Cone morphologies associated with shallow marine eruptions: east Pico Island, Azores

Eruptions in shallow water typically produce cones of volcaniclastic material. In order to identify any systematic effects of water depth and other environmental parameters on cone morphology, we have measured the heights and widths of cones in multibeam echo-sounder data from a submarine ridge extending southeast from Pico Island, Azores. XRF analyses of dredged samples show that lavas here vary compositionally from alkali basalt to trachybasalt and trachyandesite. Cones in deeper water are generally steep-sided with upper flanks close to 30°, the dip of talus at the angle of repose. However, height/width ratios of cones vary more in shallow water (200?C400-m summit depth) with extreme values below 0.1; while some shallow-water cones are steep-sided as in deep water, others are much flatter. Three such cones lie on a bench at 300-m depth immediately east of Pico Island and have flank slopes of only 10?C20°. We speculate that exceptionally shallow cone slopes here were produced by forced spreading of the erupting columns on reaching the water?Cair density barrier.     

The geologist's dilemma: predicting the future behaviour of buried radioactive wastes

The last 10–15 years have produced a considerable amount of research into the geological disposal of radioactive wastes. This has had some very beneficial spin-offs for the geosciences. There are, however, a number of areas where it is difficult for earth scientists to provide quantitative information required for the types of long-term safety assessment being performed at present. With the likely increased stringency with which we may begin to treat other industrial wastes, much is to be learned from the radioactive waste experience. This article reviews some of the geological issues in constraining long-term predictions, discusses how geological data are used, and questions exactly what it is that we are trying to achieve in the management of these wastes and in the regulations concerning their disposal.     

High-Arctic glaciers as analogues for glacial landform development in Britain

Applying the principles of structural geology and sedimentology to glacial depositional features indicates that the High-Arctic provides the best analogues for late glacial landforms in the British uplands.     

长江中游泥沙与洪水来源的网湖沉积记录

在长江中游湖北省东部网湖湖心的采取柱状沉积物,用²¹⁰Pb+¹³⁷Cs方法测定表层的沉积速率.制作连续切片,选取深度7烈-88lmm的层型沉积较完整的一段样品,用显微镜量测浅色层和暗色层的厚度,采用电子探针确定层的地球化学成份,并由此计算了纹层的主要矿物组成.认为纹层主要是由于在洪水季节长江带来的浅色泥沙与在枯水季节富水河带来的含暗色矿物泥沙交替沉积形成的.泥沙主要来源于夏季长江倒灌入湖的洪水,部分来自长江支流富水河;冬季枯水季节,泥沙基本来源于长江支流富水河.     

The genesis and tectonic control on archaean gold deposits of the Western Australian Shield — A metamorphic replacement model

Gold deposits occur in greenstone belts world wide, and contribute to anomalously high gold production from Archaean terranes. As in other cratons, Archaean gold mineralization of Western Australia represents a complex array of deposit styles. Despite this, most deposits are clearly epigenetic, and large deposits have a number of features in common, including their strong structural controls, distinctive wallrock alteration (Fe-sulphide, K-mica±albite, Ca---Mg---Fe carbonates), consistent metal associations (Au---Ag---As---Sb---W---B; low base metals), commonly Fe-rich host rocks, great depth extension and lack of appreciable vertical zonation. These shared characteristics, combined with their ubiquitous occurrence, indicate that Archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts.Auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic (porphyry) intrusions. Although these gold systems involved low-salinity, lowdensity, reduced, near-neutral H₂O---CO₂ fluids carrying gold as reduced sulphur complexes, the origin of the fluids is equivocal. Most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic (granitoid or felsic porphyry) orggins, but the lack of consistent spatial relationships between specific, volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids.The metamorphic-replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile, with high geothermal gradients inhibiting significant melting. CO₂ possibly formed by the decarbonation of early alteration, related to mantle degassing along crustal-scale, synbasinal fault zones. Auriferous fluids were channelled along greenstone-scale faults, in part developed during reactivation of crustal-scale faults in a strike-slip regime. Gold deposition occurred largely under greenschist facies conditions (about 300–400°C, 1–2 kb) in response to decreasing gold solubility with declining temperature. However, a major control on gold deposition was fluid/wallrock interaction. Many large deposits formed by sulphidation of Fe-rich host rocks, with synchronous deposition of Fe-sulphides and gold. However, the variable nature of gold-depositing reactions, including lowering of fO₂ and pH, allowed a multitude of small, and some large, deposits to form wherever that fluid circulation occurred. In consequence, several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks. There are few constraints on the source of components (Au, S, K, CO₂) added to gold deposits, but even giant deposits such as the Golden Mile, Kalgoorlie could have formed from a realistic greenstone source volume (ca. 8×8×5 km). Convective circulation of fluids could have contributed to the generation of high fluid-rock ratios.On the regional scale, the markedly heterogeneous distribution of large gold deposits, gold productivity and host rocks to deposits can be accommodated by the metamorphic-replacement model. The most favourable conditions for development of auriferous hydrothermal systems operated in younger (ca. 2.7±0.1 Ga) rift-phase greenstones where greatest extension and crustal thinning produced high geothermal gradients, crustal-scale synbasinal faults, and rapid extrusion and burial of volcanics, including abundant komatiites. Iron-rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits. The least favourable conditions existed in older (ca. 3.5-3.4 Ga) platformphase greenstones, where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites, commonly in very shallow-water environments. This allowed intense synvolcanic alteration of both gold source rocks and potential host rocks. The generally smaller gold deposits formed mainly in ultramafic or greywacke hosts. Younger (ca. 3.0 Ga) platform-phase greenstones appear intermediate in nature but, unlike other greenstones, have significant epigenetic gold deposits in originally oxide-facies BIF, which were deposited on relatively deep-water platforms. Similar controls appear to exist on a world scale, with gold mineralization peaking at ca. 2.7±0.1 Ga in response to development of major rift zones in thickened, relatively mature continental crust. Interestingly, the giant Witwatersrand goldfield formed at about the same time.     

Morphology and internal structure of sand shoals and sandbanks off the Dorset coast, English Channel

The morphology and internal structure of sand shoals and sandbanks around a coastal headland (Portland Bill, southern UK) are described on the basis of sidescan sonar and high-resolution seismic data sets. Morphological and architectural evidence, combined with the spatial distribution and nature of the bedrock surface, indicates that the evolution of these deposits, especially the sandbanks, may not only be hydrodynamically controlled but also morphologically controlled. The internal structure of the sand shoals reveals a simple pattern of clinoforms dipping in the same direction as their steeper profile. In contrast, the internal structure of the sandbanks reveals that their lower seismic units consist of a sedimentary core interpreted as the remains of a lowstand deposit or an early stage of the bank development under different prevailing hydrodynamic conditions. The present morphology of the sandbanks is represented by upper seismic units characterized by large-scale sandwave foresets dipping at 6–8°. The nature and morphology of the bedrock surface may also influence the development of sandbanks in some cases. For example, to the west of Portland Bill, the morphology of the bedrock surface (basal reflector) restricts the development of the Portland Bank. This information may explain the asymmetric evolution of sandbanks around Portland Bill.     

The theory of finite frequency body wave synthetic seismograms in inhomogeneous elastic media

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A new method is presented by means of which one can compute finite frequency synthetic seismograms for media whose velocity and density are continuous functions of two or three spatial variables. Basically, the method is a generalization of the familiar phase integral method, to which it reduces in a stratified medium. For a given source location the travel-time and distance functions needed to compute synthetics are obtained by numerically tracing rays through the model. This information is then used to evaluate a double integral over frequency and take-off angle at the source. The solution obtained reduces to the geometrical optics solution wherever that is valid but it also works in shadows and at caustics without knowing explicitly where these may be located. The method can be used as a spectral method, in which the integral over take-off angle is evaluated first, or as a slowness method, in which the frequency integral is evaluated first.