Testing the suitability of geologic frameworks for extrapolating hydraulic properties across regional scales

The suitability of geologic frameworks for extrapolating hydraulic conductivity (K) to length scales commensurate with hydraulic data is difficult to assess. A novel method is presented for evaluating assumed relations between K and geologic interpretations for regional-scale groundwater modeling. The approach relies on simultaneous interpretation of multiple aquifer tests using alternative geologic frameworks of variable complexity, where each framework is incorporated as prior information that assumes homogeneous K within each model unit. This approach is tested at Pahute Mesa within the Nevada National Security Site (USA), where observed drawdowns from eight aquifer tests in complex, highly faulted volcanic rocks provide the necessary hydraulic constraints. The investigated volume encompasses 40 mi³ (167 km³) where drawdowns traversed major fault structures and were detected more than 2 mi (3.2 km) from pumping wells. Complexity of the five frameworks assessed ranges from an undifferentiated mass of rock with a single unit to 14 distinct geologic units. Results show that only four geologic units can be justified as hydraulically unique for this location. The approach qualitatively evaluates the consistency of hydraulic property estimates within extents of investigation and effects of geologic frameworks on extrapolation. Distributions of transmissivity are similar within the investigated extents irrespective of the geologic framework. In contrast, the extrapolation of hydraulic properties beyond the volume investigated with interfering aquifer tests is strongly affected by the complexity of a given framework. Testing at Pahute Mesa illustrates how this method can be employed to determine the appropriate level of geologic complexity for large-scale groundwater modeling.     

Effects of Highly Permeable Geological Discontinuities upon Groundwater Productivity and Well Yield

It is generally understood that groundwater flow is greatly affected by linear features associated with geologic structures such as fractures, faults, lithologic boundaries, and fold axes. In crystalline rock aquifers, such geologic structures seem to be closely related to large-scale groundwater flow field and well productivity. However, quantification of how the structures are related with the flow and well productivity is not so simple, because it involves many factors of hydraulic properties and flow phenomena. This study proposes a semianalytical method to analyze the effects of highly permeable geologic discontinuities on the groundwater flow system. Effects of geological discontinuities on groundwater productivity are interpreted from the geometry of linear geologic features by the conformal mapping of the spatial distribution function. The proposed method is compared with the numerical methods, and the applicability and usefulness of the method to delineate high well yield areas from a limited number of well productivity measurements is discussed.     

Geophysical and geochemical characterization of the groundwater system and the role of Chatham Fault in groundwater movement at the Coles Hill uranium deposit, Virginia, USA

The largest undeveloped uranium deposit in the United States, at Coles Hill, is located in the Piedmont region of Pittsylvania County, south-central Virginia, and is hosted in crystalline rocks that are adjacent to and immediately west of Chatham Fault, which separates these crystalline rocks from the metasedimentary rocks of the Danville Triassic Basin (in the east). Groundwater at the site flows through a complex network of interconnected fractures controlled by the geology and structural setting. The role of Chatham Fault in near-surface (<??200?m) groundwater flow is examined using electrical resistivity profiling, borehole logging, a pumping test, groundwater age dating and water chemistry to determine if the fault represents a permeability barrier or conduit for groundwater flow. The volumetric flow per unit width flowing eastward across the fault is estimated at 0.069?C0.17?m²/day. Geochemical data indicate that groundwater in the granitic crystalline rocks represents a mixture of modern and old water, while the Triassic basin contains a possible deeper and older source of water. In regions with shallow water tables, mine dewatering during operation presents significant mining costs. The study??s results yield important information concerning the effect that Chatham Fault would have on groundwater flow during Coles Hill mining operations.     

Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa

The semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep(100 m b.g.l.)crystalline bedrock aquifers for drinking water production.Groundwater quality may be affected by high geogenic arsenic(As)concentrations(10μg/L)stemming from the oxidation of sulphide minerals(pyrite,arsenopyrite)in mineralised zones.These aquifers are still little investigated,especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry.To gain insights on the temporal aspects of As contamination,we have used isotope tracers(noble gases,~3H,stable water isotopes(~2 H,~(18)O))and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso.Results revealed a great variability in groundwater properties(e.g.redox conditions,As concentrations,water level,residence time)over spatial scales of only a few hundred metres,characteristic of the highly heterogeneous fractured underground.Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters.Arsenic concentrations are relatively stable over the course of the year,with little effect seen by the monsoon.Groundwater residence time does not seem to have an influence on As concentrations,as elevated As can be found both in groundwater with short(50 a)and long(10~3 a)residence times as indicated by ~3He/~4He ratios spanning three orders of magnitude.These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions.The existence of very old water portions with residence times10~3 years already at depths of50 m b.g.l.is a new finding for the shallow fractured bedrock aquifers of Burkina Faso,suggesting that overexploitation of these relatively low-yielding aquifers may be an issue in the future.     

Fate and transport of volatile organic compounds in glacial till and groundwater at an industrial site in Northern Ireland

Volatile organic compound (VOC) contamination of subsurface geological material and groundwater was discovered on the Nortel Monkstown industrial site, Belfast, Northern Ireland. The objectives of this study were to (1) investigate the characteristics of the geological material and its influences on contaminated groundwater flow across the site using borehole logs and hydrological evaluations, and (2) identify the contaminants and examine their distribution in the subsurface geological material and groundwater using chemical analysis. This report focuses on the eastern car park (ECP) which was a former storage area associated with trichloroethene (TCE) degreasing operations. This is where the greatest amount of volatile organic compounds (VOCs), particularly TCE, were detected. The study site is on a complex deposit of clayey glacial till with discontinuous coarser grained lenses, mainly silts, sands and gravel, which occur at 0.45–7.82 m below ground level (bgl). The lenses overall form an elongated formation that acts as a small unconfined shallow aquifer. There is a continuous low permeable stiff clayey till layer beneath the lenses that performs as an aquitard to the groundwater. Highest concentrations of VOCs, mainly TCE, in the geological material and groundwater are in these coarser lenses at ∼4.5–7 m bgl. Highest TCE measurements at 390,000 μg L⁻¹ for groundwater and at 39,000 μg kg⁻¹ at 5.7 m for geological material were in borehole GA19 in the coarse lens zone. It is assumed that TCE gained entrance to the subsurface near this borehole where the clayey till was thin to absent above coarse lenses which provided little retardation to the vertical migration of this dense non-aqueous phase liquid (DNAPL) into the groundwater. However, TCE is present in low concentrations in the geological material overlying the coarse lens zone. Additionally, VOCs appear to be associated with poorly drained layers and in peat <3.0 m bgl in the ECP. Some indication of natural attenuation as VOCs degradation products vinyl chloride (VC) and dichloromethane (DCM) also occur on the site.     

华北太行岩墙群斜锆石生长世代和U-Pb年龄及其对岩浆演化的启示

斜锆石U-Pb定年是基性岩墙时代确定的有效方法。前人对太行岩墙群高分异组岩墙(Ti O2通常1%,Mg O一般6%)进行了较多的定年,但缺少对低分异组岩墙(Ti O2约为1%,Mg O6%)的高精度测年。两者年龄是否一致是讨论两组岩墙是否来源于同一岩浆的基本前提。本研究对华北太行岩墙群丰镇地区低、高分异组代表性岩墙进行了斜锆石离子探针207Pb/206Pb定年。结果表明,低分异组车道沟岩墙207Pb/206Pb平均年龄为1768±4Ma(n=9,MSWD=2.1);而高分异组酸刺湾岩墙斜锆石207Pb/206Pb年龄可以分为两组,加权平均值分别为1780±3Ma(n=8,MSWD=0.65)和1760±3Ma(n=5,MSWD=1.7)。斜锆石成分分析表明,酸刺湾岩墙两组年龄对应的矿物成分存在差异,如晚期斜锆石比早期更富Ti,Zr/Hf比值变化范围相对较小。据此认为,酸刺湾岩墙两组斜锆石可能结晶于岩浆不同阶段:早期(~1780Ma)的斜锆石可能结晶于成分更加原始的岩浆,很可能形成于岩浆房中;晚期(~1760Ma)的斜锆石可能结晶于成分更加演化的岩浆,可能是岩浆通道(岩墙)或岩浆房中。Zr/Hf值与单点年龄的负相关趋势可能记录了单斜辉石的结晶过程。已有的太行岩墙群U-Pb年代学结果集中于1785~1760Ma。我们的研究认为,太行岩墙群岩浆房存在了约20Myr,即岩浆房形成于~1785Ma,岩墙就位于1785~1760Ma。     

Assessment of alluvial aquifer heterogeneity and development of stochastic hydrofacies models for the Hat Yai Basin in Southern Thailand

In previous studies, the groundwater flow models formulated for the Hat Yai Basin were conventional and deterministic because the geologic heterogeneity of the alluvial aquifer system in the basin had not yet been assessed. This paper describes an effort to develop hydrofacies models, such that the spatial variability of the aquifer system can be represented in a systematic way. Variogram parameters that characterize the alluvial aquifer heterogeneity were determined. Based on these variogram parameters, an indicator-based geostatistical approach was used to develop hydrofacies models using sequential indicator simulation. The hydrofacies models indicate three distinct aquifer units, namely Hat Yai, Khu Tao, and Kho Hong aquifers, which is in good agreement with a conceptual model, and incorporates spatial variability as observed in field data from borehole logs. The hydrofacies models can be used in groundwater modeling and simulations.     

青海南山当家寺花岗岩体与晚三叠世脉岩及其对早中生代构造环境的约束

当家寺花岗岩体是青海南山印支期花岗岩带中东部较大的一个岩体,形成于早三叠世(243.5±2.9 Ma、247.2±1.7 Ma)—中三叠世(240.1±2.1 Ma、241.0±2.6 Ma)。岩体内发育有众多的中-酸性脉岩,岩石类型主要为闪长玢岩、石英闪长玢岩、花岗斑岩等,本研究获得闪长玢岩脉LA-MC-ICP-MS锆石U-Pb年龄为(235.6±2.4 Ma),表明其形成于晚三叠世早期,稍晚于当家寺岩体侵位时限。脉岩作为一种浅成侵入体,其与当家寺花岗岩体分布的构造位置相似,矿物组成及地球化学特征具有近似的特点,说明二者岩石成因及构造背景类似,可能为同一源区壳-幔岩浆混合作用的产物。岩石地球化学分析表明中性脉岩SiO₂含量低(54.68%~64.75%),酸性脉岩SiO₂含量高(66.00%~77.13%),二者都具高铝(Al₂O₃=12.24%~17.11%)、高钾钙碱性准铝质-弱铝质的特点(A/CNK=0.85~1.04),具有典型的岛弧花岗岩和碰撞花岗岩的特征。脉岩富集LREE、Rb、Th、K,亏损HREE、Nb、Ta、P、Ti、Ba,(La/Yb)_N=5.05~17.61,表现出中等强度Eu的负异常。综合分析认为,脉岩形成于晚三叠世共和裂谷闭合碰撞造山后构造环境由挤压向板内稳定-伸展的变迁阶段,指示晚三叠世早期共和盆地周缘已进入后碰撞的陆内环境。     

Two Periods of Skarn Mineralization in the Baizhangyan W–Mo Deposit,Southern Anhui Province,Southeast China: Evidence from Zircon U–Pb and Molybdenite Re–Os and Sulfur Isotope Data

The recently discovered Baizhangyan skarn‐porphyry type W–Mo deposit in southern Anhui Province in SE China occurs near the Middle–Lower Yangtze Valley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with the Mesozoic Qingyang granitic complex composed of g ranodiorite, monzonitic g ranite, and alkaline g ranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones of Sinian Lantian Formation contacting with buried fine‐grained granite, and diorite dykes. There are two types of W mineralization: major skarn W–Mo mineralization and minor granite‐hosted disseminated Mo mineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the first W–Au period related to the intrusion of diorite dykes, and the subsequent W–Mo period related to the intrusion of the fine‐grained granite. In this paper, we report new zircon U–Pb and molybdenite Re–Os ages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, and W mineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted mean U–Pb ages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdenite Re–Os age determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ³⁴S values of 3.6‰–6.6‰ and their Re contents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containing Re concentrations of 50.5–50.9 ppm, and with δ³⁴S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations of Re (7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke‐related skarn W–Au mineralization took place in the Early Cretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarn W–Mo mineralization took place a little later at 135.0–136.9 Ma, and was crust‐dominated. The fine‐grained granite was not formed by fractionation of the Qingyang monzonitic granite. This finding suggests that the first period of skarn W–Au mineralization in the Baizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period of W–Mo mineralization derived from the crust at 135.0–136.9 Ma.     

U-Pb Zircon Age,Geochemical, and Sr-Nd-Pb Isotopic Constraints on the Age and Origin of Mafic Dykes from Eastern Shandong Province,Eastern China

U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).     

Arsenic in sediments, groundwater, and streamwater of a glauconitic Coastal Plain terrain, New Jersey, USA—Chemical “fingerprints” for geogenic and anthropogenic sources

Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl⁻) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust.     

北山洋晚志留世—早泥盆世构造演化:内蒙古白云山蛇绿混杂岩带南部侵入岩年代学、地球化学的制约

内蒙古北山白云山蛇绿混杂岩带南部中酸性侵入岩主要以脉岩形式产出。采用LA-ICP-MS锆石U-Pb定年和岩石地球化学分析,对脉岩的岩石类型、年代学、构造背景等进行研究,为北山洋的拼合碰撞时限提供年代学约束。研究结果显示:石英闪长岩的年龄为(420.0±1.0) Ma,花岗闪长岩的年龄为(404.6±2.6) Ma,表明脉岩形成时期为晚志留世—早泥盆世。中酸性侵入岩脉SiO₂含量为58.38%~69.46%,K₂O含量为0.74%~1.38%,Al₂O₃含量为15.54%~16.95%,Na₂O含量为4.15%~5.36%,岩石类型为钙碱性石英闪长岩和过铝质低钾拉斑系列的花岗闪长岩。石英闪长岩与花岗闪长岩源于陆缘火山弧区和同碰撞区,二者轻稀土元素相对重稀土元素明显富集,δEu负异常,高场强元素Ta、Nb亏损,显示出岛弧环境的典型特征,表明由于洋壳俯冲作用,研究区在晚志留世—早泥盆世发育了岛弧岩浆岩侵入体。随着俯冲作用深入,北山洋在晚志留世—早泥盆世早期完成闭合碰撞,并在研究区形成同碰撞花岗岩,碰撞时间应早于(404.6±2.6) Ma。     

贵州高硫酸盐地区寻找安全饮用水方法的探讨

贵州镇宁地区地下水中硫酸盐超标是典型的环境地质问题之一,而该区地下水中硫酸盐超标主要来自于三叠系中统关岭组(T2g)含水岩组。采集水样分析发现该含水岩组中地下水的硫酸盐含量达到了275.42~1 100 mg?L-1,为Ⅳ-Ⅴ类水。运用地下水中硫同位素追踪,其硫酸盐主要来源于石膏溶解,并通过实施“探采结合井”（ZK3）,初步查明 “膏岩层”分布于三叠系中统关岭组一段下亚段(T2g1a)的中下部,厚度在1～1.5 m。而分层抽水试验显示:T2g含水岩组试段水质为Ⅴ类,降深在11.50 m时涌水量为256.61 m3?d-1;当揭穿T2g膏岩层含水层并进行封隔止水后,钻孔自流量为330.05 m3?d-1,降深在10.80 m时钻孔涌水量达628.84 m3?d-1,水质属Ⅲ类水。因此,在类似因水质超标而引起的功能性缺水地区,可越过膏岩层而获取深层安全的可饮用的地下水。      

Rb–Sr and <Superscript>147</Superscript>Sm–<Superscript>143</Superscript>Nd systematics of gabbro and dolerites from fragments of ophiolite complexes of the Middle Urals

The Rb–Sr and ¹⁴⁷Sm–¹⁴³Nd age data obtained for sheeted dolerite dykes and rocks of the Platinum Belt of the Urals within the Tagil segment of the paleoceanic spreading structure (Middle Urals) are discussed. The study of the Rb–Sr isotope systematics of gabbro allowed us to reveal errochronous dependencies, which yielded ages of 415 and 345 Ma at (⁸⁷Sr/⁸⁶Sr)₀ = 0.70385 ± 0.00068 and 0.7029 ± 0.0010, correspondingly. The ¹⁴⁷Sm–¹⁴³Nd isotope age data demonstrate a specific coincidence of the chronometric ages of the sheeted dolerite dyke complex (426 ± 54, 426 ± 34, and 424 ± 19 Ma) and gabbro from the Revda gabbro–ultramafic massif (431 ± 27 Ma) and from screens between dolerite dykes in the sheeted dyke complex (427 ± 32 Ma, 429 ± 26 Ma). The proximity of the ¹⁴⁷Sm–¹⁴³Nd ages of gabbro and dolerite can be explained by the thermal effect of the basaltic melt, which is the protolith for the dyke complex, on the hosting gabbro.     

Sources of low-arsenic groundwater in the Bengal Basin: investigating the influence of the last glacial maximum palaeosol using a 115-km traverse across Bangladesh

Pollution of groundwater in the Bengal Basin (Bangladesh and West Bengal, India) by arsenic (As) puts at risk the health of more than 100 million consumers. Using 1,580 borehole lithological logs and published hydrochemistry on 2,387 wells, it was predicted that low-As (<10 μg/L) groundwater exists, in palaeo-interfluvial aquifers of brown sand capped by a protective palaeosol, beneath at least 45,000 km² of the Bengal Basin. The aquifers were predicted to be at a depth of as little as 25 m below ground level (mbgl), and typically no more than 50 mbgl. The predictions were confirmed along an east–west traverse 115 km in length (i.e. across half of Bangladesh) by drilling 28 new boreholes to 91-m depth to reveal subsurface sedimentology, and by mapping As distribution in groundwater. The aquifers identified occur at typically <40 mbgl and so are accessible with local drilling methods. A protective palaeosol that caps the palaeo-interfluvial aquifers prevents downward movement into them of As-polluted groundwater present in shallower palaeo-channel aquifers and ensures that the palaeo-interfluvial aquifers will yield low-As groundwater for the foreseeable future. Their use, in place of the shallower As-polluted palaeo-channel aquifers, would rapidly mitigate the health risks from consumption of As-polluted groundwater.     

Geochemistry of the mafic dykes in the Prakasam Alkaline Province of Eastern Ghats Belt, India: Implications for the genesis of continental rift-zone magmatism

Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.     

Mechanism of creep movement caused by landslide activity and underground erosion in crystalline schist, Shikoku Island, southwestern Japan

The mechanism of creep movement of the Zentoku landslide in crystalline schist has not been studied in detail because of the steepness of the slope, very slow movement, low population density and complex topographic and geologic characteristics. Sassa et al. (1980: Proc. INTERPRAEVENT 1, 85–106) and Sassa (1984: Proc. 4th International Symp. on Landslides, Toronto, vol. 2, pp. 179–184; 1985. Geotechnical classification of landslides, Proc. 4th International Conference and Field Workshop on Landslides, Tokyo, pp. 31–40; 1989: Landslide News, Japan Landslide Society, No. 3, pp. 21–24) monitored landslide movement and groundwater level at the Zentoku landslide on Shikoku Island, southwestern Japan, and suggested that the mechanism may be caused by underground erosion. To study the influence of underground erosion at this site, continual monitoring of suspended sediment and water discharge from a groundwater outlet (i.e. a spring) was implemented. The locations of groundwater flow paths were determined, as were the amounts of discharged sediment. Slope deformation was monitored by means of a borehole inclinometer. The conclusions were as follows: (1) the flow paths were found to be on or above the shear zones in which underground erosion has occurred; (2) in addition to being a result of precipitation and groundwater discharge, sediment discharge is affected by landslide activity; and (3) the mechanism of creep movement is an interrelated chain process that combines underground erosion caused by landslide activity with landslide activity caused by underground erosion. Thus, landslide activity increases erosion susceptibility and transportation of soils within the mass, and underground erosion causes instability of the landslide mass, in turn.

This mechanism can explain the observed phenomenon that the Zentoku landslide not only moves actively during heavy rain, but also continues to creep throughout the year.     

岩溶介质非均质性对钻孔水物化指标垂向变化的影响

岩溶介质具有较强的非均质性,其地表及地下的岩溶结构形态多样。通过对钻孔结构描述、钻孔水物化性质分析,不仅能够掌握区域上岩溶含水层的结构特征,而且对于岩溶地下水演化过程的揭示亦有重要作用。文章以桂林岩溶水文地质试验场西南部峰丛山区与峰林平原交界处的钻孔为例,通过野外便携式多参数仪原位测试钻孔垂向水物化指标（pH值、水温T、电导率EC）,探索浅部（地面以下约50 m内）地下岩溶较为发育条件下钻孔水物化指标的垂向变化特征,揭示岩溶介质非均质性对钻孔垂向水物化指标的影响。结果表明:岩溶地区小范围内不同钻孔间的水物化性质有所差异,且岩溶发育相似的钻孔（如ZK4/ZK5、ZK7/ZK8）,其水物化指标垂向变化具有一定的相似性,但不同指标（T、pH、EC）的变化幅度存在差异;钻孔水物化性质受到试验场区岩溶介质结构非均质性的控制,即岩溶介质结构影响了地下水的赋存条件和水力联系而导致水物化性质的差异;在对岩溶地区地下水物化性质进行研究时应充分考虑岩溶介质的非均质性特征,根据实际的水文地质条件选取具有代表性的钻孔含水段进行取样和监测。      

Geochemical composition of dykes along the Cameroon Line (CL): Petrogenesis and similarities with the Central Atlantic Magmatic Province

We report whole-rock geochemistry and Sr–Nd–Pb isotopic compositions of mafic dykes intruded in the Precambrian granito-gneissic basement complex, exposed at Nyos, Batibo, Dschang and Foumban on the Cameroon Line. The dykes are alkaline (Batibo), transitional (Foumban), and subalkaline (Nyos, Batibo and Dschang) with SiO₂ of 45–54 wt% and MgO of 2–9 wt%, similar to dykes reported in other areas of the Cameroon Line (CL) and the Central Atlantic Magmatic Province (CAMP). The abundances of rare earth elements (REE) and the Primitive Mantle normalised patterns for the Nyos, Batibo and Dschang dykes are similar to those of MORB, indicating that the dykes formed at shallower depths by a higher degree of partial melting relative to the Foumban dykes and the alkaline lavas of the CL. The transitional basaltic dykes with steeper REE patterns have their sources at deeper levels in the lithospheric mantle, possibly the garnet-spinel transition zone and were generated by a lower degree partial melting of the lithospheric and plume components. The Nyos and Batibo subalkaline dykes show similar isotopic compositions with a spectrum extending from depleted (DMM-like) to enriched (EM1-like) mantle, indicating the similarity in their source components. The Dschang dykes show distinct isotopic characteristics with relatively unradiogenic Nd-Pb isotope compositions compared to the Batibo and Nyos dykes. The Foumban transitional dykes with characteristic wide ranges in Sr-Nd-Pb isotopic compositions reveal varying contributions from enriched mantle components (EM1 and EM2) in addition to its plume signature similar to those of CL lavas. The Nyos and Batibo dykes alongside other dykes on the CL have low TiO₂ abundances (<2 wt%), negative PM-normalised Nb-anomalies, and moderately to strongly enriched REE patterns, and isotopic composition that overlaps with those of CAMP, suggesting a similar lithospheric origin.