Source, transport, and weathering influences upon grain-size and heavy mineral trends in glacial deposits of southwestern Sweden

Abstract

Heavy mineral variability and regional trends are interpreted in terms of sediment source, transport sorting and weathering processes at 22 sites in tills, glaciofluvial deposits and reworked sediments in southwestern Sweden. Heavy minerals make up 4-11% of the coarsesilt fraction (45-63 μm) of the tills and reworked sediments, while significant enrichment (up to 100%) has occurred in glaciofluvial deposits. The most abundant minerals are hornblende and epidote. Minor amounts of apatite, garnet, titanite, and zircon are usually present. The geographic distribution of the heavy minerals relates to three factors. The local Precambrian bedrock has considerable amounts of metamorphic minerals, such as epidote, due to repeated periods of low-grade metamorphism. Secondly, mineral sorting during sediment transport and deposition has enriched heavy minerals within individual deposits and specific fractions. Finally, chemical and physical weathering of the surficial sediments has selectively depleted certain mineral amounts now present in the upper part of the soil profiles. In contrast, the heavy mineral assemblages in the lower B horizons are only slightly different from those in C horizons in both tills and reworked sediments, allowing the combined use of these horizons in regional interpretations of source character. Consistent with regional metamorphic trends in the bedrock, the ratio of epidote to hornblende in tills is up to 10 times greater in northern parts of the former county of Älvsborg compared to south-central areas. Sediment sorting and stratigraphic heterogeneity limit validity of mineral comparison with glaciofluvial deposits. Heavy minerals are an essential source for acid neutralization and long-term nutrient supply in forest ecosystems. The correlation of mineralogy to bedrock sources and the compositional modification through sedimentologic processes are necessary parameter improvements in total ecological modelling.

Keywords:

• Heavy minerals
• provenance
• selective sorting
• weathering
• mineral stability
• till
• glaciofluvial
• southwestern Sweden