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Stratigraphy

Stratigraphy is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Stratigraphy rather than just read about it. In short: Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.

Stratigraphy — main illustration
Stratigraphy — illustration

Key takeaways

  • Stratigraphy belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Stratigraphy to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Stratigraphy from memory before moving on to harder problems.

Reference excerpt

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostratigraphy (lithologic stratigraphy), biostratigraphy (biologic stratigraphy), and chronostratigraphy (stratigraphy by age). Several principles and laws come into play when using stratigraphy such as Principle of original horizontality, Law of superposition, Cross-cutting relationships, Principle of inclusions, Principle of faunal succession.

Historical Development

Catholic priest Nicholas Steno established the theoretical basis for stratigraphy when he introduced the law of superposition, the principle of original horizontality and the principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment. In 1759 the Italian geologist Giovanni Arduino proposed the division of the earth's crust into four general and successive orders: Primary, Secondary, Tertiary and Quaternary, a classification regarded as the starting point for modern stratigraphy. The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology", Smith recognized the significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geological map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart, who studied the geology of the region around Paris.

Lithostratigraphy

Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type (lithology). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in environments of deposition (known as facies change). These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called the law of superposition, states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy. Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals (particularly carbonates), grain size, thickness of sediment layers (varves) and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates.

Biostratigraphy

Biostratigraphy or palaeontologic stratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. Biologic stratigraphy was based on William Smith's principle of faunal succession, which predated, and was one of the first and most powerful lines of evidence for, biological evolution. It provides strong evidence for the formation (speciation) and extinction of species. The geologic time scale was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating, which was based on an absolute time framework, leading to the development of chronostratigraphy. One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals, and traps of petroleum geology.

Chronostratigraphy

Chronostratigraphy is the branch of stratigraphy concerned with arranging rock units into relative temporal ordering, so that a sequence of time-relative events that created the rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth. A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity. It is called a hiatus because deposition was on hold for a period of time. A physical gap may represent both a period of non-deposition and a period of erosion. A geologic fault may cause the appearance of a hiatus.

Magnetostratigraphy

… excerpt ends here. Continue reading the full article.

Illustrations

Stratigraphy: The Permian through Jurassic strata of the Colorado Plateau area of southeastern Utah demonstrate the principles of stratigraphy.
The Permian through Jurassic strata of the Colorado Plateau area of southeastern Utah demonstrate the principles of stratigraphy.
Stratigraphy: Engraving from William Smith's monograph on identifying strata based on fossils
Engraving from William Smith's monograph on identifying strata based on fossils
Stratigraphy: Strata in Cafayate (Argentina)
Strata in Cafayate (Argentina)
Stratigraphy: Chalk layers in Cyprus, showing sedimentary layering
Chalk layers in Cyprus, showing sedimentary layering
Stratigraphy: Example of magnetostratigraphy. Magnetic stripes are the result of reversals of the Earth's magnetic poles and seafloor spreading. New oceanic crust is magnetized as it forms and then it moves away from the midocean ridge in both directions.
Example of magnetostratigraphy. Magnetic stripes are the result of reversals of the Earth's magnetic poles and seafloor spreading. New oceanic crust is magnetized as it forms and then it moves away from the midocean ridge in both directions.

Worked examples

Example 1 — a first encounter with Stratigraphy

Start with the simplest possible case. Write down what Stratigraphy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stratigraphy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stratigraphy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stratigraphy

In research
Stratigraphy appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Stratigraphy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Stratigraphy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Methods in archaeology, Petrology, Stratigraphy, so understanding it makes those chapters shorter.
In everyday life
Look for Stratigraphy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Stratigraphy in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Stratigraphy means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Stratigraphy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stratigraphy in simple terms?

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.

Why does Stratigraphy matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stratigraphy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stratigraphy.

Tags

  • Methods in archaeology
  • Petrology
  • Stratigraphy

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