Bedrock Contact Research Articles

Multivariate examination of the sediment-deficient southeast Australian continental shelf

Continental shelves are the most morphologically variable element within the source-to-sink system owing to the numerous processes that influence their formation. A recent multivariate analysis of a global compilation of modern continental shelf data showed that much of the variability is related to tectonic setting, the degree to which the shelf has been glaciated, and carbonate production. While these factors play first-order roles in determining the morphology of shelves, other controlling mechanisms such as siliciclastic sediment supply, wave and tidal energy, bedrock lithology, and sea-level fluctuations are not as well understood. Here, we report findings from a detailed investigation of the southeast Australian shelf that explored how sediment distribution, wave energy, and bedrock lithology influence shelf morphology. The high-resolution analysis suggests that the southeast Australian shelf has 11 distinct shelf types. No strong relationships exist between the shelf attributes or shelf type with their onshore catchments. However, a substantial section boundary correlates with a bedrock contact between the Sydney Basin in the south and the New England Orogen to the north. South of this boundary, we propose that the shelf morphology reflects transgression with low sediment supply, whereas to the north, the morphology reflects transgression with higher sediment input. Although several factors contributed to this difference in shelf morphology, we suggest that sediment distribution and retention due to the active wave climate during the most recent transgression likely played a vital role.

Applications of remote sensing and near-surface geophysical methods for the investigations of a morainic ridge susceptible to landslide. A case study from the high lands of the Himalayas, Pakistan

Numerous factors are necessary to comprehensively comprehend the complex phenomenon of landslides and, ultimately, facilitate effective measures for their protection and management. This paper investigates the case history of the Maicher landslide in the Nagar District (Northern Pakistan) by using a multidisciplinary combined technique approach based on conventional geophysical measurements and modern remote sensing techniques. The first technique is to identify the rate of deformations with real-time landslide monitoring sensors and satellite images. Care is devoted to data acquisition and processing using the ad-hoc methodology to minimize errors and ensure high reliability. For the second technique (electrical resistivity tomography, surface waves measurements, and ground-penetrating radar), convenient methods for landslide characterization are used and herein presented. The combination of information taken from the different techniques allows the results to be validated by conventional and modern approaches and enhances the accuracy of each method. The results suggest that shear surface planes at a depth of 35–40 m have developed over time. The analysis also provided information on controlling factors and possible expansion of the different slope sections. This work highlighted that geophysical parameters (shear wave velocity Vs, dielectric constant ε, and resistivity value ρ) could provide reliable qualitative and quantitative information about the shallow layer, bedrock contacts, possible sliding surface and subsurface water saturation, and seepage system of a landslide. The geophysical results reflect an overview of the geological model and are effective in terms of the slope stabilization process.

Drilling Overdeepened Alpine Valleys (ICDP-DOVE): quantifying the age, extent, and environmental impact of Alpine glaciations

Abstract. The sedimentary infill of glacially overdeepened valleys (i.e., structures eroded below the fluvial base level) is an excellent but yet underexplored archive with regard to the age, extent, and nature of past glaciations. The ICDP project DOVE (Drilling Overdeepened Alpine Valleys) Phase 1 investigates a series of drill cores from glacially overdeepened troughs at several locations along the northern front of the Alps. All sites will be investigated with regard to several aspects of environmental dynamics during the Quaternary, with focus on the glaciation, vegetation, and landscape history. Geophysical methods (e.g., seismic surveys), for example, will explore the geometry of overdeepened structures to better understand the process of overdeepening. Sedimentological analyses combined with downhole logging, analysis of biological remains, and state-of-the-art geochronological methods, will enable us to reconstruct the erosion and sedimentation history of the overdeepened troughs. This approach is expected to yield significant novel data quantifying the extent and timing of Middle and Late Pleistocene glaciations of the Alps. In a first phase, two sites were drilled in late 2021 into filled overdeepenings below the paleolobe of the Rhine Glacier, and both recovered a trough filling composed of multiphase glacial sequences. Fully cored Hole 5068_1_C reached a depth of 165 m and recovered 10 m molasse bedrock at the base. This hole will be used together with two flush holes (5068_1_A, 5068_1_B) for further geophysical cross-well experiments. Site 5068_2 reached a depth of 255 m and bottomed out near the soft rock–bedrock contact. These two sites are complemented by three legacy drill sites that previously recovered filled overdeepenings below the more eastern Alpine Isar-Loisach, Salzach, and Traun paleoglacier lobes (5068_3, 5068_4, 5068_5). All analysis and interpretations of this DOVE Phase 1 will eventually lay the ground for an upcoming Phase 2 that will complete the pan-Alpine approach. This follow-up phase will investigate overdeepenings formerly occupied by paleoglacier lobes from the western and southern Alpine margins through drilling sites in France, Italy, and Slovenia. Available geological information and infrastructure make the Alps an ideal area to study overdeepened structures; however, the expected results of this study will not be restricted to the Alps. Such features are also known from other formerly glaciated mountain ranges, which are less studied than the Alps and more problematic with regards to drilling logistics. The results of this study will serve as textbook concepts to understand a full range of geological processes relevant to formerly glaciated areas all over our planet.

A study of the disaster model and movement process simulation of debris flow in the Zhixi River of Deqin County

Deqin County is one of the areas suffering the most serious geological disasters in Yunnan Province. Based on the development characteristics of the debris flow in the Zhixi River, this study identifies five disaster-generating modes for the debris flow in the Zhixi River: Collapse - clastic flow, rock landslide-clastic flow, soil-rock mixture landslide-clastic flow, loose accumulation layer -bedrock contact surface landslide-clastic flow and landslide in loose accumulation layer-clastic flow. The FLO-2D model is used to simulate the debris flow movement in the Zhixi River under the rainstorm cycle of 10, 20, 50 and 100 years, and the maximum flow velocity, the maximum accumulation depth, the accumulation distance and the volume scale in different rainfall return periods are quantitatively analyzed. The results show that the debris flow is characterized by high acceleration, fast velocity, strong destructive force, and long circulation area when it erupts. When the debris flow occurs once in 100 years, its maximum velocity reaches 3.07 m/s, the maximum mud depth is 2.27 m , the volume of debris flow washed out is 84419 m3, and the disaster area reaches 91600 m2. The results provide quantitative data reference for the prevention and control of debris flow disasters in Zhixi River, and also provides an important basis for the design of disaster prevention and mitigation projects in Deqin County.

Holocene to latest Pleistocene incremental slip rates from the east-central Hope fault (Conway segment) at Hossack Station, Marlborough fault system, South Island, New Zealand: Towards a dated path of earthquake slip along a plate boundary fault

AbstractGeomorphic field and aerial lidar mapping, coupled with fault-parallel trenching, reveals four progressive offsets of a stream channel and an older offset of the channel headwaters and associated fill terrace–bedrock contact at Hossack Station along the Conway segment of the Hope fault, the fastest-slipping fault within the Marlborough fault system in northern South Island, New Zealand. Radiocarbon and luminescence dating of aggradational surface deposition and channel initiation and abandonment event horizons yields not only an average dextral rate of ∼15 mm/yr since ca. 14 ka, but also incremental slip rates for five different time periods (spanning hundreds to thousands of years) during Holocene to latest Pleistocene time. These incremental rates vary through time and are, from youngest to oldest: 8.2 +2.7/−1.5 mm/yr averaged since 1.1 ka; 32.7 +∼124.9/−10.1 mm/yr averaged over 1.61–1.0 ka; 19.1 ± 0.8 mm/yr between 5.4 and 1.6 ka; 12.0 ± 0.9 mm/yr between 9.4 and 5.4 ka, and 13.7 +4.0/−3.4 mm/yr from 13.8 to 9.4 ka, with generally faster rates in the mid- to late Holocene relative to slower rates prior to ca. 5.4 ka. The most pronounced variation in rates occurs between the two youngest intervals, which are averaged over shorter time spans (≤1700 yr) than the three older incremental rates (3700–4500 yr). This suggests that the factor of ∼1.5× variations in Hope fault slip rate observed in the three older, longer-duration incremental rates may mask even greater temporal variations in rate over shorter time scales.

Distribution and features of landslides in the Tianshui Basin, Northwest China

Landslides in Tianshui Basin, Gansu Province, Northwest China, severely affect the local population and the economy; therefore, understanding their evolution and kinematics is of great interest for landslide risk assessment and prevention. However, there is no unified classification standard for the types of loess landslides in Tianshui. In this study, we explored the landslide distribution and failure characteristics by means of field investigation, remote sensing interpretation, geological mapping, drilling exploration and shearwave velocity tests, and established a database of Tianshui landslides. Our analysis shows that shear zones in mudstone usually develop in weak intercalated layers. Landslides occur mainly along the West Qinling faults on slopes with gradients of 10° to 25° and on southeast- and southwest-facing slopes. These landslides were classified into five types: loess landslides, loess–mudstone interface landslides, loess flow-slides, loess–mudstone plane landslides and loess–mudstone cutting landslides. We discussed the evolution and failure process of each landslide type and analyzed the formation mechanism and motion characteristics of large-scale landslides. The analysis results show that the landslides in the study area are characterized by a gentle slope, long runout and high risk. The relationship between the runout L and the vertical drop H of the large-scale landslides in the study area is L > 4H. There are good correlations between the equivalent friction coefficient of largescale landslides and their maximum height, runout, area and volume. The sliding zone of large-scale landslides often develops in the bedrock contact zone or in a weak interlayer within mudstone. From microstructure analysis, undisturbed mudstone consists mainly of small aggregates with dispersed inter-aggregate pores, whereas sheared clay has a more homogeneous structure. Linear striations are well developed on shear surfaces, and the clay pores in those surfaces have a more uniform distribution than those in undisturbed clay.

Grout Curtain Construction At Bolivar Dam, Ohio

Abstract Bolivar Dam, located in eastern Ohio, is an embankment dam constructed by the U.S. Army Corps of Engineers in 1937 as part of the Muskingum River Basin Project for flood control. As a result of seepage concerns observed at the dam during several flood events over the life of the project, seepage reduction measures, including a partial-depth seepage barrier wall through the embankment and a grout curtain in the left abutment, were designed and constructed. These dam safety modifications were constructed between 2014 and 2016. During flood events, Bolivar Dam experiences excessive seepage through the glacial outwash foundation as well as through a network of open joints within the left bedrock abutment. Seepage in the bedrock abutment could erode/scour the dam embankment at the bedrock contact, potentially leading to dam failure. To lower this potential risk of dam failure in the left abutment, a grout curtain was constructed between the new seepage barrier wall and an existing grout curtain across the emergency spillway. The new grout curtain is designed to impede groundwater seepage, resulting in reduced groundwater velocity/energy downstream of the grout curtain, thereby decreasing its potential to scour or transport fine-grained embankment material. The double-line grout curtain is approximately 65 ft (19.8 m) deep and 400 ft (121.9 m) long and was completed in November 2015 as part of a major dam safety modification project. Two thin limestone units encountered during drilling proved to be problematic and posed various challenges during construction. It was common during drilling to lose water circulation within the vicinity of these limestone units, which then required the use of downstaged grouting methods. The majority of the grout volume for the project was placed within these downstaged intervals. This article presents the risk-informed decisions that were made during both design and construction of the grout curtain and includes various lessons that were learned during this process.

Influence of topography and volume on mobility of loess slides within different slip surfaces

Mobility of loess slides has been evaluated using an empirical approach based on a detailed field investigation and remote sensing image interpretation. For all the loess slides, the affected area increases with increasing loess slides volume. From the statistical analysis it is seen that the travel distance has an increasing trend, while the coefficient of friction has a decreasing trend with the increase in landslide volume. Landslide volume determine its travel distance and coefficient of friction, that can be shown with a regression equation having simple power law form, which indicates that the mobility of loess slides increases with increasing volumes. By analogy, the greater the energy of the sliding loess mass, the stronger is the mobility of loess slide. A plot of the travel distance of loess slide versus fall height shows a significant positive power law correlation indicating the increase in mobility of loess slide with fall height. The negative power law correlation observed between the travel distance and slope inclination implies that the mobility decreases with the increase of slope inclination. Three datasets of loess slides characterized by different slip surfaces were distinguished. The red clay contact landslides have the strong mobility than landslides in loess and bedrock contact.

Slope height and slope gradient controls on the loess slide size within different slip surfaces

Based on intensive landslide surveys and interpretation of remote sensing images, we established a loess slide inventory map of the Yan’an region in northern Shaanxi province, China. According to their slip surfaces, we grouped loess slides into three categories: landslides in loess, red clay contact landslides, and bedrock contact landslides. Results indicate an obvious power law relationship among loess slide length, area, and volume. Moreover, landslide size depends remarkably on the slip surface. The average area and length of red clay contact landslides are 5.52 and 2.45 times larger than those of landslides in loess. Intermediate-slope relative height and slope gradient have a prominent role in landslide formation. The size distribution of loess slides was examined with respect to the slope height and slope gradient. The analysis revealed that slope height and slope gradient were the most dominant controlling factors for loess slide size. The loess slide became larger with the increasing relative height of the slope, and there is an obvious linear or power law relationship between loess slide size and slope relative height. On the contrary, landslide size gradually decreases as the slope gradient increases at a certain level.

Incorporating ancillary data into the inversion of airborne time-domain electromagnetic data for hydrogeological applications

Abstract Helicopter time-domain electromagnetic (HTEM) surveys often suffer from significant inaccuracies in the early-time or near-surface data—a problem that can lead to errors in the inverse model or limited near-surface resolution in the event that early time gates are removed. We present an example illustrating the use of seismic data to constrain the model recovered from an HTEM survey over the Spiritwood buried valley aquifer in Manitoba, Canada. The incorporation of seismic reflection surfaces results in improved near-surface resistivity in addition to a more continuous bedrock interface with a sharper contact. The seismic constraints reduce uncertainty in the resistivity values of the overlying layers, although no a priori information is added directly to those layers. Subsequently, we use electrical resistivity tomography (ERT) and borehole data to verify the constrained HTEM models. Treating the ERT and borehole logs as reference information, we perform an iterative time-shift calibration of the HTEM soundings to achieve regional-scale consistency between the recovered HTEM models and the reference information. Given the relatively small time-shifts employed, this calibration procedure most significantly affects the early-time data and brings the first useable time gate to a time earlier than the nominal first gate after ramp off. Although time shifts are small, changes in the model are observed from the near-surface to depths of 100 m. Calibration is combined with seismic constraints to achieve a model with the greatest level of consistency between data sets and, thus, the greatest degree of confidence. For the Spiritwood buried valley, calibrated and constrained models reveal more structure in the valley-fill sediments and increased continuity of the bedrock contact.

Looking through drumlins: testing the application of ground-penetrating rada

AbstractGround-penetrating radar (GPR) is becoming a commonly applied technique in geomorphology. However, its use in the study of subglacial bedforms has yet to be fully explored and exploited. This paper presents the results of a GPR feasibility study conducted on a drumlinized terrain in Cumbria, UK, where five drumlins were investigated using multiple radar antenna frequencies. The site was selected for the presence of nearby bedrock outcrops, suggesting a shallow drumlinized diamict–bedrock contact and a permeable lithology. Despite the clayey sediment and unfavourable weather conditions, a considerable penetration depth of ~12 m was achieved when using a 50 MHz antenna, with a separation of 1 m, trace spacing of 1 m and 128-fold vertical stack. Results indicate that the drumlinized diamict is in direct erosional contact with the bedrock. While the internal drumlin geometry is generally chaotic on the stoss side, evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side. The inter-drumlin areas comprise ~4 m of infill sediment that masks part of the original drumlin profile. Overall, this study indicates that GPR can be deployed successfully in the study of glacial bedform sedimentary architecture.

Oxidation of Nonaqueous Phase Trichloroethylene with Permanganate in Epikarst

In situ chemical oxidation (ISCO) is one of the effective technologies used for source zone remediation of dense nonaqueous phase liquids (DNAPLs) such as chlorinated solvents in the subsurface environments. In karst systems, DNAPL source zones reside in epikarst where the contaminant is generally trapped in the soil or at the carbonate bedrock contact. The efficiency of oxidation of residual trichloroethylene (TCE) masses trapped in such environments by potassium permanganate was characterized via a series of batch, column, and flow cell experiments. The performance of oxidation reaction was evaluated by considering numerous factors, such as soil oxidant demand, mineralogy, heterogeneity and kinetics of dissolution, and oxidation processes. Batch experiments showed that at the low permanganate concentrations (2-10 mM), the rate of TCE oxidation by permanganate (0.86–5.1 × 10−3 s−1) was in a range similar to the rate of soil oxidant consumption (0.55–2.1 × 10−3 s−1); however, at the high permanganate concentrations (40–120 mM), this rate was about two orders of magnitude higher than the rate of soil oxidant consumption (2.4–2.9 × 10−4 s−1). In addition, at the high oxidant concentrations, the oxidation kinetics of pure phase TCE (1.1–1.5 × 10−3 s−1) was limited by the dissolution kinetics (1.4–1.6 × 10−3 s−1). Column and flow-cell experiments showed that significant fraction of residual TCE masses trapped in the soil and at the bedrock contact were oxidized by ISCO technology using permanganate, however the efficiency of ISCO was limited by the rate-limited kinetics of TCE dissolution and the desorption. The results of flow-cell experiments also indicated that instead of continuous permanganate flushing of the TCE source zones, periodic permanganate injections providing sufficient time and oxidant dosage would enhance the performance of ISCO in epikarst environment much efficiently.

Progradational Holocene carbonate tidal flats of Crooked Island, south‐east Bahamas: An alternative to the humid channelled belt model

AbstractThe stratigraphic record of many cratonic carbonate sequences includes thick successions of stacked peritidal deposits. Representing accumulation at or near sea‐level, these deposits have provided insights into past palaeoenvironments, sea‐level and climate change. To expand understanding of carbonate peritidal systems, this study describes the geomorphology, sedimentology and stratigraphy of the tidal flats on the Crooked‐Acklins Platform, south‐east Bahamas. The Crooked Island tidal flats extend continuously for ca 18 km on the platformward flank of Crooked Island, reaching up to 2 km across. Tidal flats include four environmental zones with specific faunal and floral associations and depositional characteristics: (i) supratidal (continuous supratidal crust and pavement); (ii) upper intertidal, with the mangrove Avicennia germinans and the cyanobacteria Scytonema; (iii) lower intertidal (with the mangrove Rhizophora mangal) and (iv) non‐vegetated, heavily burrowed subtidal (submarine). These zones have gradational boundaries but follow shore‐parallel belts. Coring reveals that the thickness of this mud‐dominated sediment package generally is <2 m, with depth to Pleistocene bedrock gradually shallowing landward. The facies succession under much of the tidal flat includes a basal compacted, organic‐rich skeletal‐lithoclast lag above the bedrock contact (suggesting initial flooding). This unit grades upward into rhizoturbated skeletal sandy mud (subtidal) overlain by coarsening‐upward peloid‐foraminifera‐gastropod muddy sand (reflecting shallowing to intertidal elevations). Cores from landward positions include stacked thin indurated layers with autoclastic breccia, root tubules and fenestrae (interpreted as supratidal conditions). Collectively, the data reveal an offlapping pattern on this prograding low‐energy shoreline, and these Holocene tidal flats may represent an actualistic analogue for ancient humid progradational tidal flats. Nonetheless, their vertical facies succession is akin to that present beneath channelled belt examples, suggesting that facies successions alone may not provide unambiguous criteria for prediction of the palaeogeomorphology, lateral facies changes and heterogeneity in stratigraphic analogues.

Effectiveness of exceptional rainstorms on a small Mediterranean basin

A comprehensive investigation of rainstorms and their consequent impacts on landscape evolution is geomorphologically important, but only scant information may be available on exceptional events, because parameters on synoptic conditions, rainstorm, landforms and hydrology for such events may be incomparable with previous knowledge. We studied an exceptional storm on April 2, 2006, in the Ramot Menashe region, Israel. Our investigation of rainfall, landslides, debris flows and channel suggests the effectiveness of such an event on the development of basin-scale morphology.The storm caused damage and casualties although it covered relatively narrow strips. Neither direct rainfall nor runoff measurements exist for the most severely affected area of Ramot Menashe, but the geomorphologic evidence combined with high-resolution meteorological radar data provides the basic understanding of the processes and hazardous conditions which prevailed at the time. In the storm core, based on estimation from meteorological radar data, 263mm of rain fell within 3h with a maximum intensity of 220mmh−1 for 10min, triggering both sporadic landslides at the soil/bedrock contact on the upper slopes and widespread landslides at the fractured/massive bedrock contact on the lower slopes. The 1st order channels on the alternation of chalk and marl also underwent erosion, and the produced sediment deposited on alluvial fans at the confluence with the main channel. The specific peak discharges for catchment size of 0.3–10km2 were 11 to 73m3s−1km−2, higher than any recorded floods in the Mediterranean climatic region of Israel. The effectiveness of the flood for geomorphic work, represented by shear stress and stream power per unit boundary area reached 87–398Nm−2 and 212–2134Wm−2, respectively. This kind of analysis can be applied to hazard prediction in other areas under similar geomorphological conditions.

THE NATURE OF THE ALLUVIAL DIAMOND DEPOSITS OF THE VENTERSDORP DISTRICT, NORTHWEST PROVINCE, SOUTH AFRICA

Research Article| September 01, 2009 THE NATURE OF THE ALLUVIAL DIAMOND DEPOSITS OF THE VENTERSDORP DISTRICT, NORTHWEST PROVINCE, SOUTH AFRICA T.R. MARSHALL; T.R. MARSHALL Explorations Unlimited, PO Box 6578, Homestead, 1412, South Africa e-mail: marshall.tania@gmail.com Search for other works by this author on: GSW Google Scholar G.A. NORTON G.A. NORTON Rockwell Diamonds Inc, South Africa e-mail: glenn@rockwelldiamonds.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2009) 112 (2): 109–124. https://doi.org/10.2113/gssajg.112.2.109 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation T.R. MARSHALL, G.A. NORTON; THE NATURE OF THE ALLUVIAL DIAMOND DEPOSITS OF THE VENTERSDORP DISTRICT, NORTHWEST PROVINCE, SOUTH AFRICA. South African Journal of Geology 2009;; 112 (2): 109–124. doi: https://doi.org/10.2113/gssajg.112.2.109 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Numerous models have been presented over the years to explain the nature and distribution of the diamondiferous alluvial deposits on the dolomite plain in the Ventersdorp district. These have, typically, highlighted the importance of karst processes both prior to, and coincidental with, deposition of the gravels. Karst has a particularly distinctive method of landscape sculpturing arising from greater rock solubility in natural waters than elsewhere. Solution may not always be the prevalent process, or even the dominant one, but it does play a greater role than in other kinds of landscape. It’s most critical effect lies in the enlargement of underground voids, causing increased permeability of the rock. As more water passes through the rock, the voids grow to become caves. Greater solubility of a substance is accompanied by a greater liability for the dissolved load to come out of solution, so chemical deposition can contribute in an important way to landform building in karst terranes, both on the surface and underground. In karst landscapes, the generally harmonious pattern of surficial drainage is broken up and closed depressions take over the landscape to varying degrees. Superficially, it lacks organisation and the drainage system has to be sought underground.The general stratigraphic sequence of the alluvial gravel deposits of the Ventersdorp district is shown to comprise an economic lower gravel (Lower Gravel Package and the Transition Zone Gravels), a sub-economic clay-rich unit and an economic upper gravel unit (Upper Gravel Package), all of which have been overlain by Kalahari clays/sands and the Hutton soil profile (including colluvial diamondiferous gravel units). It is fundamental to the model to appreciate that the gravels were deposited in a karst system where:The dolomitic bedrock contacts with the gravels may be steeply dipping to verticalThe mode of gravel deposition is not typical fluvial alluvialPeriodic subsidence has taken place during depositionDeposition has taken place over a long time (at least since the Mesozoic), resulting in the build-up of a very thick gravel sequence.Furthermore, the runs are often controlled by cave formation along linear, fracture or fault zones which allowed the increased flow of ground water that led to higher than normal levels of dissolution of the dolomites and resulted in the formation of sinkholes and channels. Some of the sinkholes are more than 100 m deep. The sedimentary infill of the sinkholes is variable and depends on the depth and subsidence history of each individual sinkhole.Numerous issues have been raised with respect to the potential sources of both the diamonds and the gravels. Among the major problems is the limited variation in grade and quality of the diamonds over a significant area and the relatively high grade of the lower gravels that show a distinct lack of reworking and re-concentration throughout their vertical extent. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Snowmelt‐driven macropore flow and soil saturation in a semiarid forest

AbstractLateral subsurface flow is generally assumed to occur as a result of the development of a saturated zone above a low‐permeability interface such as at the soil–bedrock contact, and it is often augmented by macropore flow. Our objective was to evaluate the development of lateral subsurface flow and soil saturation at a semiarid ponderosa pine forest in New Mexico with respect to the conceptual model of saturation building above the soil–bedrock contact. At this site, we have long‐term observations of the water budget components, including lateral flow. A 1·5 m deep by 7 m long trench was constructed to observe lateral subsurface flow and development of saturation directly. Our observations are based on flow resulting from a melting snowdrift. The edge of the drift was about 7 m upslope from the trench. Lateral subsurface flow only occurred from root macropores in the Bt soil horizon. Saturation developed and grew outward from flowing root macropores, rather than growing upward from the soil–bedrock interface. This macropore‐centred saturation resulted in a highly heterogeneous distribution of water content until enough macropores began flowing and individual macropore saturated zones grew large enough to coalesce and saturate large volumes of the soil. Our observations are based on one snowmelt event and a relatively short hillslope flow path, and thus do not represent a full range of hydrologic conditions. Nevertheless, the observed behaviour did not conform to the traditional model of soil–bedrock control of saturation and lateral flow. Copyright © 2004 John Wiley & Sons, Ltd.

Mudslides Developed on Waitemata Group Rocks, Tawharanui Peninsula, North Auckland

ABSTRACTOn Tawharanui Peninsula mudslides occur in soils developed on Waitemata Group flysch similar to that underlying much of the Auckland metropolitan area. Mudslides are shallow soil failures with a bowlshaped source area, an elongated track, and an accumulation zone. Movement may continue for decades to centuries. Modelling suggests that failure of saturated soils occurs along a basal surface defined by the regolith / bedrock contact; rates of movement and time of initiation have not been established. Reactivation or increased mobility of mudslides due to climatic change represents an increased hazard for infrastructure in Auckland City.

Hillslope and wetland hydrodynamics in a tussock grassland, South Island, New Zealand

AbstractPrevious research on two experimental catchments at the Glendhu Forest (Otago) showed that runoff dynamics from this landscape differ substantially from other areas in New Zealand. In particular, the recession curves from these catchments have two distinct phases: a rapid decline for a period of several hours, followed by a slow decline that may last many days to weeks. In the current study, soil water matric potential measurements indicated that perched saturated zones develop very rapidly in the surface (0–30 cm) soil horizons. Deep (50–250 cm) loess horizons remain moist throughout the year and may be near or at saturation at the bedrock contact, and respond only slowly to rain events. Soils in the middle horizons are often drier than the surface soil above or the deep soils below. Throughflow measurements showed that storm runoff occurs primarily as interflow through a retentive but pervious mat of mosses, litter, and roots at the soil surface. Despite the presence of abundant earthworm burrows, less water is transmitted through the soil horizon just below this surface mat. Only a small fraction of stormflow was transmitted through the deep, mineral horizons. However, baseflow could be sustained for long periods (up to 4 weeks) from unsaturated flow of soil moisture stored in the deep loess horizons. Water storage in bog peats and associated riparian soils (which are numerous in this area) is insufficient to sustain baseflow for more than a few days. The results from this study are consistent with previous hydrometric, soils, and isotopic observations and suggest that any land‐use practices that disrupt the surface soil horizons are likely to change the runoff characteristics of this area. Copyright © 2001 John Wiley & Sons, Ltd.

Seismic image of the ice–bedrock contact at the Lobbia glacier, Adamello Massif, Italy

In the framework of a geophysical study to investigate the ice thickness on the Lobbia Glacier located in the Adamello Massif, Italy, we used multifold seismic data to constrain a joint gravity survey covering an area of about 3.7 km 2. Two seismic lines were recorded above the maximum ice thickness as estimated from single-fold seismic data and gravity data collected during previous surveys. A 1071-m profile was laid out in the flat part of the glacier, aligned cross glacier. A short transverse profile of 322-m was recorded for control purposes. A strong reflection arising at the contrast between the ice and the granite bedrock was recorded on all the shots. Nevertheless, seismic images produced at intermediate stages of the data processing show that an appropriate processing sequence is fundamental to obtaining a good quality image. The processing sequence includes f–k filtering, band-pass filtering, residual static corrections, and stack. Post-stack deconvolution and migration were essential to improve vertical and horizontal resolutions and therefore to image irregularities of the bedrock surface. The contact between the ice and the granite substratum appears as a strong reflection at a maximum two-way time of 105 ms, corresponding to a depth of about 186 m.

Tertiary-age diamondiferous fluid deposits of the lower Orange River valley, Southwest Africa

The Orange River, the principal conduit for transportation of diamonds from the southern African interior to the Atlantic coast, has within its lower valley two recognized suites of gravel terraces in which part of the passing diamond population has been trapped. Both suites lie on eroded bedrock and both are downstream thickening-and-fining coarse clastic wedges. The older and higher terrace suite, comprising the Arries Drift Gravel Formation, is early mid-Miocene in age (19-17 Ma) and is referred to locally as the Proto-Orange terrace. The lower and younger terrace suite, locally known as the Meso-Orange terrace, has not yet been dated but is considered to be Plio-Pleistocene in age. Middle to Late Proterozoic bedrock, underlying both suites of terrace deposits, was deeply eroded during the incision of the Orange River which began in Late Cretaceous times. The Proto-Orange terraces, richer in diamonds than the younger Mesozoic deposits, form low-grade but large-average, diamond-size, gem quality placers which are mainly discussed here. Diamonds are best concentrated in cobble-boulder basal gravels on or close to the rough bedrock contact in three main trapsite settings: scour pools hosting oversize (obstacle) clasts return grades of 10 to 40 carats per hundred ton (cpht); push bars, where the grades range from 10 to 55 cpht; and bedrock highs with their associated oversize clasts, yielding grades of 6 to 12 cpht, do not concentrate diamonds to the same extent as the two former trapsites, but occur over a wider area and thus potentially enrich a greater volume of gravel. The average diamond (stone) size in these three trapsites is also upgraded, ranging from 1.5 to 2.8 carats per stone (ct/stn). The key factors in concentrating diamonds at the bedrock-gravel interface are turbulence scale and intensity created by the rough boundary conditions and the presence of fixed bedrock sites of turbulence, which are stable enough to initiate and retain around them the slow and stable growth of gravel that hosts the concentrated diamonds. The upper, more mobile gravels have far lower concentrations of diamond than the basal gravels, with grades dropping to an average of 0.3 cpht and stone size decreasing to 1 to 1.3 ct/stn. Sites of higher concentration in the upper gravels of the Proto-terraces are bar platforms/riffles (1 to 3 cpht and average stone sizes of 1.1 to 1.3 ct/stn) and bar heads (0.7 to 1.5 cpht and average stone sizes of 1 to 1.3 ct/stn) while bar tails are practically barren. Although the vertical decline in diamond concentration may be attributed to a decrease in diamond supply during the aggradation of the Proto-Orange gravels, the younger, lower grade Mesozoic gravels also display a similar vertical profile. Hence, diamond concentration in the lower Orange River deposits is influenced positively by the occurrence and nature of the localized fixed bedrock trapsites. From an exploration point of view, these fixed trapsites are more difficult to locate, often requiring extensive drilling, whereas the areally extensive but low-grade upper gravels are relatively easy to delineate. During exploitation, the localized but higher grade fixed trapsites provide a reliable source of above average-sized diamonds in an otherwise low-grade.