Collapse Features Research Articles

New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA

Based on a methodic sedimentological analysis, the Late Jurassic (Oxfordian) Curtis Formation unravels the intricate facies variability which occurs in a tide-dominated, fluvially starved, low-gradient, semi-enclosed epicontinental basin. This unit crops out in east-central Utah, between the eolian deposits of the underlying Middle Jurassic (Callovian) Entrada Sandstone, from which it is separated by the J-3 unconformity, and the conformable overlying supratidal Summerville Formation of Oxfordian age. A high-resolution sedimentary analysis of the succession led to the recognition of eight facies associations (FA) with six sub-facies associations. Based on the specific three-dimensional arrangement of these eight facies associations, it is proposed to separate the Curtis Formation into three sub-units: the lower, middle and upper Curtis. The J-3 unconformity defines the base of the lower Curtis, which consists of upper shoreface to beach deposits (FA 2), mud-domi­nated (FA 3a) and sand-dominated heterolithic subtidal flat (FA 3b), sand-rich sub- to supratidal flat (FA 4a) and correlative tidal channel infill (FA 4c). It is capped by the middle Curtis, which coincides with the sub- to intertidal channel-dune-flat complex of FA 5, and its lower boundary corresponds to a transgressive surface of regional extent, identified as the Major Transgressive Surface (MTS). This surface suggests a potential correlation between the middle and the upper Curtis and the neighboring Todilto Member of the Wanakah Forma­tion or Todilto Formation. The upper Curtis consists of the heterolithic upper sub- to intertidal flat (FA 6) and coastal dry eolian dunes belonging to the Moab Member of the Curtis Formation (FA 7), and it conformably overlies the middle Curtis.
 The spatial distribution of these sub-units supports the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. In sector 1, the Curtis Formation is represented by its three sub-units, whereas sector 2 is dominated by the middle and upper Curtis, and sector 3 encompasses the extent of the Moab Member of the Curtis Formation.
 This study also highlights the composite nature of the J-3 unconformity, which was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. Local collapse features within the lower and middle Curtis are linked to sand fluid over­pressure within a remobilized sandy substratum, potentially triggered by seismic activity. Furthermore, the occurrence of a sub-regional angular relationship between the middle Curtis and substratum implies that the area of study was impacted by a regional deformational event during the Late Jurassic, before the deposition of the middle Curtis.
 The spatial distribution of these sub-units supports the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. In sector 1, the Curtis For­mation is represented by its three sub-units, whereas sector 2 is dominated by the middle and upper Curtis, and sector 3 encompasses the extent of the Moab Member of the Curtis Formation.
 This study also highlights the composite nature of the J-3 unconformity, which was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. Local collapse features within the lower and middle Curtis are linked to sand fluid over­pressure within a remobilized sandy substratum, potentially triggered by seismic activity. Furthermore, the occurrence of a sub-regional angular relationship between the middle Curtis and substratum implies that the area of study was impacted by a regional deformational event during the Late Jurassic, before the deposition of the middle Curtis.

Characterization and Remediation of Karst Collapse Columns for Mining Safety and Environmental Protection: A Case Study in Renlou Coal Mine, China

Karst collapse columns are unique collapse structures in karst terranes. Coal mining in China has exposed numerous such features of tens of meters in diameter and hundreds of meters in height. Hydraulically conductive collapses functioned as groundwater pathways between underground workings and aquifers, resulting in water inrushes during coal mining. Over the last 60 years, water inrushes through these collapses have caused fatalities, economic losses, and degradation in the environment. Determination of locations and hydrogeological characteristics of the karst collapse columns are essential in preventing water inrush incidents through them. Advanced geophysical prospecting, directional drilling, aquifer testing and accompanied dye tracing are effective approaches to detecting and characterizing these structures. Five geophysical techniques consisting of both surface and underground geophysical surveys and directional drilling of three exploratory boreholes up to 986 m deep identified a concealed collapse feature of more than 135 m high in Renlou Coal Mine, China. The roof of the collapse feature was at approximately 785 m deep, and there was an open void of 1.5 - 2 m high at the top. Geotechnical properties, results from packer testing and tracer testing, monitoring of potentiometric pressures, and geochemical fingerprinting suggested that this collapse column was hydraulically conductive and still actively developing. Water in the confined thin-bedded limestone and Ordovician limestone aquifer that either overlies or underlie coal seams could flow into the underground working areas if this feature were not identified in advance but encountered during mining. A grouting program was designed and implemented to construct a water plug in the collapse that effectively cut off the hydraulic connections from the aquifers to the underground workings. Successful construction of the water plug in the collapse was confirmed by performance monitoring of the aquifers.

Should Giant Cell Hepatitis With Autoimmune Haemolythic Anaemia Be Considered a Paediatric Autoimmune Liver Disease?

Reply: We thank Dr Nastasio et al for their informative letter on giant cell hepatitis with Coombs positive haemolytic anaemia (GCH-AHA) (1). When we were commissioned by the ESPGHAN Hepatology Committee to write the Position Statement on Paediatric Autoimmune Liver Disease (2), we did consider the possibility of including GCH-AHA (3) and neonatal haemochromatosis (4), rare conditions with a possible autoimmune component to their pathogeneses. We decided not to include them for the following reasons. The major aim of the Position Statement was to formulate a diagnostic and management protocol for juvenile autoimmune liver disease on the basis of what available in the literature on conditions recapitulating classical autoimmune hepatitis, that is, sharing in common serological (positivity for anti-nuclear, anti-smooth muscle, anti-liver kidney microsomal type 1, anti-liver cytosol type 1, and/or anti soluble liver antigen) and histological (interface hepatitis, rosette formation, multilobular collapse) features. These conditions, which include autoimmune hepatitis, autoimmune sclerosing cholangitis (overlap between autoimmune hepatitis and sclerosing cholangitis) and autoimmune hepatitis after liver transplant, are not as rare as hitherto believed in the course of childhood, and, though sharing several laboratory and clinical features, differ in terms of diagnostic criteria, management, and outcome, prompting the need for a Position Statement. GCH-AHA, which most likely has an autoimmune component to its pathogenesis, is a rare disease affecting almost exclusively infants and is associated to a number of immune mediated disorders, as noted in Dr Nastasio's letter (5), raising the question as to whether the underlying problem in these children is immune dysregulation (6). The typical histological features of GCH-AHA are diffuse giant cell transformation of hepatocytes and inflammation, different from those of classical autoimmune hepatitis, rarely posing problems in regards to differential diagnosis between the 2 conditions. GCH-AHA does respond, at least partially, to immunosuppression and relapses frequently when treatment is withdrawn (5). B-cell–mediated immunity appears to be involved in both liver and systemic damage, as anti-B cell monoclonal antibodies (rituximab) are to date the most effective treatment for this condition (7), though not in all patients (8). In conclusion, we do agree that GCH-AHA has a strong immune/autoimmune component to its pathogenesis, but it differs from the more common and classical autoimmune liver diseases of childhood and adolescence. Multi-centre collaboration for a better characterisation of this serious disorder is warranted to understand its pathogenesis and devise more effective treatment.

Seafloor sealing, doming, and collapse associated with gas seeps and authigenic carbonate structures at Venere mud volcano, Central Mediterranean

Methane release from the seafloor is commonly associated with chemosynthesis-based cold seep ecosystems that facilitate the precipitation of authigenic carbonates. It has been proposed that carbonate growth results in self-sealing, but little is known regarding the evolution of cold seep structures in relation to fluid migration pathways. This study investigates structures resulting from gas seepage along ring faults peripheral to Venere mud volcano (1600 m water depth), based on multibeam bathymetry and seafloor backscatter data collected by an autonomous underwater vehicle, together with photomosaics, video observations, and samples obtained by a remotely operated vehicle. Sites of focused fluid flow are identified by gas bubble streams rising from the seafloor while anaerobic oxidation of methane (AOM) over wider areas is indicated by the occurrence of chemosynthesis-based organisms (microbial mats, vesicomyid clams, vestimentiferan tube worms). At some sites, flakes of gas hydrate were observed in the water column during sampling. A range of carbonate structures exists at these sites: 1) flat and extensive pavements; 2) mounds with disseminated nodules or centimeter-thick crusts; 3) fractured mounds with exposed, decimeter-thick crusts; and 4) seafloor depressions lined by decimeter-thick crusts. The mineralogy and stable carbon isotopic compositions of the carbonates are consistent with anaerobic oxidation of methane from thermogenic sources, and possible near-seabed influence of gas hydrate formation and dissolution. The seafloor expressions of seepage are inferred to be controlled by the interaction of fluid flow due to carbonate precipitation and gas hydrate formation. A conceptual model for mound development is proposed in the context of the known timescales of seep-colonization and rates of carbonate precipitation: (A) the onset of hydrocarbon-rich fluid seepage through hemipelagic sediments leads to (B) the establishment of microbial mats on flat seafloor over decadal timescales and is followed by (C) the growth of pavements cemented by carbonates that seal the seafloor; over longer timescales (centuries to millennia), carbonate growth and subsurface gas hydrate formation/dissolution lead to (D) upward doming and fracturing of carbonate mounds, re-sealing and stacking of carbonates and in some cases to (E) their collapse to form seafloor depressions. Gas migration through fractures in the carbonates allows re-sealing and fuels AOM to provide habitats for chemosynthesis-based fauna. This evolutionary scenario is argued to be broadly applicable to the development of ruptured mounds and collapse features described at other seepage sites in the Mediterranean Sea and elsewhere.

Geomorphological Evidence for Shallow Ice in the Southern Hemisphere of Mars

AbstractThe localized loss of near‐surface excess ice on Mars by sublimation (and perhaps melting) can produce thermokarstic collapse features such as expanded craters and scalloped depressions, which can be indicators of the preservation of shallow ice. We demonstrate this by identifying High Resolution Imaging Science Experiment images containing expanded craters south of Arcadia Planitia (25–40°N) and observe a spatial correlation between regions with thermokarst and the lowest‐latitude ice‐exposing impact craters identified to date. In addition to widespread thermokarst north of ~35°N, we observe localized thermokarst features that we interpret as patchy ice as far south as 25°N. Few ice‐exposing craters have been identified in the southern hemisphere of Mars since they are easier to find in dusty, high‐albedo regions, but the relationship among expanded craters, ice‐exposing impacts, and the predicted ice table boundary in Arcadia Planitia allows us to extend this thermokarst survey into the southern midlatitudes (30–60°S) to infer the presence of ice today. Our observations suggest that the southern hemisphere excess ice boundary lies at ~45°S regionally. At lower latitudes, some isolated terrains (e.g., crater fill and pole‐facing slopes) also contain thermokarst, suggesting local ice preservation. We look for spatial relationships between our results and surface properties (e.g., slope and neutron spectrometer water ice concentration) and ice table models to understand the observed ice distribution. Our results show trends with thermal inertia and dust cover and are broadly consistent with ice deposition during a period with a higher relative humidity than today. Shallow, lower‐latitude ice deposits are of interest for future exploration.

Simulation and analysis of collapsing vapor-bubble clusters with special emphasis on potentially erosive impact loads at walls

Cavitation is a process where a liquid evaporates due to a pressure drop and re-condenses violently. Noise, material erosion and altered system dynamics characterize for such a process for which shock waves, rarefaction waves and vapor generation are typical phenomena. The current paper presents novel results for collapsing vapour-bubble clusters in a liquid environment close to a wall obtained by computational fluid mechanics (CFD) simulations. The driving pressure initially is 10 MPa in the liquid. Computations are carried out by using a fully compressible single-fluid flow model in combination with a conservative finite volume method (FVM). The investigated bubble clusters (referred to as “clouds”) differ by their initial vapor volume fractions, initial stand-off distances to the wall and by initial bubble radii. The effects of collapse focusing due to bubble-bubble interaction are analysed by investigating the intensities and positions of individual bubble collapses, as well as by the resulting shock-induced pressure field at the wall. Stronger interaction of the bubbles leads to an intensification of the collapse strength for individual bubbles, collapse focusing towards the center of the cloud and enhanced re-evaporation. The obtained results reveal collapse features which are common for all cases, as well as case-specific differences during collapse-rebound cycles. Simultaneous measurements of maximum pressures at the wall and within the flow field and of the vapor volume evolution show that not only the primary collapse but also subsequent collapses are potentially relevant for erosion.

Simulation and analysis of collapsing vapor-bubble clusters with special emphasis on potentially erosive impact loads at walls

Cavitation is a process where a liquid evaporates due to a pressure drop and re-condenses violently. Noise, material erosion and altered system dynamics characterize for such a process for which shock waves, rarefaction waves and vapor generation are typical phenomena. The current paper presents novel results for collapsing vapour-bubble clusters in a liquid environment close to a wall obtained by computational fluid mechanics (CFD) simulations. The driving pressure initially is 10 MPa in the liquid. Computations are carried out by using a fully compressible single-fluid flow model in combination with a conservative finite volume method (FVM). The investigated bubble clusters (referred to as “clouds”) differ by their initial vapor volume fractions, initial stand-off distances to the wall and by initial bubble radii. The effects of collapse focusing due to bubble-bubble interaction are analysed by investigating the intensities and positions of individual bubble collapses, as well as by the resulting shock-induced pressure field at the wall. Stronger interaction of the bubbles leads to an intensification of the collapse strength for individual bubbles, collapse focusing towards the center of the cloud and enhanced re-evaporation. The obtained results reveal collapse features which are common for all cases, as well as case-specific differences during collapse-rebound cycles. Simultaneous measurements of maximum pressures at the wall and within the flow field and of the vapor volume evolution show that not only the primary collapse but also subsequent collapses are potentially relevant for erosion.

The mechanical behavior of a layered nanostructured Ni with an alternating growth of ultrafine grains and nano-sized grains fabricated by electrodeposition

A layered nanostructured (NS) Ni with an alternating growth from the ultrafine-grains (UFG) layer with a thickness of 5µm and the nano-sized grains (NG) layer with a thickness of 10µm along the across-section is fabricated by periodically controlling the current density during electrodeposition. It is revealed by transmission electron microscope (TEM) and X-ray diffractometer (XRD) that the alternating growth of well-defined layers consists of the ultrafine-grains with the mean grain size of about 500nm along a (200) preferential growth and the nano-grains with the mean size of 20nm with a random growth. Tensile tests reveal that in comparison with the monolithic UFG and NG Ni, the layered NS Ni exhibits a higher tensile ductility of near 13% elongation to failure while maintaining a moderate ultimate tensile strength of near 1300MPa. The improvement in the ductility for the layered NS Ni can be mainly attributed to two effects including the increased strain hardening ability sourced from the action of these specific microstructure involving the periodical distribution of ultrafine-grains, the existence of interface and the alternation of crystal orientation between the layers on the dislocation activities and alleviating the local stress concentration caused by the periodic coordination deformation of the UFG layer in the NG substrate. A larger homogeneous surface deformation trace with the uplift and collapse feature is observed for the layered NG Ni in comparison with the monolithic UFG and NG Ni, which further indicates a large uniform plastic deformation ability that the layered NG Ni holds. Additionally, the mutual restraint and coordination deformation between the UFG and NG layer also leads to the formation of a fracture feature with a smaller and uniform dimple structure.

Multiazimuth coherence

Since its introduction two decades ago, coherence has been widely used to map structural and stratigraphic discontinuities such as faults, cracks, karst collapse features, channels, stratigraphic edges, and unconformities. With the intent to map azimuthal variations of horizontal stress as well as to improve the signal-to-noise ratio of unconventional resource plays, wide-/full-azimuth seismic data acquisition has become common. Migrating seismic traces into different azimuthal bins costs no more than migrating them into one bin. If the velocity anisotropy is not taken into account by the migration algorithm, subtle discontinuities and some major faults may exhibit lateral shifts, resulting in a smeared image after stacking. Based on these two issues, we evaluate a new way to compute the coherence for azimuthally limited data volumes. Like multispectral coherence, we modify the covariance matrix to be the sum of the covariance matrices, each of which belongs to an azimuthally limited volume, and then we use the summed covariance matrix to compute the coherent energy. We validate the effectiveness of our multiazimuth coherence by applying it to two seismic surveys acquired over the Fort Worth Basin, Texas. Not surprisingly, multiazimuth coherence exhibits less incoherent noise than coherence computed from azimuthally limited amplitude volumes. If the data have been migrated using an azimuthally variable velocity, multiazimuth coherence exhibits higher lateral resolution than that computed from the stacked data. In contrast, if the data have not been migrated using an appropriate azimuthally variable velocity model, the misalignment of each image results in a blurring of the multiazimuth coherence and the coherence computed from the stacked data. This suggests that our method may serve as a future tool for azimuthal velocity analysis.

A Capstone Course in Ecuador: The Andes/Galápagos Volcanology Field Camp Program

ABSTRACTWe developed and implemented the Galapagos Volcanology Field Camp, a 3 week, 3 credit hour course for upper-level university students with a major course of study in geology. The course is offered by the South Dakota School of Mines and Technology, is open to any student, and is usually populated by students from many universities across the U.S. The course offers the essential skills of field geology, such as lithologic description, unit identification and correlation, stratigraphic logging, and geologic mapping, taught exclusively in an environment of volcanic rocks. Beginning in the subduction setting of the Andes, students are introduced to the regional volcanic and tectonic history. The course visits volcanoes that have produced andesitic, dacitic, and rhyolitic products. Students study lava flows, pyroclastic flows, ignimbrites, lahar flows, and debris avalanche deposits. Students are also introduced to the volcanic hazards and monitoring efforts in Ecuador. During the second phase of the course, the group flies to the Galápagos Islands to examine the basaltic features of hotspot volcanism. Students study recent lavas erupted from well-developed shield volcanoes, lava tunnels and collapse features, and the history of uplift, subsidence, and sea-level variation. Through design and implementation of this program, a sequence of exercises has been found that build a robust curriculum while fitting into an itinerary that is logistically feasible. Study of volcanic deposits in this classic setting leads to strong student learning, with 84% of students who have participated scoring higher than 80% on activities designed to assess the stated learning objectives.

Secondary rock structures and the regional hydrogeology of claystone-rich cretaceous strata, Williston Basin, Saskatchewan, Canada

The geometry and spatial distribution of a polygonal fault system (PFS) and collapse features within Cretaceous strata (predominantly mudstones and claystones) were investigated using three high-resolution three-dimensional seismic datasets of the Williston Basin, Saskatchewan, Canada. Mapping of the planform geometry and fault throw distributions (throw–depth (T–z) profiling) shows that the PFS present in the Colorado Group and Pierre Fm has a vertical extent of 200–330 m. Variation in the lateral planform geometry is attributed to the relative rates of stress accumulation during early development of the planar faults and is constrained using sequence stratigraphic principles. The mean fault dip is 60° ± 12° (number of measurements, n = 225). The T–z profiles appear as partial C-type profiles, demonstrating that at least half of the total height of the PFS was removed during post-Cretaceous erosion. The presence of polygonal faults in equivalent strata of the Western Interior Sedimentary Basin (WISB) suggests the PFS described in the current study (1510 km2) may be present across the WISB. Collapse features, formed in response to dissolution cavities within underlying strata, crosscut the entire Cretaceous sequence and are subcircular in plan view with typical diameters of 350–450 m. These features are present in each of the datasets at a rate of 0.02–0.11 collapses/km2. The prevalence of collapses in areas where faults display modified throw distributions may suggest post-Cretaceous fault reactivation associated with Pleistocene glacial periods. Although these secondary rock structures likely affect groundwater and solute transport at the basin scale, the impact remains to be determined.

Hydrologic connectivity and threshold behavior of hillslopes with fragipans and soil pipe networks

AbstractMany concepts have been proposed to explain hydrologic connectivity of hillslopes with streams. Hydrologic connectivity is most often defined by qualitative assessment of spatial patterns in perched water tables or soil moisture on hillslopes without a direct linkage to water flow from hillslopes to streams. This form of hydrologic connectivity may not explain the hydrologic response of catchments that have network(s) of preferential flow paths, for example, soil pipes, which can provide intrinsic connectivity between hillslopes and streams. Duplex soils are known for developing perched water tables on hillslopes and fostering lateral flows, but the connectivity of localized perched water tables on hillslopes with soil pipes has not been fully established. The objectives of this study were to characterize pipeflow dynamics during storm events, the relationships between perched water tables on hillslopes and pipeflows, and their threshold behaviour. Two well‐characterized catchments in loess soil with a fragipan were selected for study because they contain multiple, laterally extensive (over 100 m) soil pipe networks. Hillslopes were instrumented with shallow wells adjacent to the soil pipes, and the wells and pipe collapse features were equipped with pressure transducers. Perched water tables developed on hillslopes during a wetting up period (October–December) and became well connected spatially across hillslope positions throughout the high flow period (January–March). The water table was not spatially connected on hillslopes during the drying out (April–June) and low flow (July–September) periods. Even when perched water tables were not well‐connected, water flowing through soil pipes provided hydrologic connectivity between upper hillslopes and catchment outlets. Correlations between soil pipeflow and perched water tables depended on the size and location of soil pipes. The threshold relationship between available soil‐moisture index plus storm precipitation and pipeflow was dependent on the season and strongest during dry periods and not high‐flow seasons. This study demonstrated that soil pipes serve as a catchment backbone of preferential flow paths that provide intrinsic connectivity between upper hillslopes and streams.

Diffraction imaging enhancement using spectral decomposition for faults, fracture zones, and collapse feature detection in a Middle East carbonate field

Diffraction imaging is demonstrated in this case study to improve horizontal resolution over conventional reflection imaging. Edge enhancement, using spectral decomposition on diffraction imaged data, further enhances the capabilities of detecting faults, fracture zones, and collapse features improving the spatial resolution and edge detection for higher resolution seismic interpretation. Spectral decomposition also provides an effective tool for separating low-frequency reflection data noise from diffraction imaged data, improving the interpretability of the diffraction imaged data. This case study also demonstrates that edge enhancement using spectral decomposition improves the ability of spatially correlating the edges of collapse features, detected with diffraction imaged data, to drilling mud losses in carbonates. Detecting overburden collapse features with diffraction imaging could provide economic benefits, avoiding encounters with high perm zones, preventing mud loss expenses and increased drilling time, from days to weeks, and controlling the well.

Role of Fracturing in the Organization of the Karst Features of Azrou Plateau (Middle Atlas, Morroco) Studied by Remote Sensing Imagery

Karst formation geometry can be controlled by fractures and faults, and by other lithologies. Here we show the organization of kastic collapse features related to structures and to extensive basaltic lava flows in the Middle Atlas of Morocco. A lineament map of major faults and fractures has been created for the Middle Atlas using Landsat 7 ETM+ satellite images. This shows a dominant NE–SW regional direction and less prominent NNW–SSE and ENE–WSE directions. All these directions coincide with the alignments of karstic depressions that have formed in the Liassic limestones. The basaltic flows covering these formations on the Middle Atlas limestone plateau, have allowed the generation of cryptokarst, geometrically organized a long these major lineament directions. Karst landforms probabaly existed before the eruption of the lavas, but there were partly invaded by intrusions and volcanism. The extensive basaltic flows allowed for increased infiltration, and subsurface water flow, increasing the rate of kast formation after eruptions. Some basins show evidence of increased subsidence after lava emplacement (Aguelmam Sidi Ali Lake) and some maar-like craters also have subsided after eruption, by karts formation. We lay out the structural and lithological controls on Karstic formation in an intraplate volcanic field based on limestones and evaporites.

Quaternary thermokarst and thermal erosion features in northern France: origin and palaeoenvironments

A thermokarst is a collapse feature resulting from the thawing of ice‐rich permafrost or of massive ice of various origins. Little attention has been paid to the sedimentary fabric resulting from this type of collapse, except for glaciotectonic features. In western Europe, two palaeo‐forms are commonly studied: lithalsas and ice‐wedge casts. Collapsed pingos are much rarer. Very few papers have compiled present‐day and fossil data. Here, field data collected from quarries in the eastern Paris Basin were analysed, providing useful records of thermokarst collapses in alluvial calcareous silts, sands, and gravels. These forms have a circular shape when viewed on satellite images. Permafrost is attested regionally by the recurrent occurrence of meter‐sized pattern grounds at the surface of the chalk and of ice‐wedge casts. Traces of segregation and reticulate ice are common. These features are primarily connected to a major interstadial, c. 150 ka BP, orbitally forced and commonly associated with a major glacial retreat. They occur both in drained and waterlogged situations, resulting in a specific pattern of deformation. They are controlled by the brittle and plastic behaviour of sediments and resemble passive glaciotectonism. Normal and reverse faults, with the offset decreasing downward, are common, and those with local shear are reported. Lithalsas, seasonal frost blisters, spring frost blisters and perhaps pingos seem to have formed. Most of these deformations correspond to thermokarst sinkholes bordered by gravitational collapse faults. The offset of these faults increases towards the surface, and the faults have been recurrently confused with neotectonism triggered by palaeo‐earthquakes. However, there are no faults beneath the observed deformation features, and the region lacks recorded seismic activity over the last century. Our data may be helpful in interpreting similar structures elsewhere.

Unsteady characteristics of cloud cavitating flow near the free surface around an axisymmetric projectile

The effect of free surface on unsteady cloud cavitation is important for high-speed surface vehicles, However, previously published experimental and numerical works regarding this topic are limited. In this paper, a typical launching experiment is performed with the presence of free surface. A numerical approach is established by using large eddy simulation and volume-of-fluid methods. Firstly, unsteady evolutions of the cavity and re-entry jet are obtained in both experimental and numerical results, which agree well with each other. Results indicate that the cavity evolution on the upper side of projectile is remarkably different compared to the lower side under the free-surface effect. For instance, on the upper side, cavity growth is slower, the velocity of the re-entry jet is higher, the cavity sheds faster, and the position of shedding cavity collapse is closer to the main cavity. Secondly, the effect of the free surface is studied by analyzing the constraint variation. Because the flow stream around the upper surface is thin, changing its direction under the effect of pressure difference inside and outside the cavity is easy. Non-axisymmetric collapse features generate a mass of strong vortexes on the cylindrical surface, and the non-uniform distribution of high pressure region is also one of the most important factors to induce lateral and vertical forces on the projectile. Finally, the heights of wave elevation in cases with and without cavitation are compared. The presence of cavitation leads to an increase in wave height, but the increment is about half the thickness of the cavity. This finding indicates that the actual constraint effect is between the effects of the infinite water field and the fully free condition.

A Framework for Fracture Network Formation in Overpressurised Impermeable Shale: Deformability Versus Diagenesis

Understanding the formation, geometry and fluid connectivity of nominally impermeable unconventional shale gas and oil reservoirs is crucial for safe unlocking of these vast energy resources. We present a recent discovery of volumetric instabilities of ductile materials that may explain why impermeable formations become permeable. Here, we present the fundamental mechanisms, the critical parameters and the applicability of the novel theory to unconventional reservoirs. We show that for a reservoir under compaction, there exist certain ambient and permeability conditions at which diagenetic (fluid-release) reactions may provoke channelling localisation instabilities. These channels are periodically interspersed in the matrix and represent areas where the excess fluid from the reaction is segregated at high velocity. We find that channelling instabilities are favoured from pore collapse features for extremely low-permeability formations and fluid-release diagenetic reactions, therefore providing a natural, periodic network of efficient fluid pathways in an otherwise impermeable matrix (i.e. fractures). Such an outcome is of extreme importance the for exploration and extraction phases of unconventional reservoirs.

Defining the shelf edge and the three‐dimensional shelf edge to slope facies variability in shelf‐edge deltas

AbstractShelf‐edge deltas play a critical role in shelf‐margin accretion and deepwater sediment delivery, yet much remains to be understood about the detailed linkage between shelf edge and slope sedimentation. The shelf edge separates the flat‐lying shelf from steeper slope regions, and is observable in seismic data and continuous outcrops; however, it is commonly obscured in non‐continuous outcrops. Defining this zone is essential because it segregates areas dominated by shelf currents from those governed by gravity‐driven processes. Understanding this linkage is paramount for predicting and characterizing associated deepwater reservoirs. In the Tanqua Karoo Basin, the Permian Kookfontein Formation shelf‐slope clinothems are well‐exposed for 21 km along depositional strike and dip. Two independent methods identified the shelf‐edge position, indicating that it is defined by: (i) a transition from predominantly shelf‐current to gravitational deposits; (ii) an increase in soft‐sediment deformation; (iii) a significant gradient increase; and (iv) clinothem thickening. A quantitative approach was used to assess the impact of process‐regime variability along the shelf edge on downslope sedimentation. Facies proportions were quantified from sedimentary logs and photographic panels, and integrated with mapped key surfaces to construct a stratigraphic grid. Spatial variability in facies proportions highlights two types of shelf‐edge depositional zones within the same shelf‐edge delta. Where deposition occurred in fluvial‐dominated zones, the slope is sand rich, channelized with channels widening downslope, and rich in collapse features. Where deltaic deposits indicate considerable tidal reworking, the deposits are thin and pinch‐out close to the shelf edge, and the slope is sand poor and lacks channelization. Amplification of tidal energy, and decrease in fluvial drive on the shelf, coincides with a decrease in mouth bar and shelf‐edge collapse, and a lack of channelization on the slope. This analysis suggests that process‐regime variability along the shelf edge exercised significant control on shelf‐edge progradation, slope channelization and deepwater sediment delivery.

On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK

Two partially exposed infilled pipes hosted by chalk were revealed in excavations into a hillside some 50m above the modern River Thames. The pipes are interpreted as collapse features induced by voids originating at depth rather than classical stream fed sinkholes. Two kinds of infill could be distinguished, sands tentatively assigned to the early Eocene Reading Formation (Lambeth Group) and gravels associated with the mid-Pleistocene Beaconsfield Terrace. The age of development can be constrained by the Thames terrace chronology and this implies major landscape changes due to fluvial erosion since initiation of the pipes some 2Ma and a reactivation event about 1Ma.

Seismic geomorphology of cretaceous megaslides offshore Namibia (Orange Basin): Insights into segmentation and degradation of gravity-driven linked systems

This study applies modern seismic geomorphology techniques to deep-water collapse features in the Orange Basin (Namibian margin, Southwest Africa) in order to provide unprecedented insights into the segmentation and degradation processes of gravity-driven linked systems. The seismic analysis was carried out using a high-quality, depth-migrated 3D volume that images the Upper Cretaceous post-rift succession of the basin, where two buried collapse features with strongly contrasting seismic expression are observed. The lower Megaslide Complex is a typical margin-scale, extensional-contractional gravity-driven linked system that deformed at least 2 km of post-rift section. The complex is laterally segmented into scoop-shaped megaslides up to 20 km wide that extend downdip for distances in excess of 30 km. The megaslides comprise extensional headwall fault systems with associated 3D rollover structures and thrust imbricates at their toes. Lateral segmentation occurs along sidewall fault systems which, in the proximal part of the megaslides, exhibit oblique extensional motion and define horst structures up to 6 km wide between individual megaslides. In the toe areas, reverse slip along these same sidewall faults, creates lateral ramps with hanging wall thrust-related folds up to 2 km wide. Headwall rollover anticlines, sidewall horsts and ramp anticlines may represent novel traps for hydrocarbon exploration on the Namibian margin.The Megaslide Complex is unconformably overlain by few hundreds of metres of highly contorted strata which define an upper Slump Complex. Combined seismic attributes and detailed seismic facies analysis allowed mapping of headscarps, thrust imbrications and longitudinal shear zones within the Slump Complex that indicate a dominantly downslope movement of a number of coalesced collapse systems. Spatial and stratal relationships between these shallow failures and the underlying megaslides suggest that the Slump Complex was likely triggered by the development of topography created by the activation of the main structural elements of the lower Megaslide Complex.This study reveals that gravity-driven linked systems undergo lateral segmentation during their evolution, and that their upper section can become unstable, favouring the initiation of a number of shallow failures that produce widespread degradation of the underlying megaslide structures. Gravity-driven linked systems along other margins are likely to share similar processes of segmentation and degradation, implying that the megaslide-related, hydrocarbon trapping structures discovered in the Namibian margin may be common elsewhere, making megaslides an attractive element of deep-water exploration along other gravitationally unstable margins.