Virtual Outcrop Research Articles

Field geology under the sea with a remotely operated vehicle: Mona Rift, Puerto Rico

We implemented concepts of field geology at great ocean depths by constructing virtual outcrops from a string of overlapping video frames collected by remotely operated vehicles (ROVs). This lower-cost alternative to drilling boreholes allows stratigraphic extension into the offshore and regional interpretation of marine seismic profiles. The imagery was collected along a dive transect on the western wall of Mona Rift, a deep and narrow rift northwest of Puerto Rico, between water depths of 1560 m and 3927 m. The northern coast of Puerto Rico and its large offshore area are underlain by a mid-Eocene and younger forearc basin topped by a thick carbonate platform. There are no drill holes offshore, and tying seismic lines across the shoreline there is problematic. We describe our virtual outcrop and constrain its age and stratigraphy using seven rock samples collected by ROV and compare the outcrop’s stratigraphy to deep boreholes and outcrops on land. Our formation descriptions and ages agree, for the most part, with those on land, but we identified a 100-m-thick section that is represented on land by an unconformity. Our stratigraphic interpretation indicates lateral variations in formation thicknesses and establishes a cross-section for additional sampling of the Eocene–Pliocene geology. It also suggests that Mona Rift has formed since the mid-Pliocene. The presence or absence of ferromanganese (Fe-Mn) crust on rocks along the transect may be correlated with the smoothness of the rock surface.

Analysis of facies proportions as a tool to quantify reservoir heterogeneity

The geometry, orientation and stacking patterns of sedimentary geobodies play a key control on fluid migration within subsurface reservoirs, aquifers, and repositories. Outcrops and more recently virtual outcrops are commonly used as analogues to characterise geobody architecture and stacking. Traditionally, both reservoirs and their analogues are described with respect to the bulk proportion of different facies and architectural elements within the system. These are combined with geometric measurements of the different geobodies to describe heterogeneity.This study presents a new methodology for quantifying the range of facies proportions within architectural element panels taken from outcrop analogues. This method measures the facies proportions within a series of vertical profiles and considers the statistical distribution, rather than the simple mean across the entire panel. The basic premise being that within a layer cake system, the spread in facies proportions will be very narrow whereas in a more “jigsaw” type system the spread will be much wider.To test this hypothesis, a series of nine virtual outcrops from a range of clastic depositional systems including fluvial, shallow marine and deep marine settings, have been analysed. Depositional facies (architectural elements) were interpreted in LIME and a series of depositional strike-orientated orthorectified panels were extracted. The panels were then analysed, and the extracted facies proportions were translated to “net sand” distribution. Results were then compared with conventional analysis of reservoir heterogeneity.Different depositional systems show distinct patterns. When arranged in depositional dip order, there is a systematic decrease in sand distribution spread down dip with significant resets at the boundaries between the major gross depositional environments (continental, shallow marine/shelf, deep marine).

Enhanced structural analysis through a hybrid analogue-digital mapping approach: Integrating field and UAV survey with microtomography to characterize metamorphic rocks

This study focused on collecting structural data orientations of a crustal-scale shear zone (Palmi Shear Zone, PSZ, southern Calabria, Italy) by integrating various analytical and field-based techniques. The PSZ consists of deformed metamorphic rocks (migmatitic biotitic paragneiss, marbles, and skarns) showing multiple folding phases, and Hercynian tonalites and pegmatites (306-290 Ma), crosscut by Late Hercynian leucocratic dykes (ca. 290 Ma). Multi-sized clasts composed of different lithologies are preserved on clean outcrop surfaces, and are sheared into both σ - and δ -type objects that collectively suggest opposing senses of shear. The study incorporates structural analysis of folds, field and aerial surveys (UAV), digital mapping, and microcomputed tomography. Various kinematic indicators were observed in the PSZ, indicating a mix of factors influencing the shear strain patterns (e.g. fold interference patterns, different rock types with high viscosity contrast). The findings suggest a clear consistency between structural data inferred from 3D VOM (Virtual Outcrop Model) and those collected directly in the field, confirming the occurrence of both sinistral and dextral shear in the PSZ, providing important insights into the tectonic evolution of the Calabrian-Peloritani Terrane.

Best practices towards the digitization of 3D traces from virtual outcrop models

With the expanding application of virtual outcrop models (VOMs) within the geosciences, there is a growing need to establish standards for the digital measurement of geological discontinuities exposed therein. Such standards should be tailored towards the complexities of natural outcrops, where geological discontinuities often intersect the outcrop topography and are expressed as 3D traces. Digitizing geological discontinuities expressed as trace data is typically conducted manually via polyline interpolation along the exposed trace, with discontinuity orientation estimated through planar model fitting through the polyline's component nodes. Presently, establishing quality control for such measurements lacks standardization due to the absence of robust benchmarks, with the validity of the resultant orientation data heavily reliant on the experience of the interpreter.With the aim of bridging this gap, we present the results of the manual digitization and orientation estimation of bedding planes expressed as traces across seven natural outcrops. We use two digitization strategies: one employing a previewed best-fit plane during digitization and another without. The first digitization method is carried out by an expert user who visually filters data according to visual alignment with the intended bedding prior to best fit plane estimation. In contrast, the non-visually aided method mimics acquisition by a novice user, with no a priori data filtering based upon trace geometry with respect to the outcrop. Comparison of the results obtained by these ‘expert’ and ‘novice’ acquisition modes is aimed at building benchmarks and best practices. Specifically, we analyze parameters derived from the digitized traces and their corresponding best-fit planes. We compare these parameters with the deviation of the best-fit plane from the mean orientation of the bedding surface as measured using visually-aided acquisition. Comparing these datasets reveals that visually-aided digitization yields more precise and accurate bedding measurements, characterized by traces with lower vertex collinearity. Notably, comparable results can be achieved in the non-visually assisted dataset by excluding traces with high node collinearity. Consequently, we provide robust benchmarks for trace collinearity and its relationship to best fit plane quality to aid the practical implementation of the results of this study. Furthermore, we supplement quantitative comparative analysis with recommended best practices for 3D trace digitization, such as ensuring high values of coplanarity, maintaining a quasi-constant node-to-node distance relative to the model's resolution, and ensuring a minimum number of nodes to guarantee the robustness of the fitted planar model. Critically, our study highlights the critical role tacit geological knowledge plays in the robustness of 3D trace digitization from virtual outcrop models.

Photogrammetry of the Deep Seafloor from Archived Unmanned Submersible Exploration Dives

Large amounts of video images have been collected for decades by scientific and governmental organizations in deep (>1000 m) water using manned and unmanned submersibles and towed cameras. The collected images were analyzed individually or were mosaiced in small areas with great effort. Here, we provide a workflow for utilizing modern photogrammetry to construct virtual geological outcrops hundreds or thousands of meters in length from these archived video images. The photogrammetry further allows quantitative measurements of these outcrops, which were previously unavailable. Although photogrammetry had been carried out in recent years in the deep sea, it had been limited to small areas with pre-defined overlapping dive paths. Here, we propose a workflow for constructing virtual outcrops from archived exploration dives, which addresses the complicating factors posed by single non-linear and variable-speed vehicle paths. These factors include poor navigation, variable lighting, differential color attenuation due to variable distance from the seafloor, and variable camera orientation with respect to the vehicle. In particular, the lack of accurate navigation necessitates reliance on image quality and the establishment of pseudo-ground-control points to build the photogrammetry model. Our workflow offers an inexpensive method for analyzing deep-sea geological environments from existing video images, particularly when coupled with rock samples.

Rockfall susceptibility analysis of the “San Michele Arcangelo” historic trail (Central Italy) based on virtual outcrops and multiple propagation models

The “San Michele Arcangelo” historic trail (Aurunci Regional Park, central Italy), located along the southern slope of Mt. Altino, which is highly prone to rockfalls, is hiked every year by thousands of faithful on pilgrimage who are exposed to such kinds of instabilities. To contribute to a better understanding of the condition and evolution of such phenomena, providing a susceptibility scenario able to support the adoption of mitigation measures, a specific analysis was completed based on field and literature data. Three-dimensional virtual outcrop models were obtained through both photogrammetric and iPad Pro LiDAR surveys to derive geomechanical features of outcropping rocks and estimate potential detaching block volume. Possible mechanisms of detachment were then analyzed using the Markland test method. Susceptibility to rockfall propagation and block deposition was analyzed using GIS processing and rockfall propagation simulations based on deterministic/stochastic models. Such models consist of a combination of a deterministic algorithm able to simulate the physics of rockfall movement during propagation and a stochastic treatment of input parameters based on random sampling within defined interval. Two different rockfall simulations were compared in this study using the Rockyfor3D™ (ecorisQ Association) software and the newly developed RocFall3© (RocScience) code. The use of iPad Pro LiDAR survey provides accurate and high-resolution point clouds, with high speed of acquisition and real-time processing of data. Discontinuity sets identified from 3D models have represented the input for kinematic analyses of slopes, allowing to identify potential failure mechanisms among wedge and planar sliding or toppling. Rockfall simulations indicate the potential for rock blocks propagation and deposition over the whole study area. Comparing the results of rockfall simulations with the geomorphological map of the area it is evident the control exerted by the hydrographic network on rockfall propagation as demonstrated by the presence of screes and slope deposits along the main channels. Understanding the potential susceptibility to both propagation and deposition of rock blocks along the trail provides useful indication for the assessment of appropriate mitigation measures to realize for the safer touristic fruition of the site.

3D geophysical image translated into photorealistic virtual outcrop geology using generative adversarial networks

Outcrop analogues play a pivotal role in resolving meter-scale depositional facies heterogeneity of carbonate strata. Two-dimensional outcrops are insufficient to decipher the 3D heterogeneity of carbonate facies. Near-surface geophysical methods, notably ground-penetrating radar (GPR), can be employed to step into 3D and extend the dimensionality of the outcrops to behind the outcrop. However, interpreting geophysical images requires specific geophysical expertise, often unfamiliar to field geologists who are more familiar with the actual rock than the geophysical data. A novel generative adversarial network (GAN) application is presented that constructs a photorealistic 3D virtual outcrop behind-the-outcrop model. The method combines GPR forward modeling with a conditional generative adversarial network (CGAN) and exploits the apparent similarities between outcrop expressions of lithofacies with their radargram counterparts. We exemplified the methodology and applied it to the open-source GPR data acquired from the Late Oxfordian-Early Kimmeridgian Arabian carbonate outcrop. We interpret a 4 km long outcrop photomosaic from a digital outcrop model (DOM) for its lithofacies, populate the DOM with GPR properties, and forward model the synthetic GPR response of these lithofacies. We pair the synthetic GPR with DOM lithofacies and train them using CGAN. Similarly, we pair the DOM lithofacies with outcrop photos and train them using CGAN. We chain the two trained networks and apply them to construct an approximately 2 km long 2D and an approximately 60 m2 3D volume of photorealistic artificial outcrop model. This model operates in a visual medium familiar to outcrop geologists, providing a complementary instrument to visualize and interpret rock formation instead of geophysical signals. This virtual outcrop replicates the visual character of outcrop-scale lithofacies features, such as the intricate bedding contacts and the outline of reef geobodies.

Analysis of Geological Structure based on 3D Virtual Outcrop Model and Physical Properties of Rocks in Wringinanom District, Gresik Regency

The Covid-19 pandemic has caused geological mapping activities to be hampered. This study aims to create a 3D virtual rock outcrop model which is compiled from 2D rock outcrop images with Measured Stratigraphy (MS), strike & dip and the value of rock physical parameters to analyze geological structures in Wringinanom District, Gresik Regency. Analysis of geological structures will be contained in the form of folding reconstructions with rock physical parameter as a validation and descriptions of fault orientation. This research modeled 21 rock outcrops and will be tested 6 core sedimentary rock samples. Physical rock parameter tests in the form of density tests through the comparison of mass weighing results with dimensional measurements by calipers and P-wave velocity tests using the Ultrasonic Pulse Velocity (UPV) method. Petrological descriptions of core rock samples are based on direct observations. The results of 3D virtual rock outcrop integrated with measurements in the field can be used in analyzing geological structures. The study area consists of anticlines with the direction of layering the south (southwest) and north (northeast) wings and 50-degree reverse faults with southeast dipping orientation (N153°E). The folding reconstruction consists of sandstone (top), sandstone-clay interlude, and claystone (top) in the Pucangan Formation and sandstone (bottom) and claystone (bottom) in the Lidah Formation where the presence of fossil layers of bivalve mollusks is a firm correlation. The physical parameters of rocks cannot validate the reconstruction of folds due to the influence of differences in physical values of the sample supported by petrological descriptions.

Biostratigraphic investigations assisted by virtual outcrop modeling: a case study from an Eocene shallow-water carbonate succession (Val Rosandra gorge, Trieste, NE Italy)

Biostratigraphic investigations assisted by virtual outcrop modeling: a case study from an Eocene shallow-water carbonate succession (Val Rosandra gorge, Trieste, NE Italy)

Analysis of Earthquake-Triggered Landslides through an Integrated Unmanned Aerial Vehicle-Based Approach: A Case Study from Central Italy

Landslides are one of the most hazardous(危险) secondary(二次) effects of earthquakes(地震) due(由于) to the potential(潜在) for large-scale(规模) damage(损伤) and long-term(学期) alterations to landscapes. During the 2016–2017 seismic sequence(序列) in Central Italy, many earthquake(地震)-triggered landslides (EQTLs) affected the road network(网络) and mountain trails. In this study, a methodological approach(方法) for analysing EQTLs, based on data(数据) derived from Unmanned Aerial Vehicle (UAV) surveys, is shown. The approach(方法) is applied to investigate(探讨) the geometric, structural(结构), geomechanical, and kinematic features of the Foce rockslide, which is introduced in the back analysis(分析). The investigation(调查) involved three main(主要) steps: (i) set up of UAV-based Virtual Outcrop Models (VOMs) of the slope(边坡), (ii) a geomechanical characterisation of the rock mass(大众) through the VOM interpretation(解释) and conventional(常规) field data(数据), and (iii) 3D Limit Equilibrium (LE) slope(边坡) stability(稳定性) analyses(分析). This study highlights the potential(潜在) of UAV surveys for providing valuable(宝贵) data(数据) for stability(稳定性) analyses(分析), especially in emergency(紧急) conditions such(这样) as in the aftermath of seismic events.

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.

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.

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

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.

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.

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.

Contribution of High-Resolution Virtual Outcrop Models for the Definition of Rockfall Activity and Associated Hazard Modelling

The increased accessibility of drone technology and structure from motion 3D scene reconstruction have transformed the approach for mapping inaccessible slopes undergoing active rockfalls and generating virtual outcrop models (VOM). The Poggio Baldi landslide (Central Italy) and its natural laboratory offers the possibility to monitor and characterise the slope to define a workflow for rockfall hazard analysis. In this study, the analysis of multitemporal VOM (2016–2019) informed a rockfall trajectory analysis that was carried out with a physical-characteristic-based GIS model. The rockfall scenarios were reconstructed and then tested based on the remote sensing observations of the rock mass characteristics of both the main scarp and the rockfall fragment inventory deposited on the slope. The highest concentration of trajectory endpoints occurred at the very top of the debris talus, which was constrained by a narrow channel, while longer horizontal travel distances were allowed on the lower portion of the slope. To further improve the understanding of the Poggio Baldi landslide, a time-independent rockfall hazard analysis aiming to define the potential runout associated with several rock block volumetric classes is a critical component to any subsequent risk analysis in similar mountainous settings featuring marly–arenaceous multilayer sedimentary successions and reactivated main landslide scarps.