Abstract

Fluid inclusions (FI) in diagenetic minerals and fissures were studied in two boreholes drilled on the Ardeche margin, MM1 and BA1 which crosscut the Uzer fault at the surface and at depth, respectively. Most FI and their host minerals were placed in the petrostructural scheme proposed for this part of the margin, which includes three main stages. (1) At the Triassic, evaporitic pore waters were diluted, and sulfates were recrystallized, remobilized into fractures at BA1 at T< or =70 degrees C. In the latter block however, all primary FI formed at and after the deposition of the halite bed at the Carnian were saline ( nearly equal 20 wt. % eq. NaCl). (2) During burial diagenesis, monophase FI decrepitated in anhydrite above -300 m at MM1. Stratabound and fissural anhydrite was pervasively replaced by dolomite at BA1. In this borehole, diagenetic dolomite was typically zoned and enriched in FeCO 3 and MnCO 3 (up to 20 et 2.5 mole % respectively), sometimes with correlative Fe-Mn variations. It developed in tension gashes, en echelon cracks, hydraulic fractures or crack seals under high pore fluid pressures to geopressured conditions (many dolomitic joints are horizontal). Secondary FI in vertical dolomite-quartz veins at -1729.25 m and in pre-Carnian vein anhydrite are two-phased and saline ( nearly equal 20 wt. % eq. NaCl; 101 degrees <Th<163 degrees C and 125 degrees <Th<235 degrees C respectively, the FI with Th = 235 degrees C at -1630 m containing 31 wt. % eq. NaCl). They indicate that the block was supplied for hot halite-contaminated fluids, partly from the deep basin. This allowed the base of the BA1 block to be pressurized below impermeable Middle Triassic beds, and in turn, it favoured the cyclic activity of the fault [Blanpied et al., 1992]. At MM1, FI in quartz at -342.39 m and in dolomite at -295.10 m show that the hot saline fluids from BA1 migrated westward along strata. (3) Anhydrite and barite were remobilized during the late Pyreneo-Alpine compressive events, and mainly at MM1 (many horizontal gypsum-filled joints).

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