Lithostratigraphic units refer to defined bodies of rocks delineated and distinguishable based on their lithic/lithological characteristics and relative stratigraphic position. They can be bedded or unbedded and are usually layered and tabular, but not always, and obey the principle of superposition.
Some of the foremost lithological properties that distinguish and delineate bodies of rocks include composition (chemical and mineralogical), color, texture, primary structures, fossils (as rock-forming particles), fabric, and organic materials like kerogen (oil shale) or coal. As you can see, these are not time-based/age units and don’t represent the duration of time they formed, the manner of formation, or their inferred geological history.
Lithostratigraphic units may represent sedimentary, metamorphic, igneous rocks, or even lithosomes. A lithosome is a body of rock with uniform character, defined by its internal structure, geometry, and composition. It may recur multiple times in one geologic unit and interchange with adjacent bodies of rocks.
Lastly, bodies of rock that don’t obey the law of superposition are lithodermic units and include severely deformed rocks, highly metamorphosized, or predominantly intrusive rock bodies that have lost their structure.
Lithostratigraphy and lithostratigraphic classification
Lithostratigraphy is a branch of stratigraphy that describes and names the bodies of rocks based on observable lithological characteristics of the strata and their relative stratigraphic position.
The process of organizing rock bodies into units using lithic properties and relative position is known as lithostratigraphic classification.
What are the kinds of ranks of lithostratigraphic units?
The formation is the primary, fundamental, or basic unit lithostratigraphic unit. Beneath it, you can have members, submembers, and beds/flows. Above it, there are groups and supergroups.
Let us look at each of these rock units.
1. Formation
Formations are the smallest mappable bodies of rocks diagnosable by their lithologic characteristics and stratigraphic position relative to others. They are the primary, fundamental, or basic unit used in lithostratigraphic classification, appear in all stratigraphic columns, and can be less than a member to thousands of meters thick.
A formation is only useful if it can be delineated to the geological map scale of the study area, either on Earth’s surface or traced to the subsurface. The lithological contrast necessary to establish formations depends on the geologic complexity of a place and the details you need to show on the map and determine its geological history.
However, they should have unique or similar lithic features that differentiate them from adjacent layers. They may be of 1) single lithology, 2) two or more repeating lithic types, or 3) lithic heterogeneity that is extreme but unifies them compared to adjacent rock layers or units.
Lastly, a formation can be below a meter to several meters thick and be members or groups in other areas. They can include stratified volcanic rocks, low-grade metamorphic rocks with primary structures, interbed volcanic and sedimentary rocks, carbonate mounds, and organic reefs.
2. Members
Members are formal lithostratigraphic units ranking immediately below formations, named as entities due to their lithic characteristics distinguishing them from adjacent parts of a formation. They are always part of a formation, may not be present, can appear in entire or within some sections, or even extend another formation laterally.
Formations are only subdivided and named if they have some geological significance, like helping study or track certain lithology distributions within them. A good example is a sandstone formation with conglomerate beds at some lateral sections.
Members can formally or informally be subdivided into submembers or beds. Also, they can be a single bed, an unstratified body of rock, tongue, lens, carbonate mounds, or organic reefs within formations. Also, despite being in a vertical sequence, recognizable lateral equivalents in a formation may be members.
Lastly, members need not be mappable at the scale of formations; even when mappable, they cannot be raised to formations. Also, there is no fixed requirement for their thickness.
3. Submembers
Submembers rank below members and have distinctive lithic properties distinguishing them from adjacent parts of a member. They may be complete divisions within a member, certain lateral sections, or extend into another member.
Establishment or subdividing members into submembers is only necessary if important to recognize it as a heterogeneous part of a member. It doesn’t need to be mappable at the scale of formations, and even when mappable locally, they cannot be members.
Lastly, recognizably distinctive lateral equivalents parts are submembers, too, even though we recognize submembers in a vertical sequence.
4. Beds
A bed is a sedimentary layer bound by well-defined planes. It represents the smallest formal unit in lithostratigraphic classification and is only designated or named when its recognition is significant or helps to correlate rock strata. Also, its lithology or fossils should be distinctive.
Oil sands, coal beds, and other economically important beds will get names that don’t follow formal lithostratigraphic nomenclature.
Lastly, marker or key beds are usually named those thin, widely distributed beds with unique lithology or rocks. They are informal units going or traceable beyond a formal lithostratigraphic unit.
5. Flows
A flow is the smallest formal lithostratigraphic unit found in volcanic flow rocks, equivalent to a bed in sedimentary rocks. This discrete, extrusive volcanic body of rock should have a distinguishable 1) texture, 2) composition, 3) paleomagnetism, 4) superposition order, or 4) following any other objective criteria.
As a lithostratigraphic unit, formal flow establishment and naming should be limited to the widespread and distinctive ones.
5. Group
A group is a lithostratigraphic unit ranking immediately above a formation, i.e., made of two or more associated/continuous formations with certain common characteristics, entirely named or otherwise. For example, is a dominantly argillaceous facies or unconformity bound basin-wide group.
Most groups occur in the same basin and are traceable across it, and we use them for small-scale mapping and analyzing regional stratigraphy. An example is the Nanaimo Group located in Washington and British Colombia state that dates to the Cretaceous period.
Formations in a group don’t have to be the same everywhere laterally. Also, groups can drop in rank to formation. This can occur when some may wedge out, turning a group into a formation but retaining the same name. Also, it can happen if the group extends laterally to a point where it isn’t divided into formations.
Lastly, subdividing a formally established formation into two or more formations will make it a group while retaining the old name.
6. Supergroup
Two or more associated of two or more related/superposed groups or groups and formations not included in any group, sharing certain lithological traits, form a supergroup. A supergroup can have groups from different geological locations and should be named if they are significant.
Supergroups are vital regional and provincial syntheses, and their recognition and naming are necessary if they are useful. However, they are not the same as a series, a chronostratigraphic unit.
Examples of supergroups are late Proterozoic Ocoee in North Carolina, and a Mesoproterozoic to a Neoproterozoic Grand Canyon Supergroup, found on the eastern Grand Canyon, Arizona, and Newark Supergroup in East Coast USA.
7. Complex
A complex is the most inclusive lithostratigraphic unit with diverse lithology, including different rock types. It can have stratified, tabular igneous, and/or metamorphic rocks or together with sedimentary rocks. Also, its lithology is mixed or complicated structural relationships.
Any igneous rock bodies cutting through stratified rocks fall in this scheme and will be informally designated, using words like bound or included within a larger lithostratigraphic unit that is formally recognized.
8. Lithohorizon or Lithostratigraphic horizon
Lithohorizon refers to a surface where lithostratigraphic change occurs. It is commonly a lithostratigraphic unit boundary. However, it can be an extremely thin maker bed that is lithologically distinctive within a lithostratigraphic unit.
What is the nature of lithostratigraphic units?
Lithostratigraphic units are bodies of rocks like sedimentary, extrusive igneous, metasedimentary, or metavolcanic, distinguishable and delineated based on their lithology and stratigraphic position. Also, they can include unconsolidated materials like gravel, clay, till, etc.
Some things you should know about the nature of lithostratigraphic units include:
1. Type the sections
We use a type of section or possibly stratotype to define a lithostratigraphic unit and name it. It should be easy to access, such as excavation or mining sites, outcrops, or using remote sensing sampling devices. Furthermore, a section type shouldn’t change and should be valid regionally. However, you can name new subsurface units if their section differs from the surface exposures you used to establish stratigraphy.
2. Geologic history, deposition facies, and time
Lithostratigraphic units are independent of depositional facies, time (not defined by time) or geologic history, biological sequence, and depositional environment. All these aspects don’t define these units. However, well-documented geologic history may help decide lateral or vertical boundaries when no other avail exists.
3. Borehole geophysical logs
In a subsurface investigation, borehole geophysical logs like electrical, density, radioactivity, electromagnetic, etc., can or not coincide with formal lithostratigraphic units, including their boundaries. However, when you don’t have any other method, you can use some of the important geophysical markers to define boundaries. Using physical properties sensed remotely results in informal, not formal lithostratigraphic units.
4. Fossils
Fossils are a lithic property; they can help in mapping as they can distinguish strata with similar lithology in the case of noncontiguous lithostratigraphic units. However, they don’t define it.
5. Surface morphology
While secondary surface and erosional morphology help notice a lithostratigraphic unit, it should be a minor factor in defining these units. Also, the surface form of lithostratigraphic units helps in mapping. Therefore, if there is no other, you can define lithostratigraphic boundaries to coincide with where lithic changes were no other way to do so.
6. Economic valuable units
Although named, Aquifers, coal beds, quarry layers, or oil sands are unformal units. However, some units can formally be recognized as lithostratigraphic units, such as formation, member, submembers, or bed. Why? Because they help clarify regional stratigraphy.
7. Unconformities
When mapping lithostratigraphic units, those separated by unconformities (represents a hiatus – erosional or non-depositional period) should be mapped as separate units even if they are lithology looks similar.
Understanding lithostratigraphic unit boundaries
Boundaries occur where lithic characteristics of the rock body change, laterally or vertically. They can be distinctive contact or arbitrary in case of a gradation.
A single upper and lower boundary is necessary for vertical boundaries to ensure the named lithostratigraphic unit doesn’t recur in a normal stratigraphic succession. In cases of intergrading or interfingering, resulting in rock passing another vertically, assign a third unit if thick enough to map.
Suppose gradation doesn’t allow arbitrary boundary at 50% or half component of each. The best way would be using an arbitrary boundary at the highest occurrence due to boreholes caving and outcrop creep.
For the case of lateral gradation, you should pick an arbitrary boundary and name the adjacent equivalent rock body. Extensive lateral gradation or intertonguing will require a new third unit. However, if tongues are not formally named, they will have informally modifying phrases like the upper, middle, or lower tongue of Mancos Shale.
Key beds may define boundaries where internal lithic characteristics remain the same. However, a new name should apply if the key bed remains the same but the lithic character of bound rock changes.
Boundaries should correspond to a change in lithic characteristics, not the genetics of the unit or unconformities unless they coincide.
How to name lithostratigraphic units
Lithostratigraphic unit naming uses the geographical location name or permanent feature near the type of section (not a popular name far away) and predominant lithic description term (where applicable except for groups or supergroups) and/or rank name. Using identical geographical names on a single vertical sequence, such as in a cyclothem, is informal nomenclature.
Each first letter of the name is capitalist, and you should avoid descriptive adjectives like red, black, etc., or compound terms like ‘sandy and clay.’ However, don’t capitalize informal units like coal beds, oil sands, or mineralized sands. Also, note that not that capitalization doesn’t make a name formal; neither does not capitalize a formal unit name.
Here is how to name the various ranks:
- Group and supergroups = Geographical location name + Group/Supergroup
- Formation = Geographical location name + Dominant lithic or word Formation when none is dominant
- Member and submember = Geographical location name + lithic character + Member/Submember. Using position (lower or upper), numbers, or letters is informal.
- Reefs = Geographic location name + rank and only used if the reef is formally identified as a member or formation.
- Beds and flows = Geographical location name + lithic term + bed/flow
- For nonlayered volcanic rocks or highly metamorphosized rocks, we use the name of the rock (like gneiss, schist, or granite) + some appropriate terms like ophiolite, complex, or melange.
Lastly, use formal ranks like members, groups, or formations only in formally recognized names.
Significance of lithostratigraphic units
Lithostratigraphic units form the basis of geologic mapping, describing, and interpreting any region’s geology. This will help understand the geologic history of the study area and identify economic resources. We apply conventions established by the International Commission on Stratigraphy (ICS) to describe, name, and group lithological units.
Also, they help in lithostratigraphic correlation, where the outcrop has traceable lithology beyond gaps. To correlate strata, you can use key beds, composition, and position sequences. Such correlations don’t indicate time equivalence as this requires using biozones and other age dating methods.
Lastly, use cutting from drilling operations and geophysical data to correlate subsurface rocks.
References
- North American Commission on Stratigraphic Nomenclature. (2021). North American stratigraphic code. Stratigraphy, 18(3), 153-204. https://ngmdb.usgs.gov/Geolex/resources/docs/NACSN_Code_2021.pdf
- MacLeod, N (2005). Stratigraphical Principles. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5). Elsevier Academic.
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Wicander, R., & Monroe, J. S. (2010). Historical geology: Evolution of the Earth and life through time (6th ed.). Books-Cole.
- Nichols, G. (2009). Sedimentology and Stratigraphy (2nd ed.). Wiley-Blackwell.
- Brookfield, M. E. (2004). Principles of stratigraphy. Blackwell Pub.
- Murphy, M. A., & Salvador, A. (Eds.). (2000). International Subcommission on stratigraphic classification of IUGS International Commission on stratigraphy. GeoArabia, 5(2), 231–266. https://doi.org/10.2113/geoarabia0502231