Stratigraphy is a fundamental geological method with the purpose of describing, classifying and analysing geological layers (rocks) composition, age and relation in order to establish an interpretation and mapping of the Earths geological development.
The stratigraphic method subdivides and organises geological layers and successions in an area into stratigraphy units, which are fundamental when the geological development of an area is to be described and interpreted. By applying the stratigraphic method, geological layers can be identified, differentiated and correlated from one area to another area and be mapped and registered. This has great practical implications when the geological resources of a region are to be assessed and estimated, whether the resource is coal, oil, gas, water, clay, sand, gravel, geothermal heat, minerals or other resources. In addition, stratigraphy is fundamental when it comes to the understanding of the development of life on Earth.
Stratigraphy originates from Latin – stratum (layer) and –grafi (describe) and is the oldest and most fundamental element of the geological science. The first stratigraphic principle was established by the Danish Niels Stensen (Nicolas Steno; 1638 – 1686), who defined the principle of superposition, stating that in undisturbed rock successions the lowermost layers are older than the overlaying layers.
There are three principal stratigraphic disciplines including their stratigraphic units:
- Lithostratigraphic units, which are defined on the basis of the lithological composition of rocks (e.g. sandstone, claystone, limestone).
- Biostratigraphic unit, which are defined based on the content of fossils in the sedimentary rocks.
- Chronostratigraphic units, which include all rocks deposited or formed during a specific geologic time interval.
In addition, a 4. discipline – sequence stratigraphy – is widely used, which subdivides succession of sedimentary layers into genetic units, that are interpreted to be formed under continuous and successive geological processes. Sequence stratigraphic interpretations build on the application of geological (rocks, strata) and geophysical data and information (seismic data, petro physical measurements).
The different stratigraphic units (litho-, bio-, chrono- and sequence stratigraphic) are organised and named in hierarchic systems, which are practical tools for descriptions and communications; e.g. lithostratigraphic units are define as beds, members, formations, groups and super groups.
The description of the relation and contact of the geological layers is essential for an analysis of the rock forming geological processes. The term ”inconformity” is used to describe the contact between two successive layers in a rock succession, which are not formed during continuous depositional processes in contrast to an “conformity” used for contacts between layers formed during continuous processes. If the layers are parallel, the contact is named a ”disconformity”; if the contact is characterised by a distinct angle (difference in dip angle) between the layers, the contact is named a ”angular discordance”. The first mentioned contact reflects an interruption in the sedimentation processes, while the latter contact illustrates an interruption where tectonic influence (tilting) and erosion occurred prior to the deposition of the overlying layer.
As a result of an unconformity, evidences in the form of sedimentary layers are missing from a time period in the stratigraphic column; this missing part (“hole”) in the stratigraphic record is called a hiatus (pluralis: hiati), which may be of local or regional distribution.