Origins and palaeo-environments of Kalahari duricrusts in the Moshaweng dry valleys (Botswana) as detected by major and trace element composition

Autor: Bernard Vink, Chris Harris, W. Matheson, P. Huntsman-Mapila, Ali B Kampunzu, Susan Ringrose
Rok vydání: 2007
Předmět:
Zdroj: Journal of African Earth Sciences. 48:199-221
ISSN: 1464-343X
DOI: 10.1016/j.jafrearsci.2006.10.007
Popis: Geo-chemical analyses of duricrusts from the margins of the Moshaweng dry valleys (Kalahari, central Botswana) was undertaken mainly to provide a diagnostic basis for palaeo-environmental conditions and to help stimulate the development of a worldwide duricrusts database. The Moshaweng duricrusts fall into three main petrological groups: calcrete, intergrade cal-silcrete and quartz pebble conglomerate and two distinct geochemical groups for elements involved in non-pedogenic calcrete and cal-silcrete formation, comprising mainly SiO2, CaO, CO2 and Sr. For most other elements, calcretes and silcretes define a single compositional variation trend indicating their genetic link. Results of this work show that the different behaviours of Rb and Ba (no anomaly) and Na2O and CaO (important anomalies) have resulted from in situ chemical weathering likely at the aquifer scale. The source solutions for calcretes and calc-silcrete formation are also derived from the aquifer host clastic sedimentary rocks, namely the Mmamabula Formation (Karoo) and the derived Kalahari Group Sediments. Hence these sediments represent the main source of CaO and SiO2 solutes that formed the Moshaweng duricrusts. Changes in geochemical conditions within the aquifer led to changes in pH such that recharge through infiltration led to Ce oxidation generating Ce-negative anomalies in percolating water implying initially low pH values ( 8.5 during the dry season such that the groundwater was supersaturated in carbonates leading to the precipitation of calcretes. The resulting highly alkaline and saline water may have promoted silica dissolution engendering calc-silcrete formation during the subsequent wet season when a decrease of pH to ∼7–8 was triggered by the inflow of fresh water. On the basis of the hydrochemical constraints discussed in this paper, we show that calcretes precipitated at pH > 8.5 and cal-silcretes at pH
Databáze: OpenAIRE
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