Red beds
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The term red beds usually refers to strata of reddish-colored sedimentary rocks such as sandstone, siltstone or shale that were deposited in hot climates under oxidizing conditions.[1] The red color comes from iron oxide in their mineral structure. Although they have been deposited throughout the Phanerozoic, they are most commonly associated with rocks deposited during the Permian and Triassic periods.
Red beds have economic significance since many of them contain reservoirs of petroleum and natural gas.
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[edit] Primary Red Beds
Krynine (1950) suggested that the red beds were primarily formed by the erosion and redeposition of red soils or older red beds. A fundamental problem with this hypothesis is the relative scarcity of Permian red coloured source sediments to the south of Cheshire. Van Houten (1961) developed the idea to include the in situ (early diagenetic) reddening of the sediment by the dehydration of brown or drab coloured ferric hydroxides. These ferric hydroxides commonly include goethite (FeO-OH) and so called "amorphous ferric hydroxide" or limonite. In fact, much of this material may be the mineral ferrihydrite (Fe2O3 H2O).
This dehydration or "aging" process is now known to be intimately associated with pedogenesis in alluvial floodplains and desert environments. Berner (1969) showed that 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:
FeOOH (goethite)→ Fe2O3 (hematite) +H2O
Gibbs Free Energy (G) is defined as - some reactions are spontaneous because they give off energy in the form of heat (H < 0). Others are spontaneous because they lead to an increase in the disorder of the system (S > 0). Calculations of H and S can be used to probe the driving force behind a particular reaction. The Gibbs free energy of a system at any moment in time is defined as the enthalpy of the system minus the product of the temperature times the entropy of the system.
The Gibbs Free Energy for this reaction (at 250°C) is -2.76kJ/mol and Langmuir (1971) showed that G becomes increasingly negative with smaller particle size. Thus detrital ferric hydroxides including goethite and ferrihydrite will spontaneously transform into red coloured hematite pigment with time. This process not only accounts for the progressive reddening of alluvium but also the fact older desert dune sands are more intensely reddened than their younger equivalents.
[edit] Diagenetic Red Beds
The formation of red beds during burial diagenesis was clearly described by Walker (1967) and Walker et al. (1978). 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’s stability series. This is controlled by the Gibbs Free Energy (?Gr ) of the particular reaction. For example, the most easily altered material would be olivine: e.g.
FeSi4 (fayalite) + O2 → Fe2O3 (hematite) + SiO2 (quartz)with ?Gr = -27.53kJ/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. +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.
[edit] Secondary Red Beds
Secondary Red Beds are characterized by irregular colour zonation, often related to sub-unconformity weathering profiles. The colour boundaries may cross-cut lithological contacts and show more intense reddening adjacent to unconformities. Johnson et al. (1997) have also showed how secondary reddening phases might be superimposed on earlier formed primary red beds in the Carboniferous of the southern North Sea. The general conditions leading to post-diagenetic alteration have been described by Mücke (1994). Important reactions include pyrite oxidation:
3O2 + 4FeS2→ Fe2O3 (hematite) + 8S ?Gr = -789 kJmol-1
and siderite oxidation: O2 + 4FeCO3 → 2Fe2O3 (haematite) + 4CO2 ?Gr = –346 kJmol-1
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
[edit] References
American Geological Institute, Dictionary of Geological Terms, (New York: Dolphin Books, 1962), p. 416.
Berner R. A., 1969,. Goethite stability and origin of red beds. Geochimica Cosmomochimica Acta, 35, pp 267-273.
Krynine, P. D., 1950, Petrology, stratigraphy, and origin of the Triassic sedimentary Bulletin of the Connecticut Geology and Natural History Survey, 73, 239p.
Langmuir, D., 1971, Particle size effect on the reaction Goethite = Hematite + Water. American Journal of Science, 271, pp 147-156.
Mücke, A., 1994,. Part 1. Postdiagenetic ferruginization of sedimentary rocks (sandstones, oolitic ironstones, kaolins and bauxites) - including a comparative study of the reddening of red beds. [in] Wolf, K. H. and Chilingarian, G V. (eds.) pp 361-395 Diagenesis, IV. Developments in Sedimentology 5 1, Elsevier, Amsterdam.
Van Houten, F. B., 1973, Origin of red beds. A review -1961-1972. Annual Review Earth Planetary Science, 1, pp 39-61
Walker, T. R., 1967, Formation of red beds in modern and ancient deserts. Bulletin of the Geological Society of America, 78, pp 353-368.
