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Abstract "Heft 20"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 20

Friedhelm Hädrich

Das Quartär von Mengen am Oberrhein - Ergebnisse einer Forschungs-Kernbohrung

Mit einem Beitrag zur Molluskenfauna
von Klaus Münzing

Freiburg im Breisgau 1987

ISSN 0344-2691


All known loess exposures in the southern upper Rhine Valley  are commonly  situated on high slope, top or plateau  positions.  For the very first time it can be reported about a loess section in lower valley slope or valley floor position. The profile was recovered by a research core drilling north of the village of Mengen near Freiburg i.Br. in July 1983. A wide range of scientists took part in a multidisciplinary analysis of this project. The drilling reached a depth of 28,95m. The bore hole was situated in the center of the "Mengener Brucke" (bridge of Mengen), a loess ridge orographically linking the "Tuniberg" in the northwest with the "Batzenberg" in the southeast a part of the "Schon-berg-Hochfirst"-mountain group.
The drilling sinks through a sediment sequence (from top to bottom) of different facies types (loessial, lacustrine and fluvial). The loessial facies is subdivided in "SchwemmloB" (reas-sorted loess), "Fließlöß", gley loess, gleyed "FlieBloB", "Fliefi-erde" (mud) and paleosoils. The fluvial facies consists of a basal sand probably of old Pleistocene age. The paper is based on:
the description of the profile which was ascertained by  macroskopic examination of the drill core, the pedological and sedimentological data of physical and chemical analysis determined by 77 samples and the data of molluscan fauna (ascertained by selected samples).
The determined sediment types and paleosoils will be thoroughly described, genetically explained and defined. The question of gleyization predominating in many profile sections will be discussed particularly. It is attempted genetically to explain the lamination of inhomogenic and homogenic sediments created in various sediment types by the influence of ground water. Due to textural and structural differences between the laminas the inhomogenic sediments ("FlieBloB", gleyed "FlieBloB" and "FlieB-erde") are divided into reduced and oxidized bands. Another reason for the a.m. differention is the variation in the primary humus content. The "LIESEGANG"-banding ionic diffusion as well as the rhythmic precipitation of Fe-oxides are used to explain the lamination of homogenic gley loesses genetically.
Comments on the malakofaunistic results from Mr. MUNZING are following. A summary of these results concerning the molluscan fauna is given in paragraph 4.4.
Afterwards it is attempted to present a chronostratigraphy of the profile based on findings of artifacts, volcanic glasses and determinated molluscs as well as lithostratigraphic comparisons with Quaternary profiles situated in the upper Rhine region. The profile consists of at least 5 delimed, loamificated and partly humic paleosoils ana paleosoil residuals or paleosoil equivalents, respectively ("Fließerde", lacustrine sediments with high clay contents).
The topmost fossile soil (E) is a Holocene para-brown earth (luvisol) verificated by pieces of brick findings, the second paleosoil (D) is a residual of Bt-horizon of the Eemian para-brown earth with an early Würm humus layer situated above. This Eem position has been verificated by findings of volcanic glasses within the humus layer, by molluscs in the loess situated below, and above all by stratigraphical comparisons with the profile of Riegel/Kaiserstuhl in which recently paleomagnetic and thermo-luminescence measurements have been carried out.
The sequence of gley loesses and "Fließlöß" below D to a depth of at least 16,50 m is reckoned among the Rissian glacial period. The age of layers further down in the profile could not be determined.
At a depth of 25,08 m the loess facies is changing into a sequence of humic, strongly reduced, carbonate poor to lime-free and very clayey stillwater sediments (lacustrine facies); at 28,40 m the basal sand in fluvial facies follows directly. In comparison with the "Altere Schwarzwaldschotter" (older gravels of the Black Forest) of the "Markgräflerland" (profiles Heiters-heim and Buggingen) it can be substantiated that this sand is of old Pleistocene age.
The drilling also produced geological and geomorphological consequences for the constitution and the conservation of the "Mengener Brücke". Contrary to the expectation that it consists of a core of solid rock, the bedrock of loess is formed by lacustrine and fluvial Pleistocene unconsolidated rocks. Because of the low located fluvial sand the "Mengener Brucke" is a loess trap in the wind shadow of the "Tuniberg". The "Mengener Brucke" is located in the marginal deep between the Pleistocene-Holocene gravel fan of the "Dreisam" in the north and the "Möhlin-Neumagen”-system in the south. Here the fluvial-lacustrine and later on the aeolic sedimentation have presumably been lasting since the old- or middle Pleistocene. However, on the edge of these facies no sedimentation at all but frequent erosion has taken place; the fluvial sedimentation of crystalline unconsolidated rock of the Black Forest in the gravel-fan-facies has lasted until the Holocene.

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