Upper mantle structure of the Northern Eurasia from peaceful nuclear explosion data

Several long-range seismic profiles were carried out in Russia with Peaceful Nuclear Explosions (PNE). The data from 25 PNEs recorded along these profiles were used to compile a 3-D upper mantle velocity model for the central part of the Northern Eurasia. 2-D crust and upper mantle models were also constructed for all profiles using a common methodology for wavefield interpretation. Five basic boundaries were traced over the study area: N1 boundary (velocity level, V = 8.35 km/s; depth interval, D = 60–130 km), N2 (V = 8.4 km/s; D = 100–140 km), L (V = 8.5 km/s; D = 180–240 km) and H (V = 8.6 km/s; D = 300–330 km) and structural maps were compiled for each boundary. Together these boundaries describe a 3-D upper mantle model for northern Eurasia. A map characterised the velocity distribution in the uppermost mantle down to a depth of 60 km is also presented. Mostly horizontal inhomogeneity is observed in the uppermost mantle, and the velocities range from the average 8.0–8.1 km/s to 8.3–8.4 km/s in some blocks of the Siberian Craton. At a depth of 100–200 km, the local high velocity blocks disappear and only three large anomalies are observed: lower velocities in West Siberia and higher velocities in the East-European platform and in the central part of the Siberian Craton. In contrast, the depths to the H boundary are greater beneath the craton and lower beneath in the West Siberian Platform. A correlation between tectonics, geophysical fields and crustal structure is observed. In general, the old and cold cratons have higher velocities in the mantle than the young platforms with higher heat flows.Structural peculiarities of the upper mantle are difficult to describe in form of classical lithosphere–asthenosphere system. The asthenosphere cannot be traced from the seismic data; in contrary the lithosphere is suggested to be rheologically stratified. All the lithospheric boundaries are not simple discontinuities, they are heterogeneous (thin layering) zones which generate multiphase reflections. Many of them may be a result of fluids concentrated at some critical P–T conditions which produce rheologically weak zones. The most visible rheological variations are observed at depths of around 100 and 250 km.     

An investigation of upper mantle heterogeneity beneath the Archaean and Proterozoic crust of western Canada from Lithoprobe controlled-source seismic experiments

Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas.Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background).The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.     

Stratigraphic evolution of a fluvial–eolian succession: The example of the Upper Jurassic—Lower Cretaceous Guará and Botucatu formations, Paraná Basin, Southernmost Brazil

The Guará and Botucatu formations comprise an 80 to 120 m thick continental succession that crops out on the western portion of the Rio Grande do Sul State (Southernmost Brazil). The Guará Formation (Upper Jurassic) displays a well-defined facies shift along its outcrop belt. On its northern portion it is characterised by coarse-grained to conglomeratic sandstones with trough and planar cross-bedding, as well as low-angle lamination, which are interpreted to represent braided river deposits. Southwards these fluvial facies thin out and interfinger with fine- to medium-grained sandstones with large-scale cross-stratification and horizontal lamination, interpreted as eolian dune and eolian sand sheets deposits, respectively. The Botucatu Formation is characterised by large-scale cross-strata formed by successive climbing of eolian dunes, without interdune and/or fluvial accumulation (dry eolian system). The contact between the Guará and the Botucatu formations is delineated by a basin-wide deflation surface (supersurface). The abrupt change in the depositional conditions that took place across this supersurface suggests a major climate change, from semi-arid (Upper Jurassic) to hyper-arid (Lower Cretaceous) conditions. A rearrangement of the Paraná Basin depocenters is contemporaneous to this climate change, which seems to have changed from a more restrict accumulation area in the Guará Formation to a wider sedimentary context in the Botucatu Formation.     

Biofacies of Upper Jurassic and Lower Cretaceous sediments of central West Siberia

Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater.     

New data on Sr-and C-isotopic chemostratigraphy of the Upper Riphean type section (Southern Urals)

New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (Karatau Group of the southern Urals) are obtained for several southwestern sections of the Bashkirian meganticlinorium, which have not been studied before. The results obtained supplement the Sr-and C-isotopic information for the group upper horizons thus detailing chemostratigraphic characterization of the entire succession. Limestone and dolostone samples used to analyze the Sr isotope composition satisfy strict geochemical criteria of the isotopic system retentivity and have been subjected to preliminary treatment in ammonium acetate to remove secondary carbonate phases. Data on 255 samples of carbonate rocks (171 studied for the first time) show that δ¹³C value varies in the Karatau Group succession from ?2.8 to +5.9 ‰ V-PDB with several in-phase excursions from the general trend in all the sections studied in the area 90 × 130 km. The δ¹³C variation trend demarcates several levels in the carbonate succession of the Karatau Group suitable for objectives of regional stratigraphy and for C-isotope chemostratigraphic subdivision of the Upper Riphean. The results of Sr isotopic analysis of 121 samples (51 unstudied before) from the Karatau Group imply that rocks in its lower part (the Katav Formation and basal horizon of the Inzer Formation) experienced considerable secondary alterations, while limestones and dolostones of the overlying interval of the group are frequently unaltered. In the “best” samples satisfying geochemical criteria of the isotopic system retentivity, the ⁸⁷Sr/⁸⁶Sr initial ratio increases from 0.70521–0.70532 in the lower Inzer deposits to 0.70611 in the upper Min’yar carbonates, decreasing to <0.70600 near the top of the latter. Above the regional hiatus separating the Min’yar and Uk formation, this ratio grows from 0.70533 to 0.70605–0.70609 in the limestone succession of the last formation.     

Preliminary evaluation of hydrochemistry of the Kalambaina Formation, Sokoto Basin, Nigeria

Hydrochemical investigations in the Kalambaina Formation have been initiated to determine potability and suitability of the shallow groundwater for domestic and agricultural uses. This limestone formation is an extensive aquifer supplying water to livestock and domestic wells in its outcrop areas. The aquifer is recharged by rainfall and discharges mainly into the Sokoto–Rima River system and lakes at Kware, Gwadabawa and Kalmalo in Nigeria. Because recharge to the aquifer is mainly from rainfall, the quality of the groundwater is controlled essentially by chemical processes in the vadose zone and locally by human activities. Water samples were taken at 11 sites comprising boreholes, dug wells and a spring and were chemically analysed for their major ion components. Hydrochemical results show water of fairly good quality. It is, however, hard and generally of moderate dissolved solids content. Concentration of the total dissolved solids is between 130 and 2,340 mg/l. Concentrations of ions vary widely but a high concentration of K⁺ is found in places. NO₃⁻ is on the higher side of the World Health Organization (WHO) permissible limits, indicating pollution in such areas. Groundwater chemistry is predominantly of two facies, namely the calcium–magnesium–bicarbonate and calcium–magnesium–sulphate–chloride facies. These facies probably evolved primarily as a result of dissolution of calcium and magnesium carbonates as well as some human/land-use activities.     

DENDROGEOMORPHOLOGICAL ANALYSES ON EXPOSED ROOTS ALONG TWO MOUNTAIN HIKING TRAILS IN THE CENTRAL ITALIAN ALPS

ABSTRACT. Mountain landscapes show rapid evolution, especially at high altitudes, in response to current climate changes. In addition, the greater number of routes and trails made available and the increase in tourism have caused some impacts on mountain areas. Gradual soil erosion has been observed along some hiking trails, with the latter tending to deepen, and with stronger erosion where the trail cuts the slope. Where trails cross forested areas, trees can suffer root damage from foot trampling as roots become exposed. A dendrogeomorphological study was conducted on trees along two hiking trails in upper Valtellina (Italian Alps). On the first trail, in Valle dei Forni, European larches and stone pines, mostly with exposed roots, were sampled. A mean erosion rate of 2.7 mm/a, related to deepening of the footpath, was obtained, and various degrees of root growth disturbance along the trail were observed. In particular, the growth of many sampled roots shows an increase in ring width corresponding with the moment when root exposure occurred, followed, after 3–5 years, by rapid growth suppression. The exposure of many roots has taken place since the 1980s. No significant variations in stem growth were observed, even when there were exposed and damaged roots. Along the second trail, in Valle Alpisella, exposed roots of mountain pines were analysed. A mean erosion rate of 3.2–3.7 mm/a, related to the escarpments bordering the footpath, was obtained, while no significant changes were detected in root growth.     

吉林伊通盆地莫里青断陷古近系双阳组储集层成岩作用研究

古近系双阳组是在吉林省伊通盆地莫里青断陷中，油气赋存的主要层位,储层主要为湖底扇和扇三角洲的砂体。依据双阳组储层的大量薄片、扫描电镜、粘土矿物分析的资料研究了该储集层的成岩作用。研究结果表明双阳组储集层：成岩作用经历了压实、压溶、胶结、交代和溶解6方面的作用。双储集层中的自生粘土矿物蒙皂石向伊利石转化具有明显的特征：主要演化过程经历了蒙皂石渐变带，第一迅速转化带和第二迅速转化带，相应的成岩作用阶段可划分为早成岩阶段B期、晚成岩阶段A1期和晚成岩阶段A2期。依据在垂向上的成岩变化,建立了莫里青断陷双阳组储集层的成岩作用演化序列。在垂向上成岩作用的类型和强度均存在明显的差异，造成了储层物性的垂向分带。     

桂北地区早石炭世杜内期的长身贝类的发现及其生物古地理意义

在广西北部兴安县高尚镇金子山剖面下石炭统鹿寨组底部，新发现了长身贝类腕足化石，包括1新种、1个相似种、6个未定种和2个属种未定类型。这些长身贝类以Productellidae和Productidae科的分子为主，另有少量Echinoconchidae和Monticuliferidae科的分子。这些长身贝类全部属于生活在静水环境中的薄壳类型，无一与华南同期地层中已知类型相同。它们之间的差异可能反映了不同沉积环境条件对腕足动物群属种的控制。在鹿寨组底部发现的长身贝类以及近年在桂林南边村和黔南睦化剖面腕足动物群的发现显示出华南台缘斜坡―盆地相区杜内阶中的腕足动物群具有极高的属种分异度，不但有大量见于世界其他地方同期动物群中的分子，也包含有不少原本认为只在维宪阶或更高层位出现的类型或全新的类型，说明华南台缘斜坡―盆地相区不但与世界其他地区的腕足动物群之间存在密切的生物地理区系联系，而且华南台缘斜坡―盆地相区是杜内期腕足动物起源和辐射演化的重要策源地之一。许多属种可能是在这里起源，并迁移扩散到其他地方。     

福建省大田县广平含煤区推覆构造特征及找煤预测

通过对区内推覆构造及地层等的研究，认为广平含煤区推覆体下蕴藏着丰富的煤炭资源，童子岩组煤系地层埋藏深度主要受发育于童子岩组上部的F1、F2推覆断层控制，并圈定出煤炭资源预测前景较好的3个区作为煤炭勘查后备基地。