Lithostratigraphy
Definition of Lithostratigraphy
Lithostratigraphy is a branch of stratigraphy focused on the recognition, subdivision, and correlation of sedimentary rocks based on their lithology.
Definition of Lithology
Lithology involves the study and description of the physical characteristics of rocks, including:
- Rock type
- Color
- Mineral composition
- Grain size
Lithostratigraphic Units
Lithostratigraphic units are bodies of sedimentary, extrusive igneous, metasedimentary, or metavolcanic rock, defined by their lithologic characteristics. These units are commonly stratified and tabular, recognized, and defined based on observable rock characteristics. Boundaries between different units may be placed at clearly identifiable or distinguished contacts or may be drawn arbitrarily within a zone of gradation. Lithostratigraphic units are defined strictly based on lithic criteria as determined by descriptions of actual rock materials. They carry no connotation of age. They cannot be defined based on paleontologic criteria, and they are independent of time concepts.
Terminology
- Lithosomes: The term lithosome refers to masses of rock of essentially uniform character and having intertonguing relationships with adjacent masses of different lithology. The shape of a lithosome is largely controlled by its relationship to adjacent lithosomes.
Classification of Lithostratigraphic Units (Formal)
There is a hierarchical framework of terms used for lithostratigraphic units, and from largest to smallest, these are: ‘Supergroup’, ‘Group’, ‘Formation’, ‘Member’, and ‘Bed’.
Formation
The basic unit of the lithostratigraphic division of rocks is the formation, which is a body of material that is mappable and can be identified by its lithological characteristics and its stratigraphic position. A formation should have some degree of lithological homogeneity and its defining characteristics may include mineralogical composition, texture, primary sedimentary structures, and fossil content in addition to the lithological composition. It may encompass a single lithosome, or part of an intertonguing lithosome, and thus consist of a single lithology. Alternatively, a formation can be composed of two or more lithotomies and thus may include rocks of different lithology. All other lithostratigraphic units are defined as either assemblages or subdivisions of formations. A formation is not defined in terms of its age either by isotopic dating or in terms of biostratigraphy. A formation may be, and often is, a diachronous unit, that is, a deposit with the same lithological properties that was formed at different times in different places.
Group / Supergroup
The major division of rock formations is called groups, each group includes a thick succession of rock formations extending over a large period. Large unconformities separate one group from another. A Supergroup is formed when two or more groups are joined together.
Member
Members are smaller stratigraphic units within the formation.
Bed
The smallest lithological unit is a single set of sedimentary rock units that has a distinct set of mineralogical or fossil characteristics.
Hierarchy of Lithostratigraphic Units
- Supergroup: A formal assemblage of related or superposed groups or of groups and formations.
- Group: Consists of assemblages of formations, but groups need not be composed entirely of named formations.
- Formation: A body of rock, identified by lithic characteristics and stratigraphic position, that is prevailingly but not necessarily tabular and is mappable at Earth’s surface and traceable in the subsurface. It must be of sufficient areal extent to be mappable at the scale of mapping commonly used in the region where it occurs. The fundamental lithostratigraphic unit – formations are grouped to form higher-rank lithostratigraphic units and are divided to form lower-rank units.
- Member: The formal lithostratigraphic unit next in rank below a formation and always part of some formation. A formation need not be divided entirely into members. A member may extend laterally from one formation to another.
- Lens (or lentil): A geographically restricted member that terminates on all sides within a formation.
- Tongue: A wedge-shaped member that extends beyond the main boundary of a formation or that wedges or pinches out within another formation.
- Bed: Distinctive subdivisions of a member; the smallest formal lithostratigraphic unit of sedimentary rock. Members commonly are not divided entirely into beds.
- Flow: The smallest formal lithostratigraphic unit of volcanic rock.
Major Types of Stratigraphic Units (North American Stratigraphic Code/Nomenclature, 1983)
Material Categories Based on Content or Physical Limits
- Lithostratigraphic Units: Conform to the law of superposition and are distinguished based on lithic characteristics and lithostratigraphic position.
- Lithodemic Units: Consist of predominantly intrusive, highly metamorphosed, or intensely deformed rock that generally does not conform to the law of superposition.
- Magnetopolarity Units: Bodies of rock identified by remnant magnetic polarity.
- Biostratigraphic Units: Bodies of rock defined and characterized by their fossil content.
- Pedostratigraphic Units: Consist of one or more pedologic (soil) horizons developed in one or more lithic units now buried by a formally defined lithostratigraphic or allostratigraphic unit and are identifiable based on bounding discontinuities.
- Allostratigraphic Units: Mappable stratiform (in the form of a layer) bodies defined and identified based on bounding discontinuities.
Categories Expressing or Related to Geologic Age
Material Categories to Define Temporal Spans
- Chronostratigraphic Units: Bodies of rock established to serve as the material reference for all rocks formed during the same periods.
- Polarity-Chronostratigraphic Units: Divisions of geologic time distinguished based on the record of magnetopolarity as embodied in polarity-chronostratigraphic units.
Temporal (Nonmaterial) Categories
- Geochronologic Units: Divisions of time distinguished based on the rock record as expressed by chronostratigraphic units.
- Polarity-Chronologic Units: Divisions of geologic time distinguished based on the record of magnetopolarity as embodied in polarity-chronostratigraphic units.
- Diachronic Units: Comprise the unequal periods represented by one or more specific diachronous rock bodies, which are bodies with one or two bounding surfaces that are not time-synchronous and thus “transgress” time.
- Geochronometric Units: Isochronous units (units having equal time duration) that are direct divisions of geologic time expressed in years.
Categories and Ranks of Stratigraphic Units as Defined in North American Commission on Stratigraphic Nomenclature Note 63
| Lithostratigraphic | Lithodemic | Magnetopolarity | Biostratigraphic | Pedostratigraphic | Allostratigraphic | |
|---|---|---|---|---|---|---|
| Supergroup | Supergroup | Supersuite | Polarity Superzone | Allogroup | ||
| Group | Group | Suite | Polarity Zone | Alloformation | ||
| Formation | Formation | Lithodeme | Complex | Biozone (Interval, Assemblage, Abundance) | Geosol | |
| Member | Member (or Lens, or Tongue) | Polarity Subzone | Subbiozone | Allomember | ||
| Bed(s) or Flow(s) | Bed(s) or Flow(s) |
| Table IIA: Material categories used to define temporal spans | |
| Chronostratigraphic | Polarity Chronostratigraphic |
| Enothem | Polarity Superchronozone |
| Erathem (Supersystem) | |
| System (Subsystem) | Polarity Chronozone |
| Series | |
| Stage (Substage) | Polarity Subchronozone |
| Chronozone | |
| Table IIB: Nonmaterial categories related to geologic age | |||
| Geochronologic | Polarity Chronologic | Diachronic | Geochronometric |
| Eon | Polarity Superchron | Episode | Eon |
| Era (Superperiod) | Phase | Era (Superperiod) | |
| Period (Subperiod) | Polarity Chron | Span | Period (Subperiod) |
| Epoch | Diatchron | Epoch | |
| Age (Subage) | Polarity Subchron | Cline | Age (Subage) |
| Chron | Chron | ||
