Prediction in hydrogeology: two case histories     

J. J. de Vries 《International Journal of Earth Sciences》1997,86(2):354-371

An inherent aspect of hydrogeology is the dynamic character of groundwater flow within the more passive lithospheric medium. Prediction therefore requires insight into the spatial pattern of aquifer properties as well as the dynamic character of groundwater flow systems. Both aspects together determine hydrogeological system behaviour in space and time. The spatial characteristics of an aquifer are mainly based on geological and geomorphological structure, whereas the dynamics of the system result predominantly from climate and topography; knowledge of paleoconditions are a prerequisite for sound understanding of present systems. The integration of exploration and forecast is illustrated by two case studies. The first study focuses on an assessment of groundwater resource and their sustainable development in semi-arid Botswana. The second study concerns the impact of land and water management in the Netherlands coastal area on the groundwater regime and its future development, including the redistribution of salt and fresh groundwater on a long time scale.  相似文献   

Preliminary palaeomagnetic results of an Archaean dolerite dyke of west Greenland: geomagnetic field intensity at 2.8 Ga     

Chiyo Morimoto  Yo-ichiro Otofuji  Masako Miki  Hidefumi Tanaka  Tetsumaru Itaya 《Geophysical Journal International》1997,128(3):585-593

The geomagnetic field intensity during Archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 Ma Nuuk Gneisses at Nuuk (64.2°N, 51.7°W), west Greenland. Plagioclase from the dolerite dyke yields a mean K-Ar age of 2752 Ma. Palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked-contact test, indicating that the high-temperature-component magnetization of the dyke is primary. Thellier experiments on 12 dyke specimens yield a palaeointensity value of 13.5±4.4 μT. The virtual dipole moment at ca. 2.8 Ga is 1.9±0.6 × 10²² Am², which is about one-quarter of the present value. The present study and other available data imply that the Earth's magnetic field at 2.7 ∼ 2.8 Ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca. 2.6 Ga.  相似文献   

The effects of the spinel—garnet phase transition on the formation of rifted sedimentary basins     

Tadashi Yamasaki  Masao Nakada 《Geophysical Journal International》1997,130(3):681-692

We have examined the effects of the spinel-garnet phase transition on subsidence of extensional sedimentary basins. For a constant positive Clapeyron slope ( dP/dT ), the phase boundary moves downwards in the syn-rift and upwards in the post-rift phase. For a non-linear Clapeyron curve ( dP/dT > 0 above 900°C and dP/dT < 0 below 900°C), theory predicts for the reaction of the spinel-garnet phase transition, the direction of phase boundary movement is dependent on the stretching factor, the position of the Clapeyron curve and the lithospheric thickness. A smaller syn-rift and larger post-rift subsidence are predicted for a deeper phase boundary and a thicker lithosphere. The model with a non-linear Clapeyron curve is applied to the subsidence histories of a young extensional basin (Gulf of Lion) and an old continental margin (eastern Canada). The observed syn-rift uplift and the larger post-rift subsidence can be reasonably explained by this model, where the optimum depth of the phase boundary for eastern Canada (˜90 km) is consistent with the estimate from seismic observations and is larger than that for the Gulf of Lion (˜ 50 km). The depth of the spinel-garnet phase boundary is sensitive to the composition of mantle rocks and increases with the extraction of basaltic components from the lithosphere, compatible with our result that the phase boundary is deeper for an older and thicker lithosphere. Thus the surface movement associated with the rifting for these areas may reflect the chemical evolution of the continental lithosphere.  相似文献   

Middle Miocene thermal metamorphism due to the infiltration of high-temperature fluid in the Sanbagawa metamorphic belt,southwest Japan     

M. Sakakibara  Y. Isono 《Contributions to Mineralogy and Petrology》1996,125(4):341-358

 The Middle Miocene Tobe hornfels in the Sanbagawa metamorphic belt, western Shikoku, southwest Japan, is characterized by an abnormally steep metamorphic gradient compared with other hornfelses associated with intrusive bodies. The basic hornfels, originally Sanbagawa greenschist rocks, is divided into the following three metamorphic zones: plagioclase, hornblende, and orthopyroxene. The plagioclase zone is defined by the appearance of calcic plagioclase, the hornblende zone by the assemblage of hornblende+calcic plagioclase+quartz, and the orthopyroxene zone is characterized by the assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz. Calcic amphibole compositions change from actinolite to hornblende as a result of the continuous reactions during prograde metamorphism. Petrographical and thermometric studies indicate a metamorphic temperature range of 300–475°C for the plagioclase zone, 475–680°C for the hornblende zone, and 680–730°C for the orthopyroxene zone. The temperature gradient based on petrological studies is approximately 5°C/m, which is unusually high. Geological and petrological studies demonstrate that the hornfelses were formed by the focusing of high-temperature fluids through zones of relatively high fracture permeability. The steep thermal gradient in the Tobe hornfels body is consistent with a large fluid flux, greater than 8.3 × 10^–7 m³ m^–2S^–1, over the relatively short duration of metamorphism, approximately 100 years. Received: 10 October 1995 / Accepted: 28 May 1996  相似文献   

Nd and Sr isotopic constraints on the origin of igneous rocks resulting from the opening of the Japan Sea, southwestern Japan     

Y. Terakado  T. Fujitani  R. J. Walker 《Contributions to Mineralogy and Petrology》1997,129(1):75-86

Strontium and Nd isotopic compositions and trace element abundances were determined for Cretaceous to late Cenozoic igneous rocks from the Japan Sea side of Southwest Japan in order to investigate the effect of the opening of the Japan Sea on igneous activity. The ⁸⁷Sr/⁸⁶Sr ratios for both high and low silica rocks decrease with decreasing age since the middle Miocene, when the opening occurred. Similarly, ¹⁴³Nd/¹⁴⁴Nd values for these rocks increase with decreasing age, and are negatively correlated with ⁸⁷Sr/⁸⁶Sr ratios. A two-component mixing process can best account for these isotopic and chemical characteristics. One end-member is likely the subcontinental lithospheric mantle (SCLM) and its derivative mafic to intermediate materials which had ɛ_Nd values of around +3. The other endmember consists of mafic to intermediate rocks with low ɛ_Nd values (e.g., −8), probably located in the lower crust. The mantle upwelling associated with the opening of the Japan Sea did not supply typical MORB or MORB-source materials to the crust, but did provide the heat that caused the melting of lithospheric mantle and lower crust. Received: 29 August 1996 / Accepted: 6 May 1997  相似文献   

Geotechnical Hazards Associated with Desert Environment     

Shehata  W.M.  Amin  A.A. 《Natural Hazards》1997,16(1):81-95

The aridity of the Arabian Peninsula's deserts ranges between arid to hyperarid with hot dry climate, scarce precipitation and sparse vegetation. These harsh environmental conditions enhance some geomorphologic processes more than others, cause specific geotechnical problems, and increase desertification.From west to east, the general physiography of Saudi Arabia shows the Red Sea coastal plains and the escarpment foothills called Tihama followed by the Arabian Shield mountains, the Arabian Shelf plateau and finally the Arabian Gulf coastal plains. Sand moves by wind either as drifting sand or migrating dunes in four major sand seas, over the Arabian Shelf, and in the inter-mountain valleys, in the Arabian Shield causing problems of erosion and deposition. Human activities in the deserts may cause more instability to the sand bodies, enlarging the magnitude of the problem. Fine silty soil particles also move by wind, depositing loess mainly in selected areas downwind in the Tihama. These loess deposits subside and may form earth fissures by the process of hydrocompaction upon wetting. The addition of water can be either natural through storms or man-made through human agricultural or civil activities. Extensive sabkhas exist along the coastal plains of both the Red Sea and Arabian Gulf. The sabkha soil may also heave by salt re-crystallization or collapse by wetting. The shallow groundwater brines present in sabkhas also attack and corrode civil structures. Urbanization and excessive groundwater pumping may also deplete the fresh groundwater resources and may cause subsidence, ground fissuring and surface faulting as observed in some locations in the Arabian Shield. Although the average annual precipitation is very low, rain usually falls in the form of torrential storms, collected by dry valley basins and causing floods to unprotected downstream areas on the coastal plains of the Red Sea.The desert environment, being a fragile echo system, needs to be treated with care. Intercommunications between different national and international agencies and education of the layman should help to keep the system balanced and reduce the resulting environmental hazards. In addition, any suggested remedial measures should be planned with nature and engineered with natural materials.  相似文献   

Multi-phase water-rhyolite interaction and ore fluid generation at Aljustrel, Portugal     

F. J. A. S. Barriga  W. S. Fyfe 《Mineralium Deposita》1997,33(1-2):188-207

Detailed study of the Aljustrel volcanic rocks, through petrography, mineral chemistry, whole-rock geochemistry, and previously published stable isotope data, show that the rocks experienced extreme metasomatism. The metasomatic phenomena presented are explained accurately by a sea-water convective model. The evolution of the sea-water-derived fluid as it progresses downwards through the rocks is evident, from cold, oxidising, Mg-bearing, to hot, reducing, Mg-free, and enriched in Si, Fe, Cu, Zn. Several lines of evidence (feldspar mineralogy, paragenetic relations between regional and ore-zone alteration, and stable isotope data) show that the temperature of the hydrothermal circulation increased through time, and that the water-to-rock ratio evolved from very high to low values. According to our data, about 30 ppm Cu and some 100 ppm Zn were removed from the Aljustrel Volcanics. This implies, for both metals, the same amount of leached rock to produce the 2 Mt Cu and 6 Mt Zn contained in the massive sulphide deposits of the Aljustrel area. With regard to Fe, about 1% was on average removed from the rocks, producing enough metal to account for all the Fe in the MS deposits and overlying chemical sediments. This study documents semi-quantitatively the provenance of the main metals in major massive sulphide deposits via sea water leaching of the immediate host rocks.

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Resumen (translated by E. Pascual) Un estudio detallado de las rocas volcánicas de Aljustrel, mediante petrografia, química mineral, geoquímica de roca total y datos de isótopos estables previamente publicados muestra que estas rocas han sufrido metasomatismo en grado extremo. Los fenómenos metasomáticos presentados se explican con precisión mediante un modelo convectivo con agua de mar. Es evidente la evolución del fluido derivado del agua de mar conforme progresa en profundidad a través de las rocas, desde un fluido frío, oxidante y con magnesio a otro reductor, sin magnesio y enriquecido en Si, Fe, Cu y Zn. Varias líneas de evidencia (mineralogía de los feldespatos, relaciones paragenéticas entre la alteración regional y de las zonas mineralizadas, isótopos estables) muestran que la temperatura y la circulación hidrotermal se incrementaron con el tiempo y que la relación agua-roca evolucionó de valores muy altos a bajos. De acuerdo con nuestros datos, de las rocas volcánicas de Aljustrel se han extraído unas 30 ppm de Cu y unas 100 ppm de Zn. Esto implica, para ambos metales, que la misma roca lixiviada ha producido los 2 Mt de Cu y los 6 Mt de Zn contenidos en los depósitos de sulfuros masivos del área de Aljustrel. En cuanto al Fe, una media de alrededor de 1% fue extraída de las rocas, produciendo metal suficiente para explicar todo el Fe de los depósitos de sulfuros masivos y de los sedimentos químicos suprayacentes. Este estudio documenta de forma semicuantitativa la procedencia de los principales metales en depósitos mayores de sulfuros masivos, mediante lixiviado por agua de mar de sus rocas huéspedes inmediatas.

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Received: 1 February 1997 / Accepted: 30 July 1997  相似文献   

Causes of Land Subsidence in the Kingdom of Saudi Arabia     

Amin  Ammar  Bankher  Khalid 《Natural Hazards》1997,16(1):57-63

The occurrence of land subsidence in the Kingdom Saudi Arabia is either natural or man-made. Natural land subsidence occurs due to the development of subterranean voids by a solution of host rocks in carbonate and evaporite terrains, over many areas of Saudi Arabia. Man-induced land subsidence is either due to the removal of groundwater in the agricultural areas or to wetting of unstable soils. Therefore, earth fissures and a lowering of the ground surface in unconsolidated sediments took place in alluvial plains and volcanic vent terrains. Unstable soils include Sabkha soils and loess sediments. These types of soils occur in coastal plains, desert areas and volcanic terrains. When this soil is wetted either during agricultural activities, waste disposal or even during a rain storm, subsidence takes place due to either the removal of salts from the Sabkha soil or the rearrangement of soil particles in loess sediments.  相似文献   

Secondary siderite-oxide-sulphide and carbonate-andalusite assemblages in cordierite granulites from Sri Lanka: post-granulite facies fluid evolution during uplift     

D. J. Ellis  Yoshikuni Hiroi 《Contributions to Mineralogy and Petrology》1997,127(4):315-335

We report here that some of the pelitic rocks from the Wanni and Highland Complexes of Sri Lanka reacted with CO₂-rich fluids to produce a wide range of unusual secondary carbonate-silicate-oxide-sulphide assemblages. These enable the depth, temperature and fluid compositions of CO₂ reactions to be calculated more rigorously than is generally possible for the patches of arrested charnockite that have been described from Sri Lanka. Magnesite-andalusite-quartz has partially replaced primary cordierite, and siderite-rutile replaced ilmenite. Paragenetic sequences involving primary pyrrhotite, ilmenite and magnetite and secondary pyrite-siderite-rutile-magnetite-(hematite) demonstrate the control which carbonate equilibria have upon evolving fluid compositions during cooling. Direct evidence for the role of graphite as a source of CO₂ is found in the Highland Complex where primary graphite partially reacted with silicates to form secondary siderite assemblages. It is proposed that following peak metamorphism, continued uplift along a clockwise P-T-t path was accompanied by a series of devolatilization reactions involving breakdown of graphite and the continuous production of secondary CO₂-rich fluids. The limited extent of disseminated secondary carbonate reflects the small amount of graphite inferred to have been present in the source rocks. These rocks demonstrate that CO₂-rich fluids, as found in disseminated fluid inclusions, need not form during peak granulite metamorphism but may be an inevitable consequence of continued uplift along a clockwise P-T-t path. The arrested charnockite which overprinted some of the hornblende-bearing felsic-intermediate composition rocks in Sri Lanka most likely formed by the same process. Received: 4 May 1994 / Accepted: 25 October 1996  相似文献   

Karst Hazard Assessment of Eastern Saudi Arabia   总被引：1，自引：0，他引：1  

Amin  Ammar A.  Bankher  Khalid A. 《Natural Hazards》1997,15(1):21-30

Karst phenomena exist in areas in the eastern part of Saudi Arabia, forming solution features such as sinkholes, collapsed dolines and solution caverns, as a result of the chemical leaching of the carbonate and evaporite formations by percolating water. The instability of these karst phenomena could produce land subsidence problems. This paper reviews the geology of documented karstic rock units in Saudi Arabia and proposes a simple engineering classification of the solution features characteristic of limestone. Two case histories in the Dhahran area, eastern Saudi Arabia, will be used as examples for the application of a modified engineering classification.  相似文献