Structurally controlled hydrothermal dolomites in Albian carbonates of the Asón Valley, Basque-Cantabrian Basin, northern Spain

Shallow marine carbonate sedimentation dominated during the Albian in the western part of the Basque Cantabrian Basin in Northern Spain, forming the large Ramales Carbonate Platform. This platform originated on a less subsiding tectonic block facing deeper subsiding areas to the south and east whose origin is related to intense tectonic activity due to the opening of the Bay of Biscay. The Asón valley area shows excellent outcrops of fracture-related hydrothermal dolomites hosted mainly in these Albian carbonates. Mapping in the studied area shows several dolomite bodies related to main faults that cut the platform stratification almost at right angles. The bodies show a main vertical development along fault-strike up to 900 m thick from which kilometre-scale branches expand following the stratification. Dolomitization is usually pervasive and independent of the limestone facies. Main dolomite facies are fine replacive, sucrosic and saddle. Burial analysis of the host limestone supports maximum burial temperatures of 80°C and an intense tectonic activity from Albian to Turonian with the highest subsidence interval in the latest Albian. Albian stretching of the crust and subsequent ascent of the isotherms in the area is suggested to produce sufficient heat to the dolomitizing fluids. The structural analysis indicates a strong extensional to transtensional tectonic activity in the studied area during Albian to Turonian with the creation of a overstep between W-E trending Cabuérniga and Ruahermosa faults and N-S structures like the Ramales fault. In this scenario, fluids could have moved from subsiding deeper areas to fractured uplifted parts of the Ramales platform enhanced by diapiric activity. Petrography, C, O and Sr isotope-study and fluid inclusion analysis support a polyphase hydrothermal dolomitization at fluid-temperatures between 75°C and 240°C and highly variable salinity of up to 22wt.% NaCl. Fine dolomite replaced limestone firstly and then, sucrosic and saddle dolomites replaced part of the first dolomite and cemented newly created fracture-porosity together with different calcite cements. Zebra dolomites and hydroclastic breccias are products of this later stage.