science

Red beds

Red beds 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 Red beds rather than just read about it. In short: Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate, marl, limestone, or some combination of these sedimentary rocks.

Key takeaways

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

Reference excerpt

Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate, marl, limestone, or some combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of red beds, typically occur as a coating on the grains of sediments comprising red beds. Classic examples of red beds are the Permian and Triassic strata of the western United States and the Devonian Old Red Sandstone facies of Europe.

Primary red beds Primary red beds may be formed by the erosion and redeposition of red soils or older red beds, but a fundamental problem with this hypothesis is the relative scarcity of red-colored source sediments of suitable age close to an area of red-bed sediments in Cheshire, England. Primary red beds may also form by in situ (early diagenetic) reddening of the sediment by the dehydration of brown or drab colored ferric hydroxides. These ferric hydroxides commonly include goethite (FeO-OH) and so-called "amorphous ferric hydroxide" or limonite. Much of this material may be the mineral ferrihydrite (Fe2O3 H2O). This dehydration or "aging" process has been found to be intimately associated with pedogenesis in alluvial floodplains and desert environments. Goethite (ferric hydroxide) is normally unstable relative to hematite and, in the absence of water or at elevated temperature, will readily dehydrate according to the reaction:

2FeOOH (goethite) → Fe2O3 (hematite) +H2O The Gibbs free energy (G) for the reaction goethite → hematite (at 250 °C) is −2.76 kJ/mol and G becomes increasingly negative with smaller particle size. Thus detrital ferric hydroxides, including goethite and ferrihydrite, will spontaneously transform into red-colored hematite pigment with time. This process not only accounts for the progressive reddening of alluvium but also the fact that older desert dune sands are more intensely reddened than their younger equivalents.

Diagenetic red beds Red beds may form during diagenesis. The key to this mechanism is the intrastratal alteration of ferromagnesian silicates by oxygenated groundwaters during burial. Walker's studies show that the hydrolysis of hornblende and other iron-bearing detritus follows Goldich dissolution series. This is controlled by the Gibbs free energy of the particular reaction. For example, the most easily altered material would be olivine: e.g.

Fe2SiO4 (fayalite) + O2 → Fe2O3 (hematite) + SiO2 (quartz) with E = −27.53 kJ/mol A key feature of this process, and exemplified by the reaction, is the production of a suite of by-products which are precipitated as authigenic phases. These include mixed layer clays (illite – montmorillonite), quartz, potassium feldspar and carbonates as well as the pigmentary ferric oxides. Reddening progresses as the diagenetic alteration becomes more advanced, and is thus a time-dependent mechanism. The other implication is that reddening of this type is not specific to a particular depositional environment. However, the favourable conditions for diagenetic red bed formation i.e. positive Eh and neutral-alkaline pH are most commonly found in hot, semi-arid areas, and this is why red beds are traditionally associated with such climates.

Secondary red beds Secondary red beds are characterized by irregular color zonation, often related to sub-unconformity weathering profiles. The color boundaries may cross-cut lithological contacts and show more intense reddening adjacent to unconformities. Secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous of the southern North Sea. Post-diagenetic alteration may take place through reactions such as pyrite oxidation:

3O2 + 4FeS2→ Fe2O3 (hematite) + 8S E = −789 kJ/mol and siderite oxidation:

O2 + 4FeCO3 → 2Fe2O3 (hematite) + 4CO2 E = −346 kJ/mol Secondary red beds formed in this way are an excellent example of telodiagenesis. They are linked to the uplift, erosion and surface weathering of previously deposited sediments and require conditions similar to primary and diagenetic red beds for their formation.

See also Red Beds of Texas and Oklahoma Chugwater Formation Red Hills, Kansas Old Red Sandstone New Red Sandstone

References

External links Pictures of Permo-Triassic red beds in Palo Duro Canyon

Illustrations

Worked examples

Example 1 — a first encounter with Red beds

Start with the simplest possible case. Write down what Red beds 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 Red beds 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 Red beds 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 Red beds

In research: Red beds 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 Red beds 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: Red beds is common in secondary-school and first-year university syllabi. It links to neighbouring topics Sedimentary rocks, so understanding it makes those chapters shorter.
In everyday life: Look for Red beds 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 Red beds in 20 minutes

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

Frequently asked questions

What is Red beds in simple terms?

Why does Red beds 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 Red beds?

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 Red beds.