Comparative scanning electron microscopy study of oriented till blocks, glacial grains and Devonian sands in Estonia and Latvia   总被引：7，自引：0，他引：7  

WILLIAM C. MAHANEY  VOLLI KALM 《Boreas: An International Journal of Quaternary Research》2000,29(1):35-51

Samples of Middle Devonian (Eifelian age; 387–380 Ma) indurated and non‐cemented sandstone were compared with Pleistocene basal tills in Estonia and Latvia to test a hypothesis that glacial SEM (scanning electron microscopy) microtextures are distinctly different from those produced in a fluvial depositional environment. The deposits of Middle Devonian Aruküla Stage were emplaced in a continental water basin close to sea level and well away from any glacial source. Therefore, the SEM microtextures on quartz grains from the Aruküla Stage should show mainly the effect of stream transport. The basal tills are of Late Weichselian age deposited as ground moraine directly over the sandstone. Additional glaciofluvial and glaciolacustrine samples were included with the tills to determine whether glacial and fluvial‐lacustrine transport could be differentiated by the SEM microtextures. Samples of oriented blocks of till from a limited number of sites were studied without pretreatment to determine whether sand clast orientation could provide a method for determining glacial flow vectors. While there are some microtextural similarities between grains from glacial and glaciofluvial‐lacustrine depositional environments, the vast majority of grains from till deposits (50%–60%) are faceted, sharp edged, angular to subangular, and comprised of numerous and distinct microfeatures including abraded surfaces over microfractures, deep linear and curved troughs (striations), step features, and a preponderance of conchoidal and linear microfractures. Glaciofluvial and lacustrine grains contain abundant abrasion features and v‐shaped percussion cracks that make them very distinct from glacial grains. Fluvial transport produces primarily rounded grains, well abraded, with v‐shaped percussion scars dominating. Thus, it is possible to use microtextural differences between the three sample suites to identify particular depositional environments. Oriented till blocks provide information on sand clast orientation. Although carbonate coatings often obscure sand clasts in untreated blocks, it is possible to determine some microfabric information that can be useful in determining flow direction of the ice.  相似文献   

The vertical turbulence structure of experimental turbidity currents encountering basal obstructions: implications for vertical suspended sediment distribution in non‐equilibrium currents     

WILLIAM D. McCAFFREY 《Sedimentology》2012,59(3):1101-1120

Large roughness features, caused by erosion of the sea floor, are commonly observed on the modern sea floor and beneath turbidite sandstone beds in outcrop. This paper aims to investigate the effect of such roughness elements on the turbulent velocity field and its consequences for the sediment carrying capacity of the flows. Experimental turbidity currents were run through a rectangular channel, with a single roughness element fixed to the bottom in some runs. The effect of this roughness element on the turbulent velocity field was determined by measuring vertical profiles of the vertical velocity component in the region downstream of the basal obstruction with the Ultrasonic Doppler Velocity Profiling technique. The experiments were set up to answer two research questions. (i) How does a single roughness element alter the distribution of vertical turbulence intensity? (ii) How does the altered profile evolve in the downstream direction? The results for runs over a plane substrate are similar to data presented previously and show a lower turbulence maximum near the channel floor, a turbulence minimum associated with the velocity maximum, and a turbulence maximum associated with the upper flow interface. In the runs in which the flows were perturbed by the single roughness element, the intensity of the lower turbulence maximum was increased between 41% to 81%. This excess turbulence dissipated upwards in the flow while it travelled further downstream, but was still observable at the most distal measurement location (at a distance ca 39 times the roughness height downstream of the element). All results point towards a similarity between the near bed turbulence structure of turbidity currents and free surface shear flows that has been proposed by previous authors, and this proposition is supported further by the apparent success of a shear velocity estimation method that is based on this similarity. Theory of turbulent dispersal of suspended sediment is used to discuss how the observed turbulent effects of a single large roughness element may impact on the suspended sediment distribution in real world turbidity currents. It is concluded that this impact may consist of a non‐equilibrium net‐upwards transport of suspended sediment, counteracting density stratification. Thus, erosive substrate topography created by frontal parts of natural turbidity flows may super‐elevate sediment concentrations in upper regions above equilibrium values in following flow stages, delay depletion of the flow via sedimentation and increase their run‐out distance.  相似文献   

Geological investigation and optical dating of Quaternary siliciclastic sediments near Apalachicola,North‐west Florida,USA     

WILLIAM J. RINK  GLORIA I. LÓPEZ  DAVID J. MALLINSON  PETER R. PARHAM  EDUARD G. REINHARDT 《Sedimentology》2012,59(6):1836-1849

Ground penetrating radar and single‐aliquot regenerative‐dose optically stimulated luminescence were used to determine the depositional environments and age of unconsolidated siliciclastic sediments near Apalachicola, Florida. Five direct‐push cores, five vibracores and 28 optically stimulated luminescence samples were collected, as well as 7 km of ground penetrating radar data. A new model of cosmic dose rate calculation, which removes the effect of a much younger aeolian cap, was utilized to calculate more representative optically stimulated luminescence ages. Five radar facies were identified based on reflector amplitude and orientation. The resulting data indicate that the Tertiary/Quaternary Shelly Sediments were deposited before marine isotope stage 6, the Quaternary Alluvium was deposited during marine isotope stage 6 and the Quaternary Beach Ridge and Dune was deposited during the marine isotope stage 5e sea‐level highstand, which peaked at approximately 2·5 m above present sea‐level in this area.  相似文献   

Geographical information systems and planning in the USA: selected municipal adoption trends and educational concerns     

WILLIAM R. GORDON JR  NADER M. SOUBRA 《International journal of geographical information science》2013,27(4):267-278

Abstract

Practitioners and academics from many fields of study are currently facing numerous issues involving the diffusion of technologies from geographical information systems within their respective professions. This paper has two aims. The first is to report the partial results of a postal survey of selected municipal planning departments in the U.S.A. In measuring trends related to the adoption of computer and geographical information system technologies, the survey focuses on the availability of trained geographical information system and computer literate staff and on alternative means of training existing staff, as well as on the adequacy of collegiate education. The presentation of survey results is used to establish a framework for the principal discussion of this paper. The second part examines the relationship between planning and education in geographical information systems. Parallels within other disciplines and professions are derived. Curricula and training considerations within academic programmes and professional practice are suggested.  相似文献   

Development of a technique to decompose heterogeneous mapping units in a categorical map of the biotic environment using correlated categorical data     

WILLIAM DE GENST  FRANK CANTERS  WOLFGANG JACQUET  SOPHIE VERMEERSCH 《International journal of geographical information science》2013,27(6):591-614

In this article a technique is presented to estimate the proportions of different map categories in a series of heterogeneous mapping units, using information on the degree of spatial correlation with other categorical data. The technique has been applied to decompose ecotope complexes in a categorical map of the biotic environment in Flanders, using secondary information on land cover and soil type. Because the conditional probability of an ecotope occurring given a certain soil type depends on the frequency with which the ecotope occurs in an area, determining the probability of occurrence of an ecotope from the conditional probabilities can lead to predictions that contradict prior knowledge about the composition of the different mapping units. A measure expressing the affinity of an ecotope for a soil type is proposed and is used as an alternative to conditional probability in the estimation procedure. The proposed method has been tested in a study area for which detailed field observations were collected, and proves to work well if reliable a priori knowledge about the composition of complex mapping units is available.  相似文献   

CONTESTING PISCO: CHILE,PERU, AND THE POLITICS OF TRADE*     

JERRY T. MITCHELL  WILLIAM C. TERRY 《Geographical review》2011,101(4):518-535

This article examines the geography of pisco, a South American brandy claimed as the national drink of both Peru and Chile. The historic tension between these two nations—based notably on the nineteenth‐century War of the Pacific—is used as a backdrop to understanding the cultural and political contestation over the “ownership” of this beverage. Although both countries see exportation of pisco as a means of agricultural development, much of the animosity of this trade dispute is couched in cultural and historical terms. International trade policies, including the use of geographical indications (labels of geographical origin) for the purposes of trademarking, and the physical geography that makes pisco production possible are discussed.  相似文献   

Evidence for a transgressive barrier within a regressive strandplain system: Implications for complex coastal response to environmental change     

DUNCAN M. FitzGERALD  WILLIAM J. CLEARY  JOAO THADEU De MENEZES  ANTONIO H. da F. KLEIN 《Sedimentology》2013,60(2):469-502

Clastic, depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution: climate, sea‐level, and the frequency of major meteorological and oceanographic events. This study seeks to use one such record from a southern Brazilian strandplain to highlight the potentially‐complex nature of coastal sedimentological response to small changes in these drivers. Following a 2 to 4 m highstand at ca 5·8 ka in southern Brazil, falling sea‐level reworked shelf sediment onshore, forcing coastal progradation, smoothing the irregular coastline and forming the 5 km wide Pinheira Strandplain, composed of ca 500 successive beach and dune ridges. Sediment cores, grab samples and >11 km of ground‐penetrating radar profiles reveal that the strandplain sequence is composed of well‐sorted, fine to very‐fine quartz sand. Since the mid‐Holocene highstand, the shoreline prograded at a rate of ca 1 to 2 m yr^?1 through the deposition of a 4 to 6 m thick shoreface unit; a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies; and an uppermost beach and foredune unit. However, the discovery of a linear, 100 m wide barrier ridge with associated washover units, a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3·3 to 2·8 ka during the otherwise regressive developmental history of the plain. The protected nature of Pinheira largely buffered it from changes in precipitation patterns, wave energy and fluvial sediment supply during the time of its formation. However, multiple lines of evidence indicate that a change in the rate of relative sea‐level fall, probably due to either steric or ice‐volume effects, may have affected this coastline. Thus, whereas these other potential drivers cannot be fully discounted, this study provides insights into the complexity of decadal‐scale to millennial‐scale coastal response to likely variability in sea‐level change rates.  相似文献   

L'AISH DANS LA FAMILLE INTERNATIONALE DES ORGANISATIONS SCIENTIFIQUES / IAHS IN THE INTERNATIONAL FAMILY OF SCIENTIFIC ORGANIZATIONS     

WILLIAM C. ACKERMANN 《水文科学杂志》2013,58(4):352-355

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The elemental signature of transported Asian dust at Mauna Loa observatory     

JENNIFER HOLMES  WILLIAM ZOLLER 《Tellus. Series B, Chemical and physical meteorology》1996,48(1):83-92

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Satellite observation and climate system model simulation of the St. Lawrence Island polynya     

AMANDA H. LYNCH  MARY F. GLUECK  WILLIAM L. CHAPMAN  DAVID A. BAILEY  JOHN E. WALSH 《地球，A辑:动力气象学与海洋学》1997,49(2):277-297

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