Genetic modelling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia   总被引：3，自引：0，他引：3  

R.C. Morris   《Precambrian Research》1993,60(1-4)

The banded iron-formation (BIF) of the Hamersley Group, Pilbara Craton, Western Australia, particularly from the well studied Dales Gorge Member, is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km², enabling reasonable estimates for the annual input of components to the depository. In the model of this paper, varying supply of materials for the medley of mesoband types, particularly of iron and silica in the oxide BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents and (2) convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied input of pyrochastic material. (1) The surface currents were saturated in silica and carried minimal iron due to photic precipitation, but were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves, whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. (2) Precipitation from convection driven upwelling of high iron solution from MOR or hot-spot activity periodically overwhelmed the delicate seasonal patterns of (1) to produce the iron-dominated mesobands. A wide range of intermediate mesoband types resulted where the deep water supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by the present El Niño). During these periods of oxide-dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photosynthetically produced oxygen.Superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources, changing the meteorological and depositional conditions. Occasional input of extremely fi ash during BIF precipitation produced mesoband (cm) scale variations involving increased carbonate-silicate precipitation. Sustained volcanic periods resulted in S-macroband deposition (chert-carbonate-silicate BIF, with shale), gradually returning to the dominant hematite-magnetite-chert BIF as the volcanic input waned. During volcanic periods, the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron, and by photosynthetic production of oxygen, was modified by turbidity in the atmosphere (aerosols and dust) and in the water (colloids from reactive ash). S Surface-precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone, augmenting the iron content of the zone. Precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub-photic zone exceeded their respective solubilities. During volcanism, the increased availability of nutrients, particularly phosphorus, to surface waters increased the organic contribution despite lower light values, leading to an almost total absence of ferric iron oxides in the S macrobands (i.e. no magnetite or hematite). Cooling of warm, silica-saturated sea-water during these periods of “olcanic winter” increased the ratio of precipitation of silica to iron, which, however, was still controlled by seasonal conditions. Intermediate concentrations of organic matter, insufficient to totally convert the ferric compounds either during precipitation or diagenesis, resulted in overgrowths of magnetite on hematite, and eventually in the substantial conversion of hematite to magnetite, where higher temperatures were achieved during low-grade regional metamorphism.Changes in sea-level to explain facies changes in BIF are not required in this model, but are not excluded. The preferred conditions are for a very low oxygen to anoxic atmosphere, a much higher level of MOR activity than at present, the presence of photosynthetic plankton, the absence of si silica-secreting organisms, and a deep sea-water temperature higher than 20°C. However, none of these conditions is essential to the model.A narrow carbonate bank is postulated for part of the Fortescue River Valley area during Marra Mamba Iron Formation times (basal Hamersley Group), with BIF precipitation on either side. The reef is postulated to have grown northward becoming a major shallow-water carbonate platform on the Pilbara continent during upper Marra Mamba Iron Formation and Wittenoom Dolomite times, but ceased to play an important role in subsequent periods.  相似文献   

A combined boundary-profile and automated data-reduction and analysis system     

Richard Deloach  Alvin L. Morris  Robert B. McBeth 《Boundary-Layer Meteorology》1976,10(4):455-463

A portable boundary-layer meteorological data-acquisition and analysis system is described which employs a small tethered balloon and a programmable calculator. The system is capable of measuring pressure, wet- and dry-bulb temperature, wind speed, and temperature fluctuations as a function of height and time. Other quantities, which can be calculated in terms of these, can also be made available in real time. All quantities, measured and calculated, can be printed, plotted, and stored on magnetic tape in the field, during the data-acquisition phase of an experiment.The National Center for Atmospheric Research is Sponsored by the National Science Foundation.  相似文献   

Evaluation of a Chimney Drain Design in an Earthfill Dam     

Morris L. Maslia  Mustafa M. Aral 《Ground water》1982,20(1):22-31

  相似文献   

On the location of acid-hydrolysable carbon in lunar soil fines     

A.E. Fallick  I.P. Wright  C.T. Pillinger  A. Stephenson  R.V. Morris 《Earth and Planetary Science Letters》1982,59(1):28-32

  相似文献   

Acute toxicity of kepone to selected freshwater fishes     

Morris H. Roberts  Robert E. Bendl 《Estuaries and Coasts》1982,5(3):158-164

The acute toxicity of Kepone in freshwater was determined with three fish species,Ictalurus punctatus (channel catfish),Lepomis macrochirus (bluegills), andAnguilla rostrata (American eel). Elvers ofA. rostrata were most sensitive with a 96 h lethal concentration for 50% of the animals tested (LC50) of 35 μg per. 1. Bluegills were slightly less sensitive with a 96 h LC50 of 50 μg per 1. Catfish were most tolerant with a 96 h LC50 of 514 μg per l. Bluegills and catfish exposed to comparable concentrations of Kepone accumulated equivalent amounts in 96 h. This observation in conjunction with the markedly different 96 h LC50's for these species suggest a difference in the ways these fish cope with Kepone.  相似文献   

Electron paramagnetic resonance study of the site preferences of Gd³⁺ and Eu²⁺ in polycrystalline silicate and aluminate minerals     

Richard V. Morris 《Geochimica et cosmochimica acta》1975,39(5):621-634

Electron paramagnetic resonance (EPR) measurements were made on Gd³⁺ and Eu²⁺ ions in polycrystalline samples to determine the nature of the sites occupied by those ions in mineral structures. Both Gd³⁺ and Eu²⁺ ions were incorporated at Ca²⁺ structural sites in β-Ca₂SiO₄, pseudo-CaSiO₃, CaMgSiO₄, CaMgSi₂O₆, hex-CaAl₂Si₂O₈, CaAl₂O₄, and Ca₃Al₂O₆. For tri-CaAl₂Si₂O₈, Eu²⁺ was incorporated at a Ca²⁺ site and Gd³⁺ was incorporated at a site where the crystalline electric field was disordered. That difference in behavior may contribute to the anomalous behavior of Eu in plagioclase feldspar. Both Gd³⁺ and Eu²⁺ were incorporated as aggregates or clusters of those ions in Mg₂SiO₄ and clino-MgSiO₃.  相似文献   

Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway – reply     

Emmanuelle Arnaud  Carolyn H. Eyles† 《Sedimentology》2004,51(6):1423-1430

  相似文献   

Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida     

Joel S. Steward  Robert W. Virnstein  Lori J. Morris  Edgar F. Lowe 《Estuaries and Coasts》2005,28(6):923-935

Seagrass protection and restoration in Florida’s Indian River Lagoon system (IRLS) is a mutual goal of state and federal programs. These programs require, the establishment of management targets indicative of seagrass recovery and health. We used three metrics related to seagrass distribution: areal coverage, depth limit, and light requirement. In order to account for the IRLS’s spatial heterogeneity and temporal variability, we developed coverage and depth limit targets for each of its 19 segments. Our method consisted of two steps: mapping the union of seagrass coverages from all availabe mapping years (1943, 1986, 1989, 1992, 1994, 1996, and 1999) to delineate wherever seagrass had been mapped and determining the distribution of depth limits based on 5,615 depth measurements collected on or very near the deep-edge boundary of the union coverage. The frequency distribution of depth limits derived from the union coverage, along with the median (50th percentile) and maximum (95th percentile) depth limits, serve as the seagrass depth targets for each segment. The median and maximum depth targets for the IRLS vary among segments from 0.8 to 1.8 and 1.2 to 2.8 m, respectively.Halodule wrightii is typically the dominant seagrass species at the deep-edge of IRLS grass beds. We set light requirement targets by using a 10-yr record of light data (1990–1999) and the union coverage depth limit distributions from the most temporally stable seagrass segments. The average annual light requirement, based on the medians of the depth limit distributions, is 33 ± 17% of the subsurface light. The minimum annual light requirement, based on of the 95th percentile of the depth distributions, is 20 ± 14%; the minimum growing season light requirement (March to mid September) is essentially the same (20 ± 13%). Variation in depth limits and light requirements, is probably due to factors other than light that influence the depth limit of seagrasses (e.g., competition, physical disturbance). The methods used in this study are robust when applied to large or long-term data sets and can be applied to other estuaries where grass beds are routinely monitored and mapped.  相似文献   

Water and Radioactive Tracer Flow in a Heterogeneous Field-Scale System   总被引：1，自引：0，他引：1  

Frank M. Dunnivant  Meredith E. Newman  Carolyn W. Bishop  Dave Burgess  John R. Giles  Brian D. Higgs  Joel M. Hubbell  Erick Neher  Greg T. Norrell  M. Cathy Pfiefer  Indrek Porro  Robert C. Starr  Allan H. Wylie 《Ground water》1998,36(6):949-958

  相似文献   

Megadevelopment in Malaysia: From Heritage Landscapes to "Leisurescapes" in Melaka's Tourism Sector     

Carolyn Cartier 《Singapore journal of tropical geography》1998,19(2):151-176

In Malaysia, state promotion of tourism as a main component of service sector expansion and foreign exchange earnings has compelled government at all levels to facilitate leisure industry development. Even in the relatively small state of Melaka, where its historic landscapes constitute significant heritage tourism resources, development projects for theme parks and golf resorts have proliferated. This analysis examines Malaysian tourism development through an evaluation of the diverse tourism sectors in Melaka town and state – heritage, theme park, golf resort, and ecotourism – to demonstrate how the developmental contexts of national and international economic restructuring, combined with regional tourism demand for activity-oriented leisure, have rationalised large-scale, high-cost, environmentally-sensitive real estate development projects. In the process, authentic natural and historic landscapes have been rapidly forsaken for ersatz "leisurescapes" where both domestic and foreign tourists consume representations of diverse symbolic worlds.  相似文献