Modelling of turbidity currents on Navy Submarine Fan, California Continental Borderland   总被引：2，自引：1，他引：2  

ANTHONY J. BOWEN  WILLIAM R. NORMARK†  DAVID J. W. PIPER‡ 《Sedimentology》1984,31(2):169-185

Several Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain-size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep-tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain. On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s^?1 and flow velocities of about 0.75 m s^?1. Using this flow velocity and channel dimensions, sediment concentration (～2×10^?3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2. On the mid-fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross-flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand-rich late Pleistocene and early Holocene turbidity currents on the fan.  相似文献   

Petrogenesis of Victorian Cambrian Tholeiites and Implications for the Origin of Associated Boninites     

CRAWFORD  ANTHONY J.; KEAYS  REID R. 《Journal of Petrology》1987,28(6):1075-1109

In Victoria, tholeiitic basalts and subvolcanic dolerites andgabbros in the Heatheote and Mount Wellington Greenstone Beltsoccur stratigraphically above boninites, and in the latter beltcontain interbedded arc-derived andesitic detritus. Intrusiverocks form concordant, sill-like bodies up to 1 km thick whichare often strongly differentiated, chemically and mineralogically.The volcanic section in both greenstone belts, between 1000and 1500m thick, is dominated by aphyric or sparsely augitephyrictholeiites which trend toward ferrotholeiite compositions andare comagmatic with underlying intrusives. The tholeiites are a low-K₂O suite with chondritic REE patterns,initial _Nd= +4?5 to +5?0, and trace element characteristicsmost like those shown by modern basalts erupted during the earlieststages of arc rifting and backarc basin opening. From virtuallyidentical parental magmas, the basalts in the Heathcote GreenstoneBelt show a very rapid increase in TiO₂, Zr, Y, and La withincreasing differentiation relative to those in the Mount WellingtonGreenstone Belt. This is explained using O'Hara's (1977) modelfor open system fractionation, in which magma supply and eruptionrate is significantly higher for the Heathcote belt magma chambers. The proposed petrogenetic model for the Victorian Cambrian tholeiitesinvolves subduction of an oceanic spreading centre beneath anearly Cambrian forearc region. Continued magma generation atthe subducted spreading centre resulted in diapirs of 1400?CMORB-source mantle piercing into the over-riding plate and providingheat to locally generate boninites from shallow, refractory,hydrous upper mantle beneath the forearc. Subsequent partialmelting of the MORB-source diapirs yielded tholeiitic magmas,which ascended to erupt upon the very slightly older boninites,and eventually split the arc and open a backarc basin as magmasupply became more voluminous. In this forearc setting, thesource-to-eruption crustal thickness through which the earliesterupted tholeiites had to traverse was notably greater thanat typical spreading centres, and basaltic magmas were ableto differentiate through to ferrobasaltic compositions. Theboninite-tholeiite sections of the Heathcote and Mount WellingtonGreenstone Belts are therefore considered to represent a recordof the earliest stages of magrnatism accompanying ridge subductionand splitting of an arc to form a backarc basin.  相似文献   

Multiple time level adjustment for data assimilation   总被引：1，自引：0，他引：1  

S. ZHANG  J. L. ANDERSON  ANTHONY ROSATI  MATTHEW HARRISON  SHREE P. KHARE  ANDREW WITTENBERG 《地球，A辑:动力气象学与海洋学》2004,56(1):2-15

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A statistical assessment of tropical cyclone activity in atmospheric general circulation models   总被引：1，自引：0，他引：1  

SUZANA J. CAMARGO  ANTHONY G. BARNSTON  STEPHEN E. ZEBIAK 《地球，A辑:动力气象学与海洋学》2005,57(4):589-604

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Spherical and columnar, septarian, ¹⁸O-depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids     

VICTOR A. MELEZHIK  ANTHONY E. FALLICK†  RICHARD A. SMITH‡  DANTA M. ROSSE§ 《Sedimentology》2007,54(6):1389-1416

Calcite septarian concretions from the Permian Beaufort Group in the Maniamba Graben (NW Mozambique) allow controls on the composition and nature of diagenetic fluids to be investigated. The concretions formed in lacustrine siltstones, where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm long) forms within three closely spaced, discrete beds totalling 2·5 m in thickness. Cementation began at an early stage of diagenesis and entrapped non‐compacted burrows and calcified plant roots. The cylindrical concretions overgrew calcified vertical plant roots, which experienced shrinkage cracking after entrapment. Two generations of concretionary body cement and two generations of septarian crack infill are distinguished. The early generation in both cases is a low‐Mn, Mg‐rich calcite, whereas the later generation is a low‐Mg, Mn‐rich calcite. The change in chemistry is broadly consistent with a time (burial)‐related transition from oxic to sub‐oxic/anoxic conditions close to the sediment–water interface. Geochemical features of all types of cement were controlled by the sulphate‐poor environment and by the absence of bacterial sulphate reduction. All types of cement present have δ¹³C ranging between 0‰ and −15‰(Vienna Peedee Belemnite, V‐PDB), and highly variable and highly depleted δ¹⁸O (down to 14‰ Vienna Standard Mean Ocean Water, V‐SMOW). The late generation of cement is most depleted in both ¹³C and ¹⁸O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, ¹⁸O‐depleted, reducing, ice‐meltwaters accumulated in the underlying coal‐bearing sediments during the Permian deglaciation. The invariant δ¹³C distribution across core‐to‐rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates. Syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks. After cracking, the concretions retained a small volume of porosity, allowing infiltration of anoxic, Ba‐bearing fluids, resulting in the formation of barite. The results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift‐bound, lacustrine depositional systems.  相似文献   

Quantifying the Building Blocks of Igneous Rocks: Are Clustered Crystal Frameworks the Foundation?   总被引：6，自引：6，他引：6  

JERRAM  DOUGAL A.; CHEADLE  MICHAEL J.; PHILPOTTS  ANTHONY R. 《Journal of Petrology》2003,44(11):2033-2051

Most phenocryst populations in volcanic rocks, and those preservedin shallow-level igneous intrusions, are clustered (variouslyreferred to as clots, clumps or glomerocrysts). These clustersof crystals are the building blocks that accumulate to formthe high-porosity, touching crystal frameworks from which igneouscumulates form. Examination of touching crystal frameworks inolivine- (komatiite cumulates and experimental charges) andplagioclase-dominant crystal populations (Holyoke flood basalt,Connecticut, USA) reveal complex, high-porosity, clustered crystalarrangements. Olivine touching frameworks in komatiite flowsare interpreted to form in hundreds of days. Plagioclase frameworksare calculated to have formed in less than 17 years for a crystalgrowth rate of 1 x 10^-10 mm/s to less than 3 years for a growthrate of 5 x 10^-10 mm/s based on crystal size distributions.The origin of crystal clusters is likely to involve either (ora combination of) heterogeneous nucleation, remobilization ofcumulate mushes or crystals sticking together during settlingand/or flow. The spatial distribution pattern of clustered crystalframeworks from both natural and experimental examples constrainsfields on spatial packing diagrams that allow the identificationof touching and non-touching crystal populations, and furtherimprove our understanding of crystal packing arrangements andcluster size distributions. KEY WORDS: cumulates; CSD; komatiite; basalt; spatial packing; textural analysis  相似文献   

The Petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica   总被引：3，自引：3，他引：0  

HARRIS  CHRIS; MARSH  JULIAN S.; DUNCAN  ANDREW R.; ERLANK  ANTHONY J. 《Journal of Petrology》1990,31(2):341-369

The 420 m thick sequence of Kirwan basalt crops out along thesouthernmost 50 km of the Kirwanveggen Escarpment (74?S, 6?W).There is little variation in major element chemistry of thesebasalts (SiO₂ 49?3–51?6 wt.%; MgO 5?1–6?6 wt.%),but the concentrations of certain incompatible elements (e.g.,Zr) vary by factors of approximately two or more. Most interelementplots show rather poor correlation (r<0?78), but rocks fromopposite ends of the data array can be related by 30% fractionationof plagioclase, clinopyroxene, olivine, and magnetite in theproportions 51:35:11:3. Plagioclase is much more abundant inphenocryst assemblages (85%) and it appears that selective transportof plagioclase to the surface occurred. The range in incompatible element concentrations cannot be explainedby crystal fractionation and is most probably a result of theparent liquids of these basalts being derived by slightly differentdegrees of partial melting of a common source, or alternativelyof open-system (RTF) magma processes. The strontium isotopedata for the freshest rocks (R₀=0?7049–0?7065) may beexplained by 7% contamination by crustal material with an R₀of 0?709 and bulk Sr of800 ppm, but there is little supportingevidence from other trace element variations for this hypothesis.Oxygen isotope determinations on whole-rock-plagioclase pairsshow that alteration has resulted in a 0?5%o shift in (¹⁸O.Alteration also appears to have resulted in a greater spreadof data, particularly for the LIL elements and Sr isotopes.The Sr and Nd isotopic composition of the suite is close tobulk Earth at 172 Ma and this, together with REE and other traceelement data, shows these basalts to be similar in compositionto the more primitive basalts among the Karoo basalt lavas.It is suggested that the Kirwan basalts were derived from asource which was similar to that of the southern Lebombo variantof the Sabie River Basalt Formation of the Karoo Volcanic Province.This part of the Karoo was closest to the Kirwanveggen beforethe break-up of Gondwanaland.  相似文献   

A comparison between planetary-wave flow regimes and blocking   总被引：1，自引：0，他引：1  

ANTHONY R. HANSEN  ALFONSO SUTERA 《地球，A辑:动力气象学与海洋学》1993,45(4):281-288

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Probability density distributions of large-scale midlatitude circulation parameters during Northern Hemisphere summer     

ANDREA BUZZI  ANTHONY R. HANSEN  ENNID TOSI 《地球，A辑:动力气象学与海洋学》1990,42(3):366-377

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