Mineralogy and geochemistry of basal till and their relationship to till-forming processes

In Åstadalen, southeastern Norway, the size distribution of minerals in basal till is dependent on the processes of glacial grinding and on the influence of meltwater. An experimental crushing gave a size distribution of minerals corresponding to that in the source rocks and insignificant comminution of
separate mineral grains. An experimental abrasion resulted in a more intensive comminution, and in particular the feldspars and sheet silicates were significantly reduced in size and became enriched in the silt fraction. Lodgement till is characterized by a significant silt component formed by glacial abrasion.
The silt fractions are therefore relatively rich in feldspars and sheet silicates and have high contents of Al2O3, Fe2O3, MgO Na2O and K2O. Basal mett-out till from lee-side localities is mainly formed by crushing and has a low silt content. Compared with lodgement till, the content of feldspars and sheet silicates is significantly lower in the silt, resulting in a higher content of SiO2. No differences were found in the bulk sand + silt + clay geochemistry, demonstrating that the variations were only due to a different size distribution of minerals and not to removal of fines by water. The distribution of minerals in the till is a function of the grain sizes. With decreasing particle sizes, the relative quartz content decreases while the sheet silicate content increases. A corresponding variation is found in the SiO2 content relative to the contents of Al2O3 Fe2O3, MgO and K2O. If fines are removed by meltwater a significant increase of the Si02 content and decrease of Al2O3, Fe2O3, MgO and K2O results in the remaining till material. The investigalion shows that mineralogy and geochemistry of tills are clearly dependent on till-forming processes and not only on bedrock variations. This is particularly important in geochemical investigations of tills where only a limited particle-size interval is studied.