Diagenetic transformations and silcrete–calcrete intergrade duricrust formation in palaeo‐estuary sediments

Abstract

The Boteti palaeo‐estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcrete intergrade deposits while developing insight into pene‐contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO3 is derived from the dissolution and precipitation of micro‐fossils, while more general CaCO3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro‐fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO2/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO2/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene‐contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO2/CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO3 may act as a catalyst leading to pene‐contemporaneous bSiO2/CaO aggregate formation. However the processes involved require further work. Copyright © 2013 John Wiley & Sons, Ltd.

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

“…Composition and mineralogy of host sediments in the Okavango system The mineralogy of the surface sediments across the entire area covered by the Kalahari Group is dominated by quartz, with small amounts of fine-grained calcite and dolomite, clays (kaolinite, illite, smectite) and feldspars (plagioclase, microcline and K-feldspar) (McCarthy et al, 1991;McCarthy and Ellery, 1995;Chatupa and Direng, 2000;Huntsman-Mapila et al, 2005;Ringrose et al, 2014). The heavy minerals present vary according to the mineralogy of (often deeply) underlying bedrock: zircon, ilmenite, tourmaline, staurolite, kyanite, epidote, zoisite, andalusite, apatite, brookite, rutile, sillimanite, sphene, amphibole and garnet (Moore and Dingle, 1998;Nuumbembe, 2016).…”

“…The Okavango River presently delivers an estimated 360 000 tonnes of solutes to the Okavango Delta per year, ~50% of which can be dissolved silica, with concentrations up to 20 mg/L (McCarthy and Ellery, 1998; McCarthy, 2006); there is negligible solute input from rainfall (Milzow et al, 2009). Silica is removed biogenically as diatom frustules and phytoliths, which accumulate in stream and pan sediments in the Okavango Delta and its outflows (McCarthy and Ellery, 1995; Ringrose et al, 2014 ; Struyf et al, 2015).…”

“…The high pH groundwater present in many parts of the Okavango system dissolves the diatom frustules and phytoliths in the upper layers of floodplain sediment (Ringrose et al, 2014; Struyf et al, 2015), because silica solubility increases greatly at pH > 9. As a result, this groundwater is silica‐rich.…”

“…It is notable that Kalahari silcretes contain more Al than Cape silcretes even though their parent material has less (Table 1); this is probably because Al was retained by glauconite precipitation in the Kalahari silcretes, whilst no new Al‐bearing mineral formed in the Cape silcretes. Silicification of the Kalahari silcretes was driven by evaporation and periodic input of floodwater under an arid to semi‐arid climate; these processes are still occurring at the present day (McCarthy et al, 1991; McCarthy and Ellery, 1995; Ringrose et al, 2014). The crucial factor is that the evapotranspiration of groundwater in the Kalahari produces groundwater with a high pH that facilitates the mobility and precipitation of Si.…”

“…In the centre of the subcontinent, silcretes occur within Kalahari Group sediments, with outcrops described from the presently arid to semi‐arid Kalahari Desert (e.g. Summerfield, 1982, 1983c; Nash et al, 1994a; Nash et al, 1994b; Nash and Shaw, 1998; Shaw and Nash, 1998; Nash et al, 2004; Ringrose et al, 2005; Ringrose et al, 2009; Nash et al, 2013a; Ringrose et al, 2014; Nash et al, 2016) and the tropical southern Congo Basin (e.g. Veatch, 1935; Cahen and Lepersonne, 1952; Linol et al, 2015; Linol et al, 2019).…”