Sorting of heavy minerals in sediments deposited at a high accumulation rate, with examples from sandurs and an ice-marginal valley in NW Poland

Abstract

Density differences among the various heavy-mineral species appear to be the main factor responsible for their sorting in glaciofluvial and fluvial environments. Sediments from two sandurs (stretching perpendicular to the Weichselian ice-sheet front) and a terrace of the Toruń-Eberswalde ice-marginal valley (extending parallel to the Weichselian ice-sheet front) in NW Poland support this idea clearly. The deposits investigated were formed during the Pomeranian phase of the Weichselian glaciation. Heavy-mineral and sedimentological analyses of these sediments indicate that most changes in the proportion of the various heavy-mineral species in gravelly and sandy sediments can be ascribed to changes in the flow regime, which is, in turn, related to the density of the various heavy-mineral species. In addition, the turbulence of the current and the bedform played a role in the sorting. The sedimentation rate also affected the heavy-mineral composition of the sandur and ice-marginal valley sediments. Furthermore, the proportions of some heavy minerals depend on the transport distance, which is well expressed by differences that occur over the investigated distance of 90 km of the ice-marginal valley. It appears that some heavy-mineral species are good tools for reconstructing sorting processes. Heavy minerals deposited by streams with a fast-changing discharge and a high accumulation rate (which is common for glaciofluvial streams) can, by combining data about the composition of the assemblages with data about the characteristics (size, rounding, colour, etc.), help in reconstructing palaeo-flow regimes and in estimating the turbulence, transport distance and mode of sediment transport in the streams.