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81.
DAMASCENO DIMITRI; SCOATES JAMES S.; WEIS DOMINIQUE; FREY FREDERICK A.; GIRET ANDRE 《Journal of Petrology》2002,43(7):1389-1413
Phenocryst compositions and mineralmelt 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 ± FeTi 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 (03 vol. %), its non-quadrilateral components,principally Al (1·98·6 wt % Al2O3), varywidely. The results of clinopyroxeneliquid thermobarometryand clinopyroxene structural barometry indicate that the Croziermagmas crystallized at pressures ranging from 相似文献
82.
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
83.
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
84.
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
85.
Molar tooth structures of the Neoarchean Monteville Formation, Transvaal Supergroup, South Africa. II: A wave-induced fluid flow model 总被引:3,自引:0,他引:3
Sedimentological, morphological, and geochemical characteristics of molar tooth (MT) structures in the ca 2·6 Ga Monteville Formation suggest a new fluid flow model for MT formation: (i) intercalated shales and carbonate sands were deposited near to above storm wave base; (ii) sediments cracked, forming an interconnected network of MT cracks that were also open to pores in sand lenses; (iii) storm waves pumped sea water into open MT crack networks, causing rapid microcrystalline carbonate nucleation, Ostwald ripening of nuclei, and growth of granular carbonate cores; some of these cores were transported by water flowing through the cracks; (iv) unfilled MT cracks collapsed, and filled MT ribbons deformed plastically as host sediments compacted and dewatered; (v) carbonate cores were overgrown by polygonal rims; and (vi) MT structures deformed brittlely with additional compaction and produced pebbly lags if reworked. MT cracks may have formed by multiple mechanisms; however, expansion of gas from organic decay and sediment heaving due to wave loading best explain MT crack morphology and are most consistent with the fluid flow model for MT CaCO3 presented here. 相似文献
86.
87.
88.
Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism 总被引:5,自引:0,他引:5
Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·7292·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, FeTi oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H2O at 1100900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite 相似文献
89.
90.
Application and evaluation of universal kriging for optimal contouring of groundwater levels 总被引:4,自引:0,他引:4
B V N P KAMBHAMMETTU PRAVEENA ALLENA JAMES P KING 《Journal of Earth System Science》2011,120(3):413-422
This paper deals with the application of universal kriging to interpolate water table elevations from their measurements at
random locations. Geographic information system tools were used to generate the continuous surface of water table elevations
for the Carlsbad area alluvial aquifer located to the southeast of New Mexico, USA. Water table elevations in the 38 monitoring
wells that are common to 1996 and 2003 irrigation years follows normal distribution. A generalized MATLAB? code was developed to generate omni-directional and directional semi-variograms (at 22.5° intervals). Low-order polynomials
were used to model the trend as the water table profile exhibits a south-east gradient. Different theoretical semi-variogram
models were tried to select the base semi-variogram for performing geostatistical interpolation. The contour maps of water
table elevations exhibit significant decrease in the water table from 1996 to 2003. Statistical analysis performed on the
estimated contours revealed that the decrease in water table is between 0.6 and 4.5 m at 90% confidence. The estimation variance
contours show that the error in estimation was more than 8 m2 in the west and south-west portions of the aquifer due to the absence of monitoring wells. 相似文献