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   

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA     

P. J. LANCASTER  E. F. BAXTER  J. J. AGUE  C. M. BREEDING  T. L. OWENS 《Journal of Metamorphic Geology》2008,26(5):527-538

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.  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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Relations between dune generations in the Gran Desierto of Mexico     

N. LANCASTER 《Sedimentology》1992,39(4):631-644

Multiple generations or genetic units of dunes characterized by different morphologies, sand grain size and sorting, and degree of post-depositional alteration of sediments occur in the Gran Desierto sand sea of Mexico. Dune generations are separated by super bounding surfaces. Episodic accumulation of dunes is a result of a complex response of different sediment sources to regional climatic and eustatic changes. Based on examples from three parts of the sand sea, relations between different generations of dunes suggest that there are two end-member modes of sand sea accumulation: (1) stacking or superposition of dune generations, and (2) accumulating mosaics of dunes of different morphological units. The mode of accumulation is determined by sand supply and accumulation rate: stacking occurs where sand supply is high and/or the sand sea area is restricted by topography; accumulation of mosaics is the preferred mode of accumulation where sand supply is low.  相似文献   

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