Sedimentary Overburden Research Articles

Palynology, micropalaeontology and geochemistry of the Eocene Rusayl Formation of Northern Oman

New palaeontological and geochemical data are presented for the Rusayl Formation of Northern Oman. Eocene palynomorphs (97 taxa) including many new to the Arabian Peninsula and a distinctive new ostracod assemblage (21 species including 11 new taxa) are described and key taxa illustrated. This represents the most diverse palynomorph assemblage yet recorded from the Eocene within the Arabian Peninsula area. 'Ostracods of this age are poorly known from the Middle East with only one species from this area found in the Oman material and a further three from western Pakistan. The palynology indicates a dominantly restricted shallow marine palaeoenvironment with distinct periods of strong marine influence during which dinocyst abundances increase markedly. The palynofloras are dominated by Nypa palm pollen typical of mangroves ( Spinizonocolpites and Proxapertites ). The palynofacies are dominated by cuticle and membrane material (leaf debris) and brown pseudoamorphous wood, indicating a marginal marine palaeoenvironment. The associated ostracod and foraminiferal assemblages broadly support these interpretations indicating a lagoonal to infra-littoral environment of deposition. Nannofossils are recorded for the first time from the formation and support a Middle Eocene age for the upper part of the Rusayl Formation. Inorganic geochemistry indicates that the fine-grained siliciclastic intervals were largely derived from ophiolitic material and pre-Permian basement. Shales and coals contain common oil-prone lipid rich cuticle and locally abundant gas-prone woody kerogen with pyrolysis S2 yields of up to 15.13 mg/g. Although thermally immature the sequence is sufficiently buried by a sediment overburden exceeding 5 km in the offshore to generate hydrocarbons.

Emplacement and landscape controls on ancient submarine volcanism: 3D seismic geomorphological analysis of Cretaceous volcanic activity in the Browse Basin, Australian North West Shelf

Ancient submarine volcanic systems preserved within sedimentary basins provide opportunities to investigate the time and spatial distributions of intrusive and extrusive rocks, though existing work has largely focused on the geometrical features of submarine volcanic products, with limited interrogation of their interactions with sedimentary processes. This study investigates a buried submarine volcanic system within the Browse Basin, Australian North West Shelf. Through the integration of three-dimensional seismic reflection survey with borehole data, our study reveals intricate geometric features of a submarine volcano and lava channels, and their relationship to an associated intrusive network of Early Cretaceous age that has been preserved beneath ∼3.5 km of sedimentary overburden. In planform, the volcano has a diameter >4.5 km with a preserved height reaching 650 m. Meandering lava flow channels extend ∼20 km southward of the volcanic edifice, forming lava lobes with pressure ridges at the flow termini. Abundant sheet intrusions, identified within underlying Jurassic and older strata, indicate a magma transport from the north. The localised accumulation of sills led to variations in paleo-seafloor topography, influencing the direction of lava flows and post-volcanic sedimentation patterns. Our findings have a broad range of implications encompassing submarine volcanism and their impacts on basin dynamics.

A Correlation Relating the Residual Strength Parameters to the Proportions of Clay Fractions and Plasticity Characteristics of Overburden Sediments from the Open-Pit Mine Drmno

One of the prerequisites for the safe exploitation of surface mines is the stability of the working and final slopes of the mine. In order to ensure this, it is necessary to carry out detailed field and laboratory geomechanical tests of the soil and, based on the obtained results, make calculations related to stability analyses. The results obtained in this way are used for dimensioning the slope of exploitation slopes (excavation). Landslides occur when the ultimate shear strength is reached, and therefore, the adequate definition of shear strength parameters is one of the essential prerequisites for successfully solving the stability problem. Unlike earlier tests in Serbia, when the residual shear strength parameters were determined based on the usual conventional methods (direct shear apparatus, triaxial apparatus), this time, in addition to the direct shear apparatus, a ring shear apparatus was also chosen for testing. The paper shows the method of determining the residual shear strength parameters of high plasticity gray clays and siltstones of roof sediments from open pit mine Drmno, using direct and ring shear apparatus. The results show that the residual angle of internal friction for gray clays obtained with the ring shear apparatus is 9.9–10.8°, and for the siltstone, it is 11.8–12.9°, both of which are lower than the values obtained with the direct shear apparatus. In addition, correlations between the residual parameters of soil shear resistance and some physical indicators (plasticity index, clay content) are provided, showing high correlation coefficients. The proposed correlations should be used only when time and financial constraints prevent the execution of actual tests to determine residual shear strength, as concrete experimental procedures provide a much more reliable assessment of the residual strength properties of the soil.

Mesozoic clastic provenance during post-rift evolution of the Essaouira Agadir Basin, Northern Morocco

This study combines several provenance tools, analysis of published structural and geodynamic data, integrated with Low-Temperature Thermochronology (LTT) and time-Temperature Modelling (tTM) to reconstruct the evolution of source-to-sink systems feeding the Essaouira-Agadir Basin (EAB) during the Jurassic (Toarcian, Bathonian, and Kimmeridgian) and Lower Cretaceous (Hauterivian and Barremian).LTT and tTM define timing and rate of subsidence and exhumation of the hinterland and allows modelling of the predicted age and lithology of eroding rock units from the most-likely source locations through time. Extrapolation of predicted surface geology allows recognition of the lithology of sedimentary overburden in the hinterland, much of which has been subsequently eroded and is not preserved in the modern surfical geological record.Heavy mineral, petrography and detrital zircon data analysis was carried out on fluvial and shallow marine sandstones sampled from Jurassic and Cretaceous sections across in the EAB. The results document changing sediment source terrains through time. In the Early and Middle Jurassic, the heavy mineral and detrital zircon signature correlates with a Palaeozoic source, suggesting provenance was dominantly from erosion of Cambrian and Ordovician sandstone in the Central and Western Anti-Atlas. From the Late Jurassic to Early Cretaceous, the heavy mineral and zircon signatures have a strong affinity with Triassic sediments. This indicates a provenance switch to the exhuming West Moroccan Arch (MAM and Western Meseta), interpreted to have been largely covered by Triassic continental red beds at the time.The results help in predicting sediment delivery offshore, into the deep-water basin, where sandstones are a target for hydrocarbon exploration. Defining timing of input, location and composition helps to de-risk exploration. All the intervals examined contained discrete fluvial systems entering in the EAB, suggesting multiple periods of clastic delivery. Results suggest the Middle Jurassic and Hauterivian and Barremian intervals offer the optimum time for delivery of coarse clastics to the shelf margin, and potentially into the deep basin. The source to sink maps developed in this study further characterize these systems, their provenance and timing.

Active seismic surveys for drilling target characterization in Ossola Valley: International Continental Scientific Drilling Program (ICDP) project Drilling the Ivrea–Verbano zonE (DIVE) phase I

Abstract. Drilling target locations of the International Continental Scientific Drilling Program (ICDP) project Drilling the Ivrea–Verbano zonE (DIVE) have been initially proposed based on geological knowledge of surface outcrops and the structural context of the Ivrea–Verbano zone (IVZ) and of the Insubric Line. For the determination of the exact locations of drilling sites as well as for drilling geometry planning, we have carried out a series of active seismic experiments to image the subsurface at high resolution. The two drilling sites of project DIVE in Ossola Valley, one near Ornavasso and the other in Megolo di Mezzo, in the central part of the Ivrea–Verbano zone have been surveyed with site-specific velocity models and a seismic data processing chain. The findings have been interpreted in relation with the outcropping structures. These suggest a reasonable continuity from the surface. They also guide the planned borehole orientations: near-vertical at DT-1B (Ornavasso) into the tightly folded Massone Antiform and at 15–20° from the vertical in Megolo across a flank of the broad Proman Anticline. The seismic surveys indicate that the sedimentary overburden is up to 50 m deep at the specific drill sites and can be minimized by relocating the proposed locations. The seismic surveys also indicate that the center of the Ossola Valley contains about 550 m of sedimentary infill, defining the interface of bedrock and Quaternary glacial sediments at about 300 m below sea level.

Active and passive salt diapirs: a numerical study

Summary Salt diapirs dominate the structure in many sedimentary basins and control the preservation and migration of hydrocarbon. The formation of salt diapirs generally falls into two endmember models: active (up-building) and passive (down-building) diapirism. In the active model, salt diapirs rise from salt buoyancy to pierce through the sedimentary overburden, whereas in the passive model, salt diapirs result from differential loading of sediments during deposition. These endmember models are mostly conceptual or kinematic, the mechanics of active and passive diapirism, and their relative roles and interactions in the formation of salt diapirs, remain uncertain. Here, we use two-dimensional high-resolution numerical models to investigate the primary factors and critical conditions for active and passive diapirism. Our results indicate that it is improper to use driving mechanisms to classify salt diapirs, because the buoyancy-driven active salt diapirism involves differential loading, while the passive diapirism requires salt buoyancy. The rise of salt diapirs is more sensitive to the effective viscosity of the overburden than to the salt viscosity. Stiff overburdens could prevent the rise of salt diapirs, but they could be pierced by salt diapirs if plastic yield of the overburden is allowed. During deposition, the coupled salt-sediment deformation, driven by both salt buoyancy and differential loading of sediments, can lead to various diapiric salt structures and minibasins. Regional tectonic stress generally promotes salt diapirism by enhancing strain weakening of salts and overburdens. We suggest that the classification of active and passive salt diapirism is an oversimplification in most cases. We propose a general model of the formation of salt diapirs that usually begins with dome initiation driven by salt buoyancy, followed by syndepositional down-building controlled by sedimentation and differential loading, and ends with canopy formation when sedimentation stops.

Timing and Source of Recharge to the Columbia River Basalt Groundwater System in Northeastern Oregon.

Recharge to and flow within the Columbia River Basalt Group (CRBG) groundwater flow system of northeastern Oregon were characterized using isotopic, gas, and age-tracer samples from wells completed in basalt, springs, and stream base flow. Most groundwater samples were late-Pleistocene to early-Holocene; median age of well samples was 11,100 years. The relation between mean groundwater age and completed well depth across the eastern portion of the study area was similar despite differences in precipitation, topographic position, incision, thickness of the sedimentary overburden, and CRBG geologic unit. However, the lateral continuity in groundwater age was disrupted across large regional fault zones indicating these structures are substantial impediments to groundwater flow from the high-precipitation uplands to adjacent lower-precipitation and lower-elevation portions of the study area. Recharge rates calculated from the age-depth relations were <3 mm/yr and independent of the modern precipitation gradient across the study area. The age-constrained recharge rates to the CRBG groundwater system are considerably smaller than previously published estimates and highlight the uncertainty of prevailing models used to estimate recharge to the CRBG groundwater system across the Columbia Plateau in Oregon and Washington. Age tracer and isotopic evidence indicate recharge to the CRBG groundwater system is an exceedingly slow and localized process.

Geospatial analysis of groundwater arsenic and fluoride in Quaternary aquifers of southern Ontario, Canada

The geological setting of an area plays a critical role in the transfer and ultimate distribution of hydrochemical constituents present in groundwater. In southern Ontario, Canada, the present physiography was significantly influenced by glacial processes during the Quaternary period. The heterogeneous nature and complex pattern of shallow subsurface glacial overburden sediments, likely affect the fate of different groundwater constituents. In this study, arsenic (As) and fluoride (F−) concentrations from 515 water wells, that are constructed within overburden sediment, were analyzed with the physiographic map of southern Ontario along with other related variables. Geospatial mapping and several spatial statistical analyses were performed to examine the possible geological influence on As and F− distribution and water-well susceptibility at a regional scale. Key findings suggest four physiographic settings were significant variables influencing the distribution of As and F− in differently constructed bored/dug and drilled wells. Bored/dug wells in Bevelled Till Plains and bored/dug wells in Undrumlinized and Bevelled Till Plains were found to be relatively susceptible to As and F− contamination respectively. In contrast, bored/dug and drilled wells in Drumlinized Till Plains and Drumlins and drilled wells in Sand Plains seemed to be relatively safe from F− and As respectively. The statistical regression analyses suggested that other variables, such as the application of phosphate fertilizer and the textures of till, influenced the spatial distribution of As and F− as well as which types of wells (bored/dug or drilled) were impacted. The geospatial mapping and statistical cluster analysis indicated that the possible sources of elevated As and F− in drilled wells are the clasts of underlying bedrock. The relationship between physiographic settings and impacted overburden wells in southern Ontario provides planners with an approach to water-well susceptibility assessments at the regional scale, which in turn can guide further local analysis for water resource management.

Fluid dynamics, structural, micro-structural, and kinematic analyses on the Kuh-e-Namak (Dashti) salt diapiric rectangular cuboid channel flow, Iran

A study was conducted to investigate the Kuh-e-Namak (Dashti) salt diapir using fluid dynamics, structural, and microstructural, analyses. The study focused on transient flow caused by salt diapirism within the restraining bend of the Kazerun fault system. The salt diapir is located in Bushehr Province, Iran, and it was found that it represents Newtonian viscous flow within a rectangular cuboid channel, rather than a circular channel as previously thought. The study discovered that the salt diapir underwent laminar irrotational flow (Re = 1147.31) upward, with the symmetric parabola velocity profile dominated by Poiseuille flow. When loaded by sedimentary overburden, the salt tended to migrate. The upper part of the salt dome and diapiric flow transferred from irrotational laminar Poiseuille flow to transient Poiseuille-Couette and turbulent Poiseuille-Couette flows with a non-uniform asymmetric parabola profile. The drag sheath folds from diapiric flow showed monoclinic symmetry patterns, which led to a relative increase in the ellipticity of individual sheath fold rings to define the analogous eye-folds and cat ‘s-eye-folds. The presence of porphyroclasts or vortexes of σ, δ, and φ types in sheath folds indicates that the upper sections of the salt dome and diapir went through a Poiseuille-Couette flow or transient and turbulent flow during a later stage. The salt transient and turbulent flow can gradually lead to increased shear stress and high strain rates that are superposed by sub-simple shear, resulting in activation of the faults and consequently, the earthquake as we have seen in the destructive Kaki-Shonbe earthquake (Mw = 6.3).

Study of diagenitic processes and surface texture of the oldest river terrace deposits in Shirqat area- northern Iraq

The recent project deals with the diagenitic processes and surface textures of the oldest river terraces deposits in Shirqat area, the diagenitic processes compiled with Cementation, Compaction and Dissolution. The carbonate minerals represent the cementic materials while the point contact compaction represent the degree of compaction in this sediments because the small thickness of the overburden sediments and the secondary effects of differential dissolution. The surface texture is very developed in the large size of sediment included pebble, cobble and boulders which form very developed surface texture formed order and disorder forms as well as smooth surface texture.

Syn‐depositional halokinesis in the Zechstein Supergroup (Lopingian) controls Triassic minibasin genesis and location

AbstractSalt tectonics is typically caused by the flow of mobile evaporites in response to post‐depositional gravity gliding and/or differential loading by overburden sediments. This situation is considerably more complex near the margins of salt basins, where carbonate and clastic rocks may be deposited at the same time as and be interbedded with more mobile, evaporitic strata. In these cases, syn‐depositional salt flow may occur due to density differences in the deposited lithologies, although our understanding of this and related processes is relatively poor. We here use 3D seismic reflection and borehole data from the Devil's Hole Horst, West Central Shelf, offshore UK to understand the genesis, geometry, and kinematic evolution of intra‐Zechstein Supergroup (Lopingian) minibasins and their effect on post‐depositional salt deformation. We show that immobile, pinnacle‐to‐barrier‐like, carbonate build‐ups and anhydrite are largely restricted to intra‐basin highs, whereas mobile halite, which flowed to form large diapirs, dominates in the deep basin. At the transition between the intra‐basin highs and the deep basin, a belt of intra‐Zechstein minibasins occurs, forming due to the subsidence of relatively dense anhydrite into underlying halite. Depending on primary halite thickness, these intra‐Zechstein minibasins created topographic lows, dictating where Triassic minibasins subsequently nucleated and down‐built. Our study refines the original depositional model for the Zechstein Supergroup in the Central North Sea, with the results also helping us better understand the style and distribution of syn‐depositional salt flow within other layered evaporitic sequences and the role intra‐salt heterogeneity and related deformation may have in the associated petroleum plays.

Mapping critical mineral resources using airborne geophysics, 3D joint inversion and geology differentiation: A case study of a buried niobium deposit in the Elk Creek carbonatite, Nebraska, USA

ABSTRACTCritical minerals are an integral part of the energy transition because of their important and, sometimes irreplaceable, uses in solar panels, wind turbines, electrical vehicles, etc. Mapping critical mineral resources is, therefore, essential to achieving the net‐zero emission goal by 2050. We present a case study on using airborne geophysical data, borehole and physical property measurements to characterize the Elk Creek Carbonatite complex located in the southeast of Nebraska, USA. It hosts the largest known niobium deposit in the United States and contains a high level of rare earth element mineralization. Our goal is to develop a better understanding of the three‐dimensional structures and composition of the Elk Creek Carbonatite complex as well as its critical mineral resource potential. We performed three‐dimensional joint inversion of the airborne gravity gradiometry and magnetic data to produce structurally similar density and susceptibility models. We carried out geology differentiation, a process of classifying recovered physical property values into distinct units, and obtained a three‐dimensional quasi‐geology model for the Elk Creek Carbonatite complex. We identified 12 geologic units, each of which is characterized by a distinct range of physical property values. Our quasi‐geology model, especially Units 9 and 11, shows a remarkable consistency with the borehole assay measurements. Unit 9 is spatially coincident with the known niobium ore zone. Unit 11 represents a significant volume of highly dense and magnetic materials below the deepest boreholes. These materials are likely associated with unexplored niobium mineralization. Our study demonstrates the added value of three‐dimensional geophysical joint inversions and geology differentiation in the context of critical mineral exploration under a thick sedimentary overburden.

Organic Geochemical Signatures of the Upper Miocene (Tortonian—Messinian) Sedimentary Succession Onshore Crete Island, Greece: Implications for Hydrocarbon Prospectivity

The definition of pre-Messinian source rocks in the eastern Mediterranean is of paramount importance for hydrocarbon exploration because of the ability of salt to act as a high-quality seal rock. This research evaluates the organic geochemical features of the Upper Miocene (Tortonian—Messinian) sedimentary succession onshore Crete Island, Greece. The study employs original (Messinian, Agios Myron Fm) and published (Tortonian, Viannos Fm, Skinias Fm, Moulia Fm, and Messinian Ploutis section) results from organic geochemical analyses of mudstone samples. One hundred and one samples were examined using standard organic geochemistry methodology (Rock-Eval II and VI-TOC) to define the origin, type, and degree of organic matter maturity. The data indicate that the studied samples have poor to fair gas-prone source rock potential. These possible source rock units have not experienced great temperatures during burial, and, thus, their organic matter is thermally immature. The sub-salt (Tortonian—Messinian) source rock units are likely to be of higher thermal maturity in the western and eastern south Cretan trenches because of tectonic subsidence and a thicker sedimentary overburden. Several traps can grow in these regions, associated with normal faults, rotated blocks and unconformities (both below and above the unconformities). This research provides a basis for the further evaluation of the hydrocarbon potential in Crete Island. It is an area that shares geological similarities with the surrounding regions that contain proven reserves and is of crucial economic and strategic importance.

Characteristics, relationships and precision of direct acoustic-to-seismic coupling measurements from local explosions

SUMMARY Acoustic energy originating from explosions, sonic booms, bolides and thunderclaps have been recorded on seismometers since the 1950s. Direct pressure loading from the passing acoustic wave has been modelled and consistently observed to produce ground deformations of the near surface that have retrograde elliptical particle motions. In the past decade, increased deployments of colocated seismometers and infrasound sensors have driven efforts to use the transfer function between direct acoustic-to-seismic coupling to infer near-surface material properties including seismic velocity structure and elastic moduli. In this study, we use a small aperture (≈600 m) array of broadband seismometers installed in different manners and depths in both granite and sedimentary overburden to understand the fundamental nature and repeatability of seismic excitation from 1 to 15 Hz using horizontally propagating acoustic waves generated by 97 local (2–10 km) explosions. In agreement with modelling, we find that the ground motions induced by acoustic-to-seismic coupling attenuate rapidly with depth. We confirm the modelled relation between acoustic and ground motion amplitudes, but show that within one acoustic wavelength, the uncertainty in the transfer coefficient between seismic and acoustic energy at a given seismic station increases linearly with separation distance between the seismic and acoustic sensor. We attribute this observation to the rapid decorrelation of the infrasonic wavefield across small spatial scales and recommend colocating seismic and infrasound sensors for use in studies seeking to invert for near-surface material properties. Additionally, contrary to acoustic-to-seismic coupling theory and prior observations, we find that seismometers emplaced in granite do not record retrograde elliptical particle motions in response to direct pressure loading. We rule out seismometer tilt effects as a likely source of this observations and suggest that existing models of acoustic-to-seismic excitation may be too simplistic for seismometers placed in high rigidity materials.

Crustal structure across the Teisseyre-Tornquist Zone offshore Poland based on a new refraction/wide-angle reflection profile and potential field modelling

This paper presents a 230-km long refraction/wide-angle reflection profile that was acquired in the southern Baltic Sea across the Teisseyre-Tornquist Zone (TTZ), the boundary between the East European Craton (EEC) and the West European Platform (WEP). This profile is nearly parallel to the western Polish coast, halfway between Poland and the Danish island of Bornholm. The data acquisition was conducted with 15 ocean bottom seismometers (OBS) and 2 land stations. We applied the trial-and-error iterative ray-tracing technique using all identified seismic phases to derive crustal models by minimizing misfit between calculated and observed P-wave travel-times for individual layers. Final velocity model was further verified by forward potential field modelling, testing various P-wave velocity (Vp) to density relationships. The Moho boundary was inferred at 33–38km depth, deepening towards the EEC, with the local ~3km rise in a 40-km-wide zone north of the Caledonian Deformation Front, corresponding to the elevated middle-crust velocities Vp > 6.5 km/s. The lower and middle crust are mostly continuous along the BalTec profile with only minor perturbations between the EEC and WEP. Nevertheless, the thickness of these crustal layers is poorly constrained by seismic data along the SW section of the profile. In contrast, the upper crust and sedimentary cover can be subdivided into three domains, corresponding, from the NE to SW, to the EEC, TTZ and WEP, respectively. The EEC is characterised by the flat top of the basement, uniformly inclined towards the SW. The TTZ shows rapid thinning of cratonic upper crust and thickening of sedimentary layer. The WEP reveals, despite limited seismic coverage, a 13-km thick sedimentary overburden. The lateral changes of seismic structure within the upper crust and sedimentary cover along the BalTec profile can be attributed to protracted phases of late Palaeozoic-Mesozoic extension punctuated by the Carboniferous and Late Cretaceous inversion phases.

Tectonics characterization of the salt influenced structures: Insights from a seismic data interpretation in the eastern part of the Persian Gulf, Iran

Salt tectonics can influence the petroleum systems via the development and distribution of ductile migration pathways through the structural and stratigraphic traps. In this study, the effects of base-salt relief on the development of salt décollement horizon and the salt-influenced structures were investigated in the eastern part of the Persian Gulf, NE of the Afro-Arabian Plate. For this purpose, data from hydrocarbon exploration wells and seismic lines of the region have been used. To set the position in a time of the main stratigraphic and acoustic markers, and to compare well and seismic data, the processing of synthetic seismogram and the conversion of logs in the time domain has been performed for the selected well.Interpretation of seismic sections revealed the occurrence of a NE-SW trending horst structure and development of a natural model of the salt tectonics on its overburden of the Oligocene to Miocene sedimentary layers. The presence of Oligo-Miocene saliferous beds as a décollement has provided suitable conditions for overburden sediments gravitational sliding and development of thin-skinned extension which started during the Miocene time and it was evolved up to the present time. This condition controlled the geometry of salt-influenced structures in the eastern parts of the Persian Gulf and resulted in the formation of special structural features namely hereafter as Curly Bracket Structures in the overburden.

Analysis of high-resolution aeromagnetic (HRAM) data of geological transition zones of southeastern part of Bauchi, N.E Nigeria

The study investigated the subsurface tectonic framework and structural disposition of the basement/sedimentary transition zone in the southeastern part of Bauchi, Northeastern Nigeria, to delineate the basal and lateral contacts and unravel some of the hydrogeological and engineering challenges in such areas. The method deployed involved analysis of high-resolution aeromagnetic (HRAM) data, through the application of several filtering techniques. The transformation techniques applied include reduction to pole, second vertical derivative techniques analytic signal, and Euler deconvolution. Euler depths result range of 1 to 200 m coincides with region where basement rocks outcrop on the surface while depth range between 0 and −800 m correspond to sedimentary terrain (Kerri-Kerri Fm.) or part of the area where the basement rocks are overlain by thick overburden sediments. The 2-D depth model around the transition zone showed high variability as it ranged from 0 to -500 m owing to the presence of basement highs and depressions. The basement highs are usually characterized by impermeable rocks with poor aquifer characteristics and are responsible for the failure of boreholes in these zones. The basement depressions, however, constitute good groundwater prospects depending on the hydrological properties of the sedimentary infill. The general orientation of lineaments in the area revealed NW-SE, NE-SW, and ENE-WSW, with minor N-S and E-W trends, suggesting a pan-African origin. The occurrence of lineaments in parts of the contact zones also contributes to the groundwater prospects. The erratic bedrock relief exhibited by the study area is also detrimental to foundation of pavement structures as it undermines their durability. Therefore, a detailed site investigation is crucial in such geological contact zones prior to the erection of structures. Additionally, vertical electrical soundings are recommended before borehole drilling to ascertain the suitability of these hydrogeologically complex zones.

Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration

Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high-resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 m below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2.

Soil water repellency characteristic curve of Spolic Technosols from the region of Maritsa-Iztok coal mine in Bulgaria

Soil water repellency is phenomenon which is caused by compounds of organic origin, type of clay minerals and wetting/drying cycles. Hydrophobicity is usually characterized by a great spatial and temporal variability, especially in mine spoils where different types of overburden materials are mixed with coal particles and other waste products. The aim of this study was to investigate the changes of hydrophobicity with soil moisture content of Heavy Clay textured Spolic Technosols in the region of Maritsa-Iztok open cast coal mine. The sampling was done in a stubble field at 8 points from two soil depths 10–15 cm and 30–35 cm. The samples were analyzed for particle size distribution, total organic carbon content, soil bulk density, soil moisture content at field sampling, soil particle density, soil moisture content and water-drop-penetration-time (WDPT) at different matric potentials (SWRCC). The irregular presence of coal particles and different overburden sediments lead to spatial variability of studied properties, especially of total organic carbon content. In depth the average clay content, organic carbon content, water retention properties and soil water repellency (SWR) increased and correspondingly the density properties decreased. The distribution of SWR classes of the studied samples at the applied suctions during the drying process showed that the hydrophilicity of all samples at saturation persisted at suction pF 1.0 in 73% of the samples Most of the samples at suctions pF 1.7 and pF 2.0 were with slight and strong water repellency, but still there were hydrophilic samples 27% and 18%, correspondingly. At pF 2.5, which is often accepted as field capacity of fine textured soils, all samples were water repellent. The further drainage of the samples at pF >3.3 led to domination of extreme SWR and above pF3.7 all samples were extremely water repellent. Besides the unfavorable consequences of water repellency, the dry hydrophobic topsoil at the time of sampling formed a capillary barrier that reduced the evaporation and led to nearly 3 times higher water content stored in the subsoil.

Initiating Salt Tectonics by Tilting: Viscous Coupling Between a Tilted Salt Layer and Overlying Brittle Sediment

AbstractSalt basins often exhibit a shelf/slope region of extension and a deeper domain of contraction. The up‐slope normal faults in such salt tectonics systems are often associated with the pinch‐out edge of the buried salt layer. Although the spatial correlation between the normal faults and the salt pinch‐out was previously observed, the mechanism was not fully explained. The Levant basin, which is a young and mildly deformed salt basin with a relatively thin overburden, provides an opportunity to analyze a simple salt tectonics system driven by basin margin tilt, and the formation of the normal faults. This work presents analytical and numerical modeling of the coupled viscous salt and overlying visco‐plastic sediment layer. Results suggest the viscosities of both the sediment overburden and salt, as well as their thicknesses, control the deformation of the coupled layers. The visco‐plastic deformation mechanism explains quantitatively the position of faulting and observations of the temporal evolution of brittle deformation in the Levant basin margin. It predicts that the largest stress in the overburden arises above the salt edge, driving normal faulting at that location. Our model also places quantitative constraints on the effective viscosity of the overburden (&gt;2 × 1020 Pa s), which is consistent with experimentally‐determined creep laws. Our results contribute to the understanding of halokinematics in salt basins during the early stages of deformation and will allow better assessment of geological hazards related to salt related deformation.