Lower Freshwater Molasse

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. 

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. 

UFM Upper Freshwater Molasse UMM Upper Marine Molasse Ll-M Lower Freshwater Molasse Kim. Kimmendgean... 

The dominant detrital constituent in both the Lower Freshwater Molasse and the Upper Marine Molasse samples is monocrystalline quartz (ranging from 19.2 to 50.3%). The dark blue and brown CL indicates mostly plutonic and/or metamorphic origin polycrystalline quartz is much less abundant. 

Sheet silicates (muscovite, biotite and chlorite) are absent or occur only in minor amounts in most of the samples from the Upper Marine Molasse, whereas in the Lower Freshwater Molasse they can be locally more abundant. Minor or trace amounts of opaque and accessory minerals occur in each sample. In the Lower Freshwater Molasse the accessories are mostly heavy minerals, whereas in the Upper Marine Molasse they are typically glauconite grains. 

Kaolinite occurs only in traces in a few samples from the Lower Freshwater Molasse in Altishofen. It occurs as tightly clustered, vermicular aggregates of pseudohexagonal crystals in pores or between muscovite plates. 

The measured carbon and oxygen stable isotopic ratios of calcites are given in Table 1. In general, the 6 C values of calcites reveal no major variations between the different locations and lithostratigraph-ical units, although calcites from the Upper Marine Molasse are slightly heavier than those from the Lower Freshwater Molasse. The 6 0 values of calcites from the Lower Freshwater Molasse are... 

Among calcites of the Lower Freshwater Molasse (Fig. lOA), the Murgental samples are the most depleted in C. 

Fig. 9. Probable paragenetic sequence of major diagenetic events in (A) the Lower Freshwater Molasse and (B) the Upper Marine Molasse.

Sulphur isotopic ratios were measured on two samples from Murgental and Altishofen (Lower Freshwater Molasse). In the pervasive pyrite cements from Murgental, both the edge and the centre of a patch were measured, yielding cdt (Canyon Diablo Troilite) values of -14.4%o and +1.1 %o, respectively. In Altishofen, two spots of mica replacement were measured, yielding 6 S cdt values of-28.1%0 and -22.1%o. 

LFM, Lower Freshwater Molasse UMM, Upper Marine Molasse Mci, mudstone, coastal, isolated Mfi, mudstone, freshwater, isolated Sci, sandstone, coastal, isolated Sfc, sandstone, freshwater, connected Sfi, saiidstone, freshwater, isolated Smc, sandstone, marine, connected Stmi, siltstone, marine, isolated. 

Fig. 10. Cross-plots showing the stable isotopic compositions of (A) Lower Freshwater Molasse calcites and (B) Upper Marine Molasse calcites. Note the distinctly different cluster of some Tiefenbrunnen and Sonnenberg samples. This is referred to in the text as high trend.

Figure 12 demonstrates the results of pore water oxygen isotopic composition determination, calculated using Friedman O Neils (1977) fractionation equation for the Upper Marine Molasse (Fig. 12A) and for the Lower Freshwater Molasse (Fig. 12B). The temperature constraints used in the calculations were those obtained from textural data and microthermometry.

In the Lower Freshwater Molasse almost the same range was obtained for 8 0 ater from the caicites in Bassersdorf and Altishofen (-6 to +l%o SMOW, and -5.5 to +2%o smow, respectively). Textural data predict a slightly wider range (-9 to +4%o smow) for the caicites from MurgentaL The presence of all-liquid fluid inclusions, however, sets the maximum temperature of formation at 50 C,... 

In summary, within the resolution of the applied temperature constraints, the oxygen isotopic composition of pore waters was broadly the same at the time of calcite cementation in both the Upper Marine Molasse and the Lower Freshwater Molasse. 

In spite of the very different depositional environments, little difference was found between the diagenetic evolution of the Upper Marine Molasse and that of the Lower Freshwater Molasse. The following similarities can be pointed out ... 

Most diagenetic minerals are the same in the sandstones of the Lower Freshwater Molasse and the Upper Marine Molasse, apd there is no significant difference between the paragenesis of the two units. Minor variations are detected in the diagenetic history preceding the first calcite generation. 

Fig. 13. Cross-plot showing the relationship between 6 C and 6 0 of calcites from different facies associations in the Lower Freshwater Molasse and Upper Marine Molasse. See the text for an explanation of the facies associations.

Clusters of clean, connected sands of the Upper Marine Molasse and Lower Freshwater Molasse within the main trend samples overlap, and the 6 0 vs. 6 - C values correlate this supports the mixing model. 

Carbonate cements, mostly calcites, are volumetri-cally the most important authigenic minerals in sandstones of the Lower Freshwater Molasse and Upper Marine Molasse. Authigenic clays are locally significant other authigenic minerals occur only in minor or trace amounts. 

Textural evidence and fluid inclusion microthermometry suggest that most calcites formed at low temperatures, probably at around 50 °C. The oxygen isotopic composition of formation waters ranged from -9 to +2%o SMOW in the Lower Freshwater Molasse and from -5 to +l%o smow in the Upper Marine Molasse. Final ice melting temperatures suggest the presence of moderately saline waters in both formations. 

Platt, N.H. Keller, B. (1992) Distal alluvial deposits in a foreland basin setting—Lower Freshwater Molasse (Lower Miocene), Switzerland sedimentology, architecture and palaeosols. Sedimentology, 39, 545-565. 

Platt, N.H. (1992) Fresh-water carbonates from the Lower Freshwater Molasse (Oligocene, western Switzerland) sedimentology and stable isotopes. Sediment. Geol, 78, 661-669. 

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