Definition of Stratigraphy
Understanding the stratigraphy meaning becomes quite simple when you get familiar with the stratigraphy principle of geology. Stratigraphy is a branch of geology that deals with the description of rock or interpretation of geologic time scale. It also renders insight into the geologic history of strata. As a geological discipline, stratigraphy takes into account the spatial location and temporal sequence of rock bodies. With the help of stratigraphy, bodies of rock are dated and interlinked with each other. Stratigraphic studies are mainly conducted to study sedimentary and volcanic layered rocks.
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The Stratigraphic Principle
Stratigraphy organizes bodies of rock chronologically and spatially in accordance with their contained characteristics. It puts indirectly connected rock units together in a relationship. Stratigraphy is the substructure for remodelling the Earth’s history. It also plays a part in solving general geological questions.
The stratigraphic principle was initially introduced in 1669 by Nicolaus Steno in his documented work ‘Dissertations prodromus’. It has been founded and formulated on the foundation that, with a smooth sequence of sedimentary layers, the layer in the footwall (below) is older (matured) than the layer in the hanging wall (above).
Types of Stratigraphy
Following are the most important sub-disciplines in stratigraphy with their elements of study:
Lithostratigraphy (Lithostratigraphic unit)
Biostratigraphy (Zones)
Chronostratigraphy [(Chronostratigraphic units) {Age, Period, Epoch}]
Magnetostratigraphy (Reversals, chrons)
Sequence stratigraphy (Allostratigraphic units)
Pedostratigraphy – (Pedostratigraphic unit)
Geo Chronostratigraphy — (Geochronostratigraphic unit)
Chemostratigraphy (Isotope zones)
Stratigraphic Relationship
There are two types of contact in stratigraphy i.e.: conformable and unconformable.
Conformable: Non-fragmented accumulation, no breakages or hiatus (interruption or impairment in the continuity of the geological record). The resulting surface strata are known as conformity. Further, there are two forms of contact between conformable strata: abrupt contacts (directly isolated beds of distinctly distinguished lithology, minor depositional break, referred to diastems) and gradational contact (steady change in deposition, mixing zone).
Unconformable: Period of weathering/non-deposition. The surface stratum resulting is known as an unconformity. There are further
Four Kinds of Unconformity:
Angular Unconformity: younger sediment lies upon a weathered surface of folded or slanted older rocks. The older rock steeps at a different angle from the younger.
Disconformity: the contact between older and younger beds is noted by apparent, non-uniform weathering surfaces. Paleosol might form right above the disconformity surface due to the non-deposition setting.
Nonconformity: comparatively young sediments are accumulated right above older igneous or metamorphic rocks.
Paraconformity: the bedding planes below and above the unconformity run parallel to one another. A time gap exists, as depicted by a faunal break, but there is no weathering, just a period of non-accumulation.
Stratigraphic Subdivision
Concept of Zone
With respect to stratigraphic subdivision, there is the concept of stage. A stage is a crucial subdivision of strata, each minutely following each other while bearing a unique, distinctive assemblage of fossils. Thus, stages can be described as a group of strata consisting of the same major fossil assemblages. French palaeontology Alcide d’Orbigny is felicitated for the introduction and implementation of this concept. He named stages after geographic localities with specifically finer sections of rock strata that carry the characteristic fossils on which the stages are established.
Concept of Zone
The zone is an elementary biostratigraphic unit. The thickness of the unit ranges from a few to hundreds of metres, and its extent ranges from local to global. Biostratigraphic units are further classified into 6 principal kinds of biozones that are as below:
Assemblage Biozones: These are strata that consist of a special correlation of three or more taxa.
Abundance Biozones: These are strata in which the abundance of a specific taxon or group of taxa is considerably higher than in the adjacent part of the section.
Concurrent Range Biozone: It includes the coincident, concurrent or overlapping part of the range of two particular taxa.
Interval Biozone: It includes the strata between two particular biostratigraphic surfaces. It can be established on either the highest or lowest occurrences.
Lineage Biozone: These are strata consisting of species demonstrating a particular segment of an evolutionary lineage.
Taxon Range Biozone: It depicts the known stratigraphic and geographic range of formation of a single taxon.
Sequence Stratigraphy
Sequence stratigraphy is an evaluation of sedimentary deposits in a time-stratigraphic aspect. It generally includes subdividing a sedimentary basin fill into individual sequences of accumulation (thus the name), which can then be associated with alterations in the two elementary parameters of sediment supply and shelter (the amount of space available for accumulation). A key purpose of this is to reconstruct how sediments filled a basin and thus, how the stratigraphy occurred through time and space. This can enable geologists and scientists to identify many significant aspects like finding out where fine and coarse-grained sediments are located and how sea level changed.
Uses of Sequence Stratigraphy
Over the years, sequence stratigraphy has transpired to be an extensively used, methodological framework that confines many contexts of sedimentology and stratigraphy and has different useful applications and anticipating capacities.
FAQs on Stratigraphy
1. What is stratigraphy in geography?
Stratigraphy is a fundamental branch of geology and physical geography that focuses on the study of rock layers (strata) and their sequencing. It involves interpreting the order, age, and distribution of stratified rocks to understand Earth's geological history. The primary goal is to correlate rock layers from different areas to build a comprehensive timeline of past events.
2. What are the basic principles of stratigraphy?
Stratigraphy is governed by several core principles that help geologists interpret rock layers. The most important ones include:
- The Law of Superposition: In an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest are at the top.
- The Principle of Original Horizontality: Sedimentary layers are initially deposited in horizontal sheets. Tilted or folded layers indicate that they were disturbed after deposition.
- The Principle of Lateral Continuity: Strata extend in all directions until they thin out or are terminated by a geological feature.
- The Principle of Cross-Cutting Relationships: A geological feature (like a fault or an intrusion) that cuts across another is younger than the feature it cuts.
3. What are the main types of stratigraphy?
There are several specialised types of stratigraphy, each using different evidence to correlate rock layers. The main types include:
- Lithostratigraphy: The study and correlation of strata based on their physical rock characteristics, such as composition, grain size, and colour.
- Biostratigraphy: Uses the fossil content within rock layers to correlate them and determine their relative ages.
- Chronostratigraphy: Focuses on establishing the absolute age of rock units, organising them into a global geologic time scale.
- Seismic Stratigraphy: Interprets seismic data to map and understand the geometric relationships of sedimentary layers in the subsurface.
4. How are fossils used as a tool in stratigraphy?
Fossils are a crucial tool in biostratigraphy, primarily through the use of index fossils. An index fossil is a remnant of an organism that was geographically widespread but existed for a relatively short period of geological time. When geologists find the same index fossil in rock layers at different locations, they can confidently conclude that the layers are of the same age, even if the rock types are different.
5. How does stratigraphy help in determining the relative age of rock layers?
Stratigraphy is the cornerstone of relative dating in geology. By applying the Law of Superposition, geologists know that lower layers are older than upper layers. They then use biostratigraphy (correlating with index fossils) and lithostratigraphy (matching rock types) to link sequences across vast distances. This allows them to build a chronological framework and determine whether a rock layer in one continent is older or younger than a layer in another, without knowing their exact numerical age.
6. What is the importance of stratigraphy in fields like archaeology and petroleum geology?
Stratigraphy's importance extends far beyond pure geology. For example:
- In archaeology, the principles of stratigraphy are used to date human artefacts. An object's position within layers of soil and debris helps determine its age relative to other objects found in layers above or below it.
- In petroleum geology, stratigraphy is essential for finding oil and gas. Geologists use seismic and well data to map subsurface strata and identify formations, such as porous sandstones (reservoirs) capped by impermeable shales (seals), that are likely to trap hydrocarbons.
7. Can stratigraphic layers be overturned, and how do geologists identify this?
Yes, immense geological forces associated with mountain-building can cause folding and faulting, which may tilt, or even completely overturn, entire sequences of strata. Geologists can identify overturned layers by looking for specific sedimentary structures that have a clear 'up' direction. Examples include graded bedding (where sediment grains go from coarse at the bottom to fine at the top), ripple marks, or fossil burrows, which would appear upside down in an overturned sequence.
8. What is the difference between lithostratigraphy and biostratigraphy?
The key difference lies in the evidence used for correlation. Lithostratigraphy correlates rock layers based on their physical attributes, such as being composed of sandstone or limestone. It answers the question, 'Is this the same rock formation?' In contrast, biostratigraphy correlates layers based on the fossils they contain, regardless of the rock type. It answers the question, 'Was this layer deposited at the same time?' Often, a layer of sandstone in one location can be the same age as a layer of shale in another, which can only be determined using the fossils within them.
