Micromorphology of sepiolite occurrences in recent lacustrine deposits affected by soil development

Abstract

Deposits in dry lake basins of the Aminuis region, south-western Kalahari, Namibia, contain abundant sepiolite. Thin-section observations show that it occurs as part of the groundmass, for which an authigenic origin is implied by the lack of a source for detrital sepiolite. The deposits also contain common sepiolite coatings, which are nearly exclusively illuvial rather than the result of mineral authigenesis in pores. Other sepiolitic pedofeatures are: (i) carbonate depletion hypocoatings, locally disturbed by development of porostriation; (ii) surface crusts, formed by re-suspension and settling of fine material after flooding, occurring at sites where low salinity allows their preservation; and (iii) common fragments of coatings, produced by physical or biogenic disturbance, in deposits with an often strongly fragmented microstructure. This study illustrates the need to consider an illuvial or depletion-related nature of sepiolite and palygorskite occurrences along pores in soils, as an alternative for the possibility of clay neoformation.