VÝZNAM VALOUNOVÝCH ANALÝZ LEDOVCOVÝCH SEDIMENTŮ PRO PALEOGEOGRAFICKÉ REKONSTRUKCE PLEISTOCENNÍHO KONTINENTÁLNÍHO ZALEDNĚNÍ JESENICKA

Abstract

Sediments of Middle Pleistocene continental glaciation at the northern foothill of Rychleby Mts. and Zlaté Hory Highland contain clasts of local to Nordic provenance. Local clasts originating from crystalline units of the above-mentioned mountains prevail. These clasts have palaeogeographical importance for a local reconstruction of ice sheet advance directions. General advance direction from NW to SE has been reconstructed basing on local clasts in earlier studies (mainly Gába 1981a, b; Gába – Pek 1999). This interpretation has been based on the fact that the shares of clasts of rocks cropping primarily in NW part of the Rychleby Mts. decrease towards the SE. New, in this contribution presented, interpretation reconstructs the ice sheet advance generally from the North to the South, with variations conditioned by local landscape. Glacial sediments have at each site in the NW–SE direction petrological composition, which corresponds to the lithology of a mountain part south of the site of concern. Gierałtow orthogneiss clasts predominate in glacial deposits of the NW part of the main ridge forefield of Rychleby Mts. The share of amphibolites rises significantly in the forefield of the central part of the Rychleby Mts. main ridge. Feldspar and muscovite quartzites dominate in the area of Sokol Ridge and Zlaté Hory Highlands. The new interpretation presumes the colluvial, alluvial and fluvial transport of the debris towards the northern and north-eastern mountain forefi eld before the ice sheet advance. Ice sheet advancing from the North eroded and transported this debris towards the South. Preglacial sediments corresponded petrologically to the mountain parts, from which they originated. Thus, glacial sediments have petrological composition, which corresponds to the lithology of those mountain parts, which lies south of the sediment occurrence. Part of the debris has been transported by Nisa Kłodzka River from the West towards the East already before the glaciation.The following pattern could be found in the petrological composition of the glacial sediments gravel fraction. Sediments with monotonous composition of local clasts contain low shares of Nordic and Poland clasts (~2–4 %). On the contrary, sediments with polymict composition of local clasts contain relatively high shares of Nordic and Poland clasts (up to 27 %). Monotonous and distant provenance poor sediments originated at places, where the source preglacial deposits must have been petrologically monotonous considering the lithology of source areas. Concurrently, morphologically conditioned preglacial accumulation of vast lithologically monotonous deposits took place at some places (proximal parts of mountain ridges and saddles foothill). Rather polymict and distant provenance clast rich sediments originated during the later phase of ice sheet decay. Debris from the whole ice sheet body, not only from the glacier base or its front, released to the depositional system at that time. Sites with these sediments are located beyond the mountain foothill, where mixing of debris originating from alluvial fans or rivers flowing form the mountain range took place. Petrologically by far more monotonous sediments have been deposited closer to the mountain foothill. Quartz clasts are mostly of local origin and have together with other clasts been part of preglacial sediments. Part of quartz clasts has been reworked from fluvial deposits of present Poland or they might originate from the Nordic areas.