A modern oceanic hardground on the Carnegie Ridge in the eastern Equatorial Pacific     

BRUCE T. MALFAIT  TJEERD H. VAN ANDEL 《Sedimentology》1980,27(5):467-496

Fossil surfaces of erosion and non-deposition are common in limestone sequences from the Mesozoic in western Europe and in the Tethys and have been described under the name ‘hardgrounds’. They are of shallow water as well as of oceanic origin. A modern example in the Pacific is described in this paper. The Carnegie Ridge, an east-west trending shallow ridge between South America and the Galapagos Islands, has a central, deeper saddle where erosion has removed most of the sediment cover down to a hard chalk and chert bed (acoustic basement), and has cut intricate channel patterns on the south flank and two deep canyons on the north. The erosion has produced a karst-like relief of steep-walled channels, cliffs, and corroded chalk remnants. The floors of the channels are covered with ferromanganese oxide crusts or crust fragments over which loose sediment is being transported. In the two canyons on the north flank, this sediment consists of foraminiferal sand travelling downslope in the form of barchan dunes. All sediment down to acoustic basement has been stripped from the Carnegie Ridge crest except where it is protected behind basement ridges and pinnacles. Surface features of the eroded chalk are strongly reminiscent of features observed in Mesozoic hardgrounds. Current measurements over several days indicate a net northward movement, slow but possibly adequate to keep the sea floor free of fresh deposits. The rates, however, seem inadequate to explain the formation of the deep channels, and there is no evidence for the southward flow which is implied by the southern channel system. A process of combined carbonate dissolution and removal by the current of fresh sediment and dissolution residues can account for the required erosion in about two million years. Regional unconformities identifiable in seismic reflection profiles and dated in cores are of middle to late Pliocene age, suggesting that the formation of the erosion surface began 2-3 million years ago. Buried Miocene unconformities of local extent show that the present erosion period had minor precursors possibly related to short-lived increases in current action and carbonate dissolution.  相似文献   

ELECTRONIC ACCELERATION-SENSITIVE GEOPHONE FOR SEISMIC PROSPECTING***     

K. B. KLAASSEN  J. C. L. VAN PEPPEN 《Geophysical Prospecting》1983,31(3):457-480

Previously ignored characteristics of the seismic recording instrument are presently experienced as limitations as more sophisticated interpretive methods using wider frequency ranges are developed to extract stratigraphic information from seismic land data for hydrocarbon and mineral exploration. Most of these limitations arise from inadequate characteristics of the first element of the seismic instrument: the geophone. A geophone does not faithfully follow the motion of the earth for higher frequencies due to poor geophone-earth coupling. This filtering effect brings about time shifts that are dependent on the frequency and the soil type. A geophone can also produce spurious outputs, brought about by the motion of the suspended part of the geophone, with a magnitude comparable to that of the desired output. The suspension is made very compliant to obtain the required sensitivity. A compliant suspension, however, gives a large sag. The geophone can therefore only be used in one position, tolerating little tilt. A compliant suspension also widens the traveling range of the movable part. Minor sensitivity changes with travel are then noticeable as nonlinearity, since the surface wave is large with respect to the reflected wave. A compliant suspension is usually realized in the form of thin, spirally shaped spring-spiders. Such suspensions exhibit transverse or rotational resonances that are in or close to the seismic frequency band. Excited by ground roll, they can produce considerable undesirable output. The novel geophone we describe is a light-weight (17 g) acceleration-sensitive transducer which gives good ground coupling and partial correction for the increasing damping in the earth with increasing frequencies. It employs internal hybrid electronics for a magnetodynamic velocity-nulling feedback system. Velocity nulling makes the movable part of the geophone virtually rigid with respect to the housing. This makes the geophone characteristics independent of the suspension. The springs used are stiff in a transverse and rotational direction so that the suspension resonances are well outside the useful frequency band. This suspension also allows the geophone to be used in any orientation while being only sensitive to the vibration component along the main axis. The feedback system makes the sensitivity flat within 1 dB from 2 Hz to 500 Hz, with a phase tolerance smaller than 5°. The geophone is robust, has no moving internal wires, employs a current output [sensitivity 1 mA/(m s^?2)] and internal gain so that the signal-to-cable-noise ratio is improved. This type of output allows parallel connection without any interaction between the geophones.  相似文献   

SOME RESULTS OF THE GEO-ELECTRICAL RESISTIVITY METHOD IN GROUND WATER INVESTIGATIONS IN THE NETHERLANDS*     

J. C. VAN DAM  J. J. MEULENKAMP 《Geophysical Prospecting》1967,15(1):92-115

In the year 1958 the Service for Water Management of the ?Rijkswaterstaat” started its program of geo-electrical resistivity prospecting in the western part of the Netherlands. The aim of this program was to obtain data on the salinity distribution of the ground water. The ground water regime in this part of the Netherlands is most intricate. This is due to the geological and geo-hydrological conditions and to the low elevation of the land. Many reclaimed areas are up to several metres below mean sea level. The resistivity data obtained are closely related to the salinity of the ground water. On the basis of bore hole data it was even possible to arrive at calibration curves for the salinity of the ground water in sand deposits. Under special conditions it was also possible to draw conclusions with respect to the presence of less permeable formations as e.g. clay layers. Some remarks are given on the practical performance and the interpretation of the measurements. A review is given of the work done until now. Some results are shown by means of maps of the salinity distribution of the ground water in the western and northern parts of the Netherlands. Two examples are described of the use of the data obtained during the survey in the province of Zuid-Holland. Another two examples are presented of detailed investigations for special purposes in relatively small areas.  相似文献   

西藏纳木错地区约120kaBP以来的古植被、古气候与湖面变化   总被引：13，自引：4，他引：13       下载免费PDF全文

吴中海赵希涛  吴珍汉周春景严富华  麦学舜朱大岗 《地质学报》2004,78(2):242-252

西藏纳木错湖相沉积的U系和^11C测年结果表明，湖泊沿岸的6级湖岸阶地及拔湖约48～139．2m的高位湖相沉积形成于约120ka BP以来的晚第四纪。本文根据该套湖相层的发育和其中的孢粉组合特征对纳木错地区约120ka BP以来的湖面变化与古植被、古气候变迁进行了探讨。结果表明，纳木错地区晚更新世以来经历了频繁的湖面波动、气候的冷暖与干湿变化以及森林—草原与草原植被的交替演化。其总体特征是：约115．9ka BP时，纳木错湖面最高。在116～78ka BP期间，该区气候温和凉爽或温和偏湿，植被以疏林草原与森林草原或森林的交替出现为特征，湖面经历了较大幅度的波动，但基本保持在拔湖140～88m之间。在78～53ka BP期间，该区气候干冷，植被以疏林草原为主，湖面大幅度下降，并在拔湖约36～48m之间波动。约53～32ka BP期间，气候转为温暖偏湿或温暖湿润，湖面波动于拔湖约15～28m之间，波动较为频繁。与阶地的发育相对应，该时期内包含了3次明显的暖期和湖面波动过程，区域植被主要以松、蒿、桦为主，为含一定量的冷杉的森林。其中36ka BP左右气候最温暖湿润，区域内可能出现针叶林或针阔混交林。约32～12ka BP期间，该区气候最为干冷，古植被以草原和疏林草原为主，湖面再次发生较大幅度的下降，最低可至拔湖约8m处，但通常维持在拔湖约12～17m之间。约11．8～4．2ka BP期间，气候整体较为暖湿，其中在约8．4～4．2ka BP期间气候最温暖湿润，该区可能发育针叶林或针阔混交林，湖面波动于拔湖2～9m之间，整体波动幅度较小，但波动最为频繁。区域气候对比发现，纳木错地区的冷、暖气候变化过程与整个青藏高原乃至北半球的气候变化基本是一致的，特别是阶地下切所反映的湖面退缩过程与北大西洋的Henrich冷事件之间具有很好的对应关系。  相似文献   

Sedimentation in an actively tilting half-graben: sedimentology and stratigraphy of Late Tournaisian–Viséan (Mississippian, Lower Carboniferous) carbonate rocks in south County Wexford, Ireland     

ZSOLT R. NAGY  IAN D. SOMERVILLE†  JAY M. GREGG  STEPHEN P. BECKER  KEVIN L. SHELTON‡  ANDREW G. SLEEMAN§ 《Sedimentology》2005,52(3):489-512

The Wexford Basin (south-eastern Ireland) is a NE–SW-trending sedimentary basin containing carbonates and evaporites deposited during the Late Tournaisian and Viséan. Two separate depositional areas are defined on the basis of facies and facies associations. Sediments were deposited in inner ramp, lagoonal and peritidal environments near Rosslare, and in a more open-marine, shallow- to moderately deep-water, mid to outer ramp environment in the western area around Duncormick. Thick breccia deposits that occur in the Wexford Basin formed as a result of (i) fault movement that produced syn-sedimentary debris flows in the Late? Chadian (Breccia type I); (ii) dissolution of anhydrite/gypsum and subsequent collapse of sedimentary strata (Breccia type II); and (iii) fracturing and brecciation of porous rock caused by the movement of high temperature, late diagenetic fluids along fault planes (Breccia type III). The NE–SW facies polarity displayed by both sedimentary successions was the result of NW–SE extension and the reactivation of the NE–SW-trending Wexford Boundary Fault during the Chadian. Extension at the SE margin of the basin with downthrow to the NNW gave the basin a half-graben character. Thickening of the debris flow deposits to the SW suggests that while the half-graben was being tilted it also underwent a NE–SW block rotation due to an axial component of that normal fault.  相似文献   

A new method to calculate end-member thermodynamic properties of minerals from their constituent polyhedra II: heat capacity, compressibility and thermal expansion   总被引：1，自引：0，他引：1  

V. J. VAN  HINSBERG  S. P. VRIEND  J. C. SCHUMACHER 《Journal of Metamorphic Geology》2005,23(8):681-693

Thermodynamic calculations in petrology are generally performed at pressures and temperatures beyond the standard state conditions. Accurate prediction of mineral equilibria therefore requires knowledge of the heat capacity, thermal expansion and compressibility for the minerals involved. Unfortunately, such data are not always available. In this contribution we present a data set to estimate the heat capacity, thermal expansion and compressibility of mineral end‐members from their constituent polyhedra, based on the premise that the thermodynamic properties of minerals can be described by a linear combination of the fractional properties of their constituents. As such, only the crystallography of the phase of interest needs to be known. This approach is especially powerful for hypothetical mineral end‐members and for minerals, for which the experimental determination of their thermodynamic properties is difficult. The data set consists of the properties for 35 polyhedra in the system K–Na–Ca–Li–Be–Mg–Mn–Fe–Co–Ni–Zn–Al–Ti–Si–H, determined by multiple linear regression analysis on a data set of 111 published end‐member thermodynamic properties. The large number of polyhedra determined allows calculation of a much larger variety of phases than was previously possible, and the choice of constituents together with the large number of thermodynamic input data results in estimates with associated uncertainty of generally <5%. The quality of the data appears to be sufficiently accurate for thermodynamic modelling as demonstrated by modelling the stability of margarite in the CASH system and the position of the talc–staurolite–chloritoid–pyrope absent invariant point in the KMASH system. In both cases, our results overlap within error with published equivalents.  相似文献   

Perceptions of water in a changing hydrological and ecological context: the case of the Logone flood plains in Cameroon     

JAN WILLEM MOLENAAR†  JOSÉ C M VAN SANTEN‡ 《The Geographical journal》2006,172(4):331-347

We look at the beliefs of the population in North Cameroon concerning water, fish and water spirits in the context of the construction of a dam (1979) and of a flood embankment for a rice irrigation scheme financed by the World Bank. These operations caused a drought that had severe effects on the environment and the inhabitants' economic activities. Acknowledging the new approaches within development and ecological development thinking, we emphasize that local people are part of their environment, and that the environment and people's use of the ecosystem ought to be regarded as a functional unit. Our main argument is that the success of environmental strategies requires that the unequal power relations between the different actors and agencies and the perceptions of policymakers and NGOs be analysed and examined regularly in the course of a project to test their aims and integrity. In addition, the positions of beings and cosmological entities, which in the eyes of the population are 'actors of power to be reckoned with', like the water spirit, should also be included. The hidden agendas of all actors can be as difficult as the water spirit ( Maama Waata ) itself, by which we mean that hidden agendas are equally a part of reality as is the water spirit, even if they are not visible at first.  相似文献   

The Role of Advective Fluid Flow and Diffusion during Localized, Solid-State Dehydration: Sondrum Stenhuggeriet, Halmstad, SW Sweden     

HARLOV  DANIEL E.; JOHANSSON  LEIF; VAN DEN KERKHOF  ALFONS; FORSTER  HANS-JURGEN 《Journal of Petrology》2006,47(1):3-33

A localized dehydration zone, Söndrum stone quarry, Halmstad,SW Sweden, consists of a central, 1 m wide granitic pegmatoiddyke, on either side of which extends a 2·5–3 mwide dehydration zone (650–700°C; 800 MPa; orthopyroxene–clinopyroxene–biotite–amphibole–garnet)overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet).Whole-rock chemistry indicates that dehydration of the graniticgneiss was predominantly isochemical. Exceptions include [Y+ heavy rare earth elements (HREE)], Ba, Sr, and F, which aremarkedly depleted throughout the dehydration zone. Systematictrends in the silicate and fluorapatite mineral chemistry acrossthe dehydration zone include depletion in Fe, (Y + HREE), Na,K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusionchemistry is similar in all three zones and indicates the presenceof a fluid containing CO₂, NaCl, and H₂O components. Water activitiesin the dehydration zone average 0·36, or X_H2O = 0·25.All lines of evidence suggest that the formation of the dehydrationzone was due to advective transport of a CO₂-rich fluid witha minor NaCl brine component originating from a tectonic fracture.Fluid infiltration resulted in the localized partial breakdownof biotite and amphiboles to pyroxenes releasing Ti and Ca,which were partitioned into the remaining biotite and amphibole,as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, andF. At some later stage, H₂O-rich fluids (H₂O activity >0·8)gave rise to localized partial melting and the probable injectionof a granitic melt into the tectonic fracture, which resultedin the biotite and amphibole recording a diffusion profile forF across the dehydration zone into the granitic gneiss as wellas a diffusion profile in Fe, Mn, and Mg for all Fe–Mgsilicate minerals within 100 cm of the pegmatoid dyke. KEY WORDS: charnockite; fluids; CO₂; brines; localized dehydration; Söndrum  相似文献   

Wind tunnel and field testing of a simple sand catcher for sampling inhomogeneously saltating sand in desertified environments     

A. E. MOHAMMED  O. D. VAN DE VEER†  H. J. OLDENZIEL†  C. J. STIGTER‡ 《Sedimentology》1996,43(3):497-503

Problems of vertical alignment and vibrations of disposable coffee cups used in a modified version of the De Ploey saltating sand catcher were solved. The new version was tested in a sediment wind tunnel. Its catches appeared linearly related to amounts of eroded sand, largely independent of wind speed and wind direction, and depended logarithmically on height. The catch efficiency may therefore be taken to be approximately independent of wind speed and direction. The instrument performed well under conditions of inhomogeneously saltating sand in a strongly desertified environment in Central Sudan. Use and results show the improved simple catcher to be easy to assemble, reproducible and cheap, suitable for multipoint use to cover all inhomogeneities in outdoor saltating sand fields.  相似文献   

Integrating dynamic environmental models in GIS: The development of a Dynamic Modelling language   总被引：3，自引：0，他引：3  

CEES G WESSELUNG  DERC-JAN KARSSENBERG  PETER A BURROUGH  WILLEM P A VAN  DEURSEN 《Transactions in GIS》1996,1(1):40-48

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