鄂尔多斯盆地北缘砂岩型铀矿板状矿体形成机制：来自含矿层不同分带砂岩中高岭石含量研究的约束

鄂尔多斯盆地北缘是中国北方最重要的砂岩型铀矿分布区之一。与层间氧化带砂岩型铀矿的卷状矿体不同,研究区矿体以板状、似层状为特征。国内中- 东部盆地砂岩型铀矿矿带砂岩中高岭石含量相对偏高指示含矿层至少经历一期与成矿密切相关的偏酸性流体,这与国外学者在研究板状铀矿体的过程中关注含矿层上覆泥岩压实作用下排出富含有机酸的酸性流体具有相似的特点。因此,本文为探讨酸性流体活动与板状矿体形成的内在关系,选择纳岭沟铀矿床为例,通过采取含矿层不同地球化学分带砂岩样品,应用扫描电镜、X- 衍射、U元素含量等分析手段,发现矿带与还原带砂岩中高岭石含量相对偏高这一特殊现象;系统研究后认为上述现象与含矿层下伏延安组煤层埋藏演化过程中产生的大量有机酸性流体垂向渗入含矿层有关;结合含矿层埋藏演化史,将板状矿体的形成过程分为封闭体系- 有机酸性流体发育、开放体系- 大规模铀成矿、油气大规模逸散- 叠加改造保矿三个阶段,并认为板状矿体的形成是偏碱性含铀含氧大气降水与有机酸性流体长期稳定相互作用的结果,产于两种流体混合接触面上,其中含矿层砂岩厚度是控制矿体形态的关键因素。     

巴音戈壁盆地塔木素铀矿床紫红色铀矿石矿化特征

巴音戈壁盆地塔木素铀矿床是我国北方特大型砂岩型铀矿床,但矿化特征特殊,具有品位高、矿层多等特征,其矿床成因一直存在争议,亟需新的证据来破解铀矿化成因机制。项目组野外地质调查中,发现产于氧化带内的紫红色铀矿石,具有明显的热液改造的痕迹,是一种新类型的铀矿化。笔者通过岩矿鉴定、蚀刻径迹、电子探针等实验,系统分析了该地区紫红色铀矿石的矿物蚀变特征,并探讨了铀的富集成因。研究认为,塔木素紫红色铀矿石与热液蚀变关系密切,其蚀变带可划分为萤石脉带,紫红色含萤石胶结物带和灰白色高岭石化带;铀矿物主要呈分散吸附状与胶磷矿范围分布一致。该类型铀矿石是深部还原性富U、P热流体上移进入含矿目的层与氧化带弱碱性流体相互作用的产物,pH、Eh条件的改变造成U、P同时沉淀和富集。结合矿体地质特征分析,认为塔木素矿床为表生氧化流体和深部热流体相互作用下形成的具复合成因砂岩型铀矿床,表生氧化流体造成蚀源区和地层中的铀迁移并在氧化还原过渡界面富集沉淀,而热流体携带了深部铀源进入氧化带内形成富磷型铀矿石,这是造成塔木素矿床铀品位高且矿体多层的主要原因。     

新墨西哥州格兰茨矿带铀矿勘查中的粘土矿物地球化学

对新墨西哥州格兰茨矿带中的含矿岩石和无矿岩石的粘土矿物进行了对比研究,结果表明,绿泥石(玫瑰花状)、伊利石、混合层状伊利石-蒙脱石、(±Mg蒙脱石)组合与板状矿体中的铀矿物是准同生的;这些粘土矿物以及高岭石与格兰茨矿带中靠近主要氧化还原前锋的卷型矿体关系密切。无矿岩石中的粘土矿物特征是含有较丰富的钠蒙脱石、高岭石和面-棱状(face-to edge)绿泥石。矿带中的绿泥石所含的钒要比无矿岩石中的多得多。板状矿     

鄂尔多斯盆地镇原地区洛河组黏土矿物特征及找铀意义

近年经过深入的铀矿勘查工作,在鄂尔多斯盆地西南部洛河组发现了具有工业意义的铀矿体,实现了深部铀矿勘查的重要突破。通过薄片鉴定、扫描电镜、X射线衍射、蚀变矿物光谱扫描等分析手段,对镇原地区洛河组黏土矿物的组成、含量及特征进行了系统研究。结果表明,洛河组赋矿砂岩与盆地东北部、西南部最重要的含铀岩系——直罗组具有较明显的差别。洛河组赋矿砂岩中黏土矿物总量较低,不同类型砂体中黏土矿物组合略有差异,其中泥岩、氧化砂岩、钙质砂岩、矿化砂岩中主要黏土矿物为伊蒙混层和伊利石,其次为高岭石和绿泥石。富矿砂岩和灰绿色围岩中的黏土矿物则主要是高岭石和伊蒙混层,其次为伊利石和绿泥石。扫描电镜下可见绿泥石、伊蒙混层、高岭石、少量伊利石与沥青铀矿伴生现象,但黏土矿物与铀矿物相关性不明显,表明在洛河组成矿过程中,黏土矿物可以吸附铀,但不是铀矿富集最主要的影响因素。通过对洛河组砂岩中黏土矿物赋存关系的研究,提出研究区至少存在两期绿泥石和一期高岭石的生成,揭示了由碱性→酸性→碱性流体的作用过程。黏土矿物特征可以作为铀成矿过程中后生流体示踪的重要标志。     

十红滩地浸砂岩铀矿层间氧化带蚀变矿物群

十红滩砂岩型铀矿床是我国大型层间氧化带型砂岩铀矿床之一。层间氧化带型砂岩铀矿含矿砂层在含氧含铀水的渗入径流过程中,由于水介质性状的变化,在与砂体发生水岩作用时形成了完全氧化带、不完全氧化带、还原带和原生带等不同地球化学亚带及其相对应的蚀变矿物群,即完全氧化带为褐铁矿(针铁矿、水针铁矿)、伊利石、蒙脱石、少量黄钾铁矾的蚀变矿物群;不完全氧化带为褐铁矿(针铁矿、水针铁矿)、黄钾铁矾、蒙脱石、伊利石、少量绿泥石、高岭石的蚀变矿物群;还原带为沥青铀矿、黄铁矿、高岭石、绿泥石、少量蒙脱石、伊蒙混层粘土、伊利石和碳酸盐等新生蚀变矿物群;原生带的新生蚀变矿物群以黄铁矿、绿泥石、高岭石为主,有时出现少量碳酸盐、蒙脱石和伊利石等。     

基于近红外岩心光谱扫描技术研究鄂尔多斯盆地彭阳铀矿床矿物分布特征

砂岩型铀矿作为一种以表生流体作用为主的后生矿床,发育较多低温矿物。甘肃省彭阳铀矿是近年来新发现的一处深部砂岩型铀矿床,具有砂体规模大、铀矿化面积广、厚度大、等特点。在含矿层及周边发育方解石、石膏、黄铁矿、黏土矿物等低温成因矿物,研究矿物的空间分布特征及其与铀矿物的关系,对判别成矿流体的主要来源、性质以及对铀成矿的控制作用具有重要意义。近红外岩心光谱扫描技术可无损、快速、批量地识别出岩心中的层状硅酸盐矿物(如高岭石、蒙脱石、绢云母等),硫酸盐矿物(如石膏、明矾石等),碳酸盐矿物(如方解石、白云石等)等矿物信息,本文基于该技术对彭阳铀矿床的矿物类型及其组合进行研究。结果表明：洛河组发育高岭石、蒙脱石、伊利石(伊/蒙混层)、绿泥石、碳酸盐、石膏、铁氧化物等矿物,铀矿段的矿物组合为“伊利石(伊/蒙混层)+石膏+碳酸盐”,局部可见高岭石。根据矿物组合特征反演流体作用,洛河组含铀矿段矿在沉积期主要是一套碱性环境,成矿期存在还原性酸性流体的注入。     

伊犁蒙其古尔铀矿床含矿层砂岩中黄铁矿形成机制及对铀成矿的指示意义

蒙其古尔铀矿床为伊犁盆地南缘大型层间氧化带砂岩型铀矿床,为查明该矿床含矿层中黄铁矿成因及其形成机制,探讨微生物参与铀成矿过程。文章对含矿层砂岩中黄铁矿与铀矿物矿物学特征、黄铁矿S同位素与碳酸盐胶结物的C-O同位素开展细致研究。研究表明:①蒙其古尔铀矿床中铀主要以铀矿物与吸附铀形式存在,吸附铀主要为有机质吸附铀,铀矿物以沥青铀矿为主,多与黄铁矿、炭屑共生;②蒙其古尔铀矿床含矿层砂岩中黄铁矿主要以自形晶、草莓状和不规则状集合体产出,多与沥青铀矿、碳酸盐胶结物共生,其中黄铁矿S同位素(δ~(34)S_(V-CDT)=-68.4‰～22.1‰)与碳酸盐胶结物的C-O同位素(δ~(13)C_(V-PDB)=-10.2‰～-7.4‰,δ~(18)O_(V-PDB)=-9.6‰～-5.8‰)分析表明黄铁矿具有细菌硫酸盐还原(BSR)与有机物热解2种成因,并探讨了这2种不同成因黄铁矿的形成机制。③结合前人研究成果,认为硫酸盐还原菌(SRB)参与蒙其古尔铀矿床铀成矿过程,以间接还原方式为主,在有机质、黏土矿物与颗粒表面吸附U(Ⅵ)的基础上,通过硫酸盐还原菌(SRB)还原SO_4~(2-)产生的H_2S将U(Ⅵ)被还原成U(Ⅳ),形成铀矿物。     

二连盆地恩格日音砂岩型铀矿床地质特征及成矿作用初探

恩格日音铀矿床是近年在二连盆地马尼特坳陷南缘新发现的一处大型砂岩型铀矿床。矿体在平面上呈近EW向展布,矿带长度超过7.6 km,宽度400～800 m,垂向上可分为三层,产状较平缓,主要赋存于下白垩统赛汉组冲积扇相及辫状河道相灰色粗砂岩、细砂岩、泥质砂砾岩中,呈层状、透镜状产出。矿石以低品位含铀碎屑岩型为主,铀主要以吸附态赋存于杂基中,其次以沥青铀矿和铀石形式存在。矿区主要蚀变类型包括黄铁矿化、褐铁矿化、绿泥石化、黏土化等,其中黄铁矿化与铀矿化关系密切。恩格日音铀矿床上部矿体成矿类型属于层间氧化带型,下部矿体形成时代为早白垩世晚期115.1±5.9Ma,与赛汉组下段沉积时期接近,成矿类型属于沉积同生型,该结论深化了马尼特坳陷南缘铀成认识,拓宽了该区铀矿找矿方向。     

伊犁蒙其古尔铀矿床含矿层砂岩中高岭石形成机制及与铀成矿关系

伊犁盆地蒙其古尔铀矿床为典型的层间氧化带型铀矿床,虽然前人对该矿床含矿层砂岩中高岭石含量、成因及与铀成矿关系等方面开展了研究,但对高岭石及其共生矿物形成机制、高岭石与铀成矿关系等方面研究依然较为薄弱。本文为了研究蒙其古尔铀矿床含矿层砂岩中高岭石形成机制及与铀成矿关系,综合的应用了偏光显微镜、X-衍射、扫描电镜等手段,对其含矿层中高岭石含量、形貌特征进行了系统的研究,并对高岭石H- O同位素与碳酸盐胶结物C- O同位素进行了研究。研究表明:①含矿层普遍发育有强烈的高岭石化,其占黏土总量的58. 0% ~ 93. 0%,并以弱氧化带砂岩中高岭石含量最高;②高岭石主要呈泥晶状、层片状与蠕虫状的形式分布于砂岩粒间孔隙和长石溶蚀空隙中,共生矿物为自生石英、石膏、黄铁矿、铀矿物等;③高岭石D V- SMOW 为-119. 4 ‰~-48. 3‰, 18 O V- SMOW 为10. 9 ‰~13. 7 ‰,碳酸盐胶结物 δ 13 C V- PDB 为-10. 9～-7. 2‰, δ 18 O V- SMOW 为17. 6‰～24. 9‰,表明高岭石是由富含CO 2 大气降水（无机）和煤系地层有机质脱羧基及热降解作用产生的有机酸及CH 4 等还原性气体（煤层气）（有机）与砂岩相互作用的产物,并认为高岭石及其共生矿物是在含矿层埋藏成岩过程中流体与岩石相互作用的结果;④高岭石与铀矿化除了存在直接吸附、间接还原关系,还存在成因上的联系,即高岭石和铀成矿皆是有机-无机流体与砂岩相互作用的结果。     

东胜砂岩型铀矿床低温热液流体的证据及意义

东胜地区砂岩型铀矿床是近年来发现的大型砂岩型铀矿床。笔者首次发现了低温热液流体成矿作用的证据：矿体中有后生的钛铀矿和锐钛矿组合。矿石流体包裹体均一温度为58～176℃，平均114．9℃。研究表明，鄂尔多斯盆地在中晚侏罗世-白垩纪发生过显著的构造热事件，造成砂岩型铀矿含矿层之下产生大量的微裂隙与裂隙带的出现，导致下部热液流体向上运移。低温热液流体的成矿作用是形成东胜铀矿床的重要因素。     

Crustal thickness and VP/VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions

We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average V_P/V_S ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km² region of the Grenville orogen, a 1.2–0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with Lithoprobe and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4 ± 1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by V_P/V_S ranging from 1.78 to 1.85. In other parts of the Grenville orogen, V_P/V_S is found to be generally less than 1.80. The thinnest crust (34.5–37.0 km) occurs northeast of the 0.7 Ga Ottawa–Bonnechere graben and correlates with areas of high intraplate seismicity.     

Equation of state,structural behaviour and phase diagram of synthetic MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, as a function of pressure and temperature

The behaviour of synthetic Mg-ferrite (MgFe₂O₄) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch–Murnaghan equation of state result in K₀=181.5(± 1.3) GPa, K=6.32(± 0.14) and K= –0.0638 GPa^–1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by _a0+_a1*(T–298)+_a2/(T–298)², where _a0=9.1(1) 10^–6 K^–1, _a1=4.9(2) 10^–9 K^–2 and _a2= 5.1(5) 10^–2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the ONeill model, whose parameters are = 22.2(± 1.8) kJ mol^–1 and =–17.6(± 1.2) kJ mol^–1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe₂O₄, which shows that the high-pressure transition from spinel to orthorombic CaMn₂O₄-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe₂O₃.     

High-T phase transition of synthetic ANaB(LiMg)CMg5Si8O22(OH)2 amphibole: an X-ray synchrotron powder diffraction and FTIR spectroscopic study

The synthetic amphibole Na_0.95(Li_0.95Mg_1.05)Mg₅Si₈O₂₂(OH)₂ was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P2₁ /m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and 3,712 cm⁻¹, respectively, and two minor bands at 3,667 and 3,687 cm⁻¹. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing the presence of small amount of vacant A-site (^A□_0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T _c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P2₁ /m ⇔ C2/m phase transition at 255°C for synthetic amphibole ^ANa_0.8^B(Na_0.8Mg_1.2)^CMg₅Si₈O₂₂(OH)₂ indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T _c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy.     

Crustal transect from the North Atlantic Knipovich Ridge to the Svalbard Margin west of Hornsund

The crustal structure along a 312 km transect, stretching from the axial mountains of the North Atlantic Knipovich Ridge to the continental shelf of Svalbard, has been obtained using seismic reflection data and wide angle OBS data. The resulting seismic V_p and V_s models are further constrained by a 2-D-gravity model. The principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow-spreading counterpart of the North Atlantic Ridge Systems. V_p is estimated to be 3.50–6.05 km/s for the upper oceanic crust (oceanic layer 2), with a marked increase away from the ridge. The measured V_p of 6.55–6.95 km/s for oceanic layer 3A and 7.10–7.25 km/s for layer 3B, both with a V_p/V_s ratio of 1.81, except for slightly higher values at the ridge axis, does not allow a clear distinction between gabbro and mantle-derived peridotite (10–40% serpentized). The thickness of the oceanic crust varies a lot along the transect from the minimum of 5.6 km to a maximum of 8.1 km. The mean thickness of 6.7 km for the oceanic crust is well above the average thickness for slow-spreading ridges (<10 mm/year half-spreading rate). The areas of increased thickness could be explained by large magma production-rates found in the zones of axial highs at the ridge axis, which also have generated the off-axial highs adjacent the ridge. We suggest that these axial and off-axial highs along the ridge control the lithological composition of the oceanic crust. This approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments (the axial and off-axial highs) and mantle-derived peridotite outside these zone.     

A study of turbulent characteristics of atmospheric boundary layer over monsoon trough region using Kytoon and Doppler sodar

As a part of the MONTBLEX-90 observational programme, Kytoon and Doppler sodar observations were taken at Kharagpur. These data are analysed to study the turbulent characteristics of the atmospheric boundary layer in terms of stability, temperature structure function (C _T ² ) and velocity structure function (C _v ² ).C _T ² follows aZ ^−4/3 law on most of the days, whereas the variation ofC _V ² is not systematic.C _V ² andC _T ² values are found to vary between 10⁻⁵−10⁻¹ m^4/3s⁻² and 10⁻⁵−10⁻²°C² m^−2/3 respectively.     

Elastic properties and stability of coexisting 3T and 2M 1 phengite polytypes

The elastic properties of coexisting natural 3T and 2M ₁ phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature–high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P–V curves have been modelled by the Birch–Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M ₁ K ₀=60.4(±0.7) GPa, K′=5.79(±0.11) at V ₀=703.8851 ų, and K ₀=57.3(±1.0) GPa, K′=6.97(±0.24) at V ₀=938.8815 ų, respectively. The relative stability of 3T vs. 2M ₁ has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results presented agree with the hypothesis of stability of the 3T polytype in the high pressure regime.     

Numerical analysis as applied to the environmental interpretation of carbonate microfacies of some sub-Himalayan carbonates (lower upper precambrian) in northwestern India

Forty-three samples of (1) Sirban Limestone of Riasi, Jammu and Kashmir State, India, based on six mineralogic and petrographic presence-absence variables, and (2) 36 samples of Bilaspur Limestone based on 11 presence-absence mineralogic and petrographic variables, were numerically classified by Q-mode cluster analysis using Sokal and Michner's Simple Matching Coefficient. Sirban Limestone of Riasi yielded 10 clusters or groups indicating four depositional environments: (1) intertidal, (2) intertidal to subtidal, (3) subtidal, and (4) deep-water basinal. Similarly, Bilaspur Limestone yielded nine clusters indicative of the same four environments. A Q-mode ordination of the Riasi samples and of Bilaspur Limestone shows a generalized shelf-to-basin carbonate deposition profile. Results of this study substantiate earlier conclusions of Rao (1979) based on field, mineralogic, and petrographic study of the above-mentioned carbonates.     

俯冲带硫的地球化学行为及硫循环SCIEI北大核心CSCD

俯冲带是全球最大的物质循环系统,控制着硫(S)在地球内部圈层及表层的循环,影响着大气圈、水圈、生物圈、岩石圈的稳定性以及地球的宜居性。厘清S在俯冲带中的地球化学行为和循环特征对理解地球各储库的氧化还原状态、岩浆作用与演化、成矿物质聚集、以及地球大气成分等具有重要意义。本文首先总结了进入俯冲带之前的大洋岩石圈的S结构模型,对S在大洋板片中的分布状态和地球化学特征进行了系统归纳。随后,系统阐述了俯冲带高压-超高压变质岩记录的板片变质及脱水过程中硫的地球化学行为。岩石学研究表明俯冲板片中的S多以硫化物相存在,硫酸盐矿物在弧前深度就已被释放或分解。相较于熔体,俯冲带流体中S的溶解度更高,是运移硫的更有效方式。DEW模型计算结果显示,流体中S含量总体较低,但在俯冲板片~90km处其含量有一个峰值(浓度0.5%~1.0%)。岩相学证据、地球化学测试结果、磷灰石S近边吸收结构(S-XANES)特征以及模拟结果都显示俯冲深部流体中S多以HS^(-)及H_(2)S形式存在,不含大量的SO_(4)^(2-)及硫酸盐;中f_(S_(2))流体有利于S迁移出俯冲板片,从而促进俯冲带大规模S循环,而高f_(S_(2))流体在流-岩交换过程沿流体通道发生S的锁固作用而不利于俯冲带S循环。质量平衡计算显示全球俯冲带S输入通量为4.65×10^(13)g/yr,弧下深度板片S输出通量为2.91×10^(12)g/yr,板片-岛弧S循环效率仅6.3%。俯冲板片在弧下深度可能存在一个短暂高效的S释放窗口,释放流体的δ^(34)S值为-2.1±3.0‰。基于高压-超高压变质岩中硫化物的研究,初步厘清了俯冲板片中S的地球化学行为,首次从板片角度全面、定量地限定了俯冲带的脱硫通量、效率、种型和同位素特征,提出俯冲带循环的S不是岛弧岩浆的氧化剂,与岛弧环境的正δ^(34)S值也无直接因果联系,对解析俯冲带S循环和理解地球长期的S循环具有重要意义。最后,本文还展望了俯冲带S循环的未来发展方向,应在俯冲带流体氧化还原性质(硫酸盐的命运)、俯冲沉积物对S循环的制约、俯冲带环境下多硫同位素的分馏效应、S循环与其它挥发分(如C等)循环之间的耦合关系、地球历史上深部S循环等方向做出探索,更深入地理解俯冲带及全球S循环过程。     

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V_p/V_s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V_p/V_s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V_p/V_s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V_p/V_s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V_p/V_s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V_p/V_s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).     

Stress induced time dependent behavior of clayey soils

It is shown that time compression curve obtained from one-dimensional consolidation curve in the laboratory may include six phases. These are initial compression, first primary compression, transition from first primary compression to second primary compression, second primary compression, and transition from second primary compression to creep and lastly creep. This paper attempts to identify the quantitative beginnings and characteristics of these phases. A mathematical characteristic of all the soils that follow primary consolidation as per Terzaghi’s one dimensional consolidation theory is derived. It is known as the constant of primary consolidation. It is used to study the beginning of secondary consolidation and its effects on primary consolidation. Another characteristic of soils for creep and total absence of primary compression is derived. Methods are suggested for the determination of coefficients of Primary and Secondary consolidations and the compression index.