Clastic sediments and sedimentary rocks that are pigmented by red ferric oxide which coats grains, fills pores as cement, or is dispersed as a muddy matrix. These conspicuously colored rocks commonly constitute thick sequences of nonmarine, paralic (marginal marine), and less commonly shallow marine deposits. Clastic redbeds accumulated in many parts of the globe during the past 109 years of Earth history. Ferric oxides also pigment marine chert, limestone, and cherty iron formations and ooidal ironstones, but these chemical deposits are not usually included among redbeds.
Some redbeds contain abundant grains of sedimentary and low-grade metamorphic rocks and relatively few grains of iron-bearing minerals. Most of them, however, contain feldspar and relatively abundant grains of opaque black oxides derived from igneous and high-grade metamorphic source rocks. Clay minerals in older redbeds, as in most other ancient clastic deposits, are predominantly illite and chlorite, thus providing no specific clue to the climate in the source area or at the place of deposition.
In many of the younger redbeds the pigmenting ferric oxide mineral cannot be identified specifically because of its poor crystallinity. In most of the older ones, however, hematite is the pigment. As seen under the scanning electron microscope, the hematite is in the form of hexagonal crystals scattered over the surface of grains and clay mineral platelets. In red mudstones most of the pigment is associated with the clay fraction.
Redbeds do not contain significantly more total iron than nonred sedimentary rocks. Normally, iron increases with decreasing grain size of redbeds. Moreover, the amount of iron in the grain-coating pigments is small compared with that in opaque oxides, dark silicates, and clay minerals.
On a global scale, paleomagnetic evidence of the distribution of redbeds relative to their pole position corroborates paleogeographic data, suggesting that most redbeds, evaporites, and eolian sandstones accumulated less than 30° north and south of a paleo equator where hot, dry climate generally prevailed. But diagenetic development of red hematite may be acquired long after deposition. Moreover, continental drift reconstructions reveal that the most widespread redbeds in the geologic record developed near the Equator in late Paleozoic and early Mesozoic time when the continents were assembled in a great landmass, Pangaea. See also Continents, evolution of; Paleogeography; Paleomagnetism.
