Abstract
The Rosenholm depression is investigated as an example of the development of the conditions of sedimentation for the Late- and Postglacial freshwater deposits in the area between Aarhus and Randers. The sequence, which in places is 12 metres thick, includes creeping soil, meltwater sand, outwash clay, Allerød-gyttja, -mould, and -peat, outwash clay and Postglacial gyttja and peat.The surface of the meltwater sand was mapped by borings; the bottom topography seems to support the assumption that the depression has had a subglacial past (tunnel valley).The Allerød-oscillation can be separated into three different facies: The NW-part has euxinic gyttja, and the SE-end has Allerød-mould and -peat.In the succeeding Postglacial lake-phase gyttja was deposited in the NW-part, while the bottom sediment in the SE-stern end (separated from the rest of the depression by a sand barrier) was composed of dy and tyrfopel. After this the basin continued to be divided into two parts until peat covered the whole area.Measurements of acidity (pH) and redoxpotential (Eh) indicate that the whole series of layers are undergoing decomposition at the present time.
Highlights
An important part of the Quaternary landscape on the RANDERS MAP SHEET is formed of valley systems which are believed to have a genetically complex past (LARSEN 1972).This paper illustrates the Late- and Postglacial development of the valleys as shown by investigations carried out in the Rosenholm depression.This depression is an elongated peat-filled hollow in the young-glacial landscape in front of the Harder border moraine
The aims of the investigations were to find the limits of the Rosenholm depression in the terrain, to characterize the sediments and to give an account of the condition of sedimentation in the basin during Late- and Postglacial time
It was earlier mentioned that the opalcontent was not determined in the samples from the Rosenholm depression sufficiently precisely, but as the lime-content in the gyttja is normally fairly high the trophy measured will only be influenced to a small degree by the content of opal, which on average lies below 10 %
Summary
An important part of the Quaternary landscape on the RANDERS MAP SHEET is formed of valley systems which are believed to have a genetically complex past (LARSEN 1972). The sediments in the Rosenholm depression consist of a series of Postand Lateglacial freshwater formations including: Postglacial peat Postglacial gyttja Post-/ Lateglacial clay Aller0dgyttja, -soil and -peat Lateglacial clay and Lateglacial meltwater sand This stratification, which is often found in the bogs of Denmark, has been the subject of much research and some of the Danish contributions to boginvestigations are mentioned below. 205) asserted, as did VON PosT, that the brown coloured water is a result of humus which is supplied from areas outside the real lake-domain, and a genetically special Jake type should not be set up to account for the dystrophy. Both eutrophic and oli gotrophic lakes can periodically be brown-coloured. The depression is drained by the Rosenholm river which has its outlet near Andi in the NW part
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