Lithostratigraphy

Sedimentary layers are laid down by deposition of sediment associated with weathering processes, decaying organic matter (biogenic) or through chemical precipitation.

Igneous layers are generally devoid of fossils and represent magmatic or volcanic activity that occurred during the geologic history of an area.

The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed stratum is younger than the one beneath and older than the one above it.

The lithology of a unit includes characteristics such as chemical and mineralogical composition, texture, color, primary depositional structures, fossils regarded as rock-forming particles, or other organic materials such as coal or kerogen.

If the unit is nowhere entirely exposed, or if it shows considerably lateral variation, additional reference sections may be defined.

Geological correlation[citation needed] is the main tool for reconstructing the geometry of layering in sedimentary basins.

[7] The identification is based on comparison of physical and mineralogical characteristics of the rocks, and on general assumptions known as the Steno's principles:[8] 1.

The law of superposition is inapplicable to intrusive, highly deformed, or metamorphic bodies of rock lacking discernible stratification.

The 1983 North American Stratigraphic Code adopted the formal terms lithodeme, which is comparable to a formation; a suite, which is analogous to a group, and a supersuite, similar to a supergroup.

Also formalized is the term complex, which applies to a body of rock of two or more genetic classes (sedimentary, metamorphic, or igneous).

[10] However, the 1994 International Stratigraphic Guide regards plutons and non-layered metamorphic rocks of undetermined origin as special cases within lithostratigraphy.