Swiss Molasse basin

Carbonate cements in the Tertiary sandstones of the Swiss Molasse basin relevance to palaeohydrodynamic reconstruction... 

This paper discusses carbonate cementation in Tertiary sandstones of the Swiss Molasse basin. The prime objectives are to reconstruct the postdeposi-tional evolution of sandstones in the Lower Freshwater Molasse and in the Upper Marine Molasse, and to try to understand the relationship between porosity development and fluid flow in the basin, based on the textural and geochemical characteristics of the carbonate cements. 

The numerous outcrops, the tunnels and the extensively cored petroleum, water and geothermal exploration wells make the Swiss Molasse basin an ideal candidate for such a study by providing the opportunity to develop the necessary regional sedi-mentological framework and allowing detailed sampling in the wells and tunnels to study diagenesis in the subsurface. 

This study focuses on the two most important lithostratigraphical units of the Swiss Molasse basin, the Lower Freshwater Molasse and the Upper Marine Molasse. The present-day depth of samples ranges from 15 to 1300 m the sample locations are shown in Fig. 1. 

Fig. 11. Diagram illustrating the comparison between carbon isotopic composition of authigenic calcites and the bicarbonates of modem formation waters from the Swiss Molasse basin.

There is little doubt that in a basin with such a complex depositional and tectonic history as the Swiss Molasse basin, several processes could account for calcite cementation. Based on samples which—according to their petrographic and isotopic data—are dominated by one specific type of calcite, the following types can be recognized ... 

The volumetrically most important cements in the Swiss Molasse basin formed early, and their isotopic characteristics reflect the fluid flow history during subsidence. Furthermore, carbon in the calcite cements is isotopically heavier than that in the bicarbonate of the modem formation waters, and the variation in modern formation waters is not reflected in the diagenetic mineral assemblage. 

The above facts suggest that porosity modification by cementation is much more significant during subsidence than during uplift in the Swiss Molasse basin, therefore conclusions based on distribution and chemistry of modem formation waters concerning diagenetic overprinting in other inverted foreland basins may have to be handled with some care. 

Monnier, F. (1982) Thermal diagenesis in the Swiss Molasse basin implications for oil generation. Can. J. Earth Sci., 19, 328-342. 

Pfiffner, O.A., Erard, P.F. Stauble, M. (1997) Two cross sections through the Swiss Molasse Basin (Lines E4-E6, Wl, W7-W10). In Deep Structure of the Alps, Results ofNRP20 (Eds Pfiffner, O.A., Lehner, P., Heitz-mann, P., Mueller, St. Steck, A.), pp. 64-72. Birkhauser Verlag, Basel. 

Rybach, L. (1992) Geothermal potential of the Swiss Molasse Basin. Eclog. geol. Helv., 85, 733-744. 

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