Aquitard Units Research Articles

Middle Jurassic–Lower Cretaceous stratigraphy of the northern Great Australian Superbasin: insights from maximum depositional age constraints from the U–Pb detrital zircon record

The Great Australian Superbasin, incorporating the Great Artesian Basin, is host to one of the largest artesian groundwater systems on Earth. However, despite the crucial nature of this resource to Australian agriculture, industry and rural communities, key aquifer units remain poorly studied, apart from the exposed eastern/southeastern portions of the superbasin. In this study, maximum depositional age (MDA) estimations for stratigraphic units, derived from detrital zircon U–Pb geochronology, were undertaken across the Carpentaria and northern Eromanga basins, representing the northern Queensland sector of the Great Australian Superbasin. The objective was to better constrain the ages of key aquifer and aquitard units, thereby facilitating improved stratigraphic correlations and future hydrogeological modelling. Age dating of detrital zircon significantly enhances the confidence, accuracy and precision of age assignments based solely on palynology, especially in continental facies. Furthermore, the provision of MDA estimates permits confident correlation of endemic Australian spore-pollen zonations (used to date continental strata), and dinocyst zonations (used to date marine strata) to the international time-scale. Results indicate that calculated MDAs are commonly either equivalent to, or appreciably younger than, stratigraphic age estimates based on palynostratigraphy. Therefore, several of the most economically strategic aquifer units in the superbasin have been revised upwards in geological time. Similar geochronological campaigns are required in the central, western and southern superbasin to facilitate both high-resolution correlation of key aquifer units across the superbasin and the detailed tracking of the Jurassic to Early Cretaceous paleogeographic evolution of eastern Australia. Key points U–Pb detrital zircon maximum depositional age (MDA) constraints generated are applied to the Jurassic–Cretaceous stratigraphy of the northern Great Australian Superbasin. MDA estimates facilitate robust correlation of endemic Australian palynostratigraphic zones to the international time-scale, reducing chronostratigraphic uncertainty. Several of the key aquifer units across the northern superbasin (e.g. Hooray Sandstone) yield MDA estimates younger than age assignments based on palynostratigraphy and have been revised upwards in geological time. Several units yield divergent MDA estimates, suggesting either unit diachroneity or miscorrelation, indicating that the chronostratigraphic framework of the northern superbasin (and the superbasin at large) warrants further examination.

Evaluation of slim-hole NMR logging for hydrogeologic insights into dolostone and sandstone aquifers

This study assesses the performance and limitations of slim-hole borehole nuclear magnetic resonance (NMR) technology from a hydrogeologic perspective in fractured, porous rock. NMR logging was carried out in dolomitic and sandstone bedrock boreholes at two research test sites in Ontario, Canada, where aquifer and aquitard units provide a range of clay contents as well as a variety of primary and secondary porosity types (e.g. discrete fractures, reefal structures, vugs and karstic conduits). Results were compared to core measurements, geophysical logs, and hydrogeophysical testing. The vertical response curve of the instrument tested was found to produce 60% of the signal from within a 0.2m span surrounding the measuring point. The repeatability of the total porosity measurements in stationary mode is excellent where the porosity is greater than 0.15. Below that threshold, repeatability is scattered at ±0.05 porosity about the mean, with the variability primarily within the clay- and capillary-bound fractions. The NMR porosity estimates agreed with core measurements to within ±0.04 porosity in both the dolostone and sandstone, but the correlation deteriorates in finely bedded lithologies, and where fracturing is present. Much of the discrepancy is attributed to scaling in a finely layered geologic sequence, as the core samples are much smaller than the entire volume measured with NMR probes. Data collection with the probe in motion (continuous logging) added variability to the response when compared to stationary recordings. Although broadscale trends were comparable, the details and depth-specific insights of the bound fluid fractions varied with logging rates. Overall, NMR provides a robust measurement of the bulk matrix porosity and pore size distribution of lithologies intersected, both of which are critically important parameters in understanding hydrogeologic conditions and contaminant distributions in layered sedimentary rock systems.

Experimental study on mechanism for pumping-induced land subsidence

Abstract. Groundwater pumping can cause severe land subsidence, yet the mechanisms have not been completely clear. A laboratory physical model test was done to investigate the mechanism for pumping-induced land subsidence. In the model test, a model well was installed and pumpage through the well was taken. During and after pumping, the soil displacement and the pore water pressure were documented. The pore water pressure within the pumped sand layer decreased immediately after pumping and recovered immediately after stopping pumping, while the pore water pressure in the neighboring silty clay layers first increased and then decreased with pumping, and first decreased and then increased after pumping was stopped and groundwater level in the sand layer recovered. The duration within which the pore water pressure in the silty clay increased when pumping was increasingly great with the distance from the pumped sand layer. The compaction of the neighboring silty clay first occurred near the interface between the silty and sand layers, and the silty clay expanded vertically within some zones. The test results indicate that the mechanism for land subsidence is complex. Due to their low permeability, aquitard units may expand in a period when groundwater is withdrawn from the neighboring aquifer units, and they may compact when groundwater is recharged into the neighboring aquifer units. This is one of the reasons for the lagging compaction of aquitard units.

Assessing aquitard integrity in a complex aquifer – aquitard system contaminated by chlorinated hydrocarbons

This study investigates for the first time the integrity of multiple stacked aquitards with different degrees of contaminant degradation. Aquitard integrity was assessed in a contaminated, multi-layered, alluvial aquifer-aquitard system (Ferrara, northern Italy). The system was contaminated by mixed organic contaminants of industrial origin (mostly chlorinated ethenes) that were illegally disposed in an urban dump four to five decades ago. High spatial resolution profiles of hydraulic head, geochemistry and chlorinated hydrocarbon concentrations were determined through the multi-layered system via discrete interval sampling of continuous cores and multilevel groundwater sampling, at three locations aligned along a transect adjacent to the buried waste to a maximum depth of 53 m below the water table.The profiles revealed that the two shallow aquitards had low integrity with respect to impeding downward migration of dense non-aqueous phase liquid (DNAPL), and provided little protection to the underlying aquifers against DNAPL contamination due to preferential pathways through imperceptible fractures and/or permeable micro-beds. However, both aquitards inhibited downward DNAPL migration to some extent due to DNAPL retention along its flow paths and accumulation at lower permeability interfaces, with decreasing peak concentrations at the top of successively deeper aquitard units. Moreover, both aquitards enhanced contaminant biodegradation due to the occurrence of organic rich sub-layers, influencing the contaminant plume composition, mobility and fate in the underlying and overlying aquifers.The deepest aquitard showed evidence of DNAPL accumulation at the top and slow diffusion-dominated transport consistent with 40 years of transport, suggesting higher integrity compared to the two shallower aquitards. However, the occurrence of micro-fractures and/or discontinuities in the aquitard upgradient under the dump (source) is the most likely explanation for contamination of the deepest aquifer. Analytical 1-D simulations of the diffusion profiles in the deepest aquitard revealed that DNAPL contamination down to the top of this aquitard occurred with minimal delay after DNAPL waste disposal began.The results highlight the necessity of high-resolution vertical profiling for assessing the presence of imperceptible features relevant to DNAPL migration and integrity of individual aquitards affecting organic contaminant source zone mass and phase distributions over decades.

Variation in Saturated Hydraulic Conductivity at the Outcrop Scale, the Whanganui Basin, New Zealand.

Groundwater flow and contaminant transport are strongly influenced by hydrogeological spatial variation. Understanding the textural heterogeneity of aquifer and aquitard units is critical for predicting preferential flow pathways, but is often hindered by sparse hydrogeological data, widely spaced data points, and complex stratigraphy. Here, we demonstrate the application of a relatively new air permeameter technology, providing a cost-effective, rapid alternative for characterizing hydrostratigraphic units in the field. The aim of this research is to (1) characterize the variation of saturated hydraulic conductivity across shallow-marine hydrostratigraphic units of the Whanganui Basin, New Zealand, and (2) assess the variation of saturated hydraulic conductivity within individual hydrostratigraphic units and relate these changes to facies and depositional environments. Results suggest heterogeneity within fine-grained aquitard units is controlled by bioturbation, whereby burrowing, ingestion and defecation results in grain size segregation and differential micrite cementation. Coarse-grained heterolithic aquifer facies display sharp changes in permeability across planar to cross-bedded sets, related to current and wave energy fluctuations within shallow-marine depositional settings. Bedding plane orientation creates high permeability zones that promotes down dip subsurface flow. Down dip gradation of coarse-grained nearshore facies into fine-grained shelf facies along the paleo shoreline-shelf transect is suggested to promote lateral and vertical groundwater flow within the basin fill. Air permeameter techniques have potential for application within groundwater basins around the world, providing datasets that facilitate greater understanding of groundwater systems, informing practices and policies for targeted water quality management.

Three-Dimensional Numerical Investigation of Pore Water Pressure and Deformation of Pumped Aquifer Systems.

Excessive groundwater withdrawal has caused severe land subsidence worldwide. The pore water pressure and the deformation of pumped hydrostratigraphic units are complex. A fully coupled three-dimensional numerical simulation was carried out for different pumping plans in this paper. When groundwater is pumped from a confined aquifer, the great compaction occurs in the pumped aquifer and its upper and lower adjacent aquitard units. Land subsidence is smaller and the area affected by land subsidence is greater when groundwater is pumped from the deeper confined aquifer. The pore water pressure in the pumped confined aquifer changes immediately with pumpage. In the adjacent aquitard units, however, the pore water pressure increases in the early pumping time and decreases in the early recharging time. The decrease in the pore water pressure vertically spreads from the interface between aquitard and pumped aquifer to the other surface of the aquitard. The pumped aquifer compacts and rebounds immediately with pumping and non-pumping or recharging actions, while the compaction and rebounding of the aquitard units clearly lag behind. The compaction of the adjacent aquitard unit first occurs near the interface between aquitard and pumped aquifer units, and the compaction zone spreads outward as the pumping goes on. The aquitards may expand vertically within some zones. Due to the inelastic deformation of soil skeleton, different pumping plans result in different land subsidence. For the same net pumpage, maximal land subsidence and horizontal displacement are the smallest for constant discharge and the greatest for recharge-discharge cycle.

Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution heliborne geophysics

Abstract. We conducted a multidisciplinary study at the watershed scale of an andesitic-type volcanic island in order to better characterize the hydrogeological functioning of aquifers and to better evaluate groundwater resources. A heliborne Time Domain ElectroMagnetic (TDEM) survey was conducted over Martinique in order to investigate underground volcanic structures and lithology, characterized by high lateral and vertical geological variability and resulting in a very high heterogeneity of their hydrogeological characteristics. Correlations were made on three adjacent watersheds between resistivity data along flight lines and geological and hydrogeological data from 51 boreholes and 24 springs, showing that the younger the formations, the higher their resistivity. Correlation between resistivity, geology and transmissivity data of three aquifers is attested: within the interval 10–100 ohm m and within a range of 1 to 5.5 Ma, the older the formation, the lower its resistivity, and the older the formation, the higher its transmissivity. Moreover, we demonstrate that the main geological structures lead to preferential flow circulations and that hydrogeological watersheds can differ from topographical watersheds. The consequence is that, even if the topographical watershed is small, underground flows from an adjacent watershed can add significant amounts of water to such a catchment. This effect is amplified when lava domes and their roots are situated upstream, as they present very high hydraulic conductivity leading to deep preferential groundwater flow circulations. We also reveal, unlike basaltic-type volcanic islands, that hydraulic conductivity increases with age in this andesitic-type volcanic island. This trend is interpreted as the consequence of tectonic fracturing associated with earthquakes in this subduction zone, related to andesitic volcanic islands. Finally, our approach allows characterization in detail of the hydrogeological functioning and identification of the properties of the main aquifer and aquitard units, leading to the proposition of a hydrogeological conceptual model at the watershed scale. This working scale seems particularly suitable due to the complexity of edifices, with heterogeneous geological formations presenting high lateral and vertical variability. Moreover, our study offers new guidelines for accurate correlations between resistivity, geology and hydraulic conductivity for volcanic islands. Finally, our results will also help stakeholders toward a better management of water resources.

Three-dimensional imaging of aquifer and aquitard heterogeneity via transient hydraulic tomography at a highly heterogeneous field site

Previous studies have shown that geostatistics-based transient hydraulic tomography (THT) is robust for subsurface heterogeneity characterization through the joint inverse modeling of multiple pumping tests. However, the hydraulic conductivity (K) and specific storage (Ss) estimates can be smooth or even erroneous for areas where pumping/observation densities are low. This renders the imaging of interlayer and intralayer heterogeneity of highly contrasting materials including their unit boundaries difficult. In this study, we further test the performance of THT by utilizing existing and newly collected pumping test data of longer durations that showed drawdown responses in both aquifer and aquitard units at a field site underlain by a highly heterogeneous glaciofluvial deposit. The robust performance of the THT is highlighted through the comparison of different degrees of model parameterization including: (1) the effective parameter approach; (2) the geological zonation approach relying on borehole logs; and (3) the geostatistical inversion approach considering different prior information (with/without geological data). Results reveal that the simultaneous analysis of eight pumping tests with the geostatistical inverse model yields the best results in terms of model calibration and validation. We also find that the joint interpretation of long-term drawdown data from aquifer and aquitard units is necessary in mapping their full heterogeneous patterns including intralayer variabilities. Moreover, as geological data are included as prior information in the geostatistics-based THT analysis, the estimated K values increasingly reflect the vertical distribution patterns of permeameter-estimated K in both aquifer and aquitard units. Finally, the comparison of various THT approaches reveals that differences in the estimated K and Ss tomograms result in significantly different transient drawdown predictions at observation ports.

Hydraulic evaluation of the hypogenic karst area in Budapest (Hungary)

The Buda Thermal Karst area, in central Hungary, is in the focus of research interest because of its thermal water resources and the on-going hypogenic karstification processes at the boundary of unconfined and confined carbonates. Understanding of the discharge phenomena and the karstification processes requires clarification of the groundwater flow conditions in the area. Accordingly, the aim of the present study was to present a hydraulic evaluation of the flow systems based on analyses of the archival measured hydraulic data of wells. Pressure vs. elevation profiles, tomographic fluid-potential maps and hydraulic cross sections were constructed, based on the data distribution. As a result, gravitational flow systems, hydraulic continuity, and the modifying effects of aquitard units and faults were identified in the karst area. The location of natural discharge areas could be explained and the hydraulic behavior of the Northeastern Margin Fault of the Buda Hills could be determined. The flow pattern determines the differences in the discharge distribution (one- and two-component) and related cave-forming processes between the Central System (Rozsadomb area) and Southern System (Gellert Hill area) natural discharge areas. Among the premises of hypogenic karstification, regional upward flow conditions were confirmed along the main discharge zone of the River Danube.

Relationship between geohydrology and Upper Pleistocene–Holocene evolution of the eastern region of the Province of Buenos Aires, Argentina

The Upper Pleistocene–Holocene geological evolution, which is characterized by its landscape-forming energy and is related to geological and geomorphological complexity, has an impact on the groundwater dynamics of coastal aquifers. The geological configuration of a sector of the east coast of the Province of Buenos Aires was analyzed, as well as its connection with the geological and geomorphological history of the region during the Late Pleistocene and Holocene, and its influence on the regional and local geohydrological behaviour. This analysis was based on the application of the concept of hydrofacies. Boreholes were drilled and sampled (with depths of up to 40 m), and vertical electrical sounding, electrical tomography and pumping tests were undertaken. The description of the cutting samples by means of a stereo microscope, the interpretation of satellite images, and the construction of lithological and hydrogeological profiles and flow charts were carried out in the laboratory, and then integrated in a GIS. The identification of the lithological units and their distribution in the area allowed the construction of an evolutionary geological model for the Late Pleistocene and Holocene. Three aquifer units can be recognized: one of Late Pleistocene age (hydrofacies E) and the other two of Holocene age (hydrofacies A and C); their hydraulic connection depends on the occurrence and thickness variation of the aquitard units (hydrofacies B and D). The approach adopted allows the examination of the possibilities for groundwater exploitation and constitutes an applied conceptual framework to be taken into consideration when developing conceptual and numerical models at the local and regional scales.

Numerical simulation and prediction of regional land subsidence caused by groundwater exploitation in the southwest plain of Tehran, Iran

This study characterizes land subsidence in southwest plain of Tehran using numerical modeling and predicts the trend through 2018. Excessive groundwater withdrawal has caused severe land subsidence in Tehran; in the past 28years (1984–2012), groundwater level has decreased 11.65m. The multi-layered aquifer system in the southwestern plain of Tehran contains three aquifers and three aquitard units. The present model was developed simulation using PMWIN (MODFLOW for Windows). First, groundwater level and land subsidence were simulated for the end of 2004. The model was calibrated using hydraulic head measurements and InSAR data. The simulation results were in fairly good agreement with the measurement results. The calibrated and evaluated model was then used to assess the future evolution of land subsidence and for prediction of subsidence through the end of 2018. Numerical results show that, assuming a constant rate of pumping in the future, land subsidence in the southwestern plain of Tehran will reach 33cm by 2018. The study confirmed that land subsidence caused by groundwater pumping is a serious threat to southwest Tehran.

Land subsidence and uplift due to long-term groundwater extraction and artificial recharge in Shanghai, China

Increasing artificial water recharge and restriction on groundwater pumpage have caused land displacements in Shanghai (China) to shift from subsidence to uplift. On the basis of field and laboratory data, the characteristics and mechanism of land subsidence and uplift are analyzed and discussed. Under the condition of long-term groundwater extraction, the deformation of aquifer and aquitard units consists of elastic, plastic, visco-elastic, and visco-plastic components. The recoverable elastic and visco-elastic deformation is only a small portion of the total deformation for both aquitard and aquifer units, especially when the groundwater level in the units is lower than the historically lowest values. When the groundwater level in aquifer and aquitard units rises, whether their expansion occurs immediately or not, depends on the changing modes of groundwater level they have experienced. Even aquifer units do not always rebound closely following the rise of groundwater level in them. The lagging of the occurrence of arrested land subsidence and uplift, clearly behind the rise of groundwater level in aquifer units, can be attributed to the visco-plastic deformation of all units and the consolidation deformation of aquitard units. Artificial recharge and limitation of pumpage are efficient measures for controlling land subsidence, but earlier actions are necessary to keep groundwater levels in all aquifer units above their historically lowest values all the time, if a more effective outcome is expected.

Visco-elasto-plastic compaction of aquitards due to groundwater withdrawal in Shanghai, China

Long-term excessive groundwater withdrawal can cause aquifer systems to compact, and land subsidence ensues. The compaction of aquitards lags behind the change of groundwater level in adjacent aquifers, which is attributed to low hydraulic conductivity and great creep deformation of aquitards. Under the condition of groundwater withdrawal, aquitard clays may compact visco-elastically, elasto-plastically or visco-elasto-plastically, depending on the effective stress history they have experienced. Laboratory data manifest that aquitard clays have a similar constitutive relationship to aquifer sands, therefore the double-yielding visco-elasto-plastic mechanical model proposed for aquifer sands is applicable to aquitard clays. A fully coupled one-dimensional visco-elasto-plastic compaction model is constructed for aquitards to predict their compaction by integrating the double-yielding mechanical model with the flow governing equation of groundwater. The comprehensive compaction model not only makes the groundwater flow and soil deformation coupled, but also takes into account the complex deformation characteristics of aquitard units under long-term groundwater withdrawal, such as creep and plastic deformation under reloading.

Joint Estimation of Hydraulic and Poroelastic Parameters from a Pumping Test.

The coupling of hydraulic and poroelastic processes is critical in predicting processes involving the deformation of the geologic medium in response to fluid extraction or injection. Numerical models that consider the coupling of hydraulic and poroelastic processes require the knowledge of relevant parameters for both aquifer and aquitard units. In this study, we jointly estimated hydraulic and poroelastic parameters from pumping test data exhibiting "reverse water level fluctuations," known as the Noordbergum effect, in aquitards adjacent to a pumped aquifer. The joint estimation was performed by coupling BIOT2, a finite element, two-dimensional, axisymmetric, groundwater model that considers poroelastic effects with the parameter estimation code PEST. We first tested our approach using a synthetic data set with known parameters. Results of the synthetic case showed that for a simple layered system, it was possible to reproduce accurately both the hydraulic and poroelastic properties for each layer. We next applied the approach to pumping test data collected at the North Campus Research Site (NCRS) on the University of Waterloo (UW) campus. Based on the detailed knowledge of stratigraphy, a five-layer system was modeled. Parameter estimation was performed by: (1) matching drawdown data individually from each observation port and (2) matching drawdown data from all ports at a single well simultaneously. The estimated hydraulic parameters were compared to those obtained by other means at the site yielding good agreement. However, the estimated shear modulus was higher than the static shear modulus, but was within the range of dynamic shear modulus reported in the literature, potentially suggesting a loading rate effect.

A geological and hydrogeological assessment of the electrical conductivity information from the HiRES airborne geophysical survey of the Isle of Wight

A recent high resolution airborne geophysical survey across the Isle of Wight (IoW) and Lymington area has provided the first electromagnetic data across the relatively young geological formations characterising much of southern England. The multi-frequency data provide information on bulk electrical conductivity to depths of the order of 100 m. A GIS-based assessment of the electrical conductivity information in relation to bedrock geological classification has been conducted for the first time. The analysis uses over 104,000 measurements across onshore IoW and has established average and statistical properties as a function of bedrock geology. The average values are used to provide baseline maps of apparent electrical conductivity and the variation with depth (measured as a function of frequency). The average conductivity as a function of depth within the main aquifer units is summarised. The data indicate that the majority of the Palaeogene is characterised by values consistently in excess of 100 mS/m and with a surprisingly high degree of spatial heterogeneity. The youngest (Oligocene) Hamstead Member displays some strong edge effects and the largest localized values in conductivity. The central Upper Chalk is associated with the lowest observed conductivity values and mineral content and/or porosity appears to increase with increasing age. The large central outcrop of the Lower Greensand Group, Ferruginous Sand Formation provides persistently low (<30 mS/m) conductivity values which imply a relatively uniform distribution of clean sand content. Non-geological (e.g. environmental) responses are contained within the data set and examples of these in relation to a closed municipal landfill and an area of potential coastal saline intrusion are discussed. In the south, the Gault clay/mudstone of the Early Cretaceous appears as a distinctive conductive unit. Cross sectional modelling of the data has been undertaken across the aquifer units of the Southern Downs. The results indicate that the Gault Formation, acting as an aquitard, can be traced as a distinct unit under the more resistive Early Cretaceous Upper Greensand and Late Cretaceous Chalk formations. The conductivity modelling should therefore allow an estimation of the subsurface configuration of the aquifer and aquitard units.

Redox evolution in glacial drift aquifers: role of diamicton units in reduction of Fe(III)

High iron concentrations create water quality problems for municipal use in glacial drift aquifer units. The chemical evolution of oxic groundwater in shallow aquifer units to anoxic groundwater in deeper aquifer units, in which soluble Fe(II) is stable, is attributed to coupled reduction of Fe(III) on aquifer solids with oxidation of organic carbon. The objective of this study was to characterize the distribution of organic carbon in aquifer and aquitard sediments to determine the availability of potential electron donors to drive these reactions. To do this, four complete rotasonic cores in a glacial aquifer/aquitard system were sampled at close intervals for analyses of grain-size distribution and organic carbon content. The results indicate significantly higher organic carbon concentrations in diamicton (till) units that function as aquitards, relative to coarse-grained aquifer units. In addition, readily reducible iron content in the diamicton units and lower aquifer unit materials is sufficient to produce far more dissolved iron than is present in the aquifer. Groundwater evolves to the level of iron reduction as a terminal electron-accepting process as it moves downward through aquitard units along flow paths from upland recharge areas to downgradient discharge areas. Deeper aquifer units are therefore unlikely to contain groundwater with low iron concentration.

Characteristics of aquifer system deformation in the Southern Yangtse Delta, China

Excessive groundwater pumpage has caused regional cones of depression and severe land subsidence in the Southern Yangtse Delta, China. The characteristics of aquifer system compaction are complex because of the difference in the types, compositions and structures of the soils that the hydrostratigraphic units are composed of, and in the histories of groundwater level change the hydrostratigraphic units have experienced. Based on the data measured from extensometer groups and observation wells, the characteristics of deformation for individual hydrostratigraphic units are analyzed. The results show that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics, such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at different sites of the cone of depression or in different periods. If the groundwater level rises continuously and remains constant later, the aquifers and the aquitard units that consist of hard clay primarily exhibit elastic behaviour, but the aquitard units that consist of soft clay deform plastically and by creep and exhibit visco-elasto-plastic behaviour. If the groundwater level falls but is much higher than the previous lowest value, aquifers and aquitards consisting of hard clay exhibit elasto-plastic behaviour. If the groundwater level falls below the previous lowest value, aquifers and aquitards consisting of hard clay deform plastically and by creep and exhibit visco-elasto-plastic behaviour.

Fluid Flow during Deformation Associated with Structural Closure of the Isa Superbasin at 1575 Ma in the Central and Northern Lawn Hill Platform, Northern Australia

Proterozoic rocks of the Isa superbasin in northern Australia host globally significant Pb-Zn-Ag deposits as well as a series of Cu, Cu-Au, and U deposits. This numerical modeling study explores fluid-flow patterns during the shortening deformation event associated with the initial stages of the structural inversion of the Isa superbasin at ca. 1575 Ma. The major objective of this work is to explore a series of scenarios for the Isa superbasin. The models of coupled deformation and fluid-flow processes presented in this paper include (1) basin fluid flow in a basin-inversion environment subject to shortening, (2) syntectonic dehydration-related fluid production, (3) hydrofracturing and permeability creation, (4) aquifer permeability changes with depth, and (5) the effect of a buried fault versus an open fault. In a scenario only considering the interaction of deformation and fluid flow during basin inversion, intense upward flow along basin-scale faults and strong lateral flow along the well-connected segments of major aquifer horizons dominate the fluid-flow field. The combinations of deformation features, aquifer geometry, and fault distribution result in different fluid-flow patterns across the simulated section. This seems to suggest some fluid compartmentalization, resulting in variations in fluid chemistry and mineralization across the region. The incorporation of dehydration fluid production and hydrofracturing-permeability creation both significantly increase fluid flow, expressed as greater fluid fluxes and volumes along faults and aquifer units. Scenarios that consider hydrofracturing-permeability development also indicate more efficient fluid migration during basin inversion. Permeability creation as a result of hydrofracturing allows fluids to migrate more efficiently through rocks along newly developed high-permeability horizons and to converge more efficiently into basin-scale faults. Assuming an open fault (i.e., a fault with access to the surface) also increases upward fluid flux along the fault. The models also demonstrate the effects of changing permeability of aquifer units with depth on fluid flow. The key implication of the current modeling results is that shortening deformation during the inversion of the Isa superbasin, assisted by dehydration-related fluid release at deep levels and hydrofracturing-related permeability generation, could have created efficient multiple fluid pathways through the stratigraphic sequence with multiple aquitard units and increased fluid flow along and near faults in the region.

Relationships among sedimentology, stratigraphy, and diagenesis in the Proterozoic Thelon Basin, Nunavut, Canada: implications for paleoaquifers and sedimentary-hosted mineral deposits

The Thelon Basin, Nunavut, Canada, is host to unconformity-type uranium mineralisation and has the potential to host other economic deposits. The Thelon Formation (ca. 1750 Ma) is composed of thick (meters to tens of meters), poorly sorted, trough cross-bedded conglomerate and coarse-grained lithic arenite beds, and to a lesser extent, well-sorted, medium- to coarse-grained quartz arenite beds. Relatively rare, 1–12 cm thick, clay-rich siltstones to fine-grained sandstone layers punctuate the coarser lithofacies. Based on regional analysis of drill cores and outcrops, multiple unconformity-bounded sequences are defined in this fluvial-dominated sedimentary succession. Stratigraphic correlations are based on detailed lithofacies analysis, distinct changes in fining-upward cycle thickness, and intraformational surfaces (unconformities, transgressive surfaces, and paleosols). Diagenetic and paragenetic relationships vary systematically with sedimentology and stratigraphy of the Thelon and provide a framework for understanding the evolution of fluid-flow systems in the context of basin hydrostratigraphy. Stratigraphic units with well-sorted textures, which lacked clay and unstable framework grains, originally were aquifers (depositional aquifers) during early basin evolution. However, pervasive, early quartz cementation radically reduced the porosity and permeability of these units, occluding pore throats and transforming them into aquitards. Proximal fluvial and alluvial fan lithofacies that contained detrital, mechanically infiltrated, and diagenetic clay minerals and/or unstable detrital grains originally had low permeabilities and only experienced minor quartz cementation. In the deep burial setting (2–7 km), these units retained sufficient permeability to allow diagenetic fluid flow (diagenetic aquifers) as suggested by feldspar dissolution, quartz dissolution, and formation and recrystallization of illite and other diagenetic reactions. Tracing potential diagenetic aquifer and aquitard units across the study area allowed development of a hydrostratigraphic model. In this model, diagenetic aquifers onlap onto, and focused basinal fluids into basement rocks to the east in the Thelon Basin (in the vicinity of the Kiggavik uranium deposit).

Origin, lithology and weathering characteristics of Upper Tertiary ‐ Quaternary clay aquitard units on the Lower Murrumbidgee alluvial fan

The Lower Murrumbidgee alluvial fan at the eastern edge of the Murray Basin is comprised of high‐yielding coarse‐grained aquifers and interlayered fine‐grained deposits that exert an important control on recharge and vertical leakage of contaminants such as salt. Concerns over increasingly saline shallow groundwater, particularly in irrigation areas, has focused investigations on the depositional origin and spatial distribution of these fine‐grained deposits (aquitard units), which may constitute both a source of leachable salt and a barrier to leakage. Detailed laboratory analysis of a minimally disturbed core to 83m depth, obtained from a drillhole adjacent to an irrigation bore, was augmented with geophysical investigations from this and other boreholes near the apex of the alluvial fan. Previously mapped clay units (aquitards) are redefined as clayey silts based on clay content variation between 10% and 30%. Mineralogical and lithostratigraphic evidence for three clayey silt units is presented: a lower unit (75–83m), a middle unit (45–64m) and an upper unit (0–16m). Electrical image surveys indicate that the upper unit is discontinuous, interrupted by large palaeodrainage features probably containing sands and gravels. These palaeodrainage channels are buried beneath a veneer of clay and significantly increase recharge and leakage. Some evidence suggests an aeolian component near the surface and within the middle clayey silt unit. However, mixing with fluvial deposits and subsequent weathering has also occurred. The clayey silt units are extensively weathered and oxidised, with the degree of oxidation increasing towards the surface and adjacent to aquifers saturated with oxygenated groundwater. Post‐depositional weathering of the middle and lower units may also have been associated with leaching of salts. No salt remains in the middle and lower units, but 10.2kg/m2 is stored within 15m of the surface at the Tubbo site. The upper clayey silt unit is a significant source of leachable salt, which is associated with increasing shallow groundwater salinity at some sites.