Red beds
Red beds (or redbeds) are
Primary red beds
Primary red beds may be formed by the erosion and redeposition of red soils or older red beds,[3] 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).[4]
This dehydration or "aging" process has been found to be intimately associated with
- 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.[6]
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
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.[9] 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.[10]
See also
- Red Beds of Texas and Oklahoma
- Chugwater Formation
- Red Hills, Kansas
- Old Red Sandstone
- New Red Sandstone
References
- ISBN 0-922152-36-5. Retrieved 8 November 2020.
- ISBN 0-922152-76-4.
- ^ Krynine, P. D. (1950). "Petrology, stratigraphy, and origin of the Triassic sedimentary rocks of Connecticut". Bulletin of the Connecticut Geology and Natural History Survey. 73.
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- S2CID 129359697.
- ISBN 9780444885173.