Digital Outcrop Research Articles

Structural control on the alteration and fluid flow in the lithocap of the Allumiere-Tolfa epithermal system

Zones of advanced argillic alteration with general low permeability (i.e., lithocaps) are common place in the shallow parts of porphyry and epithermal ore deposits and active geothermal systems. The study of structural control on alteration distributions is of paramount importance for exploitation purposes as it really influences the caprock efficiency. We present the results of a structural-mineralogical study carried out in the lithocap of the Allumiere-Tolfa epithermal system (Northern Apennines). We characterized the composition, textures and physical properties (i.e., in-situ permeability; relative rock strength) of alteration facies. We then integrated field structural analysis with analysis of a virtual outcrop model to reconstruct the geometry of principal fluid-corridors. It resulted that advanced argillic alteration was promoted by circulation of highly reactive fluid(s) along a complex network of NE- and NW-striking faults and fractures dissecting the acidic volcanic dome. Such structures likely developed in response to a local disturbance of the regional stress field due to the extrusion of the Tolfa dome, which controlled the syn-extensional mineralization of the Allumiere-Tolfa area.

An integrated 3D digital model of stratigraphy, petrophysics and karstified fracture network for the Cristal Cave, NE-Brazil

Digital Outcrop Models (DOMs) are virtual representations of geological features. Although DOMs are widely used tools in geosciences, their integration with other datasets remains relatively underexplored. We combined a DOM, derived from a photogrammetric survey of a carbonate sequence, with lithostratigraphic, petrophysical (porosity, permeability and uniaxial compressive strength), fracture distribution, and karst dissolution information to compose a single integrated three-dimensional digital model. The study site is one of the entrances of the Cristal Cave (São Francisco Craton, Northeastern Brazil), which has been used as a structural and diagenetic outcrop analog for the Brazilian pre-salt carbonate reservoirs. Data from fracture distributions, measured on the exposed surfaces of the cave, were used to build a Discrete Fracture Network, based on the solution of the stereology inverse problem. Fracture apertures were then modified to generate different scenarios of karstification, thus composing Discrete Fracture and Karst Networks. This integrative approach brought relevant insights into the cave development due to dissolution along fracture clusters. Our methodology offers better geological data handling to build static models to be used in a fluid flow modeling environment, contributing to bridge the gap between geophysics/geology and engineering approaches.

3D geometry and displacement transfer of an oblique relay zone on outcropping normal faults

Relay zones on normal faults accommodate transfer of displacement between adjacent fault segments. We study vertical and horizontal displacement transfer mechanisms across a relay zone adjacent to the Moab Fault at Courthouse Rock (Utah, USA). The relay zone has a reservoir scale (map overlap length ca. 750 m, separation ca. 150 m), and is bounded by two normal fault segments with maximum throw of ca. 12 m. The relay zone is exposed on multiple, sub-parallel cliff faces and intervening rock pavements. We use photogrammetry of these exposures to build a 3D virtual outcrop model of the relay-bounding faults, subordinate faults deforming the relay zone and seven faulted stratigraphic horizons. The relay-bounding faults are right-stepping in map view and contractional in cross-section, defining a relay zone oblique to bedding in 3D. Displacement is transferred both horizontally and vertically across the relay zone. Horizontal transfer of displacement is achieved by a shallow dipping (<1°) relay ramp, whereas vertical transfer is achieved by antithetic faults within the relay ramp. Our analysis demonstrates that multiple mechanisms can work in conjunction to facilitate transfer of displacement across individual relay zones.

A Workflow for Interpretation of Fracture Characteristics Based on Digital Outcrop Models: A Case Study on Ebian XianFeng Profile in Sichuan Basin

Abstract Collecting information about fracture attributes through outcrops measurement is crucial for analyzing the scale, distribution, orientation, and spatial arrangement of fractures. The emergence of digital outcrop models (DOMs) provides a new technology for quantitative interpretation of fractures. However, large-scale DOMs pose additional challenges to the practical application, particularly in the interpretation of geological elements (e.g. fractures). This research proposes a workflow for fracture characteristics interpretation based on DOMs. First, DOMs are generated using light detection and ranging scanning technology. Then, a 3D visualization platform is developed based on OpenSceneGraph. We use level-of-detail technology to reconstruct DOMs for multiscale fast visualization of large-scale models. Finally, in order to realize the quantitative interpretation of fractures, we propose the best-plane fitting and the feature information (orientation, length, spacing, etc.) extraction methods for two types of fractures (exposed fracture walls and fracture traces). The proposed methods are applied to extract attributes of fractures in Dengying Formation (second member), Ebian, Xianfeng, southwest Sichuan Basin, China. The results provide the basis for reservoir evaluation in this area.

Using Virtual Outcrop Models as a Geological Learning Media

The Covid-19 pandemic has impacted various sectors due to a new wave of technology rising shortly, specifically geological learning models. Using research and development methods, the research aims to develop more scientific observations as a provision for pre-field work activities at Geophysical Engineering Department Institut Teknologi Sepuluh Nopember. Furthermore, we need a media that can provide learning that can emphasize process skills and students' abilities to determine and understand the objective concepts of material. Virtual outcrop models integrated with video learning platform can fulfil the learning process by providing experience in observations outcrops directly in the class. A three-dimensional (3D) Virtual Outcrop Model (VOMs) was created using the photogrammetry software Agisoft PhotoScan based on photos taken in the field using smartphones and drones. The existence of this media is expected to add direct experience to visualize how geological phenomena occur in nature. The learning activities are divided into four sequential stages: observation, data collection, interpretation, and hypothesis giving. 87% of respondents stated that learning models using VOMs and videos can boost their knowledge of scientific observation. This learning method can help improve academic achievement because it applies technology implementation in theory and practice, providing geological information to us

Fracture intensity and associated variability: A new methodology for 3D digital outcrop model analysis of carbonate reservoirs

The presence of hydraulically conductive fracture networks fundamentally affects subsurface fluid flow, the recovery factor and productivity in hydrocarbon carbonate reservoirs. However, methods to directly detect and map the 3D distribution and intensity of fracture networks in the subsurface is difficult. We present a new workflow and methodologies to overcome these limitations utilizing 3D digital photogrammetry on a prominent outcrop of the Late Jurassic (Kimmeridgian) Jubaila Formation near Wadi Laban (Riyadh, Saudi Arabia) as an example. Subsurface equivalents of the Upper Jubaila Formation host major oil and gas accumulations on the Arabian platform. The workflow includes mapping of fracture intensity, distribution and orientation along a 750 m long exposure transect using a high-resolution 3D Digital Outcrop Model (DOM). Results show two main fracture trends, which were E-W and NNE-SSW oriented. The techniques enable computation of fracture intensity directly on the 3D DOM, and quantification of lateral and vertical fracture intensity variability and fracture corridors. The new methodology of 3D DOM analysis, provides a reliable characterization of fractures and fracture intensity that are critical for enhancing 3D reservoir models.

Anatomy of a km-scale fault zone controlling the Oligo-Miocene bending of the Ligurian Alps (NW Italy): integration of field and 3D high-resolution digital outcrop model data

We report the first description of a ~ 15 km long NE-SW-striking transtensive fault network crosscutting the metamorphic units of the Ligurian Alps. The main fault zone, hereby named Horse Head Fault Zone, is up to 250 m thick, involves quarzite, metarhyolite, marble and alternation of dolostone and limestone and minor pelite. Relatively narrow (~ 1–3 m-thick) fault cores are characterized by gouge and cataclasites, surrounded by brecciated damage zones as thick as tens to hundreds of meters. Damage zones show widespread evidence for dilation in the form of dilation breccia, large calcite crystals and aggregates, and centimeter- to meter-thick veins. Moreover, the fault zone contains a multitude of polished slip surfaces with multiple sets of slickensides and slickenfibers. Oblique to strike-slip kinematics dominates over the large part of the fault mirrors and both overprint and are overprinted by down-dip slip surfaces. The fault network includes dominant NE-SW right-lateral faults with a minor normal component and NW–SE left-lateral steep faults with a negligible reverse component, consistent with a km-size dextral NE-SW-striking Riedel shear zone, in turn representing an antithetic R’ of the regional sinistral shear zone constituted by the Ligurian Alps after the nappe stacking. The Horse Head Fault Zone accommodated km-scale displacement before the Early Miocene, as it is sealed by the sedimentary deposits of the Finale Ligure Basin, thus predating the Corsica-Sardinia drifting. Results of this work constraint the bending of the Ligurian Alps as part of the Western Alpine arc as accomplished through two consecutive, late Oligocene and Early Miocene, stages driven by the combination of Adria rotation and the rollback of the Apennine subduction.

3D Digital Outcrop Model for Geological Structure Analysis in Mae Moh Coal Mine, Lampang Province, Thailand

3D Digital Outcrop Model for Geological Structure Analysis in Mae Moh Coal Mine, Lampang Province, Thailand

Holocene Deformations at the Po Plain–Southern Alps Transition (Lake Maggiore, Italy): Inferences on Glacially vs. Tectonic-Induced Origin

The investigation of deformations in Quaternary deposits holds primary importance in understanding recent geological history and natural hazards in highly populated areas, such as the Po Plain. While civil excavations and trenches possess the potential to be pivotal in identifying and characterizing these deformations, they often remain underused due to the stringent regulation framework and timetables governing civil construction works. In this study, we demonstrate how digital photogrammetry and digital outcrop modelling (DOM) are useful techniques for obtaining a permanent digital representation of a trench situated in Castelletto Ticino (Po Plain–Southern Alps transition). This trench exhibits Holocene deformational structures: (i) an overall tilting of sedimentary deposits towards the SW; (ii) folds with a NE–SW trend; (iii) slumping and other soft-sediment deformations structures; and (iv) reverse faults with NE–SW and NW–SE directions. Using radiocarbon and archeological dating, we are able to confidently constrain the age of these deformations to between 8760 and 400 years BC, suggesting recent tectonic activity related to buried thrust faults.

Natural fracture patterns at Swift Reservoir anticline, NW Montana: the influence of structural position and lithology from multiple observation scales

Abstract. Subsurface datasets typically lack the resolution or coverage to adequately sample fracture networks in 3D, and fracture properties are typically extrapolated from available data (e.g. seismic data or wellbore image logs). Here we assess the applicability of extrapolating fracture properties (orientation, length, and intensity) across observation scales in deformed, mechanically layered carbonate rocks. Data derived from high-resolution field images, medium-resolution digital outcrop data, and relatively low-resolution satellite imagery at Swift Reservoir anticline, NW Montana are leveraged to (i) assess interacting structural and stratigraphic controls on fracture development, and (ii) compare estimated fracture properties derived from multiple observation scales. We show that hinge-parallel and hinge-perpendicular fractures (i) make up the majority of fractures at the site; (ii) are consistently oriented with respect to the fold hinge, despite along-strike variability in the fold hinge orientation; and (iii) exhibit systematic increases in intensity towards the anticline hinge. These fractures are interpreted as having formed during folding. Other fractures recorded at the site exhibit inconsistent orientations, show no systematic trends in fracture intensity, and are interpreted as being unrelated to fold formation. Fracture orientation data exhibit the greatest agreement across observation scales at hinge and forelimb positions, where hinge-parallel and hinge-perpendicular fracture sets are well developed, and little agreement on the anticline backlimb, where fracture orientations are less predictable and more dispersed. This indicates that the scaling of fracture properties at Swift Reservoir anticline is spatially variable and partly dependent on structural position. Our results suggest that accurate prediction and extrapolation of natural fracture properties in contractional settings requires the assessment of structural position, lithologic variability, and spatially variable fracture scaling relationships, as well as consideration of the deformation history before and after folding.

Svalbox Digital Model Database: A geoscientific window into the High Arctic

Abstract Digital outcrop models (DOMs) have revolutionized the way twenty-first century geoscientists work. DOMs are georeferenced three-dimensional (3-D) digital representations of outcrops that facilitate quantitative work on outcrops at various scales. Outcrop digitalization has been traditionally conducted using laser scanners, but in the past decade, it has seen an exponential growth because of efficient and consumer-friendly structure-from-motion (SfM) algorithms concurrent with the rapid development of cost-effective aerial drones with high-resolution onboard cameras. While DOMs are routinely used in geoscientific research, education, and industry, enhanced DOM usage is restricted because raw data (e.g., photographs) and metadata are often incomplete and/or unavailable. In this contribution, we present the Svalbox Digital Model Database (Svalbox DMDb), a database of metadata and openly available data packages for individual DOMs. The Svalbox DMDb is a regional DOM database geographically constrained to the Norwegian High Arctic archipelago of Svalbard at 74°N–81°N and 10°E–35°E. Svalbard offers exceptional-quality, vegetation-free outcrops with a wide range of lithologies and tectono-magmatic styles, including extension, compression, and magmatism. Data and metadata of the systematically digitalized outcrops across Svalbard are shared according to FAIR principles through the Svalbox DMDb. Fully open-access and downloadable DOMs include not just the DOMs themselves, but also the input data, processing reports and projects, and other data products such as footprints and orthomosaics. Rich metadata for each DOM include both the technical and geological parameters (metadata), enabling visualization and integration with regional geoscientific data available through the Norwegian Polar Institute and the Svalbox online portal. The current release of Svalbox DMDb, documented in this contribution, covers 135 DOMs cumulatively covering 114 km of Proterozoic to Cenozoic stratigraphy.

An onshore-offshore interpretation of structures in the Devonian rocks of the Pentland Firth, Scotland using high resolution bathymetry and drone-enabled field observations.

Strong tidal currents in the Pentland Firth separating NE Scotland and Orkney have stripped recent seafloor sediments revealing geological structures in parts of the Devonian Orcadian Basin that are otherwise limited to narrow coastal outcrops. Here we interpret and analyse a 220 km2 high-resolution bathymetric dataset and combine these findings with onshore aerial imagery, field observations, and photogrammetric Virtual Outcrop Models created from coastal outcrops. This approach allows a reappraisal of the structure, stratigraphy and tectonic evolution of the Orcadian Basin, and gives new insights into fold and fault structures developed at sub-seismic to reservoir scales. A major basin-scale Devonian structure – the Brough-Brims-Risa Fault – can be traced from Caithness to Hoy and has partitioned reactivation-related deformation in the Orcadian Basin during later deformation episodes. Carboniferous inversion-related folds formed during E-W shortening are mapped and correlated between Caithness and Orkney. These structures are cross-cut by widespread highly connected fracture networks formed by steeply-dipping ENE-WSW and subordinate WNW-ESE trending structures developed during Permian NW-SE transtensional rifting. The offshore dataset demonstrates the continuity and topology of structures related to superimposed rifting and basin inversion events across a much greater scale range than was hitherto possible. [end].

Anatomy of a fluvial paleo-fan: sedimentological and architectural trends of the Paleocene–Eocene Wasatch–Colton System (western Uinta Basin, Utah, U.S.A.)

ABSTRACT Recent developments in fluvial geomorphology and sedimentology suggest that fluvial fans (also known as distributive fluvial systems) could be responsible for the accumulation of great volumes of clastic successions in continental basins. A general depositional model based on sedimentological and architectural trends has been formulated for these fluvial systems, however, their recognition in the stratigraphic record often relies on partially preserved, discontinuous successions. This study provides a sedimentological and architectural characterization of Paleogene alluvial strata of the Wasatch and Colton formations in the southwestern Uinta Basin, central Utah (U.S.A.), following an ∼ 120-km-long outcrop belt which records deposition from an alluvial wedge that prograded to the north and northwest, from the basin margin to distal lake-dominated environments, preceding the onset of ancient Lake Uinta. Lateral and vertical distribution of facies associations are presented from a dataset of field observations (thirteen logged stratigraphic sections, for a total of ∼ 2400 m) and virtual outcrop models along the proximal-to-distal extent of the Wasatch–Colton alluvial system. Four sectors are defined (proximal, medial, distal, and terminal) to mark the longitudinal heterogeneity of alluvial stratigraphy. Noteworthy trends comprise a downstream decrease in the overall thickness of the alluvial stratigraphic column, a reduction in the relative volume, architectural complexity, and amalgamation of fluvial-channel bodies away from the apex, a weak downstream-fining trend in channel sandstones, and a down-system increase in preserved overbank and floodplain deposits accompanied by increasing volume and facies complexity of preserved lacustrine and palustrine facies associations. A proximal-to-distal change in fluvial-channel architecture is noted, with proximal sections characterized by vertically and laterally amalgamated sheet-like channel fills, transitioning to a lesser degree of amalgamation towards the medial sector, whereas distal and terminal sections are dominated by floodplain fines enveloping a subordinate volume of isolated, ribbon-shaped channel-sandstone bodies. The temporal development of the stratigraphic succession is observed in its entirety throughout the field area and, albeit localized, channel-scale erosion and potential depositional hiatuses punctuate the stratigraphy. Two major system-scale trends have been described in the Wasatch–Colton System (WCS). A first-order, long-term progradational trend, especially evident in proximal and medial sections, encompasses the large-scale vertical patterns in facies and architecture vertically through most of the stratigraphy of the WCS. In contrast, the uppermost part of the stratigraphic succession is characterized by a reverse, retrogradational trend, possibly associated with the early transgression of Lake Uinta's southern margin, marking the base of the overlying Green River Formation. Albeit expressed by different vertical succession of facies, proximal-to-distal processes, and stratal patterns documented along a longitudinal transect of the WCS mirror substantially identical vertical trends through the stratigraphy, which are interpreted as Waltherian superposition of distinct depositional domains during fluvial-fan progradation. Analyses of sandstone petrography and zircon geochronology suggest a unified source for the fluvial system, also supporting an interpretation as a distributive, rather than a tributive, fluvial system. This study enhances our understanding of the current depositional model for extensive fluvial-fan successions via a regional-scale stratigraphic analysis of a fluvial paleo-fan based on integrated characterization of both vertical and down-system patterns in facies distribution and fluvial architecture, providing key insights on useful criteria for recognizing fluvial-fan successions from the rock record.

Post‐Jurassic brittle deformation of Lusitanian Basin in the context of W Iberia evolution

AbstractA detailed structural analysis of the fracture network exposed in the Jurassic strata is used to reconstruct the Lusitanian Basin's brittle tectonic history related to the Meso‐Cenozoic paleostress trajectories of the Iberian plate. Structural analysis is made by high‐resolution virtual outcrop models and orthophoto mosaics, along with information obtained in the field. The paleostress regime is determined based on the fault‐slip inversion method. Structural features are predominantly NNE‐SSW, NE‐SW and NW‐SE‐trending extensional fractures, including joints, veins, normal faults, and ~E‐W‐oriented strike‐slip faults. These structures remained active in the early basin evolution and were repeatedly reactivated by shearing and contraction. The chronological succession and paleostress reconstruction revealed three tectonic regimes (i) NE‐oriented extension, (ii) NE‐oriented strike‐slip and (iii) NW‐shortening. The first stress regime was driven by the North Atlantic rift propagation in the Iberia's west and northwest margins in the Late Jurassic–Early Cretaceous. The younger stress states involve reactivation and inversion of pre‐existing fractures by Africa–Europe convergence since the Late Cretaceous. The findings are consistent with the regional stress field which the Iberian plate has experienced since the Meso‐Cenozoic.

Pressure solution-assisted diagenesis and thrusting-related deformation of Mesozoic platform carbonates

Depositional and diagenetic processes profoundly control the structural evolution of shallow-water carbonates through time. We focus on the Lower Jurassic platform carbonates pertaining to the southern Apennines fold-and-thrusts belt, Italy, by performing 3D digital outcrop modeling, field structural analysis, and microstructural investigation. Results show that pressure solution of the platform carbonates was affected by the grain size of single beds. Since early diagenesis, pressure solution localized within the coarser-grained carbonate beds, forming wave-like solution surfaces. Crosscutting relations among blocky cements, bed-parallel solution surfaces and high-angle veins show that pressure solution occurred during burial diagenesis with formation of wave-like solution surfaces, and during Late Miocene tectonic burial with formation of seismogram-like solution surfaces. The tectonic burial postdated the thrusting-related flexural slip folding and small-scale thrusting of the platform carbonates. Small scale thrusting took place by means of shearing of the bed-parallel heterogeneities, and formation of bed-oblique slickolites resulting in the development of back thrusts characterized by flat-ramp-flat geometries. The main results of this work are synthesized in a six-stage synoptic scheme reporting the structural evolution of the platform carbonates. Outcomes are helpful for the better assessment of geofluid production/storage from/in fractured platform carbonates pertaining to fold-and-thrust belts.

New approaches to the architectural analysis of deltaic outcrops: Implications for subsurface reservoir characterization and paleoenvironmental reconstruction

This study defines a new workflow to investigate the internal facies architecture of a river-dominated delta deposit using outcrops of the Cretaceous Panther Tongue of the Star Point Sandstone in central Utah, U.S.A. Photorealistic virtual outcrop models (VOM) were created from ~13 linear-km of outcrop. The VOMs, alongside field observations, were used to identify and map facies and facies associations over the ~25 m-thick stratigraphic interval. In order to investigate this system as a potential subsurface reservoir analog, a database of measurements was constructed using 60 digital sections that were measured within the VOMs at 152 m (~500 ft) spacing. This database characterizes a total of 508 sandstone beds by their thickness, length, and dip, from which the average thickness (0.78 m), bed length (330 m), and bed dip (2˚ towards the south) were calculated. Thinning rates were also calculated in both depositional strike and depositional dip directions (1.37x10-2 and 1.01x10-2 respectively). These comparable values suggest that the sandstone beds thin equally laterally and longitudinally from the sediment source, a characteristic typical of modern-day river-dominated deltas. The dataset also includes siltstone unit thickness data, recording an average thickness of 0.81 m, and a trend of increasing siltstone abundance and thickness moving from proximal areas where siltstones comprise 3% of the succession, and 77% in the most distal areas. The impact of observational scale was also investigated by querying the database at multiple physical length scales to include more typical well spacings of 305 and 610 m (500 and 1000 ft). This additional analysis emphasizes the role of variability in lithologically heterogeneous systems and underscores the utility of outcrop analogs to better understand sources of uncertainty. From these data, a series of depositional environment maps was constructed, illustrating the evolution of eight delta lobes that encompass the preserved depositional history of the study area. These maps document the compensational stacking of individual lobes and the progressive southward directed trajectory of the Panther Tongue delta system. Results of this analysis provide insight into depositional processes and scales of heterogeneity of the Panther Tongue and, by extension, analogous river-dominated delta systems. The workflow established in this study is exportable to other sedimentary outcrops and environments, thus demonstrating that VOMs can be used as a basis for quantitative database development and reservoir modeling inputs.

DICE: An open-source MATLAB application for quantification and parametrization of digital outcrop model-based fracture datasets

An open-source MATLAB application (app) named Discontinuity Intensity Calculator and Estimator (DICE) was developed in order to quantitatively characterize the fractures, or in more general, discontinuities within a rocky outcrop in three-dimensional (3D) digital data, such as digital outcrop model (DOM). The workflow proposed for the parametrization of the discontinuities consists of the following steps: (1) Analysis and mapping of the fractures detected within the 3D DOMs; (2) Calculation of the orientation, position and dimensions of discontinuities that are represented by best-fit circular planes; (3) Determining the discontinuity parameters (dimension, distribution, spacing and intensity) by the DICE algorithm using different 3D oriented sampling techniques (3D oriented scanline, 3D oriented circular scan window and spherical scan volume). Different sampling methods were bench tested with a synthetic, as well as a natural case study, and compared in order to understand the advantages and limitations of each technique. The 3D oriented circular scan window appears to be the most effective method for fracture intensity estimation with high accuracy (error 0.4%) and stability with variations in scan radius.

High-resolution digital outcrop models of fossil-rifted margins: 3D imaging of extensional detachment systems

High-resolution digital outcrop models of fossil-rifted margins: 3D imaging of extensional detachment systems

Strike-slip influenced stratigraphic and structural development of the Foula Sandstone Group, Shetland: implications for offshore Devonian basin development on the northern UK continental shelf

The island of Foula, located 25 km SW of Shetland, preserves a gently folded, 1.6 km thick sequence of Middle Devonian sandstones spectacularly exposed in kilometre-long cliff sections &gt;350 m high. These rocks unconformably overlie likely Precambrian-age amphibolite facies basement rocks that are intruded by sheeted granites. The onshore succession is similar in age to the nearby Lower Clair Group offshore to the west. New mapping, incorporating the use of drone imagery in the inaccessible cliff sections, uses down-plunge projections to show that growth folding and faulting on Foula were contemporaneous with sedimentation during basin filling. The large-scale structural geometry is consistent with the regional constrictional strain due to the sinistral transtension associated with movements along the Walls Boundary–Great Glen fault zone system during the Mid-Devonian. Detrital zircon provenance studies indicate that the Devonian sequences of Foula (and nearby Melby in western Shetland) show similarities with the Clair Group and Orkney successions. We suggest that NE–SW transtensional fold development contemporaneous with regional subsidence in the Devonian basins of Scotland may be more widespread than previously realized. Large, kilometre-scale folds previously interpreted to be related to Permo-Carboniferous inversion may therefore have initiated earlier in the basin evolution sequence than previously realized. Supplementary material: Appendices A, Methodologies; B, Regional stratigraphic correlation table; C, Heavy mineral data; D, Detrital zircon data; and E, 3D virtual outcrop models are available at https://doi.org/10.6084/m9.figshare.c.6442552

3D Discrete Fracture Network Modelling from UAV Imagery Coupled with Tracer Tests to Assess Fracture Conductivity in an Unstable Rock Slope: Implications for Rockfall Phenomena

The stability of a rock slope is strongly influenced by the pattern of groundwater flow through the fracture system, which may lead to an increase in the water pressure in partly open joints and the consequent decrease in the rock wall strength. The comprehension of the fracture pattern is a challenging but vital aspect in engineering geology since the fractures’ spatial distribution, connectivity, and aperture guide both the water movement and flow quantity within the rock volume. In the literature, the most accepted methods to hydraulically characterise fractured rocks in situ are the single borehole packer test, the high-resolution flow meters for fractures, and the artificial tracer tests performed in boreholes. However, due to the high cost a borehole requires and the general absence of wells along coastal cliffs, these methods may not be appropriate in rockfall-prone areas. In this study, an unsaturated rocky cliff, strongly affected by rockfalls, was investigated by combining kinematic analysis, Discrete Fracture Network (DFN) modelling, and artificial tracer tests. The DFN model and potential rock block failure mechanisms were derived from high-resolution 3D virtual outcrop models via the Structure from Motion (SfM) photogrammetry technique. An artificial tracer was injected using a double ring infiltrometer atop the recharge zone of the slope to determine the infiltration rate and validate the DFN results. The DFN and tracer test methods are frequently used at different spatial scales and for different disciplines. However, the integration of digital photogrammetry, DFN, and tracer tests may represent a new step in rockfall and landslide studies. This approach made possible the identification of groundwater flow patterns within the fracture system and revealed about a 10-day tracer transit time from the injection area and the monitored slope, with similar conductivity values gathered from both the DFN and tracer test. Planar and wedge failures with volumes ranging from 0.1 and 1 m3 are the most probable failure mechanisms in the areas. The results were consistent with the delay between the intense rainfall and the slope failures previously documented in the study area and with their mechanisms.