Quaternary Gravel Research Articles

Palaeoclimatic response to Quaternary gravels: A case study from Kumaun Himalaya, India

Study of Quaternary intramontane Bhimtal palaeolake deposits and the Gaula piedmont fan deposits representing sediment source to sink scenario respectively; have revealed that gravelly sequences have directly responded to climatic turnovers creating contrasting lithofacies associations. Humid climatic conditions favoured thickly developed river generated clast-supported gravelly deposits, composed of clasts derived from a distant source. These deposits show intense lateral and vertical changes in lithofacies constitution. In contrast, arid phases are represented by extensive debris flows composed of clasts derived from adjacent mountain slopes. Gravelly deposits systematically interbedded in the stratigraphic records have potential for palaeoclimatic studies that is sometimes ignored.

Monsoon triggered formation of Quaternary alluvial megafans in the interior of Oman

A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.

Use of environmental isotopes to define the capture zone of a drinking water supply situated near a dredge lake

Summary Lake–ground water interactions in the vicinity of a dredge lake (Lake Leis; SW Germany) were studied by means of the stable isotopes ( 18 O, 2 H) mainly to determine the capture zone of a pumping well for the drinking water supply to the nearby town of Baden-Baden. The water supply is situated in a Quaternary gravel aquifer of the Rhine Valley. Numerical simulation performed using a two-dimensional water flow model was not able to characterise the capture zone uniquely. Several best fit options were possible by applying the values of hydraulic conductivities obtained from the pumping tests. As a result it was not possible to define whether Lake Leis is situated either completely inside or fully outside of the capture zone. To overcome this problem and to define the shape of the catchment area of the water supply precisely, a two-dimensional water flow and tracer transport model was established using both hydraulic heads and isotope data. The isotope data were applied to determine the proportion of lake water in down-gradient pumping and observation wells. Additionally, the isotope compositions of lake water and those in observation wells, measured as a function of time, were used to determine the mean water velocity ( v = 0.6 m/d) along some selected 200 m long flow-paths. Based on known porosity and hydraulic gradient values, a hydraulic conductivity of about k = 6 × 10 −4 m/s was determined for the central area down-gradient of the lake. This hydraulic conductivity and the proportion of lake water in the groundwater were used to calibrate a numerical two-dimensional regional scale transport model. The study demonstrates that it is possible to calibrate a water flow model in the vicinity of a dredge lake uniquely, only when stable isotope signals measured in a time series in both the lake and groundwater are taken into consideration. The calibrated model finally enables the capture zone of the production well to be precisely defined.

Further investigation over Quaternary silts using the Spatial Autocorrelation (SPAC) and Horizontal to Vertical Spectral Ratio (HVSR) microtremor methods

This study extends a previous investigation using the passive seismic Spatial Autocorrelation (SPAC) and Horizontal to Vertical Spectral Ratio (HVSR) methods on microtremor observations in the Port Melbourne area. The area has up to 80 m thickness of Quaternary gravels, sands, and silts, with the near-surface Coode Island Silt being so soft that the base of the unit could not be resolved in a previous study. In order to extend the previous work, further surveys using instrumentation more suited to low-frequency measurements (SPAC and HVSR methods) and a larger array radius (SPAC method) were undertaken.Despite the use of significantly larger array radii and long-period seismometers, coherencies in the sub 1–2 Hz range were poor and allowed only marginally deeper shear-velocity information to be obtained. We have attributed this deficiency in the SPAC spectrum to a lack of energy in the vertical component of Rayleigh wave motion, as indicated by a high-amplitude peak in the HVSR spectrum around 1–2 Hz.Despite the poor low-frequency SPAC results, a combined modelling approach using both HVSR and SPAC results, along with the results from the large array survey, allowed deeper shear-velocity values (to depths of 70–100 m) to be estimated with greater precision than the initial surveys. We note that the HVSR spectrum is more sensitive to variations in shear velocities at these depths than the SPAC spectrum, providing an additional constraint on interpreted shear-velocity profiles from SPAC data.Lack of a secondary HVSR peak below 1 Hz that could be attributed to the sediment-bedrock interface leads us to believe that this expected peak is masked by the high amplitude peak resulting from the large shear-velocity contrast closer to the surface in the sediment profile (a silt–gravel interface).

Response of grain size of Quaternary gravels to climate and tectonics in the northern Tibetan Plateau

The widely distributed thick gravel deposits along the rim of the Tibetan Plateau have been long thought to be the product of rapid tectonic uplift of the plateau. However, this has been challenged by recent works that suggest these thick gravels may be the result of climate change. In this paper we carried out a detailed field measurement of gravel grain sizes from the Jiuquan and Gobi Gravel Beds in the top of the Laojunmiao section in the Jiuxi Basin in the northern margin of Qilian Mts. (northern Tibetan Plateau). The results suggest that the grain sizes of the Jiuquan and Gobi Gravel Beds over the last 0.8 Ma are characterized by nine coarse-fine cycles having strong 100-ka and 41-ka periodicities that correlate well with the loess-paleosol monsoon record and isotopic global climatic record from deep sea sediments as well as by a long trend of coarsening in gravel grain size. The coarse gravel layers were formed during the warm-humid interglaciations while the fine layers correspond to the cold-dry glaciations. Because the paleoclimate in NW China began to get dramatically drier after the mid-Pleistocene, we think the persistent coarsening of gravel grain size was most probably caused by the rapid uplift of the northern Tibetan Plateau, and that the orbital scale cyclic variations in gravel grain size were driven by orbital forcing factors that were superimposed on the tectonically-forced long-term coarsening trend in gravel size. These findings also shed new light on the interaction results of climate and tectonics in relation to the uplift of the Tibetan Plateau.

Slip-rate estimate and past earthquakes on the Doruneh fault, eastern Iran

SUMMARY The Doruneh fault, with a length of ∼600 km, is one of the longest, and most prominent, faults in Iran. It performs an important role in the regional tectonics, but has no record of large earthquakes. The geomorphology of the Doruneh fault contains numerous indications of cumulative left-lateral slip over various scales. We describe three sites where Late Quaternary landforms are displaced by the fault. (a) An incised alluvial fan near the village of Uch Palang is displaced by 800‐850 m. (b) The Kuh-e Teagh-Ahmad fold is composed of folded Quaternary gravels and is displaced by ∼200‐400 m. (c) A sequence of three terraces of the Shesh‐Taraz river are displaced left-laterally by a maximum of 25 m. Infrared stimulated luminescence (IRSL) dating of the uppermost Shesh‐Taraz river terrace gives a deposition age of ∼10 ka, which correlates with changes in global climate ∼10‐12 ka ago, and provides a provisional slip-rate estimate of 2.4 ± 0.3 mm yr −1 . No major recent or historical earthquakes are recorded on the Doruneh fault. Relatively fresh scarps and partially infilled fractures appear to be the preserved surface ruptures from an earthquake event of unknown age. A series of small streams showing left-lateral displacements of 3 to 5.5 m (with an average slip of ∼4.7 m) record the possible magnitude of slip during this earthquake, which from scaling relationships would have had an M w of ∼7.5, and ruptured the fault over a length of >100 km. At the estimated slip-rate of ∼2.5 mm yr −1 , the average recurrence time between large-magnitude earthquakes on the Doruneh fault is ∼2000 yr.

Preliminary observations on the release of arsenic to groundwater in the presence of hydrocarbon contaminants in UK aquifers

AbstractDegradation of contaminant hydrocarbons in groundwater by microbially mediated oxidation, linked to the reduction of electron acceptors, is fundamental to the strategy of ‘monitored natural attenuation’ (MNA) for oxidizable hydrocarbons, which is increasingly being adopted at polluted aquifer sites throughout Europe and North America. Commonly, oxygen is depleted and following the reduction of nitrate, solid-phase Fe oxides become the dominant electron acceptors. Arsenic, associated with Fe and Mn oxides in soils and sediments, may therefore be mobilized to groundwater and pose an additional threat to environmental receptors. In a pilot study of three aquifers in England, we have examined the extent to which arsenic is released to groundwater under Fe(III)-reducing conditions imposed by contaminant hydrocarbons. Results show that arsenic is locally mobilized in the Chalk to &lt;10 μg/1, in Quaternary gravels to 70 μg/1 and in the Triassic sandstones to 160 μg/1. At the Chalk and Quaternary gravels sites arsenic mobilization is demonstrably linked to reduction of Fe- and Mn-oxides. This is not so at the Triassic sandstone site, where release of arsenic is related to elevated bicarbonate alkalinity. Redox-driven arsenic mobilization at other Triassic sandstone locations is possible. Further work is required on the solid-phase sources of arsenic in the aquifers, and to relate the hydrochemical observations to groundwater hydraulic conditions.

3-D sedimentary architecture of a Quaternary gravel delta (SW-Germany): Implications for hydrostratigraphy

This paper investigates a Quaternary Gilbert-type gravel delta that was formed in an ice-marginal environment at the end of the last glacial period. Outcrop, sediment core and ground-penetrating radar (GPR) studies reveal the sedimentary facies and depositional architecture of the delta that comprises three major units: (1) a 2–5 m thick, gravelly topset with an erosional base, formed by accretion of bedload sheets in a braided river; (2) an up to 40 m thick, steeply inclined (13–35°) foreset, dominated by gravelly lithofacies being the product of cohesionless debris flows and debris falls as well as gravity slides while sandy lithofacies was deposited by traction currents; and (3) a 10–20 m thick, sandy bottomset comprising low-density turbidites. Syn- to postdepositional deformation of parts of the bottomset deposits largely resulted from rapid deposition of overlying gravels and differential loading of the prograding foreset beds. The development of the delta was most likely controlled by a high sediment supply and lake level fluctuations. The overall coarsening-upward succession reflects delta progradation and aggradation into a glaciolacustrine environment. Outcrop sedimentology served as a direct analogue in order to characterise the three-dimensional sedimentary and hydraulic architecture of the nearby gravel-delta aquifer. Applying a multidisciplinary approach, sedimentological, geophysical, and hydrogeological data were integrated within the 3-D modelling package Gocad (Earth Decision Sciences) to develop high-resolution 3-D aquifer models.

Numerical modelling of chloride propagation in the quaternary aquifer of the southern Upper Rhine Graben

The deep groundwater in the quaternary gravel sequence of the southern Upper Rhine Graben locally contains high chloride concentrations near the river Rhine between Fessenheim (France) in the South and Breisach (Germany) in the North. This historical pollution is mainly due to past infiltration from the former brine storage basins of the French potash mines on the “Fessenheim Island” and—to a lesser extent—from the leaching of the salt dumps of the German potash mines in Buggingen and Heitersheim. The spreading of the salt plume was investigated by means of a groundwater model. The aim of the model was to understand the brine movement, the present distribution of chloride as defined by recent hydrochemical investigations, and to select locations for new reconnaissance boreholes. The geological structure was reproduced by a three layer model, which was calibrated for steady state flow conditions. The hydraulic conductivity of the first layer was determined by comparing measured and calculated heads in the model area. The vertical resolution was refined to simulate the density-dependent salt transport processes. The transport of the salt plumes was simulated over a 40-year period, starting at the beginning of brine storage in the 1950s. The relevant transport parameters have been estimated in a sensitivity analysis, where the simulated breakthrough curves of chloride concentration have been compared with the measured data. The results of the groundwater model indicate that brines containing approximately 1 million tons of chloride are still present at the bottom of the aquifer. These highly concentrated salt brines mix with fresh water from the upper part of the aquifer. This dispersive process leads to the formation of a plume of chloride-rich water extending downstream, where pumping wells for several local water supplies are located.

満洲における日本の石油探鉱

Japan hunted oil in Manchuria before World War II. In 1927 Japanese geologist found scattered asphalt in vugs of basalt and quaternary gravel deposits on the north western shore of Hulun Nur (lake), NW Manchuria. Geological Survey of China made a check survey of the area, and found the same asphalt in 1931.After establishment of Manchukuo, geological survey, tortion balance and refraction seismograph survey, and drilling were conducted during 1932-1941. One deep well (1,114 m TD) and 21 core holes were drilled on the northern shore of the lake, but no potential oil were found.That the core hole drilled in Fuxin coal mine hit oil shows in 1938, led to extensive oil exploration program. Geological field party mapped a broad anticline located on east of Fuxin, and the core hole drilled on the anticline recovered some 30 m thick sandstones (100-200 m in depth) impregnating with oil. To explore this oil called on massive drilling campaign, and 47 or more wells were drilled on this anticline, but the reservoir quality was not good enough to establish commercial production. It is noteworthy to point out that the geologists engaged in Fuxin oil exploration thought that the oil came from Jurassic shales deposited in fresh water lake.In 1940 oil shows were found in the water wells near Chengde, and a one-month field check survey was conducted, and mapped a small anticline in Jurassic shales, but no further exploration was conducted.

3-D mapping of segmented active faults in the southern Vienna Basin

In this paper we present novel data on the location and kinematics of seismically active fault segments of the Vienna Basin Transfer Fault System in the southern Vienna Basin. Spatial mapping of active faults and kinematical analyses are based on commercial 3-D reflection seismic data, geomorphological features such as tilted Quaternary river terraces and fault scarps, the geometry of subsided Quaternary basins, and published geodetic data. Accordingly, active faulting in the southern Vienna Basin occurs partly by reactivation of the Miocene fault system related to the formation of the Vienna pull-apart basin between c. 17 and 8 Ma. Two domains of Quaternary and active faults can be distinguished with, (1) predominantly strike-slip and (2) mainly normal faulting. (1) A seismically active NE-striking sinistral strike-slip fault zone with large negative flower structures is mapped at the south-eastern margin of the basin. Subsidence within the reflection seismically imaged flower structure is documented by up to 1000 m of throw since Pannonian times and the accumulation of up to 150 m thick Quaternary gravels. At the surface the fault zone is characterized by en-echelon faults with some prominent scarps. (2) Major E-dipping normal faults branch off from the transfer strike-slip fault system. The normal faults extend into the central and western part of the basin as well as into the urban area of Vienna. Close to Vienna, the normal offset along such a normal fault is at minimum 300 m since Pannonian times. Surface expressions of active normal faulting are tilted Quaternary terraces of the Danube river and tilted ancient land surfaces in the hanging wall of the normal faults. The mapped active normal faults are kinematically linked by a common detachment horizon, which is in contact with the seismically active strike-slip zone along the south-eastern border of the basin. Northeast of the Vienna Basin the seismically active zone continues as a straight line indicating a rather linear transfer fault zone than a pull-apart step over geometry.

Liquefaction-induced structures in Quaternary alluvial gravels and gravelly sediments, NE Brazil

We have identified numerous well-preserved elutriation and fluidization structures probably induced by liquefaction in Quaternary gravels and gravelly sediments of braided fluvial channel deposits in the Rio Grande do Norte and Ceará states, northeastern Brazil. They show evidence of upward-directed water escape after sediment deposition and before sediment compaction. Among the several types of structures observed, the most frequent are pillars, pockets and dikes. These structures range in width from a few centimeters to as much as 4 m, and in height from 60 cm to 4 m. Dikes, pillars and pockets are systematically associated. Clastic dikes vented large quantities of sand to the upper layers or the surface; pebbles and cobbles from the host rock sank into the dikes and formed pillars and pockets. Pockets form the root part; pillars form the intermediate part and dike, the upper part of the composite structure. The morphology of the structures in sectional and plan views indicates a 3D geometry composed of a tabular dike and pillar that present a downward continuous transition to a bowl-shaped pocket. This “stratigraphy” of liquefaction features is different from that usually presented in the current literature. Field data suggest that both the location and the geometry of the features were controlled by sedimentary properties rather than joints and small faults. The size and abundance of these features suggest that they were formed by great events rather than localized mechanisms. Field evidence also indicates that these features are the product of fluidization and elutriation and may have been induced by liquefaction processes associated with seismic shaking. A nonseismic origin related to elutriation processes, however, cannot be ruled out for some of the features.

Lothagam: The Dawn of Humanity in Eastern Africa

Lothagam: The Dawn of Humanity in Eastern Africa. Meave G. Leakey and John M. Harris (eds). 2003. Columbia University Press, New York, 688 p., cloth, ISBN 0-231-11870-8. The fossil site known as Lothagam lies west of the southern end of Lake Turkana (formerly Lake Rudolf) in northern Kenya. The Turkana Basin is famous for its paleontological, paleoanthropological, and archeological records from both east and west of this large lake in the eastern part of the rift system. The Plio-Pleistocene outcrops have been the focus of important research in early hominid taxonomy and evolution, subsistence behavior of early hominids, the tempo and mode of evolution in lacustrine gastropods, vertebrate taphonomy, and mammalian faunal turnover, as well as basic systematic paleontology of terrestrial and aquatic vertebrates. Lothagam is a more restricted area (∼60 km2 of outcrop) and its strata are older—late Miocene to early Pliocene, overlain unconformably by Quaternary sands, gravels, and coquinas. Only a handful of African sites document this time period that includes the Messinian salinity crisis and major floral and faunal changes in many parts of the world. Thus, this monograph is a welcome publication for those who work on the later Neogene, and for paleoanthropologists interested in the ecological context of basal hominids. Paleontological work began at Lothagam in 1967 under Bryan Patterson and continued sporadically until 1989, when Meave Leakey and John Harris co-led interdisciplinary research teams in five field seasons to document the geology, geochronology, paleontology, paleoanthropology, and isotope geochemistry of the sediments and faunas. This monograph covers primarily the results of these field seasons from 1989 to 1993, although there are frequent references to results from the earlier field work. The early field seasons retrieved over 500 specimens, including many vertebrate fossils. The fauna documented in the monograph is based on about 1,500 described …

Accumulation of carbon, nitrogen and phosphorus during soil formation on alder spoil heaps after brown-coal mining, near Sokolov (Czech Republic)

The accumulation of carbon (C), nitrogen (N) and phosphorus (P) and their vertical distribution in the soil profile in relation to site age were studied in a chronosequence of 19 sites on reclaimed spoil heaps from open-cast coal mining, near Sokolov (Czech Republic) and compared to a semi-natural alder forest in the vicinity of the mining area. The reclaimed sites were located on tertiary clay or quaternary gravel–sand spoil material and afforested with alder species ( Alnus glutinosa, A. incana) 4–65 years ago. Rapid accumulation of C and N in the soil profile was registered within 15 years after reclamation. The rate of increase in C and N contents in the whole profile and the thickness of the litter and fermentation layers was slower in 25-year-old and older reclaimed sites. The soil of the semi-natural alder forest was richer in C and N in the 5–10-cm layer compared with 40-year-old (clays) and even with 65-year-old (gravel–sand) reclaimed soils. N accumulated more slowly in comparison with C, especially in deeper (5–10 cm) parts of the profile. Accumulation of organic C resulted in a decrease in substrate pH from alkaline, which was characteristic of young sites, to slightly acidic on older sites. The pH decreased gradually with increasing site age in all soil layers but increased with depth in the soil profile. In contrast to C and N, P content did not significantly change with site age in any layer of the soil profile. The highest amount of P was found in the fermentation layer, but there was no difference to the other layers. The soil profile of the semi-natural sites was richer in P in comparison to 40-year-old reclaimed ones.

Hierarchical dynamic stratigraphy in various Quaternary gravel deposits, Rhine glacier area (SW Germany): implications for hydrostratigraphy

The approach of ‘dynamic stratigraphy’ aims to understand genetic processes that form stratigraphic units in a hierarchy of spatial and temporal scales. This approach was used to investigate Quaternary gravel deposits in terms of their sedimentology and in order to characterize the various sedimentary units in terms of their hydrogeological properties. Facies analysis within 62 gravel pits, laboratory permeability measurements of field samples and geophysical surveys (3-D georadar, 2-D seismic reflection) led to the detection and classification of sedimentary heterogeneity according to the following six scales whereby each scale can be translated into defined hydrostratigraphic units. (1) Particles and pores (micro scale) that reflect depositional and diagenetic fluid dynamics as well as source material behaviour (e.g. grain-size, roundness, lithological composition). This was found to be important for the hydrogeochemistry of groundwater in gravel aquifers (e.g. higher sorption capacity of carbon-rich limestone particles for organic pollutants). (2) Strata (meso scale) contain the recognition of sorting, fabric, texture and stratinomic features, which can give an indication of transport and depositional dynamics. Five major lithofacies groups, for example, were distinguished within fluvial gravel-bed deposits. Their variable hydraulic properties led to their subdivision into 12 hydrofacies types. They form the smallest mappable hydrostratigraphic units, which may result in either preferred pathways for fluid flow or flow barriers. (3) Depositional elements (macro scale) enable reconstruction of sedimentary/geomorphic elements and their dynamics within a depositional system (e.g. gravel-bed braided river systems are dominated by gravel sheet, gravel dunes and scour pool depositional elements). Hydrostratigraphically, the architecture of depositional elements influences the hydraulic connectivity and local permeability structure/distribution within an aquifer body. Five types of depositional elements in fluvial gravel-bed deposits were distinguished and their geometries/dimensions quantified. (4) Facies bodies (mega scale) composed of a stack of depositional elements and strata recording distinct environmental systems and their dynamics (e.g. a coarse-grained prograding delta system). Hydrostratigraphically, facies bodies represent major compartments of an aquifer. Six major types of meltwater-controlled facies bodies were identified in the study area. (5) Genetic sequences (mega scale) reflect the shifts of depositional environments caused by allocyclic changes (e.g. glacial advance recorded by a coarsening upward sequence) or autocyclic changes of landscape shaping events. These sequences may form separate hydrostratigraphic units or aquifer storeys. (6) Basin fill (giga scale) comprising the lateral and vertical stacking of facies bodies and genetic sequences controlled by either long-term glacier dynamics or short term flood events. The regional distribution of permeable gravel units and, for example, less permeable diamicts builds the larger scale hydrostratigraphy.

Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

A 65 km seismic reflection transect was shot in 1998 across Mackenzie Basin to Mount Cook Village, South Island, New Zealand, to provide a detailed image of the crustal structure in the central Southern Alps. The first 5 s (two‐way time (TWT)) of data were processed separately, with a maximum offset of 14 km for each shot, to image the upper 12–15 km of the crust. Data were processed as one continuous section, although the line was physically segmented due to an area of relatively steep topography with no vehicular access. No major, continuous regional‐scale features > 10 km are present in the data, but numerous 2–3 km scale reflections and discontinuities occur which are consistent with the known geology of monotonous greywacke sequences overlying schist. Strong, well‐defined reflections mark the active Irishman Creek Fault and confirm it to be a southeasterly dipping reverse fault with c. 1300–1700 m of Late Cretaceous‐Pleistocene sediments preserved in the footwall and an uplifted greywacke basement “high” in the hanging wall. Some evidence exists for active faults beneath latest Quaternary gravels at the Jollie valley and Tekapo River. Oppositely dipping reflections and discontinuities define a large, c. 15 km wavelength antiform beneath Tasman valley and Mount Cook that is imaged to 10 ± 2 km depths (3.5 s TWT). Two “end‐member” interpretations are consistent with the seismic data observations, velocity models, and constraining features of exposed geology, and extend existing geological cross‐sections to 10–15 km depth. One interpretation assumes imaged structures are primarily backthrusts developed in response to distributed Cenozoic deformation southeast of the Alpine Fault plate boundary, incorporating features observed in contemporary geodetic strain and numerical plate boundary models. The second interpretation assumes structures are mostly Mesozoic, either reactivated or preserved by late Cenozoic deformation. The main difference between the interpretative cross‐sections is the degree to which active structures link into basal detachment and high‐strain zones at depth.

Geology and geochemistry of groundwater within the Taieri Basin, east Otago, New Zealand

The Taieri Basin, in east Otago, New Zealand, is a tectonic depression bounded in part by active faults. The basin is floored and surrounded by Otago Schist that is locally overlain by Cretaceous‐Tertiary sediments and volcanic rocks. Basin fill is predominantly Quaternary gravels, sands, and silts derived from the Otago Schist, and these sediments are at least 150 m thick in places. Aquifers are hosted in the Quaternary sediments, principally gravels and sands. A wedge of estuarine silts and sands (c. 8000–5000 yr BP) extends for 18 km to the downstream outlet of the basin, where it is 25 m thick. Irregularly interfingered fluvial gravels and sands, interbedded with silts, make up the principal aquifers at the upstream end of the basin. This complex fluvial sequence dips beneath the estuarine wedge. The estuarine wedge acts as a confining layer for groundwater in the underlying aquifers, and artesian water bores occur in the lower reaches of the basin. Alluvial fans on the western side of the basin are conduits for recharge from adjacent schist ranges. Alluvial fans on the southeastern side of the basin have their toes truncated by active faults, and recharge from these fans is structurally inhibited. Ca, Mg, and bicarbonate contents of Taieri Basin surface waters and groundwaters indicate chemical interaction with calcite and chlorite of Otago Schist, in basement and/or aquifer sediment clasts. All waters have Na and Cl− contents dominated by rainout of NaCl in marine aerosols. Surface waters have lower Na and Cl− than groundwaters. Numerical modelling of groundwater flow at the upstream end of the basin can reproduce field‐based inferences of flow patterns and together indicate southwestward flow beneath the estuarine wedge confining layer. This is significant because it implies low‐concentration NO3+NO2 contamination of shallow aquifers at the upstream end of the basin will be carried to groundwaters in the southwest.

Sulfate Reduction at a Lignite Seam: Microbial Abundance and Activity.

In a combined isotope geochemical and microbiological investigation, a setting of multiple aquifers was characterized. Biologically mediated redox processes were observed in the aquifers situated in marine sands of Tertiary age and overlying Quaternary gravel deposits. Intercalated lignite seams define the aquitards, which separate the aquifers. Bacterial oxidation of organic matter is evident from dissolved inorganic carbon characterized by average carbon isotope values between ?18.4 per thousand and ?15.7 per thousand (PDB). Strongly positive sulfur isotope values of up to +50 per thousand (CTD) for residual sulfate indicate sulfate reduction under closed system conditions with respect to sulfate availability. Both, hydrochemical and isotope data are thus consistent with the recent activity of sulfate-reducing bacteria (SRB). Microbiological investigations revealed the presence of an anaerobic food chain in the aquifers. Most-probable-number (MPN) determinations for SRB and fermenting microorganisms reached highest values at the interface between aquifer and lignite seam (1.5 x 103 cells/g sediment dry mass). Five strains of SRB were isolated from highest MPN dilutions. Spore-forming bacteria appeared to dominate the SRB population. Sulfate reduction rates were determined by the 35S-radiotracer method. A detailed assessment indicates an increase in the reduction rate in proximity to the lignite seam, with a maximum turnover of 8.4 mM sulfate/a, suggesting that lignite-drived compounds represent the substrate for sulfate reduction.

Folding and the formation of bedding‐parallel faults on the western limb of Grey Valley Syncline near Blackball, New Zealand

Fresh roadside exposures of faults on the western limb of Grey Valley Syncline near Blackball (West Coast, South Island) provide new information on the kinematics of folding since c. 2.5 m.y. ago. These faults are parallel to, or dip within 10° of, steep bedding (60‐>90°) in Pliocene Old Man Group, and offset two gently dipping Quaternary gravel formations. Deformed gravel sequences record reverse‐fault displacements of 0.5‐c. 80 m and bed dips of 5–50°, which accumulated over the last c. 450 000 yr. The dip of Quaternary gravel beds and fault displacements decrease up‐sequence, suggesting that gravel deposition and deformation were synchronous. Bedding‐parallel faults in Old Man Group are typically located at the boundaries between mudstone beds and coarser grained sandstone or conglomerate beds. The bedding‐parallel orientations of the faults, fault slickenside striations approximately normal to the hinge of Grey Valley Syncline, and total fault displacement across the fold limb are consistent with these structures being flexural‐slip faults formed during folding. Small amounts of bedding‐parallel shortening and extension within Pliocene strata between faults indicate non‐rigid deformation of the limb and departure of folding from an ideal flexural‐slip model.

Torlesse greywacke and Haast Schist source for Pliocene conglomerates near Reefton, New Zealand

Late Miocene‐Quaternary conglomerates in the Inangahua valley record provenance changes which can be related to tectonic activity in the northern South Island. Conglomerate clasts within the early Miocene Rotokohu Coal Measures comprise mainly metamorphosed quartzose greywacke with subordinate granitoids, consistent with an essentially local clastic source from west of the Alpine Fault. In contrast, conglomerates of the late Pliocene Old Man Group (Larry Schist and Cronadun Conglomerates) are dominated by Rakaia Terrane (Older Torlesse) greywacke and Haast Schist clasts from east of the Alpine Fault, which is c. 50 km southeast of the study area. A complete range of textural and metamorphic grades is present, confirming that oligoclase zone Alpine Schist was being exhumed during Pliocene time. The 50–70°‐dipping Pliocene beds are unconformably overlain by subhorizontally dipping late Quaternary gravels (Tophouse and Speargrass Formations), which, like the Miocene conglomerates, are composed of quartzose greywacke and granitoid rock types. Significant Quaternary deformation isolated the Inangahua valley area from the Southern Alps and resulted in a return to a local clastic provenance.