An (U‐Th)/He age for the shallow‐marine Wetumpka impact structure,Alabama, USA     

Jo‐Anne WARTHO  Matthijs C. VAN SOEST  David T. KING JR.  Lucille W. PETRUNY 《Meteoritics & planetary science》2012,47(8):1243-1255

Abstract– Single crystal (U‐Th)/He dating was applied to 24 apatite and 23 zircon grains from the Wetumpka impact structure, Alabama, USA. This small approximately 5–7.6 km impact crater was formed in a shallow marine environment, with no known preserved impact melt, thus offering a challenge to common geochronological techniques. A mean (U‐Th)/He apatite and zircon age of 84.4 ± 1.4 Ma (2σ) was obtained, which is within error of the previously estimated Late Cretaceous impact age of approximately 83.5 Ma. In addition, helium diffusion modeling of apatite and zircon grains during fireball/contact, shock metamorphism, and hydrothermal events was undertaken, to show the influence of these individual thermal processes on resetting (U‐Th)/He ages in the Wetumpka samples. This study has shown that the (U‐Th)/He geochronological technique has real potential for dating impact structures, especially smaller and eroded impact structures that lack impact melt lithologies.  相似文献   

The education of hydrologists (Report of an IAHS/UNESCO Panel on hydrological education)     

J. E. NASH  P. S. EAGLESON  J. R. PHILIP  W. H. VAN DER MOLEN  V. Klemeš 《水文科学杂志》2013,58(6):597-607

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A MONTHLY WATER BALANCE MODEL INCLUDING DEEP INFILTRATION AND CANAL LOSSES / Un modèle mensuel du bilan hydrologique inclusive l'infiltration profonde et les pertes des canaux     

A. VAN DER BEKEN  J. BYLOOS 《水文科学杂志》2013,58(3):341-351

Abstract

A monthly water balance model is being successfully applied to the Grote Nete test basin (553 km²) in the North of Belgium. This low region has a complex geological structure, its boundaries are more or less unknown and deep infiltration into a deep aquifer is most likely to occur. Moreover, the area is crossed by several navigation canals which import and export an unknown volume of water.

The inputs are monthly precipitation and Penman potential evapotranspiration values. The model computes actual evapotranspiration, water storage in the basin, direct runoff and infiltration, baseflow and total stream discharge, deep infiltration loss into the underlying aquifer and constant seepage from the canals. The ‘pattern search’ procedure has been used for automatic optimization of the six model parameters. All parameters have a physical meaning and can be evaluated initially.

For the calibration period 1967–1972, total stream discharge has been calculated with a precision of 0.2 per cent compared to total measured volume. The correlation coefficient is 92 per cent for the same calibration period. Prediction for the period 1973–1974 gave a volume precision of 8.7 per cent and a correlation coefficient of 93 per cent.  相似文献   

Design of a spatial data structure using the relational normal forms     

JAN W. VAN ROESSEL 《International journal of geographical information science》2013,27(1):33-50

Abstract

In previous work, a relational data structure aimed at the exchange of spatial data between systems was developed. As this data structure was relational it was of first normal form, but compliance with the higher normal forms was not investigated. Recently, a new procedural method for composing fully normalized data structures from the basic data fields has been developed by H. C. Smith, as an alternative to the process of non-loss decomposition which is difficult to understand. Smith's method has been applied to data fields required to store points, lines and polygons in a chain-node spatial data model. When geographic domain, coverage layer and map are also considered, the procedure naturally leads to a catalogue model, needed for the exchange of spatial data. Although the method produces a fully normalized data structure, it is not as easy to identify which normal forms are responsible for the ultimate arrangement of the data fields into relations, but the benefits of these criteria for data base development also apply to spatial data structures and related ancillary data.  相似文献   

河口海岸中长时间尺度动力地貌系统模拟研究与进展     

郭磊城  何青  Dano ROELVINK  Zhengbing WANG  Mick VAN DER WEGEN 《地理学报》2013,68(9):1182-1196

我国河口海岸动力沉积和动力地貌研究以沉积学和地貌学的方法侧重静态描述,动力地貌相互作用的定量研究不多,与缺少动力地貌模型技术有关。本文介绍了中长时间尺度的河口海岸动力地貌模型技术及其发展和应用,讨论了决定动力地貌演变的泥沙余输运及引起泥沙余输运的主要动力因子,介绍了动力地貌模型的应用进展,进而分析了河口海岸地貌过程的机制和地貌平衡系统。本文指出对淤泥质及复杂动力条件下的河口海岸动力地貌过程还有待进一步研究。  相似文献   

Optical dating of relict sand wedges and composite-wedge pseudomorphs in Flanders, Belgium     

JAN-PIETER BUYLAERT  GUNTHER GHYSELS  REW S. MURRAY  KRISTINA J. THOMSEN  DIMITRI VANDENBERGHE  FRANS DE CORTE  IRENÉE HEYSE  PETER VAN DEN HAUTE 《Boreas: An International Journal of Quaternary Research》2009,38(1):160-175

We report on quartz Optically Stimulated Luminescence (OSL) dating of the infill of 14 relict sand wedges and composite-wedge pseudomorphs at 5 different sites in Flanders, Belgium. A laboratory dose recovery test indicates that the single-aliquot regenerative-dose (SAR) procedure is suitable for our samples (measured to a given dose ratio 0.980 ± 0.005; n =139). Completeness of resetting of the wedge infill of two samples was confirmed by single-grain analyses. The suite of optical ages indicates that repeated thermal contraction cracking, degradation and infilling with wind-blown sediment appear to have been commonplace in Flanders during the Late Pleniglacial (Oxygen Isotope Stage 2; OIS2); more specifically, around the Last Glacial Maximum (LGM, ∼21 kyr ago) and the transition period between the LGM and the start of the Lateglacial (∼15 kyr ago). Optical dating at one site has revealed two significantly older wedge levels, the younger inset into the older; the younger wedge has an age of 36 ± 4 kyr (Middle Pleniglacial; OIS3), the older wedge 129 ± 11 kyr, which points to formation during the Late Saalian (OIS6). Our OSL ages of the wedges and host sediments bracket formation of the BGB (Beuningen Gravel Bed: a widespread deflation horizon in northwestern Europe) at between ∼15 and 18 kyr; this is in good agreement with previous OSL dating studies. We conclude that optical dating using quartz SAR OSL establishes an absolute chronology for these periglacial phenomena and allows secure palaeoenvironmental reconstructions to be made.  相似文献   

A Marine or Continental Nature of the Deltas in the Early Cretaceous Lingshandao Formation—Evidences from Trace Elements     

YANG Renchao  FAN Aiping  HAN Zuozhen  A.J. 《《地质学报》英文版》2017,91(1):367-368

<正>Objective A succession with well-developed soft-sediment deformation structures and well-exposed gravity-flow deposits in the Early Cretaceous Lingshandao Formation has caused considerable controversy on whether the nondisputed deltaic environment was marine or continental.This dispute cannot be solved by mere sedimentological analysis and,moreover,too few fossils are present to be decisive.The main objective of the present contribution is  相似文献   

浙江省地闪活动特征分析   总被引：2，自引：0，他引：2  

潘娅英  王亚云  周良女  邓霞君  史海峰 《亚热带资源与环境学报》2010,5(3):82-88

通过分析闪电定位所取得的2007—2009年的浙江省地闪资料,初步研究其地闪特征.地闪中绝大多数是负闪,其平均强度为33.2 kA,地闪出现最多的强度范围是20~40 kA.地闪活动具有明显的日变化,总体呈单峰形式,总地闪频次峰值和负闪频次峰值出现时段都在14:00—16:00,而正闪频次落后于总地闪和负闪峰值2~4小时,23:00至次日11:00平均地闪频次每小时20次以下.地闪的月变化也呈单峰型分布,其中1月份、12月份地闪极其稀少,主要集中在6—8月份,以8月份为最多,正闪和负闪的各月变化与地闪总数的各月变化一致.全省地闪密度集中区位于山脉的夏季风的迎风坡一侧.全省的地闪密度分布和同期雷暴日分布不一致,表明在评价雷电活动中仅用雷暴日有一定的局限性.  相似文献   

CLIMATE-CONTROLLED GLACIAL EROSION IN THE UNCONSOLIDATED SEDIMENTS OF NORTHWESTERN EUROPE,BASED ON A GENETIC MODEL FOR TUNNEL VALLEY FORMATION     

J. J. VAN DIJKE  A. VELDKAMP 《地球表面变化过程与地形》1996,21(4):327-340

The development of large erosive subglacial forms in unconsolidated sediments is generally attributed to the eroding power of subglacial meltwater flowing under high pressure conditions. Most explanations, however, differ in the source of meltwater and the speed at which it erodes the subglacial bed. Based on the geometry of deep tunnel valleys and glacial basins in northwestern Europe, a reconstruction of subglacial hydrological conditions during the development of subglacial depressions is made. It is demonstrated that the flow of subglacial meltwater in subglacial channels under high glaciostatic pressures is only capable of eroding large volumes of sediment as long as there is imminent glaciohydrological instability. For the thick aquifers in northwestern Europe, this instability is achieved when large quantities of supraglacial meltwater are available. Furthermore, a theoretical definition is given for maximum depression depth to be reached by subglacial erosion. It is shown that this maximum depth is strongly related to average air temperatures during deglaciation and that glacier bed lowering is to be expected during any final phase of glaciations. The theoretical framework presented enables a tentative comparison between large-scale glacial morphology of different glaciations in northwestern Europe.  相似文献   

SURFACE ROUGHNESS EVOLUTION OF SOILS CONTAINING ROCK FRAGMENTS     

BAS VAN WESEMAEL  JEAN POESEN  TOMS DE FIGUEIREDO  GERARD GOVERS 《地球表面变化过程与地形》1996,21(5):399-411

Soil surface roughness is a dynamic property which determines, to a large extent, erosion and infiltration rates. Although soils containing rock fragments are widespread in the Mediterranean region, the effect of the latter on surface roughness evolution is yet poorly understood. Therefore, laboratory experiments were conducted in order to investigate the effect of rock fragment content, rock fragment size and initial moisture content of the fine earth on the evolution of interrill surface roughness during simulated rainfall. Surface elevations of simulated plough layers along transects of 50 cm length were measured before and after simulated rainfall (totalling 192.5 mm, I = 70 mm h⁻¹) with a laser microreliefmeter. The results were used to investigate whether systematic variations in interrill surface roughness along stony hillslopes in southeastern Spain could be attributed to rock fragment cover and rock fragment size. Soil surface elevations were measured along the contour lines (50 cm long transects) with a contact microreliefmeter. Roughness was expressed by two parameters related to the height and frequency of roughness elements, respectively: standard deviation of de-trended surface elevations (random roughness: RR), and correlation length (L) derived from exponential fits of the autocorrelation functions. The frequently used assumption that surface roughness (RR) of cultivated topsoils decreases exponentially with cumulative rain is not valid for soil surfaces covered by rock fragments. The RR of soils containing small rock fragments (1.7–2.7 cm) increased with cumulative rainfall after an initial decrease during the first 17.5 mm of rainfall. For soils containing large rock fragments (7.7 cm), RR increased with rainfall above a threshold rock fragment content by mass of 52 per cent. For a given rainfall application, RR increased non-linearly with rock fragment content. The correlation length for soils containing small rock fragments decreases with rock fragment content and is significantly lower than for soils with large rock fragments. Soils covered with small rock fragments (large RR and small L) are thus well protected against raindrop impact by a water film in the depressions between the rock fragments. On abandoned agricultural fields along hillslopes in southeastern Spain, rock fragments cover increases non-linearly with slope owing to selective erosion of finer particles on steep slopes. The increase of surface cover by large rock fragments (>25 mm) is even more pronounced. The simultaneous increase of rock fragment cover and rock fragment size with slope explains the non-linear increase of RR with slope. These relationships differ for soils covered by platy misaschists and those covered with cubic andesites. The variations in correlation length along the hillslopes are not clear, probably owing to a simultaneous increase in rock fragment cover and rock fragment size. These findings may provide a better prediction of soil surface roughness of interrill areas covered by rock fragments using slope angle and lithology.  相似文献