Publikationsdatum:
2021-03-29
Beschreibung:
Summary:
Berlin is situated in a region whose relief was formed by the Brandenburg advance of the Weichselian glaciation. The morphogenesis and age of these relief forms were investigated, especially as various authors have expressed different opinions:
1. ASSMANN (1957) established the existence of two Weichselian tills, which are also marked on the geological maps of Berlin, whereas CEPEK (1972) found only one Weichselian till south of the Pomeranian end moraines in the GDR.
2. The formation of the prominent surface forms of the Grunewald and its southern continuation east of the Havel were classified as Saale period by BERENDT (1899) and SOLGER (1958), whereas other authors (WOLDSTEDT, 1961; ASSMANN, 1957; HAGEDORN & HÖVERMANN, 1962; MOLLE & SCHULZ, 1975) consider them to be Weichselian.
Sandpits were preferred for the investigations since they offer better possibilities for methodic study and show more easily recognizable sedimentary patterns than drillings. The most important stratigraphic methods used were pollen analysis, radiocarbon dating and the analysis of pebbles and boulders.
The sandpit at Berlin-Kladow proved to be the most important exposure because of its clearly classifiable key horizons. A late Saalian periglacial drop soil was exposed over Saalian glaciofluvial sands. On top of this was a layer of mud and peat, dated as Eemian by pollen analysis. A layer of sand separated this mud/peat layer from overlying layers of Weichselian interstadial organogenic and calcareous sediments, remains of a Lower-Weichselian till, consisting of local material in a sandy facies. This series was topped by 15-m-thick glaciofluvial sands and a second Weichselian till. Both tills contained the range of gravel which, according to CEPEK (1536?), is typical for the Weichselian glacial period, and differed clearly from the underlying Saalian material. This is the first time that two Weichselian tills were found on top of interglacial sediments south of the Pomeranian ice marginal grounds.
Key horizons like the Eemian deposits at Kladow do not exist in the other sandpits, so only lithofacial and morphological investigations were possible. It was not always possible to distinguish Saalian from Weichselian sediments by means of pebble and boulder analysis. At the bottom of the sandpit at the Postfenn, on the eastern slope of the Grunewald hills, a till was found at an altitude of 35 m above sea level. This means that the Grunewald hills have a Saalian core. It seems that the overlying stratified sands are Weichselian and that the second Weichselian ice advance is documented here by these pushed sands. However, the surface till cannot be dated by pebble and boulder analyses unless the many young surface forms are morphologically interpreted.
It was not possible to date the sediments in the Potsdamer Chaussee sandpit, situated, like the Kladow sandpit, on the eastern Nauen till plain, on the basis of pebble and boulder analyses because their Saalian composition contradicts the morphological situation.
Up to now the furthest inland ice advances were dated only from radiocarbon dates from icefree areas. In order to obtain a more precise date, radiocarbon analyses were made of samples of organic material from glaciofluvial sands. At Kladow, interglacial and interstadial deposits were also analysed. The results were surprising: In the Pleistocene sands contaminations are possible up to a depth of 18 m below the surface and cannot be eliminated by separating the organogenic remains of humic acids by means of NaOH. Several separate analyses of components insoluble by NaOH and of humic acids from the same samples gave a greater age for the humic acids than for the other components. Thus all radiocarbon dates only give minimum ages and have no chronostratigraphic value.
From a methodological point of view, the following results were obtained: Scepticism seems justified with regard to all radiometric dates that cannot be checked by another method, e. g. pollen analysis of interglacial sediments. This is the case with all allochthonous material and is probably true of most 14C dates. Pollen analysis supplied a clear stratigraphic classification for the Eem interglacial, but for the interstadial it was only able to document a short period of vegetational evolution whose exact age is unknown. Pebble and boulder counts proved to be still unsatisfactory as no systematic relationship to the morphology could be established, even in a small study area. A morphological interpretation of these results indicates the existence of two ice covers in this area at the maximum of the Weichselian glaciation:
1. Under periglacial climatic conditions, the long pre-Weichselian depressions, such as the Havel and Grunewaldseen "Rinnen”, acted as spillways for the advancing ice sheet.
2. The first ice advance came from the N and NE, depositing the Grunewald sands as a frontal apron. The “Grunewaldrinne" (a long depression east of the Grunewald hills) diverted the meltwater away from the Saalian core of the hills.
3. When the ice sheet reached the far edge of the Brandenburg stage south of Berlin the entire area was covered by ice, but east of the Havel near the Grunewald hills there ran the seam joining the Brandenburg and the Berlin ice streams (BEHRMANN, 1949/50), so that the ice there was less thick.
4. During the melting phase the ice melted most quickly along this seam, and sand was deposited in kame-form between the east- and west-lying stagnant ice.
5. The ice melted over the whole area; only the depressions were still filled by dead ice.
6. A second ice advance, particularly marked on the Nauen till plain. followed a main route from NW, with a secondary push direction from NE. It compressed the kames east of the Havel into push-moraines, leaving the dead ice remains in front of them in the Grunewald channel, and formed the hills stretching from SE to NW across the Nauen plain (Zedlitz Berg, Krampnitz Berg, Langer Berg, Hitz-Berge, Schwarzer Berg). During this second advance, the Teltow till plain was either free of ice, with the glacier edge on the southern margin of the Berlin spillway. or covered by only thin ice, which soon melted again. Thus the meltwater of the bigger western ice stream flowed over the Grunewald hills and changed the sands of the Grunewald crest into a gentle slope inclining from W to E, in the opposite direction to their original deposition (MOLLE & SCHUIZ, 1975).
7. During the melting of the last ice advance. kames terraces were formed in places along the W-slope of the Grunewald hills.
Translation by A. Beck
Beschreibung:
research
Beschreibung:
DFG, SUB Göttingen, FU Berlin
Schlagwort(e):
910.02
;
Glazial
;
Stratigraphie
;
Quartär
;
Berlin
;
Havel
;
FID-GEO-DE-7
Sprache:
Deutsch
Materialart:
monograph_digi
Format:
56
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