Syn-rift geofluids in fractures related to the early-middle Miocene evolution of the Vallès-Penedès half-graben (NE Spain)

Abstract

The Vallès-Penedès half-graben, located in the central part of the Catalan Coastal Ranges, is NE–SW oriented, 10–14km wide and up to 100km long, and is filled mainly with continental to marine siliciclastic deposits ranging from late Oligocene(?)–early Miocene to late Miocene. The syn-rift Burdigalian continental deposits are well stratified (S0 approx. 090/15N) red beds, affected by NNW–SSE-trending, nearly vertical, extensive fractures, middle Burdigalian–early Langhian in age. These extensive fractures, which cut quasi-perpendicularly the stratification, are filled with calcite veins presenting two filling stages. The first stage is formed by millimetric to centrimetric thick bands of ‘crystalline’ calcite, consisting of palisade calcite cement, anhedral spar texture and rhombic calcite cement, non to dull-orange luminescent, with δ18O values ranging from −9.0 to −5.2‰ PDB, δ13C values varying from −9.3 to −6.7‰ PDB, 87Sr/86Sr ratios ranging from 0.71088 to 0.71361, relatively high values of Mg, and variable values of Mn, Sr and Fe. This stage is interpreted as subaerial speleothems (rimstones) which developed in meteoric vadose conditions.The second stage is formed by euhedral spar–megaspar cement and ‘coconut-meat’ cement with zoned bright yellow to orange luminescence, with δ18O values ranging from −16.2 to −9.9‰ PDB, δ13C values varying from −7.4 to −1.9‰ PDB, 87Sr/86Sr ratios ranging from 0.71608 to 0.71800, and relatively low values of Mg, Mn, Sr and Fe. This stage is interpreted as subaquatic cements developed in meteoric phreatic conditions.The two stages of calcite fillings reflect successive circulation of two meteoric fluids with very different geochemical compositions and represent the change from vadose to phreatic environment during the evolution of the syn-rift stage (middle-late Burdigalian to late Langhian). The meteoric fluid producing the first filling stage was a superficial fluid proceeding directly from the surface influenced by soil-CO2; the meteoric fluid generating the second stage was a more evolved fluid which had previously undergone interaction with sediments and reservoir effect. The beginning of the thermal subsidence coupled with a change from a more arid to a hotter and perhaps wetter climate could had been the ultimate reasons for the observed evolution.