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

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 Al203, Fe203, MgO Na20 and K20. Basal mett-out lill from lee-side localities is mainly formed by crushing and has a low silt content. Compared with lodgement lill, the content of feldspars and sheet silicates is significantly lower in the silt, resulting in a higher content of Si02• No differences were found in the bulk sand + silt + ela y geochemistry, de monstra ting that the variations were on! y due to a different size distribution of minerals and not to rem oval 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 white the sheet silicate content increases. A corresponding variation is found in the Si02 content relative to the contents of A1203 Fe203, MgO and K20. If fines are removed by meltwater a significant increase of the Si02 content and decrease of Al203, Fe203, MgO and K20 results in the remaining till material. The investigalion shows that mineralogy and geochemistry of tills are clearly dependent on lill-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.