uses an internationally agreed hierarchy of units to indicate the absolute age of the rock layers.