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:

1. Lithostratigraphy (Lithostratigraphic unit)

2. Biostratigraphy (Zones)

3. Chronostratigraphy [(Chronostratigraphic units) {Age, Period, Epoch}]

4. Magnetostratigraphy (Reversals, chrons)

5. Sequence stratigraphy (Allostratigraphic units)

6. Pedostratigraphy – (Pedostratigraphic unit)

7. Geo Chronostratigraphy — (Geochronostratigraphic unit)

8. Chemostratigraphy (Isotope zones)

Stratigraphic Relationship

There are two types of contact in stratigraphy i.e.: conformable and unconformable.

1. 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).

2. 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

1. 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.

2. 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: 

1. Assemblage Biozones: These are strata that consist of a special correlation of three or more taxa.

2. 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.

3. Concurrent Range Biozone: It includes the coincident, concurrent or overlapping part of the range of two particular taxa.

4. Interval Biozone: It includes the strata between two particular biostratigraphic surfaces. It can be established on either the highest or lowest occurrences.

5. Lineage Biozone: These are strata consisting of species demonstrating a particular segment of an evolutionary lineage.

6. 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.