The relation between unit weight and geochemical and mineralogical compositions in the fine fraction of till

A routine method for measuring the unit weight (UW) of the fine fraction of till was developed. The material was crammed into a 1.5 ml glass tube with a laboratory shaker for 75 sec. The upper half of the material was then poured away, and the volume and weight of the remainder were measured. The correlation coefficient of duplicate measurements is 0.995. Concentrations of Co, Cu, Fe, Mn, Ni, Pb and Zn were determined by AAS from the fine fraction of till. Abundances of chlorite, hornblende, K-feldpspar, mica (di- and triochtaedral) and quartz (CL, HB, KF, MI, QU) were determined by the X-ray diffraction (XRD) method. The grain-size distribution of the fine fraction was determined by X-ray sedigraph. Examination of the material revealed close interrelationships between the geochemical, mineralogical and physical properties of the fine fraction of till. UW increases as the abundances of HB, KF, PL and QU increase proportional to CL and MI. This is reflected in a decrease in concentrations of Co, Cu, Fe, Mn, Ni, Pb and Zn, and in the abundance of clay-sized material. The correlation between UW and mineralogical composition is related to the grain-size distribution of the fine fraction and/or different mineral shapes of phyllosilicates and quartz and feldspars. UW in the fine fraction of the weathered bedrock depends on the specific gravity of minerals, and not on the mineral shape as in till. The quartiles of UW in the weathered bedrock data are 1.43, 1.52 and in the till data 1.50, 1.70. The increase in the proportion of HB, KF, PL and QU compared to those of CL and MI in till depends mainly on the grade of maturity. Hence the bulk of the total variation in the geochemical composition is explained by glacial factors, and only a minor part by bedrock. Close to the till/bedrock interface, however, the bedrock effect to the total compositional variation of till is more important. Because the till maturity is reflected in the mineralogical composition, the grade of maturity can be estimated with the aid of UW. The parameter also enables glacial and bedrock factors to be distinguished from geochemical till data.

Abstract licence: CC BY-NC

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