A stratum is a rock layer with a lithology (texture, color, grain size, composition, fossils, etc.) different from the adjacent ones and several form strata. Each stratum is separated by a visible bedding surface or plane.
Here is an in-depth meaning of stratum, types (bed, flow, key bed, or band), and characteristics. We will also give its significance and images or examples.
What is a stratum in geology?
In geology and related studies, a stratum (plural strata) is defined as a sediment or rock layer with certain lithologic properties (texture, color, grain size, composition, etc.) distinctive from adjacent layers, separated by visible surface (bedding surface or plane).
The process of rock layering or bedding to form strata is known as stratification. It occurs via the deposition and cementation of sediment that can last thousands of years or happen quickly.
On the other hand, stratigraphy is a branch of geology that studies strata and rock layering. It mainly focuses on sedimentary rocks but can also include layered volcanic rocks.
It is worthwhile noting that the word stratum is of Latin origin and means something laid down or spread, a bed or layer.
More definitions are:
- The International Commission on Stratigraphy defines a stratum as “a layer of rock characterized by particular lithologic properties and attributes that distinguish it from adjacent layers” (Stratigraphic Guide, n.d.).
- According to the University of Illinois, a “stratum is a tabular or sheet-like mass, or a single, distinct layer of material of any thickness, separable from other layers above and below by a discrete change in the character of the material or by a sharp physical break, or by both” (Stratum, n.d.).
For above, it is clear that in whichever way you want to define it, you must mention that the rock layer has a certain lithology that makes it different from adjacent layers and has a visible bedding plane or surface that separates it from the adjacent layers.
Recognizing strata
Characteristics and appearance are key to recognizing strata. As per the basic definition, a stratum is a stratigraphic unit representing a nearly parallel layer upon or under which many others lie to form large, stacked strata or composite.
Usually, a stratum alternates or repeats to create strata, which are what you see as bands with different colors on cut roads, cliffs, river banks, quarries, etc. These bands represent certain deposition modes like beach sand, river silt, lava bed, coal swamp, dunes of sand, etcetera.
Strata can be small, lens-like, or span thousands of square kilometers on the Earth’s surface, i.e., horizontally. However, tectonic forces can fold, tilt, or squeeze initially flat beds.
Similarly, a stratum can span a large area on the Earth’s surface and may be several millimeters to several meters thick, with those very thin, known as laminae.
Also, the thickness of a stratum doesn’t depend on depositional time. For instance, tsunamis, turbidity currents, gravity slides, and underwater mudflows may result in a thick stratum deposited quickly. However, a stable and consistent environment generally produces thicker ones.
Lastly, the assumption of caking in strata formation isn’t always true. “Scientists have discovered that geologic layers are not always laid down one after another. Sometimes, a sequence of layers is laid down simultaneously from left to right, not from top to bottom.”
Related definitions
Bed, bedding plane, lithostratigraphy, and lithostratigraphic unit related to the stratum. Here is a basic definition of each of these terms.
- Lithostratigraphy: Sub-field in stratigraphy. It describes and organizes rocks systematically into distinctive units based on their lithology and stratigraphic relationship.
- Lithostratigraphic units: A formally established and distinctive rock body entity based on its properties, attributes, or a combination of these traits – lithology.
- Bedding plane or surface. It is what separates two stratum layers. It is important as it records lithology changes (grain size, color, composition, etc.), and some rocks may readily split along these planes. The plane represents a change in deposition episodes resulting from periodic erosion or conditions, periodic ceasing of deposition, or a combination of these factors.
- Lamina – Laminae (singular: lamina) are sedimentary rock strata less than 10 mm thick, showing variation in composition and/or grain texture. We call them planar laminae when nearly horizontal at the deposition time and with roughly parallel bounding surfaces (sometimes not).
Stratum types
Here are the common stratum types and what they mean:
1. Bed
It is the smallest formally accepted lithostratigraphic unit in sedimentary rock. It represents a single stratum with a layer below or above lithologically distinctive. We only consider beds if they are sufficiently distinct for geologic mapping and stratigraphic correlation.
2. Flow
The flow is the smallest, distinctive layer in a volcanic body or sequence, the equivalent of a bed. You should be able to distinguish it via composition, texture, or any other accepted criteria.
Also, a flow must be widespread, distinguishable, and shows stratigraphic correlation. Otherwise, it doesn’t qualify as the smallest lithostratigraphic unit if it doesn’t meet these criteria.
3. Key bed or marker bed
A key bed is an easily recognizable, identifiable, well-defined stratum or strata body with distinguishable fossil content or lithology for correlation and field mapping.
4. Bands
A band is a thin stratum you can distinguish by color or lithology, making them essential in correlating strata.
Stratum significance
Each rock stratum represents an event in the geological timeline when biological, chemical or physical conditions caused the deposition of the specific layer. Therefore, we study different happenings in the geological timeline by looking at the strata sequence. These events include climate, water, wind, glacial action, etc.
Besides telling more about possible events, stratum deposition may help preserve past movements on Earth’s surface, including fossils, making it possible to interpret past life forms.
Of course, some stratigraphy principles, like original horizontality, lateral continuity, and the law of superposition, are used. Other important principles are cross-cutting relationships, inclusion, faunal successions, etc.
Examples of rock strata
There are many examples of rock strata. Some of the few notable ones we would like to share with you include the following:
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References
- Stratigraphic guide. International Commission on Stratigraphy. (n.d.). Retrieved February 21, 2023, from https://stratigraphy.org/guide.
- Stratum. Illinois State Geological Survey. (n.d.). Retrieved February 21, 2023, from https://isgs.illinois.edu/outreach/geology-resources/stratum.
- Stratum. (2022, October 4). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Stratum&oldid=1113994544
- Morris, J. D. 2012, February 29. Lateral layers of geologic strata. The Institute for Creation Research. Retrieved February 21, 2023, from https://www.icr.org/article/lateral-layers-geologic-strata.