Lithostratigraphy is a branch of stratigraphy. It describes and names bodies of rocks on Earth based on their lithology or lithic properties and stratigraphic position. We use physical relationships showing the older and younger strata and geometric spot or place to establish the relative stratigraphic position of rock units.
It is the basis of geological mapping. Also, correlating lithostratigraphic units makes it possible to reconstruct, describe, and interpret the geology of a region, including paleogeography.
Lastly, the International Commission on Stratigraphy (ICS) establishes conventions of stratigraphic classification, terminologies, procedures, and grouping of various lithological units.
What is lithology?
Lithology refers to the observable physical character of hand specimens and outcrops. But it may also mean rock type, color, texture, mineral composition/assemblage, heavy metal assemblage, fossils, primary features, organic materials present, etc. Others are organic materials like kerogen and coal.
In this case, fossils are more like rock-forming particles important in identifying strata, not age. Thus, the taxonomy of fossils doesn’t define a lithostratigraphic unit.
Note that formation conditions, not origin time, influence the various lithological characteristics. Thus, time does not influence lithology.
What is lithostratigraphic classification?
Lithostratigraphy refers to organizing bodies of rocks into units using their lithologic characteristics and stratigraphic relations. When doing so, we only consider only lithology. So, we don’t use age (via isotopic dating or biostratigraphy way) or palaeontologic criteria.
Also, lithostratigraphic classifications don’t use geophysical properties. However, we can get boundaries using geophysical criteria (remotely sensed physical properties) and a hint on lithology, but not exclusively to define these units.
What are lithostratigraphic units?
Lithostratigraphic units are bodies of rocks delineated and distinguished based on 1) their lithological characteristics and 2) their relative stratigraphic.
These rock units can be bedded/ unbedded, often tabular, and obey Steno’s law of superposition ( and not overturned or deformed). They include sedimentary rocks, unconsolidated sediments, and sedimentary and volcanic rock interbedding. Others are tabular, stratified igneous rocks, low-grade metamorphized rocks with noticeable primary structures, organic reefs, and carbonate buildups.
Formations are fundamental units used in lithostratigraphic classification. Ranking below formations, we have members, submembers, and flows/beds in that order; those ranking higher are groups followed by supergroups and then complex.
Lithostratigraphic units indeed represent rock formed during a certain geologic time interval. However, they have little significance chronologically, and time doesn’t help establish their boundaries, though most boundaries are cut synchronous surfaces if traced horizontally.
Also, identical rocks can form at different times and repeat in a stratigraphic sequence. This means you can get similar lithology but separated by time and space.
Let us compare lithostratigraphic units with biostratigraphic units and magnetostratigraphic polarity units
1. Lithostratigraphic vs. biostratigraphic units
Both lithostratigraphic and biostratigraphic units refer to depositional environments. Also, both help picture lithology constitution and rock geometry in Earth’s crust, past environments, and past life development.
Again, lithostratigraphic units may coincide with biostratigraphic units but often lie on different horizons stratigraphically. However, even when they coincide, these units have no inherent relationships.
Lastly, biostratigraphic units indicate age and not just the body of rock lithology. Also, they are less repetitive as irreversible evolutionary change, unlike lithostratigraphic units that often repeat.
2. Lithostratigraphic vs. magnetostratigraphic polarity units
Yes, we determine lithostratigraphic and magnetostratigraphic polarity units directly from rock properties. However, boundaries of magnetostratigraphic polarity units are thought not to be time-transgressive and have a potential global recognition. These boundaries may coincide with lithology or be parallel to but displaced.
Depositional environment and lithostratigraphy
The depositional environment has more influence on lithology than the origin time. As you know, a formation is a body of rock with the same lithology and occupying a certain relative stratigraphic position. What implication does this have?
First, a change in the depositional environment will result in sedimentary facies or lithofacies. Lithofacies are rock bodies with varying lithologies due to changing depositional settings. Thus, you will have 1) laterally limited formations, i.e., from the beach to the inner shelf, and 2) several formations forming simultaneously.
Secondly, the depositional environment can migrate spatially. For instance, rising sea levels will cause the depositional environment to shift towards land. Such an event will create a time-transgressive (diachronous) formation on the shoreface and coastal plain.
The age difference that can be up to hundreds of millions of years affects the correlation of such rocks. Thus, there is a need to employ other techniques like chronostratigraphy or biostratigraphy.
Lastly, with the spatial migration of the depositional environment, you may have two or more similar formations formed at different times. Matching such will be incorrect. Why? Because the body of rock constituting the formation is diachronous, so are the upper and lower boundaries.
Lithostratigraphic relationships
Vertical contacts (planar or irregular surface separating lithological units) are conformable or unconformable.
A conformable contact is when there is no significant geologic time gap, i.e., a hiatus between the overlaying younger and underlying older rock beds. It happens when a deposition isn’t interrupted, and beds are often parallel. We call such contact conformity. It can be abrupt (due to minor depositional break or diastem) or gradational, where the deposition change is gradual.
On the other hand, unconformable contacts occur when there is a significant hiatus between the older lower and younger upper beds, i.e., these strata don’t succeed each other in immediate order of age. Such contacts are known as unconformities. They represent an erosional or non-depositional period and can create a huge age gap.
There are various unconformities, including nonconformity, disconformity, and angular unconformity. Others are paraconformity, buttress, and blended.
Lastly, contacts between adjacent lithostratigraphic units represent rocks of equivalent age. Their lithology changes due to variations in the depositional environment. These lateral contacts may be gradational (one rock unit gradually changes to another) or intertonguing, i.e., wedging or pinching of one unit into another.
Lithostratigraphic correlation
Lithostratigraphic correlation or lithocorrelation matches disjointed rock units of similar lithology occupying the same stratigraphic position. This correlation isn’t age equivalent as in the case of biostratigraphy or chronostratigraphy. Why? Because lithostratigraphic units are time transgressive. However, it is important when no fossils and other time-equivalent methods don’t apply.
Before any lithologic correlations, establish facies in the depositional basin as the environment shifts. Also, to successfully correlate lithologies, you need to assume that rock beds obey these Steno’s laws principles:
- Superposition assumes that in an undeformed sedimentary sequence, a rock layer is older than the one above but younger than the one beneath.
- Lateral continuity states that rock layers in a sedimentary basin extend outwards in all directions until they thin out, grade into another rock layer, or meet a barrier.
- Original horizontality says that sediment deposition occurs horizontally under gravity. Any deformation happens after lithification.
1. Lithostratigraphic correlation process
The lithostratigraphic correlation process involves establishing physical continuity of beds at different locations where beds are untraceable, i.e., are not continuous or it is not possible. We are essentially trying to demonstrate that separated rock strata were once continuous geographically, and the correlation will run as far as the diagnosable lithology is identifiable.
This process is easier when you have outcrops, cliffs, cut roads/railways, excavation sites, mining, etc. You will easily match rock units with strong lithologic similarity in a strata sequence.
For cases where lithology is hard to identify or in subsurface correlation, you can use instruments for logging wells, well cuttings, vegetation change, or rock cores. They should help have an idea of lithostratigraphic units and their boundaries. However, such a correlation isn’t strictly lithology.
Instruments for logging wells devices will measure various geophysical properties. These properties are sound wave transmissibility, seismic reflections, electrical resistivity, radioactivity, adsorption, etc. However, it reflects a change in lithology, fluid content, porosity, fluid content, etc., and can help correlate rock formations.
An important aspect of a successful lithostratigraphic correlation is considering the unit position and stratigraphic succession. You can easily tell which strata correlate if you see several individual successive matching units. The more consecutive units you have matching, the more reliable your results will be.
The other way the position in a stratigraphic succession makes correlation easy is when you have a distinctive bed, i.e., a key or maker bed. You will know which strata match if you have a key bed that occupies a near or at a certain vertical stratigraphic position. Having a top and lower key will make your correlation even more accurate.
Lastly, unconformities, especially regional or worldwide, can help correlate strata. Such are often synchronous. However, the upper and lower rocks may not match due to varying deposition or erosion of the surface and will have different ages at different places.
2. Factors that affect lithocorrelation reliability
Factors that affect the reliability degree of correlation are 1) the distinctiveness of lithological attributes and 2) the stratigraphic succession nature. Also, whether there are changes in lithology from one area to another and facies may have an impact.
Also, correlation isn’t easy in cyclic succession (cyclothems) when similar deposition conditions reappear. This is due to transgressive-regressive deposition cycles.
That is not all. The number of lithological properties matches. More lithological properties between units being correlated will give a more reliable match. A single property like color can change laterally, but they are less likely to change if many.
Lastly, high levels of deformation and high-grade metamorphism may make it hard to relate matching units.
References
- Levin, H. L., & King, D. T. (2016). The Earth through time (11th ed.). Wiley.
- 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.
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Catuneanu, O. (2006). Principles of sequence stratigraphy (1st ed.). Elsevier.
- North American Commission on Stratigraphic Nomenclature. (2021). North American stratigraphic code. Stratigraphy, 18(3), 153-204.
- Wicander, R., & Monroe, J. S. (2010). Historical geology: Evolution of the Earth and life through time (6th ed.). Books-Cole.
- 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.