A geological formation is the basic or formal mappable body of rock with similar physical characteristics that sufficiently differentiates it from adjacent rock layers and occupies a certain stratigraphic position. Also, it appears in all stratigraphic columns, has boundaries, is prevailing tubular, and is traceable laterally.
Formations help map, subdivide, and classify rocks. We use the International Commission on Stratigraphy (ICS) established conventions for lithostratigraphic unit descriptions, grouping, and naming to do so.
Most people refer to geologic features (erosional or depositional) as formations. For instance, they may call physical structures on Earth like outcroppings of rocks, caves, stalagmites, and stalactite formations. This usable shouldn’t be confused with the actual meaning.
Today, we will discuss formations, including a detailed definition and some aspects like lithology, age, and fossils. We will also tackle mapping, boundaries, contact, naming, significance, etc.
What is a geological formation?
A formation is the basic, formal, or fundamental lithostratigraphic unit with a similar appearance (lithology) distinctive from adjacent layers and occupying a certain position in a stratigraphic column. It should be mappable on the surface or subsurface, laterally traceable where possible, and have upper and lower boundaries.
Therefore, we identify formations by their lithology (physical/lithic characteristics) similarity that distinguishes them from adjacent layers, relative positions, and the fact that you can map them on a geological map. Also, most formations are tabular (not a must) and can have one or several lithologies sharing similarities that make them unique from adjacent layers.
The amount of contrast in physical characteristics (lithology) to establish a formation will vary with how complex the geology of the area of study is. Also, it will depend on the detail you want to capture in your geologic maps and the geologic history of the place.
Lithostratigraphic units, including formation, apply to sedimentary rocks and unconsolidated sediments (not lithified). Also, it applies to:
- Interbedded volcanic and sedimentary
- Mappable stratified, tabular volcanic rocks. Small intrusive components in dominant stratified volcanic rocks like dikes, stills, etc,. are treated informally as they are not lithostratigraphic units.
- Low-grade metamorphic rocks with recognizable primary structures are treated as sedimentary formations. Even if mineral facies change, names don’t change.
- Also, you can distinguish organic reefs and carbonate buildups or mounts as formation, if necessary, independent of nearby or surrounding temporal equivalents that are thinner.
Note: We don’t include intrusive igneous, highly metamorphosed, or deformed rocks. Instead, we refer to them as lithodermic units. They don’t obey the law of superposition.
Examples of geological formations include:
- Grand Canyon Formations
- Kimmswick Limestone
- Permian Supai Formation
- Schnebly Hill Formation
- Navajo Sandstone
- Chinle Formation
- Permian Supai Formation
Lastly, formations are subdivided into members, beds, or flow (volcanic only) and fall into groups or supergroups.
How to define a geological formation
How do you define formations in geology? What is essential and what is not?
1. Lithology
Lithology is the most important characteristic that defines all lithostratigraphic units. A formation must show some lithological homogeneity, i.e., it must have some similarities that distinguish it from adjacent bodies of rocks.
Formations can have a uniform (one type like Redwall Limestone), several like Morrison Formation including alternating lithics like interfingering or interbedded units. Also, in some instances, they may have heterogeneous lithology that unifies them while distinguishing them from above or below rock units.
Lithological characteristics include mineral and chemical composition, color, texture, fossil content, organic content (like coal or oil shale), and primary sedimentary and volcanic features. Some of the primary sedimentary structures include rip-up clasts, weathered surfaces, mud cracks, wave ripple crests, scours, crossbedding, normal grading, etc.
Also, you can describe lithology as a summary of gross features. For instance, you will have basalt lithic tuff, oolitic grainstone, interbedded coarse siltstone, claystone, etc.
Age, fossils, and other properties that describe a rock unit’s lithologic character don’t represent a formation. Such properties may include seismic, radioactive, or electrical.
2. Age
Whether you use isotopic dating or a biostratigraphic approach, age isn’t important in defining a formation. Why? Because formations may be diachronous units, i.e., formed in a different geological timeline and places. Thus, there will be a miscorrelation if you assume time equivalence in formations.
For instance, one lithological formation can last represent an age gap of up to hundreds of millions of years, transgressing into fossil-based rock correlation and chronological intervals. Also, it can have depositional breaks (unconformities) and have materials from one or various sources.
Although not a requirement in defining a formation, you can include formation age from fossil content, relationship, or radiometric dating
3. Fossils
Fossil content information of a mapping unit is important in describing a formation. However, going into fossil taxonomy detail to help get relative age, as you do in biostratigraphy, is not required to define any lithostratigraphic unit. So, fossils, including fossil assemblage, serve more like rock constituents.
Also, any unit only distinguishable by fossils is not a lithostratigraphic unit. Instead, it represents a biostratigraphic unit.
4. Geological formation boundaries or contacts
Formations are separated by upper and lower contacts or boundaries where a rock lithology changes. However, they can be arbitrary in gradational succession, i.e., lithology is about 50% of the bed.
As a convention, you should only define the boundary of the base of a formation on a type section for arbitrary boundaries. Thus, upper contact will be the base of the overlaying rock unit. This system makes it easier if the upper beds aren’t in a type locality but in another location. Also, you will add the top beds without redefining where the formation boundary lies.
5. Formation limits
Formation limits are defined by boundaries or surfaces with lithic change that represents the greatest homogeneity.
6. Stratotype or type section
Describes the location with the clearest lithological characteristics and, when possible, where the upper and lower boundaries of the formation are visible. We use a graphic sedimentary log to show the type section, which forms a stratotype.
7. Mappable
William Smith, an English geologist, pioneered using formations to draw a geologic map with different colors and cross-sections. The practice goes up to date and uses different colors in their depictions.
Therefore, a formation must be mappable on a geologic map, i.e., the unit should be thick enough and extensive enough to be represented or delineated in a geologic map such as those with a 1:50,000 or 1:100,000 scale. Leave any thin units thinner than those necessary from the practical surface study, as it is impossible to show them on a map showing rock distribution.
Also, where possible, but not must, you may show members, named beds, and recognized lithostratigraphic units, especially where they are significant or useful.
Lastly, there are no universal colors to show various rock formations in lithodermes or formations. Each national geological survey will have its own color scheme.
8. Geological formation thickness
Thickness is Not a determining factor in determining or subdividing rock into formation. It can range from less than a meter to several thousands of meters, such as one in delta clays, i.e., no limits, and it can vary laterally.
We measure the thickness of the type section but note variation laterally or in other locations
9. Geological formation naming
The naming of formations uses the name of the geographic location where they are well exposed (type section) or a permanent nearby physical feature. These names represent the first place where the formation was described and shouldn’t be used in other lithostratigraphic units like a member, group, subgroup, etc.
Besides location, you will use the rock type name if it has a single or dominant rock type. For instance, Navajo Sandstone, Kimmswick Limestone, or Bright Angel Shale mean sandstone, limestone, and shale are the main or only rock types.
However, if you have more than one rock type in the formation, use the ‘location name’ + the word ‘formation.’ For instance, you can have Carmel Formation, Fountain Formation, Permian Supai Formation, or Summerville Formation. Here we are not using the name of a rock type or a name combination.
Acceptance of a formation name required it to be mappable. Also, it should have a full unit description, including a scientific type locality widely available for geologists. Don’t use age, e.g., rocks of Permian age, as it will require fossil and isotopic dating to ascertain the age. However, you can provide information about facies and deposition environment information. However, they are not necessary to define formations.
Lastly, a formation in one area may be a formation or a member in another. However, names shouldn’t change.
Significance
Defining formation (i.e., those with similar lithologies) makes it possible to map rocks of an area, as it would be impossible to map every stratum as some are too thin to fit into scale.
Secondly, formations help describe and interpret the geology of a certain geographical region vital in understanding its geologic history and identifying mineral and other valuable natural resources.
Thirdly, formations may help in lithostratigraphic correlation on a wide horizontal distance between outcrops, exposed or subsurface rock strata like cut roads by applying the principle of horizontal continuity.
Geological formations and chronological geological timeline
Geological formations shouldn’t work as chronologic geological time markers based on the principle of superposition and relative ages as the 18th and 19th-century stratigraphers did it.
Why? Because the deposition environment can result in lithology deposited at different times. Also, some formations form for hundreds of millions of years, transgressing to fossil-based correlation or chronological intervals. Thus, you risk miscorrelation of strata.
References
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Prothero, D. R., & Schwab, F. (2014). Sedimentary geology: An introduction to sedimentary rocks and Stratigraphy (3rd ed.). W.H. Freeman and Company.
- Nichols, G. (2009). Sedimentology and Stratigraphy (2nd ed.). Wiley-Blackwell.
- Brookfield, M. E. (2004). Principles of stratigraphy. Blackwell Pub.
- 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.
- Levin, H. L., & King, D. T. (2016). The Earth through time (11th ed.). Wiley.