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Sedimentary Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Oolitic Limestone, or oolite, has a texture resembling the hard roe of a fish (Greek oon = egg). It is made up of rounded grains formed by the deposition of successive coats of calcium carbonate around particles such as a grain of sand or piece of shell, which serves as a ‘seed’. These particles are rolled to and fro between tide marks in limy water near a limestone coast, and become coated with calcium carbonate; they are called ‘ooliths,’ and their concentric structure can be seen in thin section (Fig. 6.16a). Sometimes the grains are larger, about the size of a pea, when the rock made of them is called pisolite. Deposition of CaCO3 may also take place through the agency of algae; each particle, or seed, which is rolled about by currents acquires a coating of calcareous mud, which is surrounded by a layer of blue-green algae. The latter can survive the rolling for a long time. Devonian oolites when carefully dissolved have been shown to contain these concentric organic coatings. Ooliths are forming today in the shallow seas off the coasts of Florida and the Bahamas.
Mineralogy and origin of Carboniferous and Cretaceous kaolins from a number of localities in Egypt
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
The Senonian kaolin deposits are represented by plastic and colloidal pisolite, white to yellowish brown in colour. The colloidal pisolite form nodules or concretion of different shapes and sizes cemented with microcrystalline kaolinite and amorphous material most probably cliochite. Gypsum and anhydrite are observed in and outside the pisolite. Subangular to rounded nepheline are detected in some sections. Accessory minerals including subangular to subrounded zircon, rutile anatase and tourmaline are also detected. Different degrees of ferrugination are observed in some of the studied specimens.
Geomorphic provinces and regolith-landform evolution of the Capricorn Orogen, Western Australia
Published in Australian Journal of Earth Sciences, 2021
R. L. Thorne, S. C. Spinks, R. R. Anand
An interpretation of the regolith evolution across Transect 4 is shown in Figure 8. Similar to Transect 3, lateritic residuum developed above saprolite, with varying thickness dependent on lithology (Figure 8a). Pisoliths are developed by accretionary process in soils and transported downhill into topographic lows (Figure 8b) (Anand et al., 2002). Subsequently, a change in climate to more arid condition or uplift results in the stripping of previously formed weathering profiles and an increase in the transport of pisoliths (Figure 8c). Arid conditions result in the formation of calcrete (Mann & Horwitz, 1979) and the replacement of pisoliths. Finally, the dissection of the landscape by valleys formed by creeks and rivers results in the present-day regolith landscape (Figure 8d).
A biological origin for gravel mounds in inland Australia
Published in Australian Journal of Earth Sciences, 2018
Five mounds and the near surrounds were partly excavated. Samples were passed through 0.5, 0.9, 2, 6, 13 and 25 mm mesh sieves and weighed to determine particle size distribution. The pebble content of three excavated OGM and two gravel malleefowl mounds is shown by Table 2. An erosion gutter at D269 (Figure 1e) and removal of gravel from five OGM including KY01 (Figure 1f) during roadwork have exposed the interior and substrate. Pebble is used to describe all rock particles >2 mm diameter. Pisoliths are small rounded pebbles that commonly exhibit internal concentric bands formed by accretion on a nucleus. Most iron oxide pisoliths are magnetic and contain maghemite (γ-Fe2O3). Gravel is used for material that comprises a mix of sand and pebbles, developed either in situ or transported. Scree is used to identify initially generally angular material on slopes that is transported by a combination of gravity and sheet flow from the source to a drainage channel. Some rounding occurs during movement. Regolith is all unconsolidated material above bedrock and includes fragmented and broken-down bedrock, transported alluvial gravel, fine-grained sediments, soil, and eolian deposits (Eggleton, 2001).