Silt production by weathering of a sandstone under hot arid conditions: an experimental study

Abstract

Various mechanisms have been proposed for the formation of silt-sized material, of which loess deposits mainly consist. These range from glacial grinding to aeolian abrasion and, more recently, salt weathering of loose, sand-sized quartz grains which relies upon salt solutions penetrating and exploiting already existing cracks. If, however, the original grains are partly constrained within a rock mass it could be argued that the effectiveness of salt weathering would be enhanced, and the requirement for pre-existing cracks might possibly be negated. To test this contention, cubes of unbedded quartz sandstone were subjected, under laboratory conditions, to 60 diurnal cycles of heating and cooling between approximately 21 and 54°C directed through one exposed face and wetting with solutions of NaCl, Na 2 SO 4 and MgSO 4 . Debris produced by the simulation was examined by scanning electron microscopy and thin sections were made from the sandstone blocks. Micrographs and particle size analysis show that most debris consists of more or less intact sand-sized grains liberated from the parent rock. However, coarse and medium silt-sized material, often characterised by fresh fracture surfaces, was also observed. Examination of the parent rock suggests that this silt-sized material originates from two sources: breaking away of silica cement and coatings of secondary silica from around sand-sized grains, and the microfracturing of quartz sand grains. Microfracture patterns observed in thin sections appear to result from point-loading by adjacent grains, which results in compressive and/or shear stresses sufficient to fracture individual grains. No attempt is made to quantify the amounts of silt produced, but it is suggested that the mechanisms described could, under suitable conditions of rock type and salt availability, provide a viable source of loessic silt.