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Thermal History Constraints from Studies of Organic Matter, Clay Minerals, Fluid inclusions, and Apatite Fission Tracks at the Ardeche Paleo-margin (BA1 Drill Hole, GPF Program), France
Maurice Pagel (1), Jean-Jacques Braun (1), Jean Robert Disnar (2), Luis Martinez (3), Christophe Renac (4), Guy Vasseur (5)
In the Triassic sandstones, primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one-phase brine inclusions entrapped below 70°C. The homogenization temperature determined on the later barite cement is about 100°C.
Aqueous two-phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults. Temperatures as high as 210°C recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the Early Jurassic.
The fluid inclusions in dolomite, most of them reequilibrated during burial, indicate a trapping temperature of 130-145°C at 1600 m and a temperature decrease towards the surface. This value is in good agreement with the MPTB (maximum paleotemperature of burial) method on organic matter, which converges to a maximum burial temperature of 130°C at the same depth. The apatite fission-track data agree with these temperatures and indicate that the temperature decreased below 120 ± 10°C during the Eocene.
These paleotemperatures, much higher than the present ones, can be explained
by the erosion of 1900 m of mainly Cretaceous sediments. This interpretation
implies a high average sedimentation rate during the Cretaceous, in agreement
with data determined on a regional scale. A time-temperature evolution
is proposed for the formations present in the Balazuc drill hole and the
eroded sedimentary rocks. The application of the dual reaction model of
the transformation of smectite to illite (Velde and Vasseur 1992) shows
that the data are in good agreement with the kinetic model. However, the
vitrinite reflectance variation trend with depth, predicted from the Burnham
and Sweeney model (1989), is different from the data variation. No definitive
explanation can be proposed for the failure of aturity assessment through
vitrinite reflectance in the BA1 drill hole.
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