Calcrete pedogenic

Sur cia.1 Deposits. Uraniferous surficial deposits maybe broadly defined as uraniferous sediments, usually of Tertiary to recent age which have not been subjected to deep burial and may or may not have been calcified to some degree. The uranium deposits associated with calcrete, which occur in Australia, Namibia, and Somaha in semiarid areas where water movement is chiefly subterranean, are included in this type. Additional environments for uranium deposition include peat and bog, karst caverns, as well as pedogenic and stmctural fills (15). 

Data has been obtained from published sources and listed data held in the library of the Geological Survey of Namibia. This data has been used to assess the geochemical environment in the vicinity of channel fill and pedogenic uraniumbearing calcrete deposits. The data has been analysed by various methods and laboratories so direct comparison of the data has to be treated with caution. A summary table of the geochemistry of waters from each of the deposits is given in Table 1. 

Candy, I., Black, S. Sellwood, B.W. (2004b) Quantifying timescales of pedogenic calcrete formation using U-series disequilibria. Sedimentary Geology 170, 177-187. 

Figure 2.1 Settings for calcrete development. In fluvial settings pedogenic calcretes can develop on floodplains and terraces, whereas groundwater calcretes may form in channel deposits or around the capillary fringe and upper part of the phreatic zone in more permeable parts of the floodplain. In alluvial fans paired calcretes may develop on the fans, with hydromorphic calcretes near discharge zones.

Radiometric dating has been applied successfully to pedogenic Quaternary calcretes (e.g. Wang et al., 1996 Amundson et al., 1998 Rowe and Maher, 2000 Deutz et al., 2002 Ludwig and Paces, 2002 Candy et al., 2004). U/Th dating has been used on Quaternary calcretes by Kelly et al. 

Figure 2.7 Models for pedogenic calcrete development (A) (i) after Gile et al. (1966), and Machette (1985), Stages 1-5 as defined by Machette (1985) (ii) after Blumel (1982) and Verrecchia et al. (1995) (B) after Rabenhorst et al. (1991) (C) after Wright et al. (1995).

Figure 2.8 Dynamic model for pedogenic calcrete development based on Alonso-Zarza et al. (1998). Existing calcrete horizons of profiles may be remobilised following some degree of exhumation (Elbersen, 1982) or by changes in rainfall.

As discussed above, pedogenic calcretes can form in a variety of climates, especially as regards temperatures. Intriguing as the presence of calcretes... 

Pedogenic calcretes form in soils and, of course, soils are the products of the interaction of the biosphere, atmosphere, hydrosphere and lithosphere as these are complex, dynamic and interrelated systems we should expect... 

Klappa, C.F. (1983) A process-response for the formation of pedogenic calcretes model for calcrete formation. In Wilson, R.C.L. (Ed.) Residual Deposits. Special Publication 11, Geological Society of London, pp. 211-220. 

Tandon, S.K. Narayan, D. (1981) Calcrete conglomerate, case-hardened conglomerate and cornstone a comparative account of pedogenic and non-pedo-genic carbonates from the continental Siwalik Group, Punjab, India. Sedimentology 28, 353-367. 

Watts, N.L. (1980) Quaternary pedogenic calcrete from the Kalahari (southern Africa) mineralogy, genesis and diagenesis. Sedimentology 27, 661-686. 

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