Proceedings Chapter
Reference
Indications for Paleogeothermal Anomalies in the Molasse Basin (Switzerland & France)
SCHEGG, Roland Albert, MORITZ, Robert
Abstract
The maturation level of organic matter, in Oligocene and Miocene sediments from the Swiss and French Molasse basin, reveals a close relationship between very low temperature metamorphism in the foreland basin and the tectonic history of the Alps. Measured mean random vitrinite reflectance of outcrop samples range from 0.21 to 0.97%. Generally, the degree of thermal maturity increases towards the Alps, probably due to increasing burial in the same direction. Second order variation of the coalification pattern in the Plateau Molasse may be related to a regional discharge of warm fluids controlled by tectonic structures. Clay mineral analysis and fluid inclusion studies support the coalification results.
SCHEGG, Roland Albert, MORITZ, Robert. Indications for Paleogeothermal Anomalies in the Molasse Basin (Switzerland & France). In: Parnell, John. Geofluids '93: contributions to an International Conference on Fluid Evolution, Migration and Interaction in Rocks:
extended abstracts . Torquay : [s.n.], 1993. p. 96-99
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PJ\RNELL .J. ET AL.
Indications for Paleogcothermal Anomalies in the Molasse Basin (Switzerland & France)
R. SCHEGG & R. MORITZ
lnstitut des Sciences de la Terre, 13 me des Marafclzers, 12 JI Geneva 4, Switzerland
Abstract: The maturation level of organic matter, in Oligoccne and Miocene sediments from the Swiss and French Molassc basin, reveals a close relationship between very low- tcmpcraturc metamorphism in the forcland basin and the tectonic history of the Alps.Measured mean random vitrinitc rencctancc of outcrop samples range from 0.21 to 0.97%.
Generally, the degree of thermal maturity increases towards the Alps, probably due to increasing burial in the same direction. Second order variation of the coalification pattern in the Plateau Molasse may be related to a regional discharge of warm nuids controlled by tectonic structures. Clay mineral analysis and nuid inclusion studies support the coalification results.
INTRODUCTION
The geothermal regime in peripheral forcland basins is generally characterized as hypothermal (Allen & Allen 1990). 111c present thermal conditions of the Swiss Molasse basin indicate a low geothermal regime on a basin-wide scale. Geothermal gradients gradually decrease from >35°/km in the north to <25°C/km towards the Alps (Rybach 1984).
Given the high heat capacity of water, the geothermal pattern of a basin is dirc.ctly inOucnccd by the hydrodynamic regime (Hitchon 1984, Lam cl al. 1982). There is growing evidence that geothermal anomalies in sedimentary basins arc often caused by uprising deep groundwater (Nesbitt 1990, Beck et al. 1989). Convective heat transport has been suggested as a cause of the actual heal flow anomaly (>150mW/m2) in the distal part of the Molasse basin in Northern Switzerland (Rybach et al. 1987).
In the German Molasse basin, most oil and gas fields are linked to hasin-parallcl synthetic and antithetic normal faults (Bachmann & Muller 1991 ). Generally gas fields are closer than oil fields to the Alpine front (op. cit., their f-ig.
20.17). This observation agrees with a statement by Oliver (1986): "In the Appalachian trough, gas is more common near the orogcn, and oil is more common at great distances, possibly as a consequence of the temperature distribution in the nuids as they travcled into the forcland basin." Deming et al. (1990) calculated that temperature perturbation resulting from possible nuid invasion into a forcland basin can be as high as 50-100°C, over restricted areas under certain conditions. If such processes were relevant in the past, they should be reflected in the sediments from the Molassc basin.
Organic matter is very sensitive to minor changes in temperature (Barker 1989). The level of organic maturity can be determined by various methods (Bustin et al. 1985).
However, vitrinite rcflcclancc (%Rr) is probably the most widely used method. In order to establish local and regional variations in vitrinite rcllcctm1cc, about 200 outcrop samples (coals, phytoclasts) have hccn analyzcd . Additional information comes from clay mineral analysis and nuid inclusion studies.
GEOLOOICAL SETrING
The study area is located in the Swiss and French part5 of the Molasse basin. The Molassc basin is a typical peripheral forcland basin which extends from the "Haute-Savoie"
(France) in the west to the Linz-Vienna area (Ausuia) in the east (Homewood cl al. 1986). It includes, from north to south, the following tectonic units: i) Plateau Mola~sc (nat- lying formations affected by gentle folds and faults), ii) Subalpinc Molasse (consisting of closely imbricated thrust sheets adjacent to the Alpine nappes) and iii) Helvetic Zone (Alpine nappcs). Sedimentation in the Molassc basin continued from Early Oligoccne to Late Miocene. The sediments consist mainly of dctritic deposiL5 derived from the rising Alps. In general, four lithostratigraphic supcrgroups arc distinguished: Lower Marine Molassc (UMM, Middle Oligocenc), Lower Freshwater Molassc (USM, Upper Oligoccnc to early Lower Miocene), Upper Marine Molasse (OMM, late Lower Miocene) and Upper Freshwater Mola~se
(OSM, Middle Miocene) . The Oligocene Subhclvctic Flysch, which has filled the most. internal part of the subsiding forcland basin, is found in the llclvctic nappcs. In the western part of the basin only Oligocenc and lower Miocene sediments are preserved: younger strata have been eroded. There arc large uncertainties in erosion estimates of the Molasse basin (Lemcke 1974, Klilin et al. 1992, Schegg in press).
VITRINITE REFLECTANCE RESULTS
A detailed description of vitrinite rcOcctancc results is given in Schcgg ( 1992, in press) and only a brief review is given here.
Rencctancc values range from 0.21 % Rr (OSM, Plateau Molassc) to 0.97% Rr (USM, Subalpinc Molassc).
The distribution of vitrinite rencctancc values in the studied sedimentary formations clearly reveals that there is some stratigraphic control on maturation. Mean average values increase wil11 age.
The thermal maturity of the coal samples varies also geographically. Figure I shows the isorencctance contours for an area between Lake Geneva and Lake Ncuchatcl. The isorcflcctancc lines in the southern part of this area indicate an increase in maturation of the organic mailer towards the Alps. This trend occurs predominantly in the Subalpinc Molasse north of Lake Geneva. In the Plateau Molassc variations in vitrinitc rcncctance arc rather sporadic.
Nevertheless, two restricted regions possessing relatively high maturation levels (Rr>0.5%) were observed. One zone is located just south of Lake Ncuchatel. It is interesting to note that the three analyzcd samples in this region lie along the "Pomy-Cuarny thrust" (PC in Fig. I). Another area of
GEOFLUIDS '9:1 EXTENDED ABSTRACTS 97
1~
10 km OMM
USM
lNJ t
I
---0.5---~-
ISOREFLECTANCE LINE (%Rr)
Fig. 1. Isorencctance map of Western Switzerland. Interpretation of mean vitrinite· renectance values by kriging. Modified map from Schegg (in press). Tectonic structures after Jorcli (1990) and Weidmann (in press): BCC=Baulmes-Chamblon-Chcvrcssy thrust, ES=Essertine fault, EP=Ependes fault, l'vl=Mentue thrust, PC=Pomy-Cuarny thrust, RB=Rances-Baulmes fault, S=La Sarraz fault, B=Broye thrust.
A similar pattern has been found in Eastern Switzerland (Schcgg in press) where a zone of increased vitrinitc reflectance is located parallel to Lake Ziirich, which is probably tectonically controlled.
Second order variations arc also superimposed on the general trend of increasing thermal maturity towards the Alpine front in the Haute-Savoie area (Fig. 2). An area oriented NE-SW, south west of the Prealps, displays reflectance values of up to 1.06% Rr. Again major tectonic structures, the "Arvc" strike-slip fault (Rigassi 1978) and the thrust front of the Subalpine Molasse (Gorin I 99 I), arc located close to this zone.
11-IERMAL MODELLING
The EASY%Ro-modcl of Sweeney & Burnham (1990) was used for this study. This model uses an Arrhenius first-order parallel reaction approach with a distribution of activation energies. It enables the calculation of the vitrinite maturation history (%Rr versus time) for a given temperature history (temperature versus time). The model is reliable for values of mean random vitrinite rencctance ranging from 0.3 to 4.5%
Rr.
Measured vitrinite reflectance values of outcrop samples from the Plateau Molasse range from 0.2 to 0.6% Rr. Modelling shows that this may correspond to maximum paleotemperaturcs of up to l l0°C (Schegg in press). For the zones with increased thermal maturities, an incrca<;e between 30°C and 70°C in comparison to the surrounding area is suggested by our calculations.
DISCUSSION
Conventially, maximum burial depth is regarded as the most important factor for coalification in hypothermal basins.
Therefore, the increase of thermal maturity towards the Alps can be explained by increasing burial depth in the same direction.
Second order variations in the coalificalion patterns of the investigated areas can only partly be explained as the result of local high overburden-th icknesses and high uplifl/erosion values. Given a paleogeothermal gradient of 30°C/km, an uplift/erosion value of l-2km would be required.
Alternatively, zones with high thermal maturity may be due to focused convective heat transport. A plausible mechanism is the discharge of warm groundwater controlled by tectonic
98 PARNELL J. ET AL.
•USM o UMM
• FLYSCH
Q
0.75--Fig. 2. Jsoreflectance map of Haute-Savoie. Interpretation of mean vitrinite reflectance values by kriging. Modified map from Gorin (1991). Th=pressure corrected fluid inclusion homogenization temperature, A="Arve" strike-slip.
structures. There is some independent evidence for this interpretation. The present day heat flow anomaly in Northern Switzerland, explained by uprising deep groundwater (Rybad1 et al. 1987), shows that convective heat transport is important for the present geothermal regime of the Molasse basin. Clendenin & Duane
(1990) confirmed a tectonic link between the Appalachian- Ouachita orogeny and the Mississippi Valley-type Pb-Zn deposits of its forcland basin. Seismic pumping and distal fluid concentration along faulls appeared to be a plausible mechanism to move hot reactive fluids Lo the sites of ore deposition.
SUPPORTING EVIDENCE
Fluid inclusion : Fluid inclusions studied in calcite veins at the eastern end of the "Salcve" mountain (Fig. 2) were used lo evaluate fluid trapping conditions in a region of increased vitrinite reflectance. The "Sa!Cve" is a thrust slice consisting of Mesozoic sedimenl~ which has been sheared off during the final phase of the Alpine orogeny (Jura folding, late Miocene and Pliocene). The calcite veins were sampled from early Cretaceous Urgonian limestones which underly Oligocene Molasse sediments in the "Haute-Savoie" area. The studied veins belong to the NW-SE running "Arve" strike-slip system. This strike-slip fault was mainly active during and after the final phase of alpine orogeny. Therefore, evaluated trapping conditions reflect post-Molasse-depositional temperatures.
Fluid inclusions yield bulk homogenization temperatures between 62.6°C and 73.8°C (mean= 68.5°C).
Last melting of ice occurs at -0.2°C and -0.3°C, thus indicating low salinities of about 1 weight% NaCl equivalent for the trapped fluid. Assuming I) lithostatic conditions, 2) a maximum thickness of 2000 m of overlying
sediments (Charollais et al. 1988, Lemcke 1974) at the time the fluid inclusions were trapped, and 3) an average rock density of2.7 g/cm3, the pressure on the fluid at the time of entrapment is estimated to have been 530 bars. This value corresponds to an average temperature correction of 50°C (Potter 1977). Thus, maximum trapping temperatures arc estimated to be around 120°C. The corresponding palcogeothcrmal gradient of 60°C/km is abnormally high for the Molasse basin (TeichmUllcr & TeichmUller 1986).
However, the temperature estimate from our fluid inclusion data is consistent with the high vitrinite reflectance values along the "Arve" strike-slip fault, and it is in line with the hypothesis that warm, probably dilute, fluids were channeled along tectonic structures during the late evolution of the Molasse basin. Finally, it must be noted that our fluid inclusion data agrees well with previous studies by Mullis (1987) suggesting the migration along tectonic structures of low-salinity fluids, possibly of meteoric origin, lo great depths into sedimentary rocks during the Tertiary in Northern Switzerland.
Clay minerals : The percentage of smectite layers in mixed- layer illite/smectite for shales and sandstones decreases towards the Alpine front (Schegg 1992) and thus confirms the coalification results. The occurrence of corrensite in the clay mineral fraction (<2µm) of feldspathic, serpentine- bcaring sandstones indicates minimal temperatures of about 90°C to 100°C (Kubler 1973). Corrensite is only found in sandstone taken in proximity to major strike-slip faulL5 (Fig.
2) which indicates that hot fluids may have migrated along these tectonic structures.
Acknowledgement~: For the critical review of the manuscript we would like to thank B.Ujetz and W.Wildi. This project is supported by the Swiss National Foundation for Scientific Research (Projects n° 20.26.218.89 and 21.30.309.90).
GEOFLUIDS ·93 EXTENDED ABSTRACTS 99
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