The prodelta environment of a fjord: suspended particle dynamics     

JAMES P. M. SYVITSKI  K. W. ASPREY  D. A. CLATTENBURG  GORDON D. HODGE† 《Sedimentology》1985,32(1):83-107

The dynamics of suspended particles within a fjord's estuarine circulation are investigated and the results compared with larger non-enclosed prodelta environments. In the upper prodelta, the seaward-flowing river plume flows over the ambient marine water depositing much of the initial riverine suspended load. Sedimentation is dominated by coarse silt and fine-grained sand particles with coarseness determined by the tidal and fluvial stage. Particles less than 10 μn have similar settling velocities regardless of size because they settle in flocs: the settling velocity at a water depth of 5 m is 30 m day^-1 and increases with depth so that at 30 m the particles settle at 100 m day^-1. For larger particles, the downward settling velocity enhancement due to flocculation decreases with increasing grain size. Hydraulic sorting allows the preferential settling of feldspar and quartz over mica. Particle dynamics in the lower prodelta are dependent on the character of the freshwater wedge that thins seaward of the upper prodelta. The vertical flux of particles is controlled by biogeochemical interactions such as pelletization of fine particles and flocculation (which occurs within rather than below the surface layer in contrast to the upper prodelta). The pellets are produced by indiscriminate filter feeding zooplankton. Across the lower prodelta the suspensate character, recognized in the composition of both flocs and pellets, changes from a dominance of mineral grains to that of autochthonous organic matter. The interaction of bacteria with the suspended particles increases with depth and seaward distance. At depth, the mucoid filaments form stable interconnecting webs. Particle concentration in the surface layer decreases at a rate proportional to the negative one-half and three-halves power of the distance seaward over the upper and lower prodelta, respectively. This relationship is hypothesized as being universal for large marine deltas dominated by buoyancy flow dynamics, regardless of the levels of initial riverine particle concentration or their composition.  相似文献   

Discrimination between depositional environments using grain-size analyses     

RAMADAN ABU EL-ELLA  JAMES M. COLEMAN 《Sedimentology》1985,32(5):743-748

Grain-size analyses were performed on 84 samples from six environments in the Burdekin River Delta, NE Queensland, Australia. The data were subjected to a stepwise discriminant function analysis and canonical analysis which showed that the grain-size characteristics of the sediments are sufficiently distinctive to distinguish the six different environments of deposition. The resulting ‘reference set’ was then tested by plotting the grain-size characteristics of 66 samples from different environments of the Mississippi River Delta, Louisiana and Galveston Island, Texas. Seventy-four per cent of the North American samples were correctly classified.  相似文献   

Assimilation of Crustal Material by Basaltic Magma: Strontium Isotopic and Trace Element Data from the Edgecumbe Volcanic Field, SE Alaska     

MYERS  JAMES D.; SINHA  A. KRISHNA; MARSH  BRUCE D. 《Journal of Petrology》1984,25(1):1-26

The Edgecumbe volcanics, which range from basalt through rhyodacite,have Sr contents between 125 and 370 p.p.m., Rb contents of1 to 70 p.p.m., Ba ranging from 50–550 p.p.m. and initial⁸⁷Sr/⁸⁶Sr ratios between 0.70291 and 0.70404. No simple correlationexists between these components and silica. The highest ⁸⁷Sr/⁸⁶Srvalues occur in a group of intermediate lavas (55–60 wt.per cent SiO₂) while the rhyodacites have initial ratios between0.7035 and 0.7038. With increasing silica, Sr increases to amaximum in the andesites and then steadily decreases; Ba andRb increase over the same compositional range. The highest ⁸⁷Sr/⁸⁶Srlavas have major and trace element concentrations which departfrom trends defined by most of the lavas. The variation in strontiumisotopic compositions suggests interaction between parentalbasaltic magma and crustal material. Attempts to model the assimilationprocess using fixed end-member assimilation and assimilation-fractionalcrystallization models have failed to produce the observed chemicaltrends. Because the parental basaltic liquid underwent littlefractionation, the variability in hybrid lavas is attributedto variation in contaminant composition. Initial melts werelow in CaO, A1₂O₃, MgO and Sr and enriched in SiO₂. K₂O, Na₂O,Rb and Ba. As melting progressed, melts became enriched in themore refractory components. Because hybrid strontium isotopiccomposition is a function of Sr concentration as well as isotopiccompositions, the Sr content of the assimilant strongly influencesresultant isotopic systematics. The development of the assimilantssuggests plagioclase was a residual phase during early melting.This model of crustal assimilation represents one end-memberin the spectrum of processes responsible for the generationof continental volcanic suites.  相似文献   

Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico     

W. J. MEYERS  A. T. JAMES 《Sedimentology》1978,25(1):105-124

Oxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co-precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non-ferroan calcite cements indicates that both are original precipitates that formed during pre-Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ¹⁸Os of megaquartz (mean =+27.0^0/00 SMOW; range =+ 24.8 to + 28.9^0/00) and calcite (mean =+ 28.0^0/00 SMOW; range =+ 27.3 to + 28.4^0/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre-Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre-Pennsylvanian opal-CT precursors, and therefore probably re-equilibrated during recrystallization in late or post-Mississippian time. We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ¹⁸Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground-water system, the seaward terminus of which moved southward during lowering of pre-Pennsylvanian sea level. The calcite cements and some of the opal-CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ¹⁸Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.  相似文献   

Zircons from Syros, Cyclades, Greece--Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism   总被引：27，自引：1，他引：27  

TOMASCHEK  FRANK; KENNEDY  ALLEN K.; VILLA  IGOR M.; LAGOS  MARKUS; BALLHAUS  CHRIS 《Journal of Petrology》2003,44(11):1977-2002

Zircons were studied from high-pressure/low-temperature metamorphosedmeta-igneous lithologies from Syros. These rocks carry severalzircon generations related to each other by dissolution–reprecipitationprocesses. One generation is pristine zircon that shows growthzoning, relatively elevated contents of trivalent cations andhigh Th/U ratios. The other end-member is a skeletal zircongeneration with negligible trivalent cation contents and lowTh/U ratios (  相似文献   

Testing the case for a Middle Pleistocene Scandinavian glaciation in Eastern England: evidence for a Scottish ice source for tills within the Corton Formation of East Anglia, UK   总被引：1，自引：0，他引：1  

JONATHAN R. LEE  JAMES ROSE  JAMES B. RIDING  BRIAN S. P. MOORLOCK  RICHARD J. O. HAMBLIN 《Boreas: An International Journal of Quaternary Research》2002,31(4):345-355

The provenance of the Happisburgh Till and Corton Till of the Corton Formation is investigated using erratic clast lithologies and allochthonous palynomorphs to test whether the long held assumption that they were deposited by ice that originated in Scandinavia is valid. The results show a wide range of lithologies including Carboniferous Limestone and Coal Measures, and Permian Magnesian Limestone that are not found in Scandinavia, and an absence of distinctive Scandinavian material such as rhomb porphyry and larvikite. Lithologies found indicate deposition by an ice sheet which flowed southwards into north-east East Anglia from central and southern Scotland eroding and transporting materials derived from outcrops in these areas and from eastern England and the western margins of the southern North Sea Basin. It is concluded that the long held assumption that the Happisburgh Till and Corton Till of the Corton Formation were deposited by a Scandinavian ice sheet is erroneous and that they were instead deposited by Scottish ice.  相似文献   

Mineral Chemistry of Mildly Alkalic Basalts from the 25 Ma Mont Crozier Section, Kerguelen Archipelago: Constraints on Phenocryst Crystallization Environments     

DAMASCENO  DIMITRI; SCOATES  JAMES S.; WEIS  DOMINIQUE; FREY  FREDERICK A.; GIRET  ANDRE 《Journal of Petrology》2002,43(7):1389-1413

Phenocryst compositions and mineral–melt equilibria inthe mildly alkalic basalts from the 25 Ma Mont Crozier sectionon the Kerguelen Archipelago are used to estimate the depthsat which magmas stalled and crystallized and to constrain therole of crustal structure in the evolution of magmas producedby the Kerguelen mantle plume. The Crozier section, of nearly1000 m height, consists of variably porphyritic flows (up to21 vol. % phenocrysts), dominated by plagioclase ± clinopyroxene± olivine ± Fe–Ti oxides. Feldspars showan extreme range of compositions from high-Ca plagioclase (An₈₈)to sanidine and variable textures that are related to extensivefractionation, degassing, and mixing in relatively low-pressure(sub-volcanic) magma chambers. Although clinopyroxene is a minorphenocryst type (0–3 vol. %), its non-quadrilateral components,principally Al (1·9–8·6 wt % Al₂O₃), varywidely. The results of clinopyroxene–liquid thermobarometryand clinopyroxene structural barometry indicate that the Croziermagmas crystallized at pressures ranging from  相似文献   

NATURAL HAZARDS RISK IN THE KULLU DISTRICT,HIMACHAL PRADESH,INDIA*     

JAMES S. GARDNER 《Geographical review》2002,92(2):282-306

ABSTRACT. Hazardous processes, including floods, landslides, soil erosion, and debris flows, are common in the Himalaya. Deforestation has been held responsible for increasing risk from such hazards in the Indian context for more than a century. The deforestation‐hazard linkage in the Kullu District of Himachal Pradesh is examined. Evidence suggests that the extent of forest cover has altered little over 150 years and that hazardous processes recur in much the same locations, with similar frequency and magnitude, except where road construction has increased slope instability. Nonetheless, population growth and economic development, especially since 1990, have increased vulnerability to hazards.  相似文献   

Cyclicity in the nearshore marine to coastal, Lower Permian, Pebbley Beach Formation, southern Sydney Basin, Australia: a record of relative sea-level fluctuations at the close of the Late Palaeozoic Gondwanan ice age     

CHRISTOPHER R. FIELDING  KERRIE L. BANN†  JAMES A. MACEACHERN‡  STUART C. TYE§  BRIAN G. JONES¶ 《Sedimentology》2006,53(2):435-463

The Lower Permian (Artinskian to Sakmarian) Pebbley Beach Formation (PBF) of the southernmost Sydney Basin in New South Wales, Australia, records sediment accumulation in shallow marine to coastal environments at the close of the Late Palaeozoic Gondwanan ice age. This paper presents a sequence stratigraphic re‐evaluation of the upper half of the unit based on the integration of sedimentology and ichnology. Ten facies are recognized, separated into two facies associations. Facies Association A (seven facies) comprises variably bioturbated siltstones and sandstones with marine body fossils, interpreted as recording sediment accumulation in open marine environments ranging from lower offshore to middle shoreface water depths. Evidence of deltaic influence is seen in several Association A facies. Facies Association B (three facies) comprises mainly heterolithic, interlaminated and thinly interbedded sandstone and siltstone with some thicker intervals of dark grey, organic‐rich mudstone, some units clearly filling incised channel forms. These facies are interpreted as the deposits of estuarine channels and basins. Throughout the upper half of the formation, erosion surfaces with several metres relief abruptly separate open marine facies of Association A (below) from estuarine facies of Association B (above). Vertical facies changes imply significant basinward shift of environment across these surfaces, and lowering of relative sea level in the order of 50 m. These surfaces can be traced over several kilometres along depositional strike, and are defined as sequence boundaries. On this basis, at least nine sequences have been recognized in the upper half of the formation, each of which is < 10 m thick, condensed, incomplete and top‐truncated. Sequences contain little if any record of the lowstand systems tract, a more substantial transgressive systems tract and a highstand systems tract that is erosionally truncated (or in some cases, missing). This distinctive stacking pattern (which suggests a dominance of retrogradation and progradation over aggradation) and the implied relative sea‐level drop across sequence boundaries of tens of metres are remarkably similar to some other studies of continental margin successions formed under the Neogene icehouse climatic regime. Accordingly, it is suggested that the stratigraphic architecture of the PBF was a result of an Icehouse climate regime characterized by repeated, high‐amplitude cycles of relative sea‐level change.  相似文献   

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis   总被引：3，自引：0，他引：3  

JAMES P. HENDRY  MICHAEL J. PEARSON†  NIGEL H. TREWIN‡  ANTHONY E. FALLICK§ 《Sedimentology》2006,53(3):537-565

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.  相似文献