Sepiolite as a multifactorial indicator of paleoenvironments in the Chobe Enclave (northern Botswana)

Abstract

The origin and evolution of clay minerals in sedimentary basins are usually influenced by changes in depositional hydrochemistry. The formation and preservation of rare fibrous magnesium-rich clay minerals, like sepiolite, are of interest in the Chobe Enclave, northern Botswana, as it occurred in a siliceous-rich intra-cratonic basin. New X-ray diffraction (XRD) data, scanning electron (SEM) and transmission electron (TEM) microscopy analyses, as well as geochemical data, are used to study the origin of authigenic sepiolite and the sub-environments related to its formation in the Chobe Enclave. The paleoenvironment record during the Late Pleistocene to Holocene demonstrates that the deposition in the region was preceded by an aluvial fan/fluvial deposits in humid conditions. The upper-basin catchment yielded a clay assemblage, including kaolinite and minor smectite amounts, now preserved in the Chobe as a detrital phase. The alluvial fan deposits were later overlain by deposits from lacustrine/palustrine environments where significant hydrochemical changes took place: during these changes, a fibrous clay mineral (authigenic sepiolite) formed in shallow lake/marshes in a closed system with low energy, under the influence of magnesium-rich and alkaline waters. The mineralogical and geometrical (diagenetic) relationships between the palustrine deposits (carbonate facies) and sepiolite point to a sequence of precipitation from an evaporitic system in which calcite, sepiolite, and finally amorphous silica follow one another. In addition, although the sepiolite parent solution was enriched from a water source concentrated in Mg2+, the presence of Ba2+ ions also emphasizes some spells of hydrothermal activity.