Conxita Taberner

Stratigraphic architecture and fracture-controlled dolomitization of the Cretaceous Khami and Bangestan groups: an outcrop case study, Zagros Mountains, Iran

Abstract The Barremian–Aptian upper Khami Group and Albian–Campanian Bangestan Group have been studied at outcrop in Lurestan, SW Iran. The upper Khami Group comprises a thin deltaic wedge (Gadvan Fm) transgressively overlain by shelfal carbonates (Dariyan Fm). The Dariyan Fm can be divided into lower and upper units separated by a major intra-Aptian fracture-controlled karst. The top of the Daryian Fm is capped by the Arabian plate-wide Aptian–Albian unconformity. The overlying Bangestan Group includes the Kazhdumi, Sarvak, Surgah and Ilam formations. The Kazhdumi Fm represents a mixed carbonate-clastic intrashelf basin succession, and passes laterally (towards the NE) into a low-angle Orbitolina-dominated muddy carbonate ramp/shoal (Mauddud Mbr). The Mauddud Mbr is capped by an angular unconformity and karst of latest Albian–earliest Cenomanian age. The overlying Sarvak Fm comprises both low-angle ramp and steeper dipping (5–10°) carbonate shelf/platform systems. Three regionally extensive karst surfaces are developed in the latest Cenomanian–Turonian interval of the Sarvak Fm, and are interpreted to be related to flexure of the Arabian plate margin due to the initiation of intra-oceanic deformation. The Surgah and Ilam Fm represent clastic and muddy carbonate ramp depositional systems respectively. Both The Khami and Bangestan groups have been affected by spectacularly exposed fracture-controlled dolomitization. Dolomite bodies are 100 m to several km in width, have plume-like geometry, with both fracture (fault/joint) and gradational diagenetic contacts with undolomitized country rock. Sheets of dolomite extend away from dolomite bodies along steeply dipping fault/joint zones, and as strata-bound bodies preferentially following specific depositional/diagenetic facies or stratal surfaces. There is a close link between primary depositional architecture/facies and secondary dolomitization. Vertical barriers to dolomitization are low permeability mudstones, below which dolomitizing fluids moved laterally. Where these barriers are cut by faults and fracture corridors, dolomitization can be observed to have advanced upwards, indicating that faults and joints were fluid migration conduits. Comparisons to Jurassic–Cenozoic dolomites elsewhere in Iran, Palaeozoic dolomites of North America and Neogene dolomites of the Gulf of Suez indicate striking textural, paragenetic and outcrop-scale similarities. These data imply a common fracture-controlled dolomitization process is applicable regardless of tectonic setting (compressional, transtensional and extensional).

Photogrammetric digital outcrop reconstruction, visualization with textured surfaces, and three-dimensional structural analysis and modeling: Innovative methodologies applied to fault-related dolomitization (Vajont Limestone, Southern Alps, Italy)

Different remote sensing technologies, including photogrammetry and LIDAR (light detection and ranging), allow collecting three-dimensional (3D) data sets that can be used to create 3D digital representations of outcrop surfaces, called digital outcrop models (DOM). The main advantages of photogrammetry over LIDAR are represented by the very simple and lightweight field equipment (a digital camera), and by the arbitrary spatial resolution, that can be increased simply getting closer to the outcrop or by using a different lens. The quality of photogrammetric data sets obtained with structure from motion (SFM) techniques has shown a tremendous improvement over the past few years, and this is becoming one of the more effective ways to collect DOM data sets. The Vajont Gorge (Belluno Dolomites, Italy) provides spectacular outcrops of jurassic limestones (Vajont Limestone Formation) in which mesozoic faults and fracture corridors are continuously exposed. Some of these faults acted as conduits for fluids, resulting in structurally controlled dolomitization. A 3D DOM study, based on a photogrammetric SFM data set, was carried out, aimed at enabling interdisciplinary characterization and reconstruction of coupled brittle deformation and fluid flow processes. For this study we used a DOM (730 m × 360 m × 270 m) consisting of continuous triangulated surfaces representing the outcrop, textured with high-resolution images. Interpretation and modeling performed on this data set include (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (ground-truthed); (3) correlation and extrapolation of realistic 3D surfaces from these 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 DOM study highlighted the close relationship between faults and dolostone geobodies, demonstrating that dolomitization was guided by fluid infiltration along Mesozoic normal faults. In order to explore the uncertainty associated with the 3D model of irregularly shaped dolostone bodies, three different 3D dolostone geobody realizations have been modeled, providing a minimum, intermediate, and maximum estimate of the dolostone/limestone volumetric facies ratio, while honoring the field constraints.

Anatomy and paleofluid evolution of laterally restricted extensional fault zones in the Jabal Qusaybah anticline, Salakh arch, Oman

The E-W−trending Jabal Qusaybah anticline, at the western termination of the Salakh arch, Oman Mountains, is characterized by a complex fault network that developed in layered Cretaceous carbonates. This network includes NE-SW left-lateral, N-S extensional, and subordinate E-W extensional fault zones. The N-S−striking extensional faults zones are roughly perpendicular to the fold axis and are best developed in the longitudinally bulged central sector of the anticlinal crest. They are likely due to along-strike outer-arc extension associated with positive fault inversion and salt migration. These extensional fault zones are confined within, and locally abut, major NE-SW left-lateral strike-slip fault zones. Extensional fault displacements range between a few decimeters and ∼60 m, whereas the maximum exposed trace lengths range between a few meters and ∼800 m. Narrow (∼1−15-cm-thick) cataclastic fault cores are surrounded by vein-dominated damage zones as thick as tens of meters. Moreover, fault zones show widespread evidence for substantial dilation in the form of (1) dilation breccias, (2) infilling by large columnar calcite crystals and aggregates, and (3) centimeter- to meter-thick veins. Dilation breccias and calcite infillings are primarily localized at fault tips, fault overlaps, and interaction zones between strike-slip and extensional fault segments. Displacement profiles along the N-S−striking extensional fault zones indicate that they are one order of magnitude shorter than values predicted by most published displacement-length scaling laws. By analyzing fault abutting geometries, detailed vein relative chronology, δ13C and δ18O signatures, and fluid inclusion data from calcite veins and calcite fault infillings, we propose a model whereby a deep-seated, regionally sized, left-lateral strike-slip fault system that was active during anticline growth inhibited the lateral propagation of late-stage transversal extensional fault zones. Our findings show that, in this geological setting, the structural position, rather than fault displacement, is the parameter controlling the location of the more dilatants (and permeable) fault segments. Results of the present work suggest that fault intersections may be more useful than fault throw for predicting zones of enhanced vertical fluid flow in structurally complex carbonate reservoirs.

Complex fault-fold interactions during the growth of the Jabal Qusaybah anticline at the western tip of the Salakh Arch, Oman

The Jabal Qusaybah anticline is located at the western end of the Salakh Arch, a major salient in the foothills of the Oman Mountains. We performed a structural and petrographical-geochemical study of vein sets and fault zones associated with the development of this anticline. Our data illustrate a complex deformation pattern both in space and time, characterized by the unusual presence of widespread NE-SW left-lateral strike-slip fault zones trending oblique to the E-W fold axial strike, and of abundant and well-developed N-S fold-perpendicular extensional fault zones associated with axial bulging and dilation, well developed in the central region of the anticlinal crest. We propose a three-stage evolution for the Jabal Qusaybah anticline, starting with prefolding jointing in the foreland of the late Cretaceous Oman Mountains, and followed by development of extensional faulting in Campanian times. Positive inversion of the Qusaybah Fault, possibly in Miocene times, caused development of a layer-parallel shortening fabric and amplification the Jabal Qusaybah Anticline, in concomitance with the activity of NE-SW left-lateral strike-slip fault zones that triggered N-S, fold-perpendicular extensional faulting, particularly in the axial bump of the anticline. The final evolutionary stage was characterized by further amplification of the axial bump and related N-S extensional fracturing and by uplift and exhumation. To explain the complex noncylindrical fault-fold interactions in the study anticline, we tentatively propose that they were triggered by near foredeep-parallel tapering of the sedimentary/tectonic overburden of the Ara evaporites.

Simulation of end-member scenarios of density-driven flow: prediction of dolostone geobody dimensions using COMSOL

Abstract Density-driven flows occur in nature and are initiated by density contrast. Salinity and temperature differences are remarkably effective in initiating density-driven flow, which may instigate convective flow. In turn, convective flow is seen by many authors as key for solute and mass transport, which are important processes controlling dolomitization. Two key dolomitization models rely on the application of density-driven flow concepts: (1) dolomitization by shallow or deep reflux of hypersaline brines in coastal ramp settings; and (2) fault-related dolomitization by the inflow of deep-seated fluids. These dolomitization scenarios are evaluated in this paper to infer plausible architecture, geobody dimensions and rock property distributions of dolomitic reservoirs. Since dolomitization processes may also alter porosity and permeability of the host lithology, changes in these properties are also discussed. This contribution focuses on the application of numerical models to assess the applicability of specific dolomitization models/mechanisms in different geological scenarios. We discuss end-member results of flow simulations using the numerical code COMSOL. This software has been chosen to model these processes as it provides a multiphysics framework for solving coupled systems, as in the case of brine reflux and geothermal convection. Our results provide a useful insight to the dynamic propagation of the limestone–dolomite front through fingering effects. The dolomite geobodies/patterns generated with the help of numerical simulation compare favourably with dolomite geobodies seen in outcrops. This study shows that during the dolomitization process the shape, the extent of propagation of the limestone–dolomite front and the rock properties (porosity and permeability) of the dolomitized reservoir are mainly influenced by the ‘contaminant’ source initiating dolomitization: that is, the magnesium-rich brine density in the case of brine reflux and the heat flux intensity in the case of geothermal reflux circulation.