Sedimentary Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

D I S T R I B U T I O N A N D T H I C K N E S S OF Q U A T E R N A R Y SEDIMENTS

IN T H E L A K E C O N S T A N C E BASIN

GERMAN MCILLER AND RUDOLF A. GEES

Laboratorium fiir Sedimentforschung, Universitdt Heidelberg, Heidelberg (Germany) Department of Geology, Dalhousie University, Halifax, N.S. (Canada)

(Received October 25, 1968)

SUMMARY

Distribution and thickness of Pleistocene and Holocene sediments are derived from a continuous seismic reflection survey of the Lake Constance Basin. The sediment distribution within the lake confirms the concept that the Lake Constance Basin has been formed by glacial erosion rather than by structural deformation.

QUATERNARY SEDIMENTS

In two recent papers MLrLLER and GEES (1968 a,b) reported on the results of a continuous seismic reflection survey of the Lake Constance Basin. The sub- bot tom profiles (Fig.l,2) revealed the existence of three main geological units, i.e., the Tertiary molasse forming the framework of the lake basin, and two rather distinct younger units representing basin fill. The older of the two units, very likely tillite material, represents parts of the Pleistocene, while the younger unit, well-bedded fine-grained lacustrine sediments, is of Holocene age. 1 Because of the rather close spacing of the 35 cross-sections and the two longitudinal sections of the survey an attempt was made to depict the distribution and thickness of the basin fill material by means of three maps. On the first map (Fig.3) the distribution of the Pleistocene basin fill material is shown. In Fig.4 the Holocene basin fill distribution is depicted, while Fig.5 shows the distribution of both units. The sediment thicknesses of both units were computed, based on a velocity of the longitudinal seismic waves of 1,633 m/sec. This value is in close agreement with values recently obtained by refraction seismic surveys carried out by the "Bundes- anstalt fiir Bodenforschung" in the vicinity of Lake Constance. Because of the absence of adequate seismic data of the eastern parts of the lake basin, the maps cover only the central and western parts of the basin.

x In two drillings in the emergent zone of the Lake Constance "Untersee", two younger units could be studied directly. Here the youngest unit overlying gravel and moraine material revealed to be of post-Warm age (MOLLER et al., 1967; STAESCHE, in prep.).

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Distribution and thickness of the Pleistocene sediments A comparatively narrow body of Pleistocene sediments extends from the

eastern edge of the map into the so called "Qberlinger See" across a fairly shallow sill reflecting the path of a glacial valley. In the eastern-most cross-section (Fig. 1) the maximum sediment thickness is approximately 80 m.

Distribution and thickness of the Holocene sediments The Pleistocene sediments are overlain by a well-bedded unit of lacustrine

sediments which reaches a maximum thickness of about 90 m in the Friedrichs- hafen-Romanshorn profile. Assuming a duration of 15,000 years for the post- Glacial period, the average annual rate of sediment deposition for this location is equal to 6 ram.

Distribution and thickness of the Quaternary sediments In the profile Friedrichshafen-Romanshorn (Fig.l) the total thickness of

the Quaternary sediments amounts to approximately 170 m. This means that the boundary between the Tertiary and the Quaternary at the bot tom of the trough is about 20 m below sea level.

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QUATERNARY SEDIMENTS IN THE LAKE CONSTANCE BASIN 87

I t stands to reason that the total thickness of the Quaternary increases in the general direction toward the Rhine delta. This is confirmed by the log of a well which was drilled in the Rhine valley near the town of Dornbirn, Austria (Fig.l,5). The thickness of the Quaternary section at this location amounted to 336 m (WAGNER, 1962).

CONCLUSIONS

As a result of their survey, Mt)LLER and GEES (1968 a,b) came to the con- clusion that the Lake Constance Basin is the result of glacial erosion rather than related to structural deformation, i.e., the formation of a graben. The sediment distribution within the lake basin confirms this concept. The Pleistocene unit shows marked changes both in thickness and extent throughout the central and western parts of the lake basin. In Fig.2, this unit appears as fill in the main valley as well as in an apparent tributary south of it.

The Holocene unit rests unconformably on the Pleistocene unit. I1 reaches its maximum thickness in the deepest parts of the lake.

ACKNOWLEDGEMENTS

We wish to express our sincerest thanks to the "Deutsche Forschungsge- meinschaft" for their financial assistance and to the "Bundesanstalt ftir Boden- forschung", Hannover, for providing the technical equipment to run the con- tinuous seismic reflection survey.

REFERENCES

MiJLLER, G. and GEES, R. A., 1968a. Origin of the Lake Constance Basin. Nature, 217: 836-837. M[JLLER, G. und GEES, R. A., 1968b. Erste Ergebnisse reflexionsseismischer Untersuchungen

des Bodensee-Untergrundes. Neues Jahrb. Geol. PalOontol., Monatsh., 1968: 364-369. MiJLLER, G., SCHREINER, A. und STAESCHE, W., 1967. Kurzprofile der wissenschaftlichen

Bohrungen "Bodensee D F G 1 und 2". Naturwiss., 54: 87-88. STAESCHE, W., in prep. Mineralogisch-sedimentpetrographische Bearbeitung der Bohrung

"Bodensee DIG 1" bei Radolfzell, Bodensee. Thesis, Univ. of Heidelberg. WAGNER~ G., 1962. Zur Geschichte des Bodensees. Jahrb. Vet. Schutze Alpenpflanzen Tiere,

27: 1-17.

Sediment. Geol., 4 (1970) 81-87