Reassessing southern African silcrete geochemistry: implications for silcrete origin and sourcing of silcrete artefacts

Abstract

A synthesis of the geochemistry of silcretes and their host sediments in the Kalahari Desert and Cape coastal zone, using isocon comparisons, shows that silcretes in the two regions are very different. Kalahari Desert silcretes outcrop along drainage-lines and within pans, and formed by groundwater silicification of near-surface Kalahari Group sands. Silicification was approximately isovolumetric. Few elements were lost; silicon (Si) and potassium (K) were gained as microquartz precipitated in the sediment porosity and glauconite formed in the sub-oxic groundwater conditions. The low titanium (Ti) content reflects the composition of the host sands. Additional elements in the Kalahari Desert silcretes were supplied in river water and derived from weathering of silicates in basement rocks. Evaporation under an arid climate produced high-pH groundwater that mobilized and precipitated Si; this process is still occurring. In the Cape coastal zone, pedogenic silcretes cap hills and plateaus, overlying deeply weathered argillaceous bedrock. Silicification resulted from intensive weathering that destroyed the bedrock silicates, almost completely removing most elements and causing a substantial volume decrease. Some of the silica released formed a microcrystalline quartz matrix, and most Ti precipitated as anatase, so the Cape silcretes contain relatively high Ti levels. The intense weathering that formed the Cape silcretes could have occurred in the Eocene, during and after the Palaeocene-Eocene Thermal Maximum, when more acidic rainfall and high temperatures resulted in intensified silicate weathering worldwide. This could have been responsible for widespread formation of pedogenic silcretes elsewhere in Africa and around the globe. Trace element sourcing of silcrete artefacts to particular outcrops has most potential in the Cape, where differences between separate bedrock areas are reflected in the silcrete composition. In the Kalahari Desert, gains of some elements can override compositional differences of the parent material, and sourcing should be based on elements that show the least change during silicification.

Effect of surface weathering on portable X‐Ray fluorescence analysis of artefacts: Evidence from Tasmanian hornfels

“…In the present study, the isocon line was drawn through the Zr value for each sample, as Zr in the hornfels appears to be present mostly as zircon grains, which are very insoluble and extremely resistant to weathering and diagenetic processes (Morton, 1984; Pettijohn, 1941; Thornber, 1992). Many studies of regolith processes have found Zr to be immobile (e.g., Shackley, 2011; Webb & Golding 1998; Webb & Nash, 2020), and it has been considered as such in this study. During hydrothermal alteration, Zr is almost invariably immobile along with Ti (Grant, 1986); the fact that in the present case Ti plots on the isocon defined by Zr supports the immobility of both elements during hornfels weathering (discussed further below).…”

A numerical model for duricrust formation by water table fluctuations

“…One possible development and use of our model would consist in adjusting the value of the key rate parameter, i.e., τ , for each type of duricrusts, namely calcretes, silcretes or other types of hardened layers that are likely to form according to the hydrological model. It is known that different types of duricrusts form in different environments, most likely in an optimum moisture/climatic gradient in time and space (Webb and Nash, 2020;Taylor and Eggleton, 2001;Khalifa et al, 2009;Momo et al, 2020;Mather et al, 2019;Watson, 1988). Using the model, we could then map the tectonic and climatic scenarios under which each type of crust forms and explore potential differences in their long-term preservation.…”

Calcretes, Silcretes and Intergrade Duricrusts

“…Nash et al, 2013. For example, the large geochemical datasets arising from this work have been used by Webb and Nash (2020) to constrain the environmental controls upon silcrete formation in the Botswana Kalahari (see section X.3.3).…”

“…For readers seeking general information about silcrete, useful reviews are provided by Milnes and Thiry (1992), Nash and Ullyott (2007), Nash (2011Nash ( , 2012 and, from an archaeological perspective, Thiry and Milnes (2017). Overviews that focus on silcretes in Botswana are limited to early studies such as Goudie (1973) and Summerfield (1982;1983b;1983a) and the more recent syntheses by Nash (2012) and Webb and Nash (2020).…”

“…Several studies of Botswana silcretes incorporate geochemical data (Summerfield, 1982;Nash et al, 1994bNash et al, , 2013Nash and Shaw, 1998). Meta-analysis of major and trace element data for silcretes within the Okavango-Makgadikgadi system (Webb and Nash, 2020) shows that the median composition comprises 94 wt% SiO2 with small amounts (<1.1 wt%) of Al2O3, K2O, Fe2O3, MgO, CaO and TiO2 (in decreasing order of abundance). Barium is the most abundant trace element (~1000 ppm), with high levels of chromium (~200 ppm), strontium and zirconium (50 and 65 ppm respectively), low levels (3-20 ppm) of rubidium, cerium, lanthanum, neodymium and yttrium (in decreasing order of abundance), and very low levels (mostly <1 ppm) of other elements.…”