Fault-related hydrothermal dolomites in Cretaceous carbonates (Cantabria, northern Spain): Results of petrographic, geochemical and petrophysical studies

The present contribution documents NW-SE oriented fault and fracture related dolomites in Aptian-Albian carbonates (Karrantza area; northern Spain). Field observations revealed two main dolomite types, namely massive and zebra dolomite. Texturally, these dolomite types are mostly planar and nonplanar and variably reworked by subsequent alterations, which resulted in neomorphism and recrystallization, cataclastic deformation and calcite filling of dolostones. Petrographic and geochemical studies demonstrate the superposition of different diagenetic events, which were involved in multiphase dolomitization. Several phases of hydrothermal calcite cement pre- and post-date the dolomitisation events. Massive dolomites show overlapping stable isotopic ratios ranging from -16.9 to -8.9{per thousand} ({delta}18 O V-PDB), and -2.6 to +3.1{per thousand} ({delta}13 C V-PDB). Zebra dolomite shows more depleted values of {delta}18O and {delta}13C as compared to massive dolomites ({delta}18O: -18.1 to -15.2{per thousand}V-PDB and {delta}13C: -8.1 to +1.6{per thousand} V-PDB). Fluid inclusion analyses show homogenization temperature (Th) values from 120 to 200°C and estimated salinities range between 10 and 24 eq. wt.% NaCl. Both dolomite types are nearly stoichiometric, with CaCO3 values between 50 and 52 mole% Limestones close to the dolomites show depleted {delta}l8O values (similar to those of the dolomites), implying isotopic resetting during dolomitization. Recrystallization appears to have decreased the bulk porosity values in the interlocking nonplanar dolomite (with negligible porosity), while late-stage calcite cements occlude most of the remaining porosity, and make petrophysical measurements difficult to interpret. The possible source of dolomitizing fluids can be deeply buried Triassic evaporitic strata in the intra-platform basin, Keuper salt diapirs and/or Mg-bearing igneous rocks