Origin of siltstones: glacial grinding versus weathering


  • D. NAHON,

    1. Laboratoire Pítrologie de la Surface, Université de Poitiers, 40 Av. Recteur Pineau, 86022 Poitiers Cedex, France, and E.R.A. CNRS 220 and
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    1. Laboratoire Géologie dynamique, Université Aix-Marseille III, Centre de St Jérôme, 13397 Marseille Cedex 4, France, and L.A. CNRS 132 Géologie Ouest-Africaine
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    • O.R.S.T.O.M., Universidade de São Paulo, Brasil.


Siltstones are unusual rocks. They are mainly made up of quartz. Glacial grinding is considered by some authors to be the main, or even the only, process generating silt. This is supported by: (1) the presence of silt in basal tills from present ice-caps and, above all, (2) by the volumetric importance of Quaternary loess. However, ancient glaciogenic sediments contain only a little silt scattered in the matrix of tillites, or forming thin intercalations in pro- and periglacial deposits. These siltstones represent less than 5% of the total volume of Upper Proterozoic and Upper Ordovician glaciogenic sequences of West Africa.

On the other hand, silt is abundant in weathering profiles of tropical, equatorial and mediterranean zones. This silt is generated by splitting of minerals, mainly quartz, inherited from the parent rock. Quartz grains are partly dissolved in situ and their fragments cemented by iron oxi-hydroxides in tropical and equatorial zones and by calcite in mediterranean zones. Silt is particularly abundant in tropical soils, comprising up to 50-75%. Secondary dissolution of the iron or calcite matrix induces disaggregation of these soils and the release of silt which is later reworked and concentrated by wind or running water.

Weathering processes, and especially those operating in tropical zones, are the main phenomena generating silt. Glacial grinding would generate only a little silt. A large part of the material of Quaternary loess may be derived from glacially reworked weathering profiles.