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1.
The formation of deep gullies (called ‘dongas’ locally) in rangeland in KwaZulu-Natal Province in South Africa is a natural phenomenon. These U-shaped, very wide gullies have considerable lateral expansion due to the episodic collapse of sidewalls. The dongas have developed in duplex soils such as Luvisols and Lixisols formed on Permian sedimentary rocks or unconsolidated Quaternary colluvium. This study combined morphological, mineralogical and chemical characterization with measurements of grain-size content, structural stability and the complete shrinkage curve to detect changes in soil properties of the different horizons located in the gully banks. The different soil horizons present clear and sharp differences in physical and mineralogical properties. The topsoil with complete grass cover is very resistant to soil detachment. However, the leached E horizon and the BC horizon have low structural stability. The soil profile down to and including the Bt horizon contains exclusively illite in the clay fraction, while the BC colluvial layer and the C horizon (mudstone) contain expandable interstratified illite–smectite. The Bt horizon has a high water content at saturation and high shrinkage, while the BC and C horizons have a high residual shrinkage and a very low water content at saturation. Because this type of gully expansion is not significantly linked to slope value or the stream power index (SPI) at the gully head, to land-use change, high rainfall intensities or the threshold of concentrated runoff being exceeded at the gully head, other causes were investigated. It was concluded that the heterogeneity between horizons with different mineralogical properties and structural stabilities, soil types and parent material, anisotropic water-saturation and shrink-swell properties are of major importance. This heterogeneity between different soil horizon morphologies and their physical properties can explain why the relationship between the critical slope and the drainage area for gully initiation showed a threshold for gullying much lower than that found elsewhere. © 2020 John Wiley & Sons, Ltd.  相似文献   

2.
The establishment of a tephra framework for the Greenland ice-cores spanning the last glacial period, particularly between 25 and 45 ka b2k, provides strong potential for precisely correlating other palaeoclimatic records to these key archives. Tephra-based synchronisation allows the relative timing of past climatic changes recorded within different depositional environments and potential causal mechanisms to be assessed. Recent studies of North Atlantic marine records have demonstrated the potential of tracing cryptotephra horizons in these sequences and the development of protocols now allows a careful assessment of the isochronous nature of such horizons. Here we report on tephrochronological investigations of a marine sequence retrieved from the Goban Spur, Eastern North Atlantic, covering ∼25–60 ka b2k. Density and magnetic separation techniques and an assessment of potential transport and depositional mechanisms have identified three previously unknown isochronous tephra horizons along with deposits of the widespread North Atlantic Ash Zone II and Faroe Marine Ash Zone III. Correlations between the new horizons and the Greenland ice-core tephra framework are explored and despite no tie-lines being identified the key roles that high-resolution climatostratigraphy and shard-specific trace element analysis can play within the assessment of correlations is demonstrated. The previously unknown horizons are new additions to the overall North Atlantic tephra framework for the last glacial period and could be key horizons for future correlations.  相似文献   

3.
Measurements have been made of unsaturated and saturated lateral soil water flow on a convex hill-slope with a good soil cover and impermeable bedrock during natural rainstorms. The hydraulics of flow are examined in detail with particular reference to the role of breaks in vertical permeability, the change from saturated to unsaturated flow and the velocity of flow. In this instance, after rainfall slope flow is dominated by vertical unsaturated movement towards the profile base. Preceding upslope moisture gradients result in the growth of a zone of soil saturation upwards from the slope base. Slope discharge, through the B and B/C horizons, is related to the form of the saturation zone, within which flow is lateral, according to Darcy's law. The time required for vertical percolation and the low hydraulic conductivity of the lower soil horizons result in a hillslope hydrograph which is delayed and attenuated and cannot be regarded as stormflow. During drainage the saturation zone contracts and is replaced by a lateral unsaturated flow system at the profile base which supplies discharge from the B/C horizon for up to 42 days without further recharge. It is concluded that, in general, either distinct soil horizons or impermeable bedrock are essential for the initiation of lateral flow. Saturated flow is likely to dominate hillslope hydrographs through non-capillary pore spaces but these may be integrated to the point where Darcy's law still holds. Although lateral soil water flow must be a widespread phenomenon, it is unlikely to provide storm runoff to the stream unless saturated conditions are generated within the organic horizons for flow within the lower soil horizons is dominated by non-Darcian flow through non-capillary spaces in the soil.  相似文献   

4.
Palaeomagnetic investigations of highly oxidized interbasaltic horizons and associated Lower Tertiary lavas from the Faeroe Islads suggest a long history of magnetization. The remanence directions are composed mostly of two deviating directions of opposite polarities aligned along an axis which palaeomagnetically appears to be younger than that of the original field. This, in addition to the observation that both the baking and baked rocks in most cases show almost identical directional trends upon demagnetization, questions the general validity of the baked contact test. It is suggested that to a major extent post-baking processes, such as low-temperature oxidation and precipitation of ferric oxides, have remagnetized the lavas as well as the tuff layers. Only two of the seven interbasaltic horizons investigated appear to have retained the ‘primary’ magnetization. It is concluded that besides a high bulk oxidation state (induced prior to, or as a result of baking) a “hornfelsic” nature of the baked sediments may be a necessary requirement for establishing primary palaeomagnetic directions in many volcanic regions.  相似文献   

5.
The Cane Run Bed is a prominent unit of micrograined limestone and shale of distal storm origin in the lower part of the Late Ordovician (late Chatfieldian; late Caradoc) Lexington Limestone (Trenton) in central Kentucky, USA. The unit's prominence relates to presence of up to three horizons of penecontemporaneous, soft-sediment deformation that form a distinctive event bed; equivalent horizons of different lithology are also deformed. Concurrence of four lines of evidence, including deformation consistent with a seismogenic origin, widespread distribution in temporally and stratigraphically constrained horizons, a pattern of increasing frequency or deformation intensity toward likely epicentral areas, and the ability to exclude other likely causes, points strongly to a seismogenic origin for each horizon. This interpretation is bolstered by the facts that the deformation contains random fold axes, crosses facies boundaries, is associated with a periodically reactivated basement structural lineament, and crosscuts undeformed beds from both above and below. Moreover, mapping the distribution of deformation intensity allows for the interpretation of possible epicentral areas. The association of Cane Run deformation with other sedimentologic-stratigraphic anomalies related to reactivated basement faults, as well as their coincidence with the inception of a Taconian tectophase, suggest that Cane Run seismites are one of several responses to the distal transmission of far-field forces from the coeval Taconian orogeny into the foreland, largely via zones of basement structural weakness. Units like the Cane Run Bed and its equivalents, whose seismogenic origins can be confidently demonstrated, suggest that seismicity must have had a substantial influence at times on epicontinental sedimentation, even in seas far removed from orogenic sources of stress.  相似文献   

6.
In eight areas of Central and Eastern Europe elevated electrical conductivity horizons determined from magnetotellurics, heat flow densities and crustal structure were identified. Based on these data, it was determined whether good conducting horizons are the consequence of elevated temperatures or not. In three areas investigated the thermal activation was detectable; these areas are Alpine subsidences. In the five other areas no thermal activation was found, the good conducting layers on these areas have been caused by other disturbing effects, such as material inhomogeneities or near surface disturbances.  相似文献   

7.
Polarity zones of sedimentary sections reflect a pattern of alternating polarity of the geomagnetic field recorded by the remanent magnetization of rocks. Unfortunately, this pattern can have been modified by the variable sedimentation rate, which complicates the identification of polarity zones against the reference geomagnetic polarity time scale. To avoid this obstacle, the present paper suggests a transform applied to both the sequence of levels of polarity reversal horizons and the sequence of ages of polarity reversals before computing their cross-correlation. This transform usually reduces the impact of the variable sedimentation rate so that a sequence of more than eight polarity reversal horizons may be identified without biostratigraphic constraints. Numerical experiments involving random processes to simulate both the duration of polarity reversals and the sedimentation rate proved, however, that not all the parts of a hypothetical stratigraphic section spanning the past 165 Ma would be equally suitable for dating by magnetic polarity stratigraphy. A program performing both the compilation of polarity zones from the directions of the primary magnetization sampled along a section and subsequent identification of these polarity zones is made available online.  相似文献   

8.
The detailed stratigraphic study of the pyroclastic surge units S1, IU, and S3 produced during the most violent phases of the 1982 eruption of El Chichón volcano, contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water. Component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates. Based on their morphology and internal structure four different types of aggregates were distinguished: (a) accretionary lapilli, (b) armored lapilli, (c) irregular aggregates, and (d) cylindrical aggregates. The first three types have been described in the volcanological literature (field and experimental studies); cylindrical forms are reported here for the first time. These hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer-grained matrix. They formed around millimeter-size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges. SEM analyses suggest different mechanisms of formation for the four types of aggregates. Irregular aggregates and armored lapilli formed nearly instantaneously, whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud.All types of ash aggregates contain fractured particles. This common feature suggests that particles ruptured during fragmentation prior to the growth of the aggregates. Broken clasts with cracks filled by a fine-grained matrix only occur inside the cylindrical ash aggregates and to a lesser degree in some types of accretionary lapilli. This suggests that small thermal contrasts at the contact of warm particles with the colder fine-grained matrix of the aggregate cause existing small fractures to propagate and open as the already weakened clasts deform slightly. The occurrence of all four types of aggregates in some horizons indicates that several mechanisms of aggregation occurred nearly simultaneously. The pyroclastic clouds therefore were not only stratified in terms of density but the content of fluid phases also were not uniform. A dark-red, Fe-rich amorphous film (locally rich in P and S) envelops the particles and fosters their preservation in the deposits by forming a hard shell. The composition of this cement reflects the abundance of these elements in acid fluids of hydrothermal systems that were intersected by the conduit during the eruption. In distal areas, fallout aggregates were incorporated by dissipating pyroclastic surges.  相似文献   

9.
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10.
Concentrations of major and trace metals were determined in eight sediment cores collected from the inter-tidal zone of the Medway Estuary, Kent, UK. Metal associations and potential sources have been investigated using principal component analysis. These data provide the first detailed geochemical survey of recent sediments in the Medway Estuary. Metal concentrations in surface sediments lie in the mid to lower range for UK estuarine sediments indicating that the Medway receives low but appreciable contaminant inputs. Vertical metal distributions reveal variable redox zonation across the estuary and historically elevated anthropogenic inputs. Peak concentrations of Cu, Pb and Zn can be traced laterally across the estuary and their positions indicate periods of past erosion and/or non-deposition. However, low rates of sediment accumulation do not allow these sub surface maxima to be used as accurate geochemical marker horizons. The salt marshes and inter-tidal mud flats in the Medway Estuary are experiencing erosion, however the erosion of historically contaminated sediments is unlikely to re-release significant amounts of heavy metals to the estuarine system.  相似文献   

11.
A complicated geological structure may make it difficult to recognize seismic horizons. This may be the case when blind zones are present, when horizons have to be identified on both sides of a fault, or when lithological facies exhibit a lateral variation. A method for the recognition of seismic reflections is described. The process takes the dynamic range of seismic information into account as well as the extensive logical possibilities of modern computers. The method implements a factorial discriminant analysis of seismic traces by means of a stepwise iterative process. The reliability of results is studied by examining the a posteriori discrimination between different horizons. If the classification is good, a percentage of well classified reflections is given for each horizon.  相似文献   

12.
The results of magnetic and X-ray studies of the magnetic extracts separated from highly magnetically susceptible horizons of the Late Pleistocene sediments from the North Caspian Basin are presented. Greigite is shown to be the major carrier of magnetic properties in these horizons. Its coercive parameters are characteristic of the predominantly single-domain state of magnetic grains. It is found that the Curie point of greigite is at least 460°C, while the specific magnetization of pure greigite is half the saturation magnetization of magnetite.  相似文献   

13.
New magnetic and paleomagnetic data from the loess-soil Volodarka section (the Ob River) are presented. In particular, a secondary magnetization aligned with the contemporary magnetic field and persisting on heating to 500–600°C is shown to be present. It is shown that the transition zone of the Matuyama-Brunhes reversal is most likely located within the soil complex, consisting of three paleosol horizons separated by two horizons of loesslike loam that belong to the same interglaciation.  相似文献   

14.
Silcretes exposed within river-marginal or valley settings have been described in a number of studies, but few models have been suggested for the development of these ‘fluvial’ silcretes. An exception is that proposed by McCarthy and Ellery (Journal of Sedimentary Research, 1995, Vol. A65, pp. 77–90) to describe mechanisms of early stage near-surface silica diagenesis in the Okavango Delta, Botswana. This paper describes the characteristics and possible origins of massive surface and sub-surface silcretes at Samedupe and Boro Junction, beyond the distal margin of the Okavango Delta and further downstream than the sites described by McCarthy and Ellery. Morphological and petrological evidence from surface exposures and three sedimentary cores suggests that other modes of formation may also be applicable. A dual model of formation is proposed: surface silcretes are suggested to have developed by silica accumulation in seasonal pools remaining after the annual Okavango flood, whilst sub-surface horizons appear to have formed under conditions of varying pH associated with fluctuating groundwater levels beneath the channel floor. This model is reviewed in the context of the fluvial silcrete debate. © 1998 John Wiley & Sons, Ltd.  相似文献   

15.
A number of samples from the A0, A2, B and C horizons have been collected from podzolic soils in the Lake Gosciaz catchment in central Poland. The catchment is very uniform consisting of 2–6-m-high sand dunes with sparse stands of pine and a grass undervegetation. Radiocarbon dates of fossil soils covered by dune sand, within the same type of environment as the Lake Gosciaz catchment, indicate that the main dune-forming phase is of Older Dryas age. Magnetic measurements have been carried out on bulk samples and on particle size fractions from the different soil horizons. With respect to our mineral magnetic study it can be concluded that the soils are very uniform over the entire catchment. Relatively high concentrations of magnetic grains are found in the humus layer, particularly in the <4 μm size fraction, as indicated by the highest X, Xfd, SIRM and HIRM values. The most likely mechanism for the magnetic enhancement in these podzolic top soils is continued burning processes. Low magnetic concentrations in the A2 horizon are due to eluviation and chelation. Local conditions can lead to a range of magnetic minerals precipitating in the B horizon.  相似文献   

16.
Prediction of the material composition of deep crustal horizons in the Earth from the geophysical data requires an algorithm to classify the rocks according to their petrophysical properties. In the present work, we propose a classification algorithm that is based on the membership function and describe the computer program, which is based on this algorithm and intended for visualization of the most typical crystalline rocks of the Voronezh massif.  相似文献   

17.
New data on the distribution of organic compounds at different horizons (0–10, 30–40, and 70–80 cm) of bottom sediments of the fresh Fyrkal Lake (Republic of Khakaßsia) and the salt Lake Mormyshanskoe (Altai Territory) have been collected. The compositions of organic compounds in the sediments in the examined lakes has been found to differ considerably, though some parameters in their composition show changes in the same direction. The concentrations of triterpenoids of the groups of lupane, oleanane, and hopane in the sediments decrease with depth. An increase in the share of saturated compounds in the composition of steroids is accompanied by a decrease in the ratio of their isomers (5β/5α). Terrestrial vegetation dominates among organic matter sources, while, in Lake Fyrkal, the contributions of terrestrial plants and macrophytes are similar. The presence of thermally transformed steroids and triterpenoids throughout the vertical profile of sediments in Lake Mormyshanskoe may be an indication to a slight oil pollution of the water body.  相似文献   

18.
南海北部为张裂大陆边缘,在新生代时期经历了复杂的地质演化,具有良好油气勘探前景.本文首次利用南海北部某区域长排列地震数据的远偏移距折射波速度计算方法获得南海北部地层层速度,并进行了分析,此外,还提出了非初至折射波的概念.本方法在共偏移距剖面上识别、拾取折射波层位,得到折射波走时的横向连续变化数据,利用不同共偏移距同一折射波层位的到达时差计算该折射层位的速度结构.该方法在南海北部陆坡某区域地震资料的应用,揭示了新生界地层层速度的整体结构,反映了新生代不同时代地层的速度变化特征,为进一步利用折射层速度资料探讨南海新生代地质演化提供了新的视角.研究表明,与常规初至折射波法相比,远偏移距折射波能够提供更多的地质信息;该方法的初步应用能够对远偏移距折射波的研究提供进一步的视角.  相似文献   

19.
In order to better understand the nature of deep crustal high electrical conductivity, we studied the electrical properties of a tilted section of a former lower continental crust exposed in the Calabrian arc of the Alpine-Apennine mountain system. Geoelectric field measurements and impedance measurements on rock samples showed that these high-grade metamorphic rocks are generally highly resistive as expected for crystalline, electrolytically conducting rocks of low porosity. This holds for graphite-free metabasites as well as for metapelites which generally contain accessory, up to 3% biogenic graphite in the form of isolated grains. Clearly as an exception, a group of thin stratiform black horizons with thicknesses of 1-15 cm within the metapelitic series was detected by means of self-potential (SP) measurements. Rock samples from these horizons exhibit high, quasi-metallic bulk conductivities of up to 50 S/m (0.02 Ωm) in agreeement with up to 20% syngenetic graphite, forming a network of interconnected streaks or crack fillings. The high amount of carbon most probably originates from organic matter of Corg-rich black shales. Relative enrichment of the low mobility graphitic matter compared to the carbon content of the assumed protoliths may have been due to pressure solution and partial melting during prograde metamorphism, without major contribution of a fluid phase, resulting in isolated graphite flakes. Although enriched, graphite in this form has little effect on electrical conductivity. For the Calabrian black horizons, microscopic analyses make conceivable that, in a further decisive step, isolated graphite grains were mechanically smeared to continuous pathways during uplift by shearing, producing hereby the observed graphitic network which is needed to generate high conductivity. As Corg-rich black shales are common members of sedimentary sequences throughout the earth’s history, good conductors of this type may be expected in the continental crust at any depth depending on tectonic and metamorphic history, with the exception of magmatic protoliths. Regarding the extremely high conductivity of the meta-black shale samples containing syngenetic sheared graphite, a total thickness of a few meters of such rocks is sufficient to explain magnetotelluric high conductivity anomalies in the deep crust.  相似文献   

20.
Carolina Ödman makes the case for UNAWE, a means to share the inspiration and wonder of the universe and modern science with the very young. Some of them will become the talented scientists of the future; all of them should have wider horizons as adults.  相似文献   

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