Gravel Lenses Research Articles

Geotechnical reconnaissance of an extensive cover-collapse sinkhole phenomena of 2020–2021 Petrinja earthquake sequence (Central Croatia)

This article presents geotechnical reconnaissance data that characterize the formation of 122 new and 49 historical cover-collapse sinkholes within 1.13 km2 area in 12 months following the MW 6.4 earthquake that occurred on 29 December 2020, in Petrinja, Croatia. Data include a geological background, seismic sequence information, sinkhole geometric characteristics, rainfall data, and results of detailed geotechnical subsurface investigation. The sinkhole geometrical features were collected using aerial and satellite imagery, terrestrial lidar, and manual measurements. Soil properties and groundwater levels were obtained from four geotechnical boreholes, accompanied by in situ geotechnical characterization and standard penetration tests (SPTs). Soil parameters were obtained from consolidated undrained conventional triaxial compression, oedometer, soil water retention, and index tests performed on 31 soil samples. Clayey cover, 4–10 m thick, with sporadic gravel lenses overlying cavernous, intensely karstified carbonate rocks, characterizes the sinkhole area. Clays are mostly overconsolidated, with varying degrees of saturation ranging from very small to fully saturated. Seasonal and climate-induced variations in the groundwater table interact with artesian/subartesian karst aquifer, thus affecting the suction and the shear strength. Soil water retention curves (SWRCs) indicate that desaturation is possible for deeper groundwater tables, thus further affecting the effective stress, shear strength, and interparticle tensile forces. Finally, the observed vertical walls that accompanied sinkholes opening can occur in the overconsolidated cohesive cover clay layer with varying degree of saturation. The presented data provide essential geomechanical information necessary to understand the associated sinkhole failure mechanism. This article will help future investigators to perform detailed analyses and provide a background for complementing future sinkhole precursor research. Geotechnical, geological, seismic, and precipitation data generally indicate that the formation of cover-collapse sinkholes in the study area is a consequence of a specific local geological setting but is significantly expedited by earthquake-induced dynamic loading and complemented by multiple hydro-mechanical factors.

Multivariate Geostatistical Analysis of CPT Readings for Reliable 3D Subsoil Modeling of Heterogeneous Alluvial Deposits in Padania Plain

Urban planning and big infrastructure designing demand two novel and seemingly contrasting approaches: 1) a continuous description of subsoil nature and behavior under natural hazards, to increase the resilience of urban areas; and 2) a reliable characterization of subsoil hydro-mechanical properties and monitoring their working behavior. Both exigences can be addressed by reconstructing 3D mechanical models at a local scale by extracting from large databases several in-situ testings, already available for several urbanized territories worldwide. In this paper, 182 cone tip resistance qc, sleeve friction fs, and pore pressure u2 profiles, drawn from CPTs performed in the Bologna district (Padania Plain, Italy), have been used. Here, the alluvial deposits are mixtures of silt, clay, and sands, and they locally show gravel lenses where the ancient fans from the Apennines can be detected. Their heterogeneous hydro-mechanical characters cannot be described only through point investigations such as CPTs. Additionally, the variability of these mechanical profiles and the uncertainties due to the limited amount of data must be assessed and used in designing and hazard mapping. Thus, to draw a continuous 3D subsoil mechanical model based on these 182 CPTs, the Partially Heterotopic Co-Kriging technique (PHCK) has been applied. This approach is a multivariate technique that can be used when only some measurements are taken at the same locations. It allows for the estimation of the distribution of qc, fs, and u2 values in the studied domain by considering the spatial variability of the preceding random functions but also their spatial correlations. Differences in variance and spatial resolution between the measurements on the horizontal plane and along the vertical direction were accounted for by considering anisotropic spatial dependence models. As a result, this study has provided horizontal maps and vertical sections of qc, fs, and u2, as well as their 3D solid models.

Acheulian artefacts and tephra from Upland Western Maharashtra (Deccan Volcanic Province), Peninsular India

Abstract The two Early Acheulian sites of Bori and Morgaon in the Deccan Volcanic Province in Upland Maharashtra, India bear some association with an acidic tephra deposit in a fluvial context. Acheulian artefacts in association with tephra were first reported in India from the Bori site and numerous efforts to date this tephra have since been undertaken. These efforts employed various dating methods and produced a total of 10 absolute dates ranging from the Early Pleistocene with a maximum age of 1.4 Ma to the Late Pleistocene with a minimum age of 23.4 ± 2.4 ka. However, field observations and a typo-technological analysis strongly suggests that these Early Acheulian artefacts occur in a semi-primary context, and thus contest the Late Pleistocene age of the tephra and redeposition of the artefacts as argued by some scholars. At Morgaon, the Acheulian artefacts have been recovered from basal fluvial sediments that contain clasts of laterite. These sediments are capped by two low-energy clay facies that are separated by high-energy gravel lenses. The tephra at Morgaon has been reported from the upper clay facies and has produced absolute dates ranging from the Matuyama period (>0.78 Ma) to the Late Pleistocene (41 ka). After more than two decades of investigation at these sites, the number of absolute dates procured through methods such as electron spin resonance (ESR), 39 Ar– 40 Ar, U–Th and palaeomagnetism, although encouraging, are inconclusive. This paper is therefore an attempt to gauge the nature of the palaeolandscapes that most probably existed during the Early Quaternary. This will be achieved by studying local geomorphological variability between the two sites, along with a preliminary analysis of lithic morphology.

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

Groundwater resources in coastal regions are facing enormous pressure caused by population growth and climate change. Few studies have investigated whether offshore freshened groundwater systems are connected with terrestrial aquifers recharged by meteoric water, or paleo-groundwater systems that are no longer associated with terrestrial aquifers. Distinguishing between the two has important implications for potential extraction to alleviate water stress for many coastal communities, yet very little is known about these connections, mainly because it is difficult to acquire continuous subsurface information across the coastal transition zone. This study presents a first attempt to bridge this gap by combining three complementary near-surface electromagnetic methods to image groundwater pathways within braided alluvial gravels along the Canterbury coast, South Island, New Zealand. We show that collocated electromagnetic induction, ground penetrating radar, and transient electromagnetic measurements, which are sensitive to electrical contrasts between fresh (low conductivity) and saline (high conductivity) groundwater, adequately characterize hydrogeologic variations beneath a mixed sand gravel beach in close proximity to the Ashburton River mouth. The combined measurements - providing information at three different depths of investigation and resolution - show several conductive zones that are correlated with spatial variations in subsurface hydrogeology. We interpret the conductive zones as high permeability conduits corresponding to lenses of well-sorted gravels and secondary channel fill deposits within the braided river deposit architecture. The geophysical surveys provide the basis for a discharge model that fits our observations, namely that there is evidence of a multilayered system focusing groundwater flow through stacked high permeability gravel layers analogous to a subterranean river network. Coincident geophysical surveys in a region further offshore indicate the presence of a large, newly discovered freshened groundwater system, suggesting that the offshore system in the Canterbury Bight is connected with the terrestrial aquifer system.

Quantification of the uncertainty in the modelling of unstable slopes displaying marked soil heterogeneity

This study presents the application of the Boolean Stochastic Generation (BoSG) approach to the case study of the Mortisa landslide, an active earth slide flow in the Dolomites (Italy). The approach relies on the stochastic generation of different soil profiles for which the interfingering of distinct layers within a prevailing matrix is randomized. With this technique, it is possible to quantify the error associated with the simplification of stratigraphic profiles in geotechnical models. This is particularly valuable in slope stability assessment because little data are usually available in comparison with the typically complex geometry of large and deep landslides. A detailed geomorphological analysis of the study area informs an interpretation of the sequence of processes which generated the stratigraphy of the Mortisa slope. It appears to be composed of interfingered layers of silty clay and gravel that originated from subsequent earth slide and debris flow events occurring since the Late Glacial. In order to reproduce the behavior of the Mortisa slope via numerical modelling and to design effective mitigation works, it is crucial to account for the influence of the gravel lenses deposited by ancient debris flow events on the dynamics of the whole landslide. A total of 1200 possible soil configurations of the central and most active section of the Mortisa landslide have been generated. The method has been used to estimate (i) the possible errors that might occur when the presence of the gravel layers is not accounted for and (ii) the variability deriving from the position of the layers. Moreover, through the analysis of the whole ensemble of configurations, it is possible to identify the areas within the slope where the presence (or absence) of gravel layers appears to have the greatest influence on the dynamics of the landslide. This information is useful to plan future investigations and to evaluate the most effective structural mitigation measures.

Lahar deposits generated after the Millennium eruption of the Changbaishan Tianchi volcano in the Erdaobaihe River system, China

After the Plinian Millennium Eruption (including the ME-I and ME-II eruptive stages) of Changbaishan Tianchi volcano in 969 ± 20 CE, two large-volume lahars were deposited mainly on the northern flank of the Erdaobaihe River, which flows through the only outlet from Tianchi Lake: (1) a lahar deposit containing gray and black pumice (Lahar (G&B) P) and (2) a lahar deposit containing lithic particles (Lahar L). Based on the sedimentary characteristics of lahar outcrops and grain-size and compositional analyses performed using standard dry sieving techniques, examination of thin sections and geochemical analyses, the origins, transport and deposit processes and fluid phase variations of the two lahar deposits are studied. These two lahar deposits are fan-like in the flat medial area (23–60 km from the crater) and fill channels in the distal mountainous area (>60 km from the crater). Lahar L in the medial area represents a complete composite lahar flow deposit with a frontal debris head (very coarse gravel texture, extremely poor sorting, massive and matrix-supported structure) and a hyperconcentrated fluid phase body and tail (coarse sandy texture, very poor sorting, faint horizontal stratification with thin lenses of gravel, clast-supported structure). Lahar L in the distal area represents the material that passed the debris flow head, and the fluid phase changes from debris flow to hyperconcentrated (poor sorting, clear thin horizontal stratification) downcurrent. The fluid phase of Lahar (G&B) P changes from debris flow in the medial area (medium gravel to fine sand, very poor sorting, thick horizontal stratification with floating pumice on each unit surface) to hyperconcentrated flow (poor sorting, thin horizontal stratification, clear normal graded bedding) and to stream flow (moderate sorting, cross-bedding) in the distal area. Lahar (G&B) P was likely formed by the mixing of water from Tianchi Lake and pyroclastic flows of ME-II. Lahar L was likely triggered by a sudden release (dam break) from Tianchi Lake. An assessment of the risk posed to the stability of Tianchi Lake is as important as an assessment of the eruption risk.

Biofacies and diagenetic alterations of the Pleistocene coral reefs, northwest Red Sea coast, Saudi Arabia

Middle to Late Pleistocene coral reefs stretch in three discontinuously elevated units above the present sea level between Duba and Sharma along the Red Sea coast, northwest Saudi Arabia. They correspond to MIS 5, MIS 7, and MIS 9 of the deep sea cores. Each unit exhibits prominent terraces as a result of the onlap during different sea levels, erosion during transgression, and tectonic uplift. Framestones, bafflestones, bindstones, packstones, grainstones and floatstones of coral, algal, and other bioclasts are the main microfacies types recorded from these reefal units and their internal sediment. The lower and the upper units exhibited a vertical pattern of a transgressive sequence, starting at the base with a coral assemblage of coral rock zone and graded into the upper reef slope community at the top, while the middle one exhibited a regressive sequence. Corals progressively changed from aragonite in the lower unit to calcite in the upper one by meteoric leaching of the trabeculae, and the cements changed from marine aragonitic needles and peloidal Mg‐calcite in the lower unit to blocky and dog teeth cements and silica in the middle and upper units. The gravel lenses within coral colonies of the upper unit with plagioclase, epidote, and fractured quartz grains are attributed to a granitic province from the hinterland mountains, driven to the reef during rainy periods.

Hidrogeološke osobenosti Babušničke (Lužničke) kotline u istočnoj Srbiji

The Babušnica basin near the city of Pirot is a mountain depression composed of Late Oligocene (Ol3) and Pliocene (Rl) sediments. There are alluvial (al) formations and terraces (t) along the Lužnica River, which flows through the depression. From a hydrogeological perspective, the Late Oligocene and Pliocene sediments have not been explored in detail. What is known is that the shallow ("first") aquifer is formed in the upper Pliocene strata (sediments up to a depth of 20 m). The hydraulic conductivity of the water-bearing interbeds and lenses of fine-grained sands and gravels is in the range from K=6,5h10-6 to 1,1h10-5 sm/s. The water-bearing capacity of the alluvial gravels and sands (al) of the Lužnica River is somewhat higher than that of the Pliocene sediments (K=1h10-5 - 1h10-3 cm/s). Hydrochemical analyses of groundwater from the Babušnica basin, conducted from 2013 to 2017, frequently revealed non-compliance with drinking water standards. In most cases, the samples exceeded threshold values for nitrites (NO2 ), the ammonium ion (NH4), oxygen demand (KMnO4 ), iron (Fe), and manganese (Mn). In addition, groundwater from the Pliocene sediments exceeded permissible boron (V) concentrations. As a result, public water supply in the town of Babušnica and 24 villages in Babušnica Municipality relies on groundwater extracted along the southwestern edge of the Babušnica basin and the Lužnica springs in Ljuberađa.

Hydrogeological characteristics influencing the occurrence of pesticides and pesticide metabolites in groundwater across the Republic of Ireland

Pesticide contamination of water is a potential environmental issue which may impact the quality of drinking water. The full extent of pesticide contamination is not fully understood due to complex fate pathways in the subsurface. Groundwater pesticide occurrence was investigated at seven agricultural sites in different hydrogeological settings to identify where pesticide occurrence dominated in temperate maritime climatic conditions. In Ireland, six cereal dominated sites in the South East and one grassland site in the West were investigated. Soil and subsoils varied from acid brown earths with high permeability to clay and silt rich tills with lower permeability. Over a 2year monitoring period, 730 samples were collected from a network of dedicated wells and springs across the seven sites. Multi-nested piezometers were installed in intergranular, fissured and karstic type aquifers to target shallow, transition and deeper groundwaters. Several springs were also sampled and the network included a confined aquifer. Groundwater was analysed for nine pesticide active ingredients and eight metabolites. Mecoprop and 2,4-D were the most frequently detected active ingredients above the instrument detection limit, accounting for 36% and 26% of the 730 samples collected and analysed. Phenoxyacetic acid was the most frequently detected and widespread metabolite found in 39% of samples collected at all seven sites. Where the European Union drinking water standard of 0.1μg/L was exceeded, metabolites accounted for the majority of exceedances with 3,5-dichlorobenzoic acid (DBA) and phenoxyacetic acid (PAC) dominating. Highest detections were encountered in sites with well drained soils underlain by gravel and limestone aquifers and within gravel lenses in lower permeability subsoil. Across the seven sites pesticide detections were mostly associated with metabolites and the environmental impact of many of these is unknown as they have received little attention in groundwater previously.

Electromagnetic induction mapping at varied soil moisture reveals field-scale soil textural patterns and gravel lenses

Knowledge of the spatial distribution of soil textural properties is important for determining soil moisture storage and soil hydraulic transport properties. Capturing field heterogeneity without exhaustive sampling and costly sample analysis is difficult. Our objective was to employ electromagnetic induction (EMI) mapping in low apparent electrical conductivity (ECa) soils at varying soil water contents to capture time invariant properties such as soil texture. Georeferenced ECa measurements were taken using a ground conductivity meter on six different days where volumetric water content (θv) varied from 0.11 to 0.23. The 50 m × 50 m field included a subsurface gravelly patch in an otherwise homogeneous silt-loam alluvial soil. Ordinary block kriging predicted ECa at unsampled areas to produce 1-m resolution maps. Temporal stability analysis was used to divide the field into three distinct ECa regions. Subsequent ground-truthing confirmed the lowest conductivity region correlated with coarse textured soil parent materials associated with a former high-energy alluvial depositional area. Combining maps using temporal stability analysis gives the clearest image of the textural difference. These maps could be informative for modeling, experimental design, sensor placement and targeted zone management strategies in soil science, ecology, hydrology, and agricultural applications.

Aquifer permeability change caused by a near‐field earthquake, Canterbury, New Zealand

AbstractThe MW 7.1 Darfield (Canterbury) earthquake, 4 September 2010, generated widespread hydrological effects in New Zealand ranging from instantaneous changes of piezometric levels, to more sustained postseismic changes in spring flow, river discharge and groundwater levels, and increased turbidity and declined yields of water abstracted from wells. Four years later, piezometric levels remained elevated in deeper (>40 m) aquifers along the north‐western (upper) side of the Canterbury Plains near the Greendale Fault, with changes in mean piezometric level reaching +13 m. Linear reservoir modeling (eigen modeling) suggests that sustained high groundwater was not the result of changes in abstraction or land surface recharge. Step‐drawdown tests at six wells within 15 km of Greendale Fault were carried out prior to the earthquake and were retested following fault rupture. Eden‐Hazel analysis of discharge/drawdown relationships discriminates potential sources of head losses, and how these changed (or otherwise) as a result of the earthquake. Objective application of Eden‐Hazel analysis provided confidence levels for the interpretation, including when step tests provide reliable/unreliable estimates of transmissivity change. Increases in both aquifer losses and well losses were observed in four wells, reflecting both a change in sediment transmissivity and decrease in well efficiency. At two locations, the data were unable to provide results that can be interpreted with confidence. As the majority of local groundwater flow occurs through high‐permeability open framework gravel lenses, we suggest that reduction in the permeability of these gravels, due to fine‐sediment incursion, is the cause of the reduction in transmissivity and increase in well losses.

A conectividade hidrogeológica de uma nascente salina de baixa vazão em turfeiras topotróficas na região de areias betuminosas de Athabasca, Canadá

Saline springs can provide clues as to the nature of groundwater flow, including how it relates to subsurface wastewater storage and the distribution of solutes in the landscape. A saline-spring peatland neighboring a proposed in-situ oil facility was examined near Fort McMurray, Alberta (Canada). The study area is situated just north of a saline groundwater discharge zone, which coincides with the erosional edge of the Cretaceous Grand Rapids Formation. Na+ (mean 6,949 mg L−1) and Cl− (mean 13,776 mg L−1) were the dominant salts within the peatland, which increased by an order of magnitude in the opposite direction to that of the local groundwater flow. Rivers and freshwater wetlands within the study area had anomalously high salinities, in some cases exceeding 10,000 mg L−1 total dissolved solids within deeper sediments. Saline-spring features were observed as far as 5 km from the study area. A low-permeability mineral layer underlying the peatland restricted vertical groundwater exchange (estimated to be less than several mm over the 4-month study period). Sand and gravel lenses underlying the fen’s high-salinity zone may function as areas of enhanced discharge. High Cl/Br ratios point to halite as a potential source of salinity, while δ18O and δ2H signatures in groundwater were lower than modern-day precipitation or Quaternary aquifers. The complex connectivity of saline-spring wetlands within the landscape has implications for industry and land-use managers, and justifies incorporating them into monitoring networks to better gauge the magnitude and flow history of natural saline discharge in the oil sands region.

The Boolean Stochastic Generation method - BoSG: A tool for the analysis of the error associated with the simplification of the stratigraphy in geotechnical models

In geotechnical modelling, some minor stratigraphic features are usually discarded in order to simplify the problem, avoiding to deal with further uncertainties about their position, thickness and lateral extent. The study proposes a new method based on the stochastic generation of different soil layers configurations, following a boolean logic: the material is either matrix or layer (i.e., gravel lenses in a clay-rich matrix).The method has been called BoSG (Boolean Stochastic Generation). The methodology allows to randomize the presence of a specific material interdigitated in a uniform matrix thus enabling to gather a dataset which could be analysed automatically, in order quantify the error associated with the adopted simplification.The commercial codes FLAC and FLAC3D were used for the geotechnical modelling. A specifically-coded MatLab program allows to generate randomly the different soil configurations and then to automate the computation with the commercial software in order to maximize the sample number.In this paper the methodology is applied with reference to a simplified slope in 2D and in 3D. Results show that within a low resistance matrix, the presence of layers with higher friction angle can significantly affect significantly the stability and the displacement pattern of an unstable slope. Therefore, a method to investigate the influence of the spatial distribution of these layers can be particularly useful.

Origin of a washboard moraine of the Des Moines Lobe inferred from sediment properties

Geometric characteristics of the washboard moraines of the Des Moines Lobe (DML) of the Laurentide ice sheet agree with their proposed origin as crevasse-squeeze ridges, but study of their sediments is required to help further test this hypothesis. A 70-m-long, 3–5m high section through a moraine ridge in central Iowa revealed till with irregular, isolated lenses of silt, sand, and gravel that dip to varying extents upglacier. The texture and density of the till are like those of the basal till of the DML studied elsewhere in Iowa, and preconsolidation pressures determined from tests on till and silt of the ridge indicate that it developed subglacially rather than at the glacier margin. Preconsolidation pressures additionally demonstrate that pore-water pressures in the bed supported most of the glacier's weight, which would have contributed to till mobility. Fabrics based on the anisotropy of magnetic susceptibility of 3125 intact till specimens collected at 125 locations in the section indicate two end-member states of strain that varied with location in the ridge and caused sediment mounding: simple shear that was directed downglacier along shear planes inclined upglacier, together with pure shear where an overlying crevasse allowed the sediment bed to extend upward and laterally. Meltwater that likely flowed along the crevasse deposited sorted sediments that were incorporated in till, deformed, and rotated. This positive test of the crevasse-squeeze hypothesis indicates that the DML was in longitudinal extension near its margin, reinforcing previous arguments that the lobe surged. The predominance of fabrics caused by simple shear demonstrates that crevasse filling was underway before the surge had fully halted. This study should prompt caution in using similar transverse ridges, such as those geophysically imaged in some submarine glacier forefields, as indicators of retreat rates.

Metamorphic CO2 production in calc-silicate rocks from the eastern Himalaya

The Mortisa landslide is located in the urban area of Cortina d'Ampezzo, north-east Italy. The landslide unit consists of three mudslides; the affected area is 3500 m long, stretching from 1750 to 1300 m a.s.l. The landslide has been investigated since 1998 as its track crosses a national route and some buildings have been severely damaged by the movements.Through radiocarbon dating it was possible to reconstruct the dynamic of the events that led to the formation of the present-day Mortisa slope. The landslide body is composed by interdigitated layers of gravel in a silty clay matrix originated from subsequent earth and debris flows events since the Lateglacial.To assess the stability conditions of the Mortisa landslide via numerical modelling it is crucial to consider the mechanical influence of the gravel lenses in the dynamic of the whole landslide body. However, their position is almost unknown inside the clay matrix with just some certain positions, where stratigraphic data are available. Therefore, in order to calibrate the model on the monitoring data through back analysis, a new procedure has been implemented. A new code permits to generate stochastically several soil configurations which are automatically elaborated by the commercial software FLAC. The results are organized and filtered based on the monitoring data, hence it is possible to select the model that fits the data best. In this way it it possible to obtain a sound back-analysis model of the Mortisa landslide, to be used in order to design countermeasure works and mitigate the risk associated with the phenomenon.

Paleogene rim gravel of Arizona: Age and significance of the Music Mountain Formation

The early Tertiary exotic rim gravel scattered across the Colorado Plateau in Arizona (USA) provides the only widespread evidence concerning the nature of the regional Paleogene drainage system that preceded the emergence of the modern Colorado River. The term “rim gravel” includes a wide range of Laramide (herein ca. 85–40 Ma) and younger reworked quartzite-dominated, arkosic sediments with diverse origins and ages that have been clarified only recently. The parent arkoses, with subordinate gravel lenses, contain volcanic clasts with ages ranging from Late Jurassic to early Eocene. The Laramide-age arkoses were shed off uplifted Precambrian terranes to the south and west; they are separated from their reworked derivatives and younger sediments by a disconformity that is best preserved in the structurally isolated paleocanyons of the Hualapai Plateau. Younger generations of reworked gravels continued to evolve during and following the widespread eruption of basalts of late Oligocene to Pleistocene age. Basaltic clasts in the reworked gravels attest to their much younger ages. Combined paleontologic, stratigraphic, paleomagnetic, K-Ar, U-Th/He, and zircon studies of the parent arkoses indicate that the tectonic framework corresponds to the Laramide orogeny, the concurrent regional erosion of the Colorado Plateau margin, and ensuing widespread deposition. The overall time frame coincides with the similar tectonic history recorded in the early Tertiary strata of southern Utah. Comparable but unrelated quartzite-dominated gravels, reworked southward from the Utah source rocks, are widespread on strath terraces north of the Colorado River. The Utah-derived gravels are products of the much younger Neogene incision of the modern Grand Canyon drainage system. The name Music Mountain Formation redefines the Laramide-age suite of the Arizona rim gravel parent sediments, best preserved on the Hualapai and Coconino Plateaus of northern Arizona, and distinguishes them from younger reworked gravels occurring both north and south of the Colorado River. A lacustrine mollusk assemblage collected from thin limestones intertonguing with Music Mountain arkose constrains the age of one Coconino Plateau exposure to early Eocene, whereas the association with the Laramide orogeny indicates that a Late Cretaceous to middle Eocene time frame comprises the broader geologic setting.

Mapping groundwater conditions in different geological environments in the northern area of UAE using 2D earth resistivity imaging survey

A water resources database was developed and used to characterize the geological and hydrogeological settings of groundwater aquifers in the northern part of the United Arab Emirates. An intensive 2D Earth Resistivity Imaging (ERI) survey was conducted in selected areas to assess the available groundwater resources and delineate the salt-water intrusion. Drilling information of the existing monitoring wells as well as their records of water table elevations and groundwater salinity were used to measure the horizontal and vertical variations in lithology, degree of saturation, and groundwater salinity and thus to improve the interpretation of ERI data. The results of the chemical analyses of water samples collected from the wells along with the 2D ERI survey profiles were used to obtain an empirical relationship between the inferred earth resistivity and the amount of total dissolved solids. This relationship was used along with the true resistivity sections resulting from the inversion of 2D ERI data to identify and map three zones of water-bearing formation (fresh, brackish, and salt-water zones) in the coastal areas. The results indicated that the depth to the fresh-brackish interface exceeded 50 m at the upstream of the wadies and was in the order of 10 m or less in the vicinity of shoreline. Because of the high resistivity contrast between dry and water saturated fractured rocks; this method was very successful in detecting water-filled fractures and cavities in the carbonate aquifer. The application of this method was unsuccessful in clayey aquifers as it was not possible to isolate lenses of gravel and/or saturated with saline or brackish water from the surrounding clayey materials.

Resistivity imaging of Pleistocene alluvial aquifers in a contractional tectonic setting: A case history from the Po plain (Northern Italy)

In this work we present the hydrogeophysical imaging of a key sector of the Quaternary Po foreland basin (northern Italy), focussing on the reconstruction of clastic aquifers and aquitards in a complex tectono-sedimentary subsurface architecture. The study area includes the relic reliefs of Casalpusterlengo and Zorlesco, two smooth morphological features involving uplifted and gently folded Pleistocene marine to alluvial sediments, plausibly linked to the buried Northern Apennines thrust and fold belt. The geophysical data include 35 Direct Current Vertical Electrical Soundings collected over a 37 km2 wide area, acquired with Schlumberger array and maximum half-spacing of 500 m. 1-D resistivity-depth profiles were computed for each VES. An integrated hydrostratigraphic approach was applied, to constrain the interpretation of the geophysical data along several cross-sections, including the comparison of resistivity soundings to stratigraphic logs, borehole electric logs and the pore-water properties. The resistivity interfaces, traceable with the same laterally continuous vertical polarity, were used to develop an electrostratigraphic model in order to portray the stacking of electrostratigraphic units down to 200 m below ground surface. Their vertical associations show a general upward increase of electrical resistivity. This assemblage mimics the regional coarsening upwards depositional trend, from the conductive units of the Plio-Pleistocene marine-to-transitional depositional systems to the resistive units of the Middle–Late Pleistocene fluvial and alluvial plain depositional systems. Middle Pleistocene depositional systems host an alternation of North-dipping, high-to-intermediate permeability aquifer systems (70–180 Ωm, thickness of 5–70 m) separated by low permeability aquitards (20–50 Ωm, thickness up to 40 m). These units pinch out against the Casalpusterlengo and Zorlesco relic reliefs, where they cover the uplifted and folded regional aquitard (20–50 Ωm) formed by Pliocene-Lower Pleistocene clays to sandy silts with gravel lenses in agreement with borehole data. In the deepest part of the local stratigraphy, a broad low-resistivity anomaly (< 10 Ωm) was clearly mapped through the study area. By comparison with electrical borehole logs in deep oil-wells, it could be interpreted as the fresh–saltwater interface due to the presence of connate waters and brines hosted by the marine-to-transitional shales.

Electrical resistivity tomography to detect the effects of tillage in a soil with a variable rock fragment content

SummaryElectrical resistivity tomography (ERT) is a promising non‐destructive tool to characterize agricultural soils where management effects are superimposed on natural variability. The aim of our study was to test whether ERT was capable of detecting stones and tillage effects in a soil with a variable rock fragment content. Field experiments were conducted by performing a set of three two‐dimensional (2D) resistivity tomographies across two management systems (tillage/no tillage) replicated twice on each transect, using dipole‐dipole configuration and 0.25‐m inter‐electrode spacing. Soil texture, bulk density and water content were measured destructively. Greater average electrical resistivity (ER) was found in tilled plots, with maximum values of up to 1700 Ohm m. However, when the spatial correlation structure was considered in a mixed‐effects model, no significant difference in ER was found between tilled and untilled plots. Empirical semivariograms showed less spatial continuity and more noise in tilled plots. Resistivity was strongly correlated with rock fragment content (r = 0.68), with greater average values in ploughed plots, which may possibly be linked to kinetic sieving after ploughing. ERT was able to identify the position of gravel lenses and was also sensitive to the presence of clay (r = −0.45): a linear trend in resistivity across the field (r = 0.80) was consistent with a decreasing clay content (r = −0.68). Resistivity was correlated with rock fragments, clay and an interaction variable (water × rock fragments). There was a poor fit for the tilled plot where resistivity peaks could be linked to the presence of voids, but their detection would have required a resolution greater than that which we adopted.

Late Quaternary glacial chronology on the eastern slope of Gongga Mountain, eastern Tibetan Plateau, China

The Gongga Mountain is the largest area of modern glaciation in the Hengduan Mountains and, with a summit elevation of 7556 m, is the highest mountain on the eastern margin of the Tibetan Plateau. During the Quaternary glacial-interglacial cycles the Gongga Mountain was extensively and repeatedly glaciated, and glacial landforms and outwash deposits from multiple glaciations are well-preserved in valleys, in basins, and on piedmonts. To constrain the glacial chronology of the eastern slope of Gongga Mountain, sample sites were selected based on the distribution and weathering of glacial tills, relationships among glacial deposits, and soil development on moraines. Dating of the tills and glaciofluvial deposits was undertaken with electron spin resonance (ESR) and optically stimulated luminescence (OSL). The ages of the glacial deposits can be divided into four clusters: 2.2±0.5, 11.9±0.6, 35.9±2.7-58.0±6.3 and 119.2±15.9-194.2±32.8 ka. Five glacial advances in this region have been identified, which are equivalent in age to the Little Ice Age (LIA), Neoglaciation, marine oxygen isotope stage (MIS) 2, mid-MIS3, and MIS6. The largest local last glacial maximum (LGML) occurred on Gongga Mountain during mid-MIS3, characterized by a cold-humid climate, rather than the global Last Glacial Maximum (LGMG) of MIS2. The Gongga, Nanmenguangou (NMGG) and Yajiageng Glaciations occurred during the late part of the last glacial cycle, the middle of the last glacial cycle and the penultimate glacial cycle, respectively. On the basis of geomorphological, sedimentological, and compositional characteristics, landforms of the Moxi Platform and terraces can be grouped by facies and geochronology. In combination with the dating results, this analysis indicates the basal part of the Moxi Platform between Xinxin and the Moxi Hotel is correlative with the till of the Nanguanmen Glaciation (mid-MIS3). This basal unit has occasional lenses of glaciofluvial sandy gravel and lacustrine sediments. The remainder of the Moxi Platform and the terraces beside the platform are glaciofluvial deposits occasionally mixed with debris flow deposits and range in age from MIS3 to Holocene.