Stratigraphy
Introduction
Stratigraphy is a branch of geology that studies rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has two related subfields: lithostratigraphy (lithologic stratigraphy) and biostratigraphy (biologic stratigraphy).
History
The concept of stratigraphy was first introduced by the Danish scientist Niels Steno in the 17th century. Steno's principles of stratigraphy include the Principle of Original Horizontality, the Principle of Superposition, and the Principle of Lateral Continuity. These principles are still used today in the interpretation of the stratigraphic record.
Principles of Stratigraphy
Stratigraphy is based on four fundamental principles:
Principle of Original Horizontality
This principle states that layers of sediment are originally deposited horizontally under the action of gravity. It is a direct consequence of the laws of physics.
Principle of Superposition
The Principle of Superposition states that in an undisturbed sequence of sedimentary rocks, the oldest rocks are at the bottom and the youngest are at the top. This principle is fundamental to the interpretation of Earth's history from the stratigraphic record.
Principle of Lateral Continuity
This principle states that layers of sediment initially extend laterally in all directions. This means that if a sedimentary rock layer is found at two separate locations, it's likely that the two outcrops were once connected.
Principle of Cross-Cutting Relationships
The Principle of Cross-Cutting Relationships states that an igneous intrusion or a fault must be younger than the rocks it intrudes or displaces.
Lithostratigraphy
Lithostratigraphy is the branch of stratigraphy that studies the distribution and organization of rock layers. Lithostratigraphic units are defined by their observable rock characteristics. These units are grouped into larger units called formations. Formations are the fundamental units in lithostratigraphy.
Biostratigraphy
Biostratigraphy is the branch of stratigraphy that uses fossils to establish relative ages of rock layers and the geologic timescale. It relies on the Principle of Faunal Succession, which states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it never reappears.
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time. The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.
Sequence Stratigraphy
Sequence stratigraphy is a branch of stratigraphy that studies rock relationships within a time-stratigraphic framework of related facies bounded by unconformities. It provides insights into the changes in sediment supply and sea level changes.
Magnetostratigraphy
Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. It uses the magnetic polarity reversal scale, which has been calibrated to the geologic timescale for the last 23 million years.
Chemostratigraphy
Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. It can be used to further refine stratigraphic correlations and is particularly useful in marine strata.
Cyclostratigraphy
Cyclostratigraphy, a branch of stratigraphy that deals with the detection and investigation of cycles in stratigraphic sequences and the relation between such cycles and Earth's orbital parameters.