REPLY     

NICHOLAS EYLES  CAROLYN H. EYLES  REW D. MIALL 《Sedimentology》1986,33(1):152-154

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Coarse-grained sediment gravity flow facies in a large supraglacial lake     

NICHOLAS EYLES  BRYAN M. CLARK  JOHN J. CLAGUE 《Sedimentology》1987,34(2):193-216

Near Williams Lake, in the central interior of British Columbia, the Fraser River exposes long sections of late Pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough. The dominant facies types are thick, normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep. Channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice. Fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi-continuous ‘surging’ turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice-cored sediments. Large-scale post-depositional deformation structures, such as synclinal folds, normal faults, sedimentary dyke swarms and dewatering structures, record gravitational foundering of sediment and pore-water expulsion caused by the melt of underlying glacier ice. Melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub-basins within the overlying sediment pile. An idealized model of ‘supraglacial’ lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography.  相似文献   

Drilling Blake Nose: the search for evidence of extreme Palaeogene–Cretaceous climates and extraterrestrial events     

KROON  NORRIS  KLAUS  IAN T. ALEXANDER  LEON PAUL BARDOT  CHARLES E. BARKER  JEAN-PIERRE BELLIER  CHARLES D. BLOME  LEON J. CLARKE  JOCHEN ERBACHER  KRISTINA L. FAUL  MARY ANNE HOLMES  BRIAN T. HUBER  MIRIAM E. KATZ  KENNETH G. MACLEOD  SANDRA MARCA  FRANCISA C. MARTINEZ-RUIZ  ISAO MITA  MUTSUMI NAKAI  JAMES G. OGG  DOROTHY K. PAK  THOMAS K. PLETSCH  JEAN M. SELF-TRIAL  NICHOLAS J. SHACKLETON  JAN SMIT  WILLIAM USSLERIII  DAVID K. WATKINS  JOEN WIDMARK  & PAUL A. WILSON 《Geology Today》1998,14(6):222-226

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A palaeo-ice stream of the British Ice Sheet in eastern Scotland     

NICHOLAS R. GOLLEDGE  MARTYN S. STOKER 《Boreas: An International Journal of Quaternary Research》2006,35(2):231-243

Terrestrial and marine subglacial landforms in eastern Scotland are used to evaluate previously unsubstantiated notions of ice streaming within the British Ice Sheet (BIS) in this area during the last glacial cycle. Employing both regional and local-scale data sets, we describe onshore landform-sediment assemblages, offshore geomorphology and stratigraphy, and reconstructed palaeo-ice flow patterns. The results and their glaciological significance are discussed in the context of stratigraphical and geomorphological frameworks established by earlier workers, and are compared with modelled reconstructions for the BIS in this area. We conclude that the Main Late Devensian ice sheet in eastern Scotland hosted a zone of fast-flowing ice at least 100 km long and 45 km wide, akin to a contemporary ice stream. This sector - the Strathmore Ice Stream - flowed through a combination of basal sliding on meltwater-lubricated rigid beds and by deforming unconsolidated basal substrates.  相似文献   

Deficiencies of sheets and tiles: building sheetless databases     

NICHOLAS R. CHRISMAN 《International journal of geographical information science》2013,27(2):157-167

A geographic information system (GIS) provides access to information for potentially large areas. Traditionally, cartographers have divided maps into sheets for technical reasons. While such units survive in digital databases, they serve a different function. The 'tile' system has limitations and deficiencies which must be understood by software developers and users. Remedies to the problems of tiles provide a basis for specifying the requirements for sheetlessness. A truly sheetless database depends on indexing algorithms, reviewed in a companion paper.  相似文献   

Oligo–Miocene seagrass‐influenced carbonate sedimentation along a temperate marine palaeoarchipelago,Padthaway Ridge,South Australia     

NICHOLAS K. RIORDAN  NOEL P. JAMES  YVONNE BONE 《Sedimentology》2012,59(2):393-418

Oligo–Miocene carbonates associated with the Padthaway Ridge form the southern margin of the Murray Basin, South Australia. The carbonates are a thin, somewhat condensed succession of echinoid and bryozoan‐rich limestones that record accumulation in the complex of islands and seaways and progressive burial of the Ridge through time. The rocks are grainy to muddy bioclastic packstones, grainstones and floatstones, composed of infaunal echinoderms, bryozoans, coralline algae and benthic foraminifera, with lesser contributions from molluscs and serpulid worms. Locally as much as half of these skeletal components are Fe‐stained, relict grains that imbue the lithologies with a conspicuous yellow to orange hue. This variably lithified succession is partitioned into metre‐scale, firmground‐bounded and hardground‐bounded beds textured by extensive Thalassinoides burrows. Dominant lithologies are interpreted as temperate seagrass facies. Limestones contain attributes indicative of both seagrass‐dominated palaeoenvironments and carbonate production and accumulation on unconsolidated, barren sandflat palaeoenvironments. Together these two depositional systems are thought to have generated a single multigenerational, amalgamated facies recording sedimentation within a complex temperate seagrass environment. Limestones overlying the Padthaway Ridge reflect a gradually warming climate, increasing water temperature and decreasing nutrient content, within the framework of a ridge gradually being buried in sediment. This succession from cool–temperate to warm–temperate to subtropical through time permits recognition of the relative influence of changing oceanography on a seagrass‐dominated shallow inter‐island sea floor. Criteria are proposed herein to enable future recognition of similar temperate seagrass facies in Cenozoic limestones elsewhere.  相似文献   

Towards a protocol for the trace element analysis of glass from rhyolitic shards in tephra deposits by laser ablation ICP‐MS          下载免费PDF全文

NICHOLAS J. G. PEARCE 《第四纪科学杂志》2014,29(7):627-640

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Alteration of a Komatiite Flow from Alexo, Ontario, Canada   总被引：5，自引：4，他引：1  

LAHAYE  YANN; ARNDT  NICHOLAS 《Journal of Petrology》1996,37(6):1261-1284

To investigate the ability of komatiites to preserve duringalteration a record of the geochemical and isotopic compositionof the Archean mantle, we studied the petrology and geochemistryof a komatiite flow from Alexo, Ontario. Although this flowis relatively well preserved, two main types of alteration arerecognized: (1) hydration of variable intensity throughout thewhole flow; (2) local rodingitization. In samples only slightlyaffected by hydration, the alteration was essentially isochemical;where more intense, hydration was accompanied by the mobilityof TiO₂, Zr and the light rare-earth elements (LREE). The mobilityof these reputedly immobile trace elements is influenced bytexture and secondary mineral assemblages. Rodingitization wasaccompanied by still greater mobility. An Nd isochron with anage of 272698 Ma and Nd_(T) =+ 2.5 is defined by wholerock samples.Because the spread of Sm-Nd ratios is far greater than can beexplained by magmatic processes, the age is interpreted as thatof the main alteration event during or soon after emplacement.Scattered initial Sr isotopic ratios and Nd _(T) (+ 3.6 to +0.7)provide evidence for several younger events. The Nd (T) of magnaticclinopyroxene ( plus;3.8) is higher than that of mafic tofelsicrocks and pyroxenes from the Abibiti belt (Nd _(T) +2.5). Thisresult implies: (1) the komatiites exchanged Nd with surroundingrocks during waterrock interaction; (2) the isotopic compositionof komatiites and their mantle source differed from that ofthe more common basalts of the Abitibi belt. KEY WORDS: alteration; Archean; Komatiite; trace elements; radiogenic and stable islotopes ^*Present address: Ore Genesis Research Group, VIEPS Department of Earth Science, Monash University, Clayton, Vic 3168, Australia. Telephone: (61) 3 9905-3881. Fax: (61) 3 9905–4903. e-mail: ylahaye{at}artemis.earth.monash.edu.au  相似文献   

Petrology of glacial sand: implications for the origin and mechanical durability of lithic fragments     

ROGER M. SLATT  NICHOLAS EYLES 《Sedimentology》1981,28(2):171-183

The petrology of first cycle (unmodified) and second cycle (reworked) sand at the termini of eleven valley glaciers eroding coarse- to fine-grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands. First cycle sands are coarse- to medium-grained, poorly sorted, fine-skewed, non-modal lithic sands with an average composition of Q₂₁F₆L₇₃. Grain-size distributions do not vary with composition or source rock types, although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources. By contrast, second cycle sands are medium-grained, poorly sorted, fine- to coarse-skewed arkosic to lithic sands with an average composition of Q₁₉F₄₀L₄₁, and contain fewer lithic fragments than do first cycle sands. We propose that the origin, mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness, and the particular stress field of the transporting medium. As a result of abrasion, glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain-size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock. Upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point-loading during saltation in addition to shear stress. Point-loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness, so the lithic fragments break into smaller monomineralic grains and/or lithic fragments. Lithic fragments of very coarse sand-size are abundant in first cycle sands, thus refuting, at least for glacial sands, the widely held view that grains of this size are deficient in nature. The presence of fewer grains of this size in second cycle sands indicates that very coarse sand-size lithic fragments can survive recycling, but in greatly reduced proportions due to breakage.  相似文献   

Planktonic foraminiferal and oxygen isotopic stratigraphy and paleoclimatology of Norwegian Sea deep-sea cores     

THOMAS B. KELLOGG  JEAN CLAUDE DUPLESSY  NICHOLAS J. SHACKLETON 《Boreas: An International Journal of Quaternary Research》1978,7(1):61-73

BOREAS Kellogg, T. B., Duplessy, J. C. & Shackleton, N. J. 1978 03 01: Planktonic foraminiferal and oxygen isotopic stratigraphy and paleoclimatology of Norwegian Sea deep-sea cores. Boreas. Vol. 7, pp. 61–73. Oslo. ISSN 0300–9483.
Three Norwegian Sea deep-sea cores, which penetrate to sediments at least 200,000 years old, were analyzed for oxygen isotope content, total calcium carbonate, and planktonic foraminifera. The oxygen isotopic stratigraphy was used to refine the time control for paleoclimatic and paleo-oceanographic events previously described for the region. Two pulses of relatively warm subpolar water entered the region between 124,000 B.P. and 115,000 B.P. (the last interglacial), and since about 13,000 B.P. The remaining portion of the last 150,000 years was characterized by extensive ice cover. The magnitude of the change in isotopic composition between peak glacial and peak interglacial conditions is larger than can be explained by the changing isotopic content of the oceans alone suggesting that large temperature and salinity effects are recorded in isotope curves from Norwegian Sea isotope curves. The magnitude of the isotopic change from substage 5e to 5d (greater than 1%) is attributed to a combination of changing oceanic isotopic composition combined with a large temperature effect due to a sudden sea-surface temperature decrease of about 6^oC. The persistence of heavy isotope values throughout substages 5d through 5a may be related to the sea-ice cover which prevented dilution of the isotopically heavy waters by isotopically light run-off. Sedimentation rates calculated for each of the isotope stages show large changes from one stage to another with some tendency for odd numbered stages to have higher rates.  相似文献