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Morphogenetic landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
(3) Silcrete. This is a silica-rich accumulation (95 per cent SiO2), often overlying a kaolinized zone, formed within the weathered zone. It is up to 3 m thick, extremely resistant and having columnar structure. It appears to form under drier conditions (250 mm mean annual precipitation), where drainage is poor, or in areas of frequent sheetwash and where the groundwater has a high pH. Like laterite, it appears to be both accumulative and residual, and may even form in association with laterite or as a secondary product of it. Silcrete sheets are widespread in central and southern Australia (e.g. southern Flinders Range and Arcoona Plateau, South Australia) and in Southern and Central Africa, where they form important topographic caprocks.
Phanerozoic history of the Pilbara region: implications for iron mineralisation
Published in Australian Journal of Earth Sciences, 2022
C. S. Perring, J. M. A. Hronsky, M. Crowe
A thin veneer of Lower Cretaceous sediments, of fluviolacustrine origin, also disconformably overlies Permian sediments of the Collie Basin in the southwest Yilgarn (Le Blanc-Smith, 1993). The Yilgarn Craton also contains major paleodrainages that manifest today as chains of salt lakes, which comprise broad valleys of Mesozoic age (Anand & Paine, 2002; de Broekert & Sandiford, 2005; McGee, 2009; see Figure 1). De Broekert and Sandiford (2005) describe a subsidiary set of Cenozoic ‘inset valleys’ that were incised into the bedrock surface of the much broader and older system of ‘primary’ (Mesozoic) valleys. Valley infill comprises Eocene fluviatile to estuarine deposits (commonly lignitic and containing palynomorphs indicative of rainforest vegetation), which are overlain by lower-energy fluviatile to lacustrine deposits of late Oligocene to Miocene age. Pliocene sediments are of limited significance except in along the western Yilgarn margin, which has been subject to local uplift. Quaternary sedimentation is represented by sand sheets, dune fields, playas and evaporites. Major phases of weathering occurred before the inset valleys were incised as well as during and after they were filled. Silcrete and ferricrete formed during both the early Oligocene and the late Miocene whereas calcrete formation is restricted to the late Miocene.
A biological origin for gravel mounds in inland Australia
Published in Australian Journal of Earth Sciences, 2018
Most OGM occur in regions underlain by deeply weathered Paleozoic sedimentary rocks. Large areas were eroded to a peneplain surface with extensive chalcedonic silcrete to silcrete cemented sand and gravel, or ferricrete developed. Erosion has removed most of the peneplain surface and excavated an extensive, largely infilled, drainage network. Surface deposits are dominated by scree derived from outcrop, and gravels of various ages that cap ridges and fill the drainage, and lag deposits derived from these. Some gravel contains abundant iron-rich material as ferricrete, variably ferruginised sandstone and siltstone or silcrete, maghemite pisoliths and minor vein quartz. Eolian sand was deposited widely and partially blankets many mounds including D400 (Figure 1g).
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
Transect 3 comprises three different mesas; silcrete, ferricrete and colluvium capped (Figure 3b). These are present over 5 km and form the prominent landforms in the region. The area is dissected with minor drainages running around low hills. In the west of the transect silcrete forms 4–6 m-high mesas (Figure 3bi) located within paleodrainage. These mesas reduce in height towards the east and the landscape becomes heavily dissected, forming small hills. Vughy silcrete with conchoidal fracture and iron-oxide staining forms the top 1 m of the mesas, and there is a sharp horizontal boundary with partially silicified siltstones below (Figure 4b). The slopes have a mix of silcrete and siltstone lag. Vegetation is dominated by spinifex.