A remote-sensing technique for the identification of aeolian fetch distance     

KEVIN LYNCH  DEREK W. T. JACKSON  J. ANDREW G. COOPER 《Sedimentology》2006,53(6):1381-1390

This paper describes a new method for assessing aeolian fetch distances in beach/dune systems. The remote‐sensing technique has advantages over currently used proxy measures of fetch distance, such as those based on tide gauges and beach profiles. The method uses a digital camera, global positioning system surveying, and a geographical information system software package to produce a rectified image of the beach surface. From this, direct measurements of the wet–dry beach boundary can be combined with wind direction measurements to determine fetch distance. The method offers an improvement on current approaches to sediment transport estimates: a 28‐day study on Magilligan Strand, Northern Ireland, showed that the commonly used high water mark approach overestimated fetch distance by 30% compared with the technique reported here.  相似文献   

Porosity evolution of the Cambrian Bonneterre Dolomite, south-eastern Missouri, USA     

JAY M. GREGG  PETER R. LAUDON†  ROBERT E. WOODY  KEVIN L. SHELTON‡ 《Sedimentology》1993,40(6):1153-1169

The Cambrian Bonneterre Dolomite of south-eastern Missouri, USA, hosts the large Mississippi Valley type (MVT) lead-zinc-copper ore deposits of the region. The Bonneterre Dolomite consists of dolomitized algal bioherms, oolitic grainstone and associated lithologies that were deposited on a carbonate platform surrounding the Precambrian age St Francois Mountains. Porosity was determined by point counting thin sections from cores and mines in the Bonneterre Dolomite and by gas porosimetry. Volumes of epigenetic cements were estimated by point counting cement filling micro- and mesoporosity using cathodoluminescence. Cement volumes were added to present porosity to estimate porosities during various stages of mineralization. Prior to the onset of mineralization, micro- and mesoporosity in the Bonneterre Dolomite averaged approximately 19%. Precipitation of early dolomite cement (roughly concurrent with the main period of sulphide deposition) reduced average porosity to approximately 7% and closed off much of the intercrystalline pore space. Later cementation by dolomite (prior to late stage sulphides) reduced porosity to approximately 5%, and late cementation by quartz further reduced porosity to the present average value of <4%. Periods of carbonate dissolution during MVT mineralization enhanced large scale megaporosity associated with fractures and breccias but did not significantly increase smaller scale porosity. Dolomite cementation associated with MVT mineralization, porosity and permeability were facies controlled. Today, as a result of mineralization, large scale fractures and breccias control porosity and permeability. This study indicates that dolomite porosity may undergo significant change during basinal brine migrations associated with MVT mineralization.  相似文献   

WEATHERING BY WETTING AND DRYING: SOME EXPERIMENTAL RESULTS     

KEVIN HALL  ALIDA HALL 《地球表面变化过程与地形》1996,21(4):365-376

A series of experiments on sandstone and dolerite was undertaken in an attempt to better understand the wetting and drying weathering process. As rock samples are frequently subjected to wet–dry cycles within the simulation of other weathering mechanisms (e.g. freeze–thaw), three common methods of moisture application were used and the influences of these evaluated. It was found that the method of moisture application could affect the nature of the weathering products resulting from wetting and drying. It was also observed that there were changes in the internal properties of the rock (e.g. porosity/microporosity) and that these could influence the synergistic operation of other weathering processes. Although not all of the observations could be explained, it is apparent that wetting and drying has both a direct and an indirect effect on the weathering of rock that has not been taken into account in simulations. Greater cognizance needs to be given to the role of this process both in the field and in laboratory simulations.  相似文献   

Long-distance marker horizons from small-scale eruptions: British tephra deposits from the AD 1510 eruption of Hekla,Iceland     

ANDREW J. DUGMORE  ANTHONY J. NEWTON  KEVIN J. EDWARDS  GUDRÚN LARSEN  JEFFREY J. BLACKFORD  GORDON T. COOK 《第四纪科学杂志》1996,11(6):511-516

  相似文献   

SMALL-SCALE SPATIAL STRUCTURE OF SHALLOW SNOWCOVERS     

KEVIN SHOOK  D. M. GRAY 《水文研究》1996,10(10):1283-1292

The results of a field study of the small-scale spatial structure of the depth of shallow seasonal snowcovers in prairie and arctic environments are presented. It is shown that the spatial distribution of snow depth is fractal at small scales, becoming random at scales beyond some limiting length. This is due to the autocorrelation of depth at small sampling distances. The transition of fractal to random behaviour is indexed by a ‘cutoff length’, which is defined by the intersection of the ‘fractal’ slope and horizontal tangent of a logarithmic plot of the standard deviation of depth versus sampling distance. The magnitude of the cutoff length is related to the degree of macroscopic variability of the underlying topography. An increase in length due to the effects of macroscopic topographic variability on snowcover accumulation is confirmed by de-trending field measurements. The de-trended data shown a cutoff length for wheat stubble and fallow surfaces of approximately 30 m, which is consistent with the distance determined from measurements on ‘flat’ fields. The implications of the transition of snow depth from fractal to random structure on the scales of snow sampling and modelling are presented. The cutoff length may provide a statistic for stratifying shallow snowcovers, by linking snowcover properties to the underlying topography.  相似文献   

PUEBLO MISSION CHURCHES AS SYMBOLS OF PERMANENCE AND IDENTITY*     

KEVIN S. BLAKE  JEFFREY S. SMITH 《Geographical review》2000,90(3):359-380

ABSTRACT. The three Pueblo mission churches of San Esteban del Rey, Nuestra Señora de Guadalupe, and San José de Laguna are the most visually striking structures in the western New Mexico pueblos of Acoma, Zuni, and Laguna. Prime examples of “structures of permanence” on the landscape, the churches define local cultural identity. Church permanence and Pueblo identity are expressed in a five‐part typology of visible characteristics: natural materials and hand labor, massive exterior form, adjoining cemeteries, syncretism of interior decorations, and structural decay and rebirth. Permanence must, however, be understood as an evolving condition, undergoing new representations as multicultural relationships evolve.  相似文献   

Quaternary coastal lithofacies,sequence development and stratigraphy in a passive margin setting,North Carolina and Virginia,USA   总被引：3，自引：0，他引：3  

PETER R. PARHAM  STANLEY R. RIGGS  STEPHEN J. CULVER  DAVID J. MALLINSON  W. JACK RINK  KEVIN BURDETTE 《Sedimentology》2013,60(2):503-547

Lithofacies analysis is fundamental to unravelling the succession of depositional environments associated with sea‐level fluctuations. These successions and their timing are often poorly understood. This report defines lithofacies encountered within the north‐eastern North Carolina and south‐eastern Virginia Quaternary section, interprets their depositional environments, presents a model for coastal depositional sequence development in a passive margin setting and uses this understanding to develop the stratigraphy and Quaternary evolutionary history of the region. Data were obtained from numerous drill cores and outcrops. Chronology was based on age estimates acquired using optically stimulated luminescence, amino acid racemization, Uranium series and radiocarbon dating techniques. Geomorphic patterns were identified and interpreted using light detection and ranging imagery. Since lithofacies occurrence, distribution and stratigraphic patterns are different on interfluves than in palaeo‐valleys, this study focused on interfluves to obtain a record of highstand sea‐level cycles with minimal alteration by fluvial processes during subsequent lowstands. Nine primary lithofacies and four diagenetic facies were identified in outcrops and cores. The uppermost depositional sequence on interfluves exhibits an upward succession from shelly marine lithofacies to tidal estuarine lithofacies and is bounded below by a marine ravinement surface and above by the modern land surface. Older depositional sequences in the subsurface are typically bounded above and below by marine ravinement surfaces. Portions of seven depositional sequences were recognized and interpreted to represent deposition from late middle Pleistocene to present. Erosional processes associated with each successive depositional sequence removed portions of older depositional sequences. The stratigraphic record of the most recent sea‐level highstands (Marine Isotope Stage 5a and Marine Isotope Stage 3) is best preserved. Glacio‐isostatic adjustment has influenced depositional patterns so that deposits associated with late Quaternary sea‐level highstands (Marine Isotope Stages 5c, 5a and 3), which did not reach as high as present sea‐level according to equatorial eustatic records, are uplifted and emergent within the study area.  相似文献   

Sediment magnetic properties of glacial till deposited since the Little Ice Age maximum for selected glaciers at Svartisen and Okstindan, northern Norway     

STEPHEN D. GURNEY  KEVIN WHITE 《Boreas: An International Journal of Quaternary Research》2005,34(1):75-83

Samples of glacial till deposited since the Little Ice Age (LIA) maximum by two glaciers, North Bogbre at Svartisen and Corneliussen-breen at Okstindan, northern Norway, were obtained from transects running from the current glacier snout to the LIA (c. AD 1750) limit. The samples were analysed to determine their sediment magnetic properties, which display considerable variability. Significant trends in some magnetic parameters are evident with distance from the glacier margin and hence length of subaerial exposure. Magnetic susceptibility (χ) decreases away from the contemporary snout, perhaps due to the weathering of ferrimagnetic minerals into antiferromagnetic forms, although this trend is generally not statistically significant. Trends in the ratios of soft IRM/hard IRM which are statistically significant support this hypothesis, suggesting that antiferromagnetic minerals are increasing relative to ferrimagnetic minerals towards the LIA maximum. Backfield ratios (IRM -100 mT/SIRM) also display a significant and strong trend towards magnetically harder behaviour with proximity to the LIA maximum. Thus, by employing a chronosequence approach, it may be possible to use sediment magnetics data as a tool for reconstructing glacier retreat in areas where more traditional techniques, such as lichenometry, are not applicable.  相似文献   

STONE‐BANKED LOBES AS A PRODUCT OF MILD FREEZE–THAW ACTION: AN EXAMPLE FROM WESTERN TASMANIA,AUSTRALIA          下载免费PDF全文

ADRIAN SLEE  JAMES SHULMEISTER  KEVIN KIERNAN  ANDREW JENKINSON 《Geografiska Annaler: Series A, Physical Geography》2016,98(2):97-109

This paper interprets a stone‐banked lobe on the upper western face of Mt Rufus, at an altitude of 1380 m in western Tasmania, Australia. The morphology of the deposit resembles that of a solifluction lobe. Field observations show vertical and downslope movement of pebbles, cobbles and small boulders over a single winter season. The movement is largely related to frost pull (10–15 cm) and shallow freeze–thaw processes promoting the downslope (up to 50 cm yr^–1) creep of material and the accumulation of coarse clasts at the lobe riser. The climate of Mt Rufus is strongly maritime and this is reflected in the limited duration and depth of penetration of frozen ground at this site during the 2013 winter. Despite the relatively mild climatic conditions, freeze–thaw processes are clearly the dominant geomorphic force operating at the site. These findings support observations of active stone‐banked lobes on sub‐Antarctic islands where intense freezing is absent. Both there and at Mt Rufus, movement is dominated by freeze–thaw processes operating in the upper c. 20 cm of the regolith. These are typical landforms of marginal freeze–thaw settings.  相似文献   

Mud diapirism: the origin of melanges in accretionary complexes?     

TONY BARBER  KEVIN BROWN 《Geology Today》1988,4(3):89-94

Chaotic melange deposits, a mixture of blocks in a clay matrix, have commonly been attributed to the mechanism of submarine slumping. In the oceans at the present day slumping does not occur on a sufficiently large scale to produce the quantities of melange seen in ancient accretionary complexes. In modern accretionary complexes, massive shale diapirism produces large volumes of melange, and is an entirely adequate mechanism to account for melanges in ancient complexes.  相似文献