Dolomitization controlled by normal faults in the Jurassic Vajont Limestone (Southern Alps, Italy): Photogrammetric digital outcrop modelling and structural analysis

The Vajont Gorge (Dolomiti Bellunesi, Italy) provides spectacular outcrops of Upper Jurassic limestones (Vajont Limestone Formation) in which Mesozoic and Cenozoic (Alpine) faults and fracture corridors are continuously exposed. The Mesozoic normal faults and fracture corridors acted as conduits for Mg-rich fluids, resulting in structurally-controlled dolomitization of the limestones, with strong impact on porosity and permeability. A 3D surface characterisation of the outcrops was carried out, based on a photogrammetric high resolution survey. This provided a quantitative framework for structural analysis and 3D modelling of the dolostone bodies, aiming to unravel the linkage between coupled brittle deformation and fluid flow processes. Our photogrammetric workflow has advantages with respect to Lidar-based projects in terms of spatial resolution, quality of imagery, acquisition and processing timing and cost. The survey resulted in a Digital Outcrop Model dataset (700 m x 350 m x 300 m thick) consisting of continuous triangulated surfaces, representing the outcrop surfaces, textured with high-resolution images. Interpretation and modelling performed on this dataset include: (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (where possible ground truthed by direct observations); (3) correlation and extrapolation of realistic 3D surfaces from their traces; and (4) development of a 3D geological model at the scale of the Vajont Gorge, including stratigraphy, faults, dolomitization fronts, and volumetric meshes suitable for the statistical analysis of structural, diagenetic and geochemical parameters. The Mesozoic normal faults which outcrop in the Vajont Gorge are relatively small-offset features (< 30 m net slip), but show relatively thick damage zones, so they represent important conduits for Mg-rich fluids. They are arranged in two orthogonal (orthorhombic?) conjugate sets, striking NW and NE. These faults can be related to larger-scale regional features, possibly controlled in their orientation and kinematics by inherited basement discontinuities. Our 3D Digital Outcrop Model (DOM) allowed reconstruction of subtle offsets (10-30 m range) across faults, even if currently the faults are heavily masked by dolomitization. Crosscutting relationships between faults and dolomitization fronts demonstrate that fluid flow and dolomitization took place during the activity of the normal faults. This is in accordance with models which predict that permeability is higher during tectonic activity (possibly enhanced by fracturing and dilatancy). The interplay of intersecting orthogonal fault sets and related damage zones resulted in complex shapes of the dolostone bodies, which can be fully appreciated only in 3D. Moreover, the 3D DOM allows for different realizations to be generated from field data, in order to explore the uncertainty e.g. in the 3D limestone/dolostone facies proportions.status: publishe