Heritage Identification, Conservation and Management     

Gillian Bladder 《New Zealand geographer》2002,58(2):81-81

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An introductory account of Suswa Volcano,Kenya     

G. J. H. McCall  C. M. Bristow 《Bulletin of Volcanology》1965,28(1):333-367

The authors have visited Suswa, a complex caldera-volcano situated thirty miles north-west of Nairobi, a feature surprisingly neglected by geologists. While they do not pretend to do more than present an introductory account of the general geology of this unique volcano, they are able to augment the brief references of earlier workers, Gregory, Spink and Richard. The principal rock types are described in general terms, and are found to include unusual rhomb-porphyry types of lava, vitrophyres of phonolitic composition (closely related to the kenytes, but devoid of modal nepheline). The earliest eruptions were of quite normal lava type, phonolites of Kenya type, erupted over a wide area in central Kenya in Plio-Pleistocene times (not later than 1.7 m.y. ago), and the rhomb-porphyries are restricted to a secondary eruptive sequence, of probable Pleistocene age. There was a minor reactivation in recent times, represented by restricted, bare, fresh flows, of type at present unknown. Chemical analyses of representative specimens of the two major suites are provided, and are supported by modal analyses of related specimens. Two summit calderas have been recognised, both apparently subsidence structures related to cauldron subsidence in depth. The earlier and larger caldera covers about 40 square miles, and is interpreted as ofGlencoe type with weakly developedKrakatoan characteristics. The inner caldera covers seven square miles, and is interpreted as aGlencoe type structure: it is not a simple caldera but contains an island — block of four square miles extent — a feature which may perhaps be reasonably compared with island features within the Lake Toba cauldron, Sumatra and Nyamlagira caldera, Congo. The terminal eruptions of the first volcano seem to have largely stemmed from a ring feeder, analogous with a body reported from Crater Lake caldera, Oregon, U.S.A. The outer caldera is now partly obscured by products of later eruption, from a secondary cone eccentric to the first caldera — Ol Doinyo Nyukie — and from minor parasitic vents. Ol Doinyio Nyukie volcano possessed an axial pit-crater, nearly a mile in diameter, now transected by the boundary fault of the inner caldera: this might reasonably be regarded as a third,Kilauean, summit caldera, since it was apparently drained by low-level, adventive eruptions. Fumarolic activity is rife within Suswa at the present time: steam is being emitted, probably derived from meteoric water but charged with CO₂ and probably nitrogen. Analogies between the Suswa pattern of calderas and certain lunar crater patterns are briefly mentioned.  相似文献   

The geology and structural relationships of the southern Lebombo volcanic and intrusive rocks,South Africa     

E. P. Saggerson  J. W. Bristow 《Bulletin of Volcanology》1983,46(2):161-181

A brief account is presented for the Lebombo volcanic succession which crops out in Natal, South Africa. The volcanic belt is of late Karoo age and is composed of a thick sequence of basaltic lavas (Sabie River Formation) overlain by an equally voluminous succession of acid-flows (Jozini Formation) erupted over a period of about 70 m.y. Field relationships indicate that the Lebombo basalt pile consists of simple and compound flow units. The rhyolite succession consists of thick (80–284 m) flows units characterised by features found in both ignimbrites and rhyolitic lavas respectively. It is postulated that they were extruded as high temperature, low volatile pyroclastic flows. The Bumbeni volcanic complex which crops out near the southern termination of the Lebombo mountains, disconformably overlies the Jozini Formation and is characterised by a suite of rocks that includes rhyolite lavas, air-fall and ash-flow tuffs, syenite intrusions and basic-intermediate lavas. Dolerite dykes are ubiquitous throughout the succession and an extremely dense concentration of basic intrusions located along the western margin of the belt gives rise to the Rooi Rand dyke swarm. Rare sill-forms are found associated with the mafic volcanies. Acid intrusives are represented by simple and composite quartz-porphyry intrusions and rhyolite dykes. The structure of the Lebombo is that of a faulted monocline, tilted to the east, developed prior to the fragmentation of eastern Gondwanaland. The volcanic belt is located at the tectonic contact between two major Precambrian elements, the 3,000 m.y. Kaapvaal craton to the west and the southerly extension of the 550 m.y. Mozambique belt to the east. It is bounded to the south by the 1,000 m.y. old Natal-Namaqua mobile belt.  相似文献   

The Table Rocks Sandstone: A fluvial, friction-dominated lobate mouth bar sandbody in the Westphalian B Coal Measures, NE England     

Brian R. Turner  Gillian N. Tester   《Sedimentary Geology》2006,190(1-4):97-119

Westphalian B (Duckmantian) alluvial Coal Measures along the Northumberland coast, NE England, comprise coal-capped coarsening-upward crevasse-splay sequences of shale, siltstone and sandstone, interbedded with a number of major distributary channel sandbodies, including the Table Rocks Sandstone. Lithofacies, architectural analysis and outcrop geometries divide the Table Rocks Sandstone into flaggy sandstone, massive sandstone, heterolithic, and mudstone facies associations, each comprising up to 7 lithofacies types. The three sandy facies associations are characterised by lenticular bed geometries on different scales producing a hierarchy of lensoid packages and associated bounding surfaces, all showing typical offset stacking patterns: (1) lenses, represent individual lenticular cross-bed sets, bounded by 1st order surfaces; (2) packages of lenses, called lens clusters are bounded by 2nd order surfaces, and are the basic architectural building block of the sandy facies associations; and (3) vertically stacked lens clusters called amalgamated lens clusters, bounded by 3rd order surfaces. The Table Rocks sandbody has a laterally extensive, irregular, lobate subsurface plan geometry, it displays a radial palaeocurrent pattern with 180° dispersion, and it forms part of a 14-m thick coarsening-upward regressive sequence. It is interpreted as a composite, lobate crevasse-splay delta system that prograded into a shallow interdistributary fresh to brackish water lake up to 14 m deep. The shallow lake water, fluvial input, and extensive development of traction structures such as cross-bedding and ripple cross-lamination suggests a friction-dominated delta, in which the four facies associations can be interpreted in terms of discrete elements of the mouth bar environment. The flaggy sandstone facies association represents the main, axial part of the mouth bar system, the erosively based massive sandstone facies association major subaqeous distributary channels, the lithologically more variable heterolithic facies association the medial mouth bar, and the mudstone facies association the distal mouth bar fringe and prodelta. Within this environmental setting amalgamated lens clusters are interpreted as small, discrete mouth bar sand lobes, whose offset, imbricate stacking pattern reflects channel spacing and bifurcation, the rate of channel shifting, or shallow depths and lack of accommodation space. Thus, lens clusters are interpreted as discrete growth elements of the mouth bar sand lobes, and lenses as individual bedforms making up these growth elements. Because of the high rate of channel shifting, lack of extensive erosion of the mouth bar lobes, and deposition of low discharge fines, the lobes retained much of their original depositional geometry, thereby providing advantageous gradients for offset deposition and stacking of adjacent sand lobes. Although the delta complex was maintained by frequent crevassing from the feeder channel, and by subsidence due to contemporaneous compaction and/or local tectonism, it was deeply incised on two occasions by subaqeous channels in response to high magnitude floods or falling lake level.  相似文献   

Ambient and cold‐temperature infrared spectra and XRD patterns of ammoniated phyllosilicates and carbonaceous chondrite meteorites relevant to Ceres and other solar system bodies          下载免费PDF全文

Bethany L. Ehlmann  Robert Hodyss  Thomas F. Bristow  George R. Rossman  Eleonora Ammannito  M. Cristina De Sanctis  Carol A. Raymond 《Meteoritics & planetary science》2018,53(9):1884-1901

Mg‐phyllosilicate‐bearing, dark surface materials on the dwarf planet Ceres have NH₄‐bearing materials, indicated by a distinctive 3.06 μm absorption feature. To constrain the identity of the Ceres NH₄‐carrier phase(s), we ammoniated ground particulates of candidate materials to compare their spectral properties to infrared data acquired by Dawn's Visible and Infrared (VIR) imaging spectrometer. We treated Mg‐, Fe‐, and Al‐smectite clay minerals; Mg‐serpentines; Mg‐chlorite; and a suite of carbonaceous meteorites with NH₄‐acetate to exchange ammonium. Serpentines and chlorites showed no evidence for ammoniation, as expected due to their lack of exchangeable interlayer sites. Most smectites showed evidence for ammoniation by incorporation of NH₄⁺ into their interlayers, resulting in the appearance of absorptions from 3.02 to 3.08 μm. Meteorite samples tested had weak absorptions between 3.0 and 3.1 μm but showed little clear evidence for enhancement upon ammoniation, likely due to the high proportion of serpentine and other minerals relative to expandable smectite phases or to NH₄⁺ complexing with organics or other constituents. The wavelength position of the smectite NH₄ absorption showed no variation between IR spectra acquired under dry‐air purge at 25 °C and under vacuum at 25 °C to ?180 °C. Collectively, data from the smectite samples show that the precise center wavelength of the characteristic ~3.05 μm v₃ absorption in NH₄ is variable and is likely related to the degree of hydrogen bonding of NH₄‐H₂O complexes. Comparison with Dawn VIR spectra indicates that the hypothesis of Mg‐saponite as the ammonium carrier phase is the simplest explanation for observed data, and that Ceres dark materials may be like Cold Bokkeveld or Tagish Lake but with proportionally more Mg‐smectite.  相似文献   

Crevasse splays from the rapidly aggrading, sand-bed, braided Niobrara River, Nebraska: effect of base-level rise   总被引：2，自引：0，他引：2  

Bristow  Skelly  & Ethridge 《Sedimentology》1999,46(6):1029-1047

Base-level rise of ≈2·35 m on the Niobrara River has resulted in aggradation of the channel belt and a recent avulsion. Overbank areas have become flooded by rising groundwaters, and more than eight crevasse splays have formed between 1993 and 1997. Two crevasse splays, situated on the west and east sides of the Niobrara, have been studied using ground-penetrating radar (GPR), shallow boreholes and topographic surveys. The vibracores and GPR profiles provide a nearly three-dimensional view of the architecture of crevasse splay deposits. The east splay was initiated in the winter of 1993/94 and has expanded to cover an area ≈200 m by 1000 m, with sediment up to 2·5 m thick. The west splay, which was initiated by the opening of a crevasse channel through a levee in the autumn of 1995, covers an area ≈150 m by 250 m, with up to 1·2 m of sand deposited in a single year. The Niobrara splays are sand dominated and characterized by bedload deposition within channels, 5–30 m wide and 0·5–2 m deep, with the development of slipfaces where splays prograde into standing bodies of water. Sedimentary structures in cores include horizontal lamination, ripple lamination and sets of cross-stratification. There is a slight tendency for splays to coarsen up, but individual beds within the splays often fine up. The abundance of crevasse splays on the Niobrara River contrasts with other braided river floodplains. In the Niobrara, crevasse splay formation followed aggradation within the channel belt, which occurred in response to base-level rise. The link between crevasse splays, channel aggradation and base-level rise has important implications for the interpretation of ancient braided river and floodplain sequences. It is suggested that crevasse splay deposits should be an important component of aggrading fluvial sediments and, hence, should be preserved within the rock record. In this case, the aggradation and crevassing have been tied to a rise in base-level elevation, and it is suggested that similar deposits should be preserved where braided rivers are affected by base-level rise, for instance during transgression and filling of palaeovalleys.  相似文献   

Time-series measurements of Th in water column and sediment trap samples from the northwestern Mediterranean Sea     

J. Kirk Cochran  Juan Carlos Miquel  Robert Armstrong  Scott W. Fowler  Pere Masqu  Beat Gasser  David Hirschberg  Jennifer Szlosek  Alessia M. Rodriguez y Baena  Elisabet Verdeny  Gillian M. Stewart 《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(18):1487-1501

Disequilibrium between ²³⁴Th and ²³⁸U in water column profiles has been used to estimate the settling flux of Th (and, by proxy, of particulate organic carbon); yet potentially major non-steady-state influences on ²³⁴Th profiles are often not able to be considered in estimations of flux. We have compared temporal series of ²³⁴Th distributions in the upper water column at both coastal and deep-water sites in the northwestern Mediterranean Sea to coeval sediment trap records at the same sites. We have used sediment trap records of ²³⁴Th fluxes to predict temporal changes in water column ²³⁴Th deficits and have compared the predicted deficits to those measured to determine whether the time-evolution of the two coincide. At the coastal site (327 m water depth), trends in the two estimates of water column ²³⁴Th deficits are in fairly close agreement over the 1-month deployment during the spring bloom in 1999. In contrast, the pattern of water column ²³⁴Th deficits is poorly predicted by sediment trap records at the deep-water site (DYFAMED, 2300 m water depth) in both 2003 and 2005. In particular, the transition from a mesotrophic to an oligotrophic system, clearly seen in trap fluxes, is not evident in water column ²³⁴Th profiles, which show high-frequency variability. Allowing trapping efficiencies to vary from 100% does not reconcile the differences between trap and water column deficit observations; we conclude that substantial lateral and vertical advective influences must be invoked to account for the differences.Advective influences are potentially greater on ²³⁴Th fluxes derived from water column deficits relative to those obtained from traps because the calculation of deficits in open-ocean settings is dominated by the magnitude of the “dissolved” ²³⁴Th fraction. For observed current velocities of 5–20 cm s⁻¹, in one radioactive mean-life of ²³⁴Th, the water column at the DYFAMED site can reflect ²³⁴Th scavenging produced tens to hundreds of kilometers away. In contrast, most of the ²³⁴Th flux collected in shallow sediment traps at the DFYFAMED site was in the fraction settling >200 m d⁻¹; in effect the sediment trap can integrate the ²³⁴Th flux over distances 40-fold less than water column ²³⁴Th distributions. In some sense, sediment trap and water column sampling for ²³⁴Th provide complementary pictures of ²³⁴Th export. However, because the two methods can be dominated by different processes and are subject to different biases, their comparison must be treated with caution.  相似文献   

Reply to: Plates, Plumes, and Planetary Processes (P 4 ) by AC Kerr     

Gillian R. Foulger  Donna M. Jurdy 《Marine Geophysical Researches》2008,29(3):219-220

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CUBES: cassegrain U-band Brazil-ESO spectrograph     

B. Barbuy  V. Bawden Macanhan  P. Bristow  B. Castilho  H. Dekker  B. Delabre  M. Diaz  C. Gneiding  F. Kerber  H. Kuntschner  G. La Mura  W. Maciel  J. Meléndez  L. Pasquini  C. B. Pereira  P. Petitjean  R. Reiss  C. Siqueira-Mello  R. Smiljanic  J. Vernet 《Astrophysics and Space Science》2014,354(1):191-204

CUBES is a high-efficiency, medium-resolution (R～20,000) ground based UV (300–400 nm) spectrograph, to be installed in the cassegrain focus of one of ESO’s VLT unit telescopes in 2017/18. The CUBES project is a joint venture between ESO and IAG/USP, and LNA/MCTI. CUBES will provide access to a wealth of new and relevant information for stellar as well as extragalactic sources. Main science cases include the study of beryllium and heavy elements in metal-poor stars, the direct determination of carbon, nitrogen and oxygen abundances by study of molecular bands in the UV range, as well as the study of active galactic nuclei and the quasar absorption lines. With a streamlined modern instrument design, high efficiency dispersing elements and UV-sensitive detectors, it will give a significant gain in sensitivity over existing ground based medium-high resolution spectrographs, enabling vastly increased sample sizes accessible to the astronomical community. We present here a brief overview of the project including the status, science cases and a discussion of the design options.  相似文献   

Seasonal POC fluxes at BATS estimated from 210Po deficits     

Gillian M. Stewart  S. Bradley Moran  Michael W. Lomas 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(1):113-124

In this study at the Bermuda Atlantic Time-series Study (BATS) site we demonstrate that the polonium–lead disequilibrium system may perform better as a tracer of organic carbon export under low-flux conditions (in this case, <2.5 mmol C m^?2 d^?1) than under bloom conditions in an oligotrophic setting. With very few exceptions, the POC flux predictions calculated from the water-column ²¹⁰Po deficit were within a factor of 2 of the POC flux caught in surface-tethered sediment traps. However, we found higher correlation between size-fractionated particulate ²¹⁰Po activity and POC concentration in November 2006 (r=0.93) than in January (r=0.79) and during the spring bloom in March 2007 (r=0.80). We suggest that this is due to the ability of polonium to distinguish between bulk mass flux and organic carbon export under oligotrophic and lithogenic-driven flux regimes. Further, we found that the POC/Po ratio on particles was largely independent of size class between 10 and 100 μm (P=0.13) during each season, supporting the notion that export in this oligotrophic system is driven by sinking aggregates of smaller cells and not by large, individual cells.  相似文献