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141.
JOHN MENZIES 《Sedimentology》1990,37(3):481-493
Examination of sediments along the north shore of Lake Erie at Mohawk Bay reveals a relationship between the formation of intensely brecciated diamictons and the presence of sand-block intraclasts. It is postulated that the sand blocks were subglacially deposited within a meltwater environment, and later frozen prior to being eroded and transported within a mobile subglacial debris layer. On immobilization the frozen sand blocks, encased within the diamicton, acted as a heat sink creating cryostatic stresses within the surrounding diamicton as a result of the advance of a frost front and related frozen ‘fringe’. The effect of these anisotropic stresses resulted in porewater migration to the frost front. Subsequent development of intense brecciation occurred as aureoles around the sand intraclasts due to localized high tensile stresses causing fracturing within the fine-grained matrix of the diamicton. 相似文献
142.
Large-scale current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current 总被引:2,自引:0,他引:2
JOHN E. HUGHES CLARKE ALEXANDER N. SHOR† DAVID J. W. PIPER‡ LARRY A. MAYER 《Sedimentology》1990,37(4):613-629
New observations concerning the degree of current-induced erosion and deposition in the path of the 1929 Grand Banks turbidity current are presented. Most of the observations are available from Eastern Valley, Laurentian Fan. Seabeam and SeaMARC I data reveal widespread current erosion along the valley over a distance of 200 km from the shelfbreak. Erosional valley-floor channels are preferentially developed adjacent to the valley margins and the flanks of intravalley highs. Asymmetric transverse bedforms (herein termed gravel waves) are moulded in a deflationary pebble and cobble lag that overlies the eroded valley floor. In contrast, at the distal limit of Eastern Valley, thick deposits of massive granule gravel indicate deposition beneath a decelerating turbidity current. Symmetrical transverse bedforms (herein termed macrodunes) are developed within these granule gravel sediments. The spatial distribution of both bedforms and the areas of erosive excavation suggest that the turbidity current in 1929 was accelerating over the first 100 km from the shelfbreak and was eroding and entraining sediment from the valley floor over a distance of at least 200 km. With the loss of lateral constraint at the distal limit of Eastern Valley the turbidity current spread laterally and started depositing sediment as it decelerated. Current-induced erosion of the valley floor represented a potential source of between 50 and 100 km3 of sediment for incorporation into the resulting turbidite. 相似文献
143.
Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin (87Sr/86Sr = 0.70255; Nd=+9.3; 206Pb/204Pb = 18.52; 208Pb/204Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone (87Sr/86Sr 0.7042; Nd= +5; 206Pb/204Pb= 18.81; 208Pb/204Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement (87Sr/86Sr 0.715; Nd 4; 206Pb/204Pb 18.86; 208Pb/204Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO2 vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K2O at 50% SiO2, Ba 600p.p.m., 910% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilationfractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements
*Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia. 相似文献
144.
SCHMIDT‐HAMMER EXPOSURE AGES FROM PERIGLACIAL PATTERNED GROUND (SORTED CIRCLES) IN JOTUNHEIMEN,NORWAY, AND THEIR INTERPRETATIVE PROBLEMS
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STEFAN WINKLER JOHN A. MATTHEWS RICHARD W. MOURNE PETER WILSON 《Geografiska Annaler: Series A, Physical Geography》2016,98(3):265-285
Periglacial patterned ground (sorted circles and polygons) along an altitudinal profile at Juvflya in central Jotunheimen, southern Norway, is investigated using Schmidt‐hammer exposure‐age dating (SHD). The patterned ground surfaces exhibit R‐value distributions with platycurtic modes, broad plateaus, narrow tails, and a negative skew. Sample sites located between 1500 and 1925 m a.s.l. indicate a distinct altitudinal gradient of increasing mean R‐values towards higher altitudes interpreted as a chronological function. An established regional SHD calibration curve for Jotunheimen yielded mean boulder exposure ages in the range 6910 ± 510 to 8240 ± 495 years ago. These SHD ages are indicative of the timing of patterned ground formation, representing minimum ages for active boulder upfreezing and maximum ages for the stabilization of boulders in the encircling gutters. Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early‐Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late‐Holocene activity clarify existing understanding of the process dynamics and palaeoclimatic significance of large‐scale sorted patterned ground as an indicator of a permafrost environment. The interpretation of SHD ages from patterned ground surfaces remains challenging, however, owing to their diachronous nature, the potential for a complex history of formation, and the influence of local, non‐climatic factors. 相似文献
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148.
JOHN D. COLLINSON 《Sedimentology》1968,10(4):233-254
In the Kinderscout Grit (Namurian, R1c) of the southern half of the Central Pennine Basin, England, there occur tabular, isolated, cross-bedded sets of coarse sandstone, between 4 and 40 m thick. The sets are up to 1 km wide and the foresets, in plan, are convex in the direction of dip. The sets interfere laterally to form extensive sheets. Foresets dip at up to 27° and are sometimes themselves internally cross-bedded in trough-shaped sets, here termed “intrasets”. The large sets are thought to be deltaic sedimentation units rather than sandwaves. In the delta top environment in which these sets were deposited, a high rate of bed-load sediment supply and a sudden deepening were required to initiate the sets. Medium-scale cross-bedding, thought to have been laid down in migrating, fluviatile channels overlies the large-scale sets and represents the topset member of the sedimentation units. From the spatial arrangement of the large- and medium-scale cross-bedding, it is possible to distinguish those areas where deposition took place during the deepening, from those where it took place essentially after the deepening. The causes of the deepening, which must have been on at least a basinal scale, may have been eustatic or tectonic. 相似文献
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150.
JOHN STUART GLADWELL 《水文科学杂志》2013,58(4):515-516
ABSTRACT Assistance to developing nations in the area of water resources has shown mixed successes and failures. A major factor is the capacity of the receiving nations to make good use of the resources provided at their stage of development—their “absorptive capacity”. Programmes must be tailored to the situations. This requires planning that is specific to the national and cultural context, as well as careful sensitivity to positive interaction with those who will be expected to implement programmes whose real purpose should be to allow the entire population, especially its very poor, to become socially effective and self-aware as well as more productive and comfortable. It also requires careful preparation in the way of education, science and institutional responsiveness. Experts and government leaders must reconsider their approaches to the problems so that the nations can learn and think for themselves. 相似文献