What is stratigraphy?
The word ‘stratigraphy’ is used to describe the study of rock layers or strata. The most fundamental principals employed within this discipline are:
- The law of superposition, which states that, within any un-deformed suite of rock layers, the oldest will be at the bottom and the youngest at the top. In this way, a relative chronology between connected rock units can be established.
- The principal of faunal succession, which states that different rock layers from different times will contain unique fossil assemblages.
- The principal of uniformitarianism, which states ‘the present is the key to the past’. That is, by looking at sedimentological processes today and the features produced by them, we can draw inferences that tell us about the conditions in which ancient rocks were laid down. For example, we’ve all seen ripple marks on beaches – these are caused by sand particles being moved about by the ebb and flow of a shallow tidal environment. Therefore, it follows that rocks from Hastings, for instance, which show fossilised ripple marks, must have been laid down in a similar environment.
The principal of uniformitarianism can also be applied to biological entities. We can compare the characteristics of extinct organisms with those of organisms living today. From these comparisons, a huge amount of information about the lifestyles of these ancient organisms can be gleaned. One example of this can be seen in the dinosaur, Edmontosaurus. Scientists believe that an inflatable sac on its snout would have been used as a resonator, enabling calls to be made to other individuals, in much the same way that elephant seals use a similar nasal sac.
Types of stratigraphy
There are a variety of ways in which rock layers can be split up, united into groups or correlated. Three of the most important are set out below:
Lithostratigraphy. This is the study of the physical properties of rock layers, which include grain size, grain roundness, colour, chemical composition and so on. Changes in the lithology of rock sections are the most obvious form of stratigraphy that the amateur geologist or palaeontologist can employ. For example, if you can see distinct layers in a cliff, you are looking at the changing lithology of that section and can be said to be practicing lithostratigraphy.
Biostratigraphy. This is based on the fact that, across sizeable distances, the fossils that a rock layer contains will often be the same. Therefore, locations remote from each other, but with similar fossil assemblages, can be shown to be of the same age. However, biostratigraphy cannot always be used for correlations, as all species are restricted (to varying extents) by their environments.
Chronostratigraphy. Unlike the previous two branches of stratigraphy mentioned, which only provide relative ages for rocks and fossils, chronostratigraphy is used to assign an actual age to them. This branch of stratigraphy came into being much later than the other two, due to the high level of technology needed to date rocks. However, using chronostratigraphy in conjunction with lithostratigraphy and biostratigraphy, the world’s rocks can be dated and correlated.
The vast stretch of time that preceded the present day on Earth has now been divided and sub-divided into manageable chunks. These divisions have been made according to changes in the stratigraphy of the rock column; most importantly biostratigraphical changes. Definitions of these time divisions are given below:
Eon: This is the coarsest division of geological time. For example, the Phanerozoic eon began some 550mya and continues today. This covers the time during which abundant life that readily fossilised has been present on our planet.
Era: The Phanerozoic eon is split up into the Palaeozoic, the Mesozoic and the Cainozoic eras. These eras are separated by mass extinctions. At the end of the Palaeozoic era and the start of the Mesozoic, some 90% of marine species and 70 % of terrestrial species were wiped out. At the end of the Mesozoic era and the start of the Caenozoic, about 50% of species died out – most notably the dinosaurs and the ammonites. Eras are sub-divided into periods.
Period: This is the division of geological time most commonly encountered by the amateur palaeontologist. If nothing else, most can confidently state that the rocks of their favourite fossil hunting ground were laid down during the Jurassic, Cretaceous and Silurian Periods, and so on.
Epoch: An epoch is a division of a period. Many are simply called the Lower/Early, Middle or Upper/Late part or the period they describe, for example, the Lower Cretaceous, the Upper Triassic and so on. However, some have names relating to where the type sections of the epoch are found, for example, the Ludlow, Wenlock and Llandovery epochs of the Silurian period, which are named after locations in Shropshire, England and Dyfed, Wales. Interestingly, the Palaeocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene are not periods as many think, but epochs of the Palaeogene (Palaeocene to Oligocene) and Neogene (Miocene to Holocene) periods.
Stage: The stage sub-division is used by palaeontologists to denote a suite of rock layers within an epoch that all contain easily identifiable and abundant index fossil(s), for example, the Cenomanian of the Upper Cretaceous. However, one stage may be represented by numerous different environments around the world, so stage index fossils are regional in nature.
Zone Stages: Stages can be divided into zones. These are named after fossil species that are abundant, often short lived and relatively restricted to that particular zone.
Bed: This is a specific layer or stratum of rock that that can be separated from the proceeding and succeeding beds by some definite change in stratigraphy.