Sand analysis as a method of estimating bedrock compositions in Greenland. Illustrated by fluvial sands from the Fiskenæsset region

F Kalsbeek,B Thomsen,M Ghisler

doi:10.34194/bullggu.v111.6652

Abstract

Mineralogical and chemical composition, heavy and opaque mineral content and grain size distribution of the sands show that they mainly consist of locally derived material. Glacially transported material has little influence on the composition of the sands. By comparing sands of various grain sizes, it is shown that the sedimentary processes do not appreciably change the composition of the sands, except that biotite is largely winnowed out. Thus, the sands are suitable for a regional study of the composition of the country rocks and for prospecting. Estimates are made of the average mineralogical and chemical composition of the area and the abundance of the different rock types. The amount of hypersthene gives an impression of the metamorphic grade of the rocks, and the average chemical composition of the high-grade rocks in the northern part of the area is the same as that of the lower grade rocks in the southern part.

Highlights

The Fiskenæsset region belongs to the Archaean gneiss block of SouthWest Greenland (PULVERTAFT, 1968) which consists mainly of gneisses with bands and lenses of amphibolite
If the study of sand samples is to be used as a tool to estimate the average composition of the bedrock in an area, it is of importance to know whether or not the composition of the sands is dependent on the grain size
The same differences appear for these mixtures and these differences in heavy mineral content of the sands must be due to differences in metamorphic grade of the source rock

Summary

Introduetion

The Fiskenæsset region belongs to the Archaean gneiss block of SouthWest Greenland (PULVERTAFT, 1968) which consists mainly of gneisses with bands and lenses of amphibolite. If the study of sand samples is to be used as a tool to estimate the average composition of the bedrock in an area, it is of importance to know whether or not the composition of the sands is dependent on the grain size. The same differences appear for these mixtures (which were chosen so that they contained different amounts of hypersthene) and these differences in heavy mineral content of the sands must be due to differences in metamorphic grade of the source rock. B. Results of the grain counts an average the opaque minerals form 6.4 volume per cent of the heavy fraction, which amounts to 1.1 volume per cent or approximately 2.0 weight per cent of the whole sample.

The Provenance of the Sands

Findings

Summary and Conc1usions