Glaciotectonic effects on a Middle-Wisconsin boreal fenland peat in Michigan, USA   总被引：2，自引：0，他引：2  

RICHARD L. RIECK  JOHNS S. KLASNER  HAROLD A. WINTERS  PERRY A. MARLETTE 《Boreas: An International Journal of Quaternary Research》1991,20(2):155-167

A variety of glaciotectonic structueres are associated with a distinctive Middle- Wisconsin organic formation that is exposed at thirteen places along Michigan's Pine River. The carbonaceous material, with a ¹⁴C age of about 46,000 years BP, accumulated in an extensive, weakly geogenous, oligotrophic fen that was buried by sand before being deformed. Numerous faults and folds, the latter with observed amplitudes that may exceed 5 m, have a consistent sense of easterly structural vergence indicating glacial movement from the west. Gelogic relationships here and generally accepted regional chronology indicate that deformation was produced by Late Wisconsin ice that flowed from the Lake Michigan basin. This unequivocal marker bed provides detailed information on the topography, environment, and climate associated with an ice-free Middle-wisconsin landscape. Equally important and unprecedented for the area, it is an especially precise measure of multiple glaciotectonic effects upon an incompetent Pleistocene organic formation of considerable extent.  相似文献   

Lake shoreline development, frost weathering and rock platform erosion in an alpine periglacial environment, Jotunheimen, southern Norway   总被引：2，自引：0，他引：2  

JOHN A. MATTHEWS  ALASTAIR G. DAWSON  RICHARD A. SHAKESBY 《Boreas: An International Journal of Quaternary Research》1986,15(1):33-50

Ice-dammed lake Boverbrevatnet existed for 75–125 years in the 'Little Ice Age'. After about A.D. 1826, glacier retreat led to a fall in lake level and to exposure of the former shoreline, which includes well-developed platforms cut in metamorphic bedrock. The rock platforms, up to 5.3 m wide and backed by cliffs up to 1.55 m high, are partially covered by large angular boulders which form pavements. Accurate levelling has permitted correlation of platform fragments, overflow cols and related features of the shoreline, such as benches eroded in moraines, ice-push ridges, a perched delta, vegetation trim-lines, lichen limits and a 'lichen-kill' zone. The evolution of the lake, the chronology of deglaciation and the period of formation of the rock platforms have been dated by lichenometry, supported by ¹⁴C dating, Schmidt hammer 'R'-values and historical data. The morphology of the rock platforms, together with estimates of their rate of erosion ranging from 1.4 to 7.1 cm/year, indicate the importance of frost shattering (frost riving, frost wedging or macrogelivation) at the lake margin under a periglacial climate, while the permanence of such platforms as landscape features suggests their use in the reconstruction of former periglacial environments. A semi-quantitative model is outlined for the development of rock platforms which emphasises deep penetration of the annual freeze-thaw cycle, the movement of unfrozen lake water towards the freezing plane, and the growth of segregation ice in fissures and cracks at the interface between lake ice and bedrock. Ice-push and ice-pull processes are involved primarily as transporting agents in the formation of boulder pavements and in the removal of debris from the platforms. Analogous processes may occur on polar coasts producing coastal rock platforms.  相似文献   

Eddy correlation measurements of size-dependent cloud droplet turbulent fluxes to complex terrain     

RICHARD J. VONG  ANDREW S. KOWALSKI 《Tellus. Series B, Chemical and physical meteorology》1995,47(3):331-352

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Petrology of the Great Abitibi Dyke, Superior Province, Canada     

ERNST  RICHARD E.; BELL  KEITH 《Journal of Petrology》1992,33(2):423-469

The Great Abitibi Dyke (GAD) which can be traced northeast,for >> 700 km, across the Abitibi Belt in the southeasternSuperior Province of the Canadian Shield, is composed of olivinegabbro to monzodiorite, weakly saturated to undersaturated insilica. All rocks of the GAD can be derived by mainly plagioclase andolivine fractionation from a parental magma corresponding incomposition to chilled margin samples. Two units can be distinguished,a marginal unit (Unit 1) representing 0–50% crystallizationand a central unit (Unit 2), found over about half of the dykelength, representing 50–70% crystallization. Modelling,using Pearce elemental ratio analysis, quantifies the fractionationhistory and allows mass–balance calculations over thepresent exposure level of the dyke. The approximate balancebetween the amounts of cumulate and fractionated rocks suggeststhat Unit 1 differentiated essentially in situ as a closed system.In contrast, Unit 2 rocks were formed by loss of substantialplagioclase and olivine from the parental magma. This fractionatemust have either been lost to depth or left behind in an externalchamber. Unit 2 rocks show depletion in plagioclase and enrichmentin mafic minerals along strike towards the southwest (deeperexposure level?), a trend explained by density stratification.Regional variation in Unit 1 chemistry is interpreted in termsof lateral magma injection towards the northeast from the locusof Keweenawan rift volcanism. Feldspar, olivine, and augitecompositions are linearly correlated with equilibrium temperatureand extent of magma evolution. The parent magma had a trace element chemistry correspondingto a ‘within–plate’ setting and was probablyderived from an incompatible–element enriched mantle similarto the source for ocean island basalts (OIBs).  相似文献   

The Burnwell,Kentucky, low iron oxide chondrite fall: Description,classification and origin     

SARA S. RUSSELL  TIMOTHY J. McCOY  EUGENE JAROSEWICH  RICHARD D. ASH 《Meteoritics & planetary science》1998,33(4):853-856

Abstract— The Burnwell, Kentucky, meteorite fell as a single stone on 1990 September 4. The Burnwell meteorite has lower Fa in olivine (15.8 mol%), Fs in orthopyroxene (13.4 mol%), Co in kamacite (0.36 wt%), FeO from bulk chemical analysis (9.43 wt%), and Δ¹⁷O (0.51 ± 0.02%), and higher Fe, Ni, Co metal (19.75 wt% from bulk wet chemical analysis) than observed in H chondrites. The Burnwell meteorite plots on extensions of H-L-LL chondrite trends for each of these properties towards more reducing compositions than in H chondrites. Extensions of this trend have been previously suggested in the case of other low-FeO chondrites or silicate inclusions in the HE iron Netschaëvo, but interpretation of the evidence in these meteorites is complicated by terrestrial weathering, chemical disequilibrium or reduction. In contrast, the Burn-well meteorite is an equilibrated fall that exhibits no evidence for reduction. As such, it provides the first definitive evidence for extension of the H-L-LL ordinary chondrite trend beyond typical H values towards more reducing compositions.  相似文献   

The Petrology of Plutonic Blocks and Inclusions from the Lesser Antilles Island Arc   总被引：20，自引：12，他引：20  

ARCULUS  RICHARD J.; WILLS  KEVIN J. A. 《Journal of Petrology》1980,21(4):743-799

Three main groups of plutonic nodules are present in the LesserAntilles arc and are interpreted as (a) phenocryst clusters,(b) metamorphosed wallrock xenoliths and (c) cumulate texturedxenoliths. Large cumulate blocks display felsic-mafic layering,slump structures and auto-intrusive features. The majority ofspecimens are ad- and heteradcumulates with fewer ortho- andcrescumulates. Interstitial scoria and glass are present ina variety of samples. Plagioclase, amphibole, cino- and orthopyroxene,olivine, magnetite, biotite, ilmenite, quartz and apatite arepresent in various proportions in individual blocks. Plagioclaseand amphibole are modally predominant. Significant variationalong the arc is displayed in the rarity of orthopyroxene andabundance of amphibole in the southern islands compared withthe common presence of two pyroxenes in the northern islands. Plagioclase varies from An_100–36 with very low orthoclasecomponent, and usually precedes amphibole in a given crystallizationsequence. Only on Grenada are plagioclase-free blocks present.Olivine is restricted to assemblages where coexisting plagioclaseis more calcic than An₈₉ and its composition range is Fo_90–59.Clinopyroxene is predominantly calcic augite and cinopyroxene,olivine and plagioclase all coexist stably with amphibole. Ageneral trend of decreasing Ca content in clinopyroxene fromsouth to north in the arc is present. Orthopyroxene ranges from En_73–49 and is most common inassemblages where the coexisting plagioclase is more sodic thanAn₈₃ Coexisting pyroxenes define temperatures in the range 800–1050?C. Amphibole compositions include pargasite, magnesiohastingsite,magnesio-hornblende and tschermakitic hornblende. The K contentof the amphiboles increases from north to south in the samesense as the general tholeiitic-calcalkalic-alkalic variationof parental magmas. Magnetite is the dominant spinel phase but ferrian chromiteand chromian magnetite are present in some Grenada cumulatesand pleonaste is found in rare St. Kitts samples. Ilmenite ispresent in blocks from several islands; coexisting Fe-Ti oxidesdefine a temperature range of 710?-950 ?C at of 15.5 to 10.0 bars. Biotite, quartz and apatiteare restricted to evolve cumulate types. Some modification of interstitial scoria/glass compositionsfrom equilibrium melts has occurred in the majority of samples,but general similarity with erupted lava types is importantevidence for the cognate relationship of the cumulate assemblages.The role of H₂O is crucial in determining the calcic natureof island arc plutonic plagioclase when compared with relativelydry, layered tholeiitic plutons. Some modal and chemical featuresof cumulate-lava comparisons suggest plagioclase flotation maybe significant. A variety of thermodynamic calculations indicate temperaturesand pressures of crystallization in the range 850–1050?C, 4–10 kb. No evidence exists for systematic along-arcvariations in these parameters. Standard amphibole crystallinesolution models give unsatisfactory results for calculations. Some distinctive contrasts between cumulate and phenocryst modesare present. The abundance of amphibole in equilibrium withbasaltic melts in the plutonic situation compared with its rarityin lavas is striking. Plagioclase coexisting with a given meltis more anorthitic in the plutonic than the phenocryst mode. Least squares fractionation tests demonstrate the possibilityof relating basalt-andesite-dacite suites by fractional crystallizationof the cumulus phases. Trace element systematics of cumulate-lavasuites for individual islands also generally support this hypothesis.The suggestion of sole amphibole fractionation for the generationof andesite from basalt is discarded. ^* Present address: CRAE, Box 39598, Darwin, N.T. 5798, Australia.  相似文献   

Aspects of the large-scale tropical atmospheric circulation     

BRIAN HOSKINS  RICHARD NEALE  MARK RODWELL  GUI-YING YANG 《Tellus. Series B, Chemical and physical meteorology》1999,51(1):33-44

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Quaternary chronostratigraphy: the nomenclature of terrestrial sequences     

PHILIP L. GIBBARD  RICHARD G. WEST 《Boreas: An International Journal of Quaternary Research》2000,29(4):329-336

There is a long-standing tradition of dividing Quaternary stratigraphical sequences on the basis of climate. This climatostratigraphical approach, begun on terrestrial sequences in the last century, led to the widespread adoption of glacial, interglacial, stadial and interstadial subdivisions as standard. However, the limitations of the approach resulted in its replacement by local sequences of chronostratigraphical stages from the 1950s. The subsequent establishment of the detail and complexity of ocean-core sequences has encouraged increasing use of the terminology developed for these records to the terrestrial and shallow marine succession. The view is expressed that it is advisable to separate and retain regional chronostratigraphies for each sequence-type, and that these should be correlated using event-based stratigraphy where possible. This should ensure both the highest precision and lack of ambiguity and a securely-based synthesis of the relations between the terrestrial and ocean isotope sequences, a synthesis which is fundamental to understanding the nature and detail of regional and global climate change.  相似文献   

Humans,megafauna and environmental change in tropical Australia     

MICHAEL I. BIRD  LINDSAY B. HUTLEY  MICHAEL J. LAWES  JON LLOYD  JON G. LULY  PETER V. RIDD  RICHARD G. ROBERTS  SEAN ULM  CHRISTOPHER M. WURSTER 《第四纪科学杂志》2013,28(5):439-452

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Subglacial bedforms and conditions associated with the 1991 surge of Skeiðarárjökull, Iceland     

RICHARD I. WALLER  THAIENNE A. G. P. VAN DIJK  ÓSKAR KNUDSEN 《Boreas: An International Journal of Quaternary Research》2008,37(2):179-194

Much previous research at surge-type glaciers has sought to identify features diagnostic of surge-type behaviour. However, in comparatively little work have subglacial landform–sediment characteristics been used to reconstruct changing basal processes and conditions during surge events. Subglacial bedforms described in this article are associated with the 1991 surge of Skeiðarárjökull, Iceland, and include a series of drumlins with superimposed flutes and basal crevasse-fill ridges. The drumlins were formed by the subglacial erosion of ice-contact fans. Sedimentary evidence indicates a shift from rigid-bed to soft-bed conditions during the surge. The presence of eroded but undeformed fan sediments suggests that they acted as a rigid bed when initially overridden. Subsequent deposition of a layer of deformation till resulted in a change to soft-bed conditions and the generation of flutes and subglacial crevasse-fill ridges. The lack of mixing between this till and the underlying stratified sediments indicates that subglacial sediment deformation was restricted to a thin layer and that its deposition resulted in a cessation of subglacial erosion. The drumlin is therefore a composite of both rigid-bed and soft-bed processes that illustrates changes in basal conditions and processes during the course of the event. The limited time frame in which the drumlin formed and the presence of kettleholes across its surface are distinctive features that may warrant further investigation in the search for features diagnostic of past surge events.  相似文献