Reply to Comment by R. Higgs on ‘Hybrid sediment gravity flows– classification,origin and significance’     

Peter Haughton  Christopher Davis  William McCaffrey  Simon Barker 《Marine and Petroleum Geology》2010

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Contributions from the 9th International Conference on Gas in Marine Sediments, University of Bremen, 15–19 September 2008     

Gerhard Bohrmann  Bo Barker Jørgensen 《Geo-Marine Letters》2010,30(3-4):151-155

Following the first International Conference on Gas in Marine Sediments in Edinburgh, UK (1990), another eight successful conferences have provided a continuous forum for scientists from a variety of disciplines, organisations and countries. The 9th meeting of the Shallow Gas Group was held in September 2008 in Bremen, a hanseatic city more than 1,200 years old in northern Germany. The Shallow Gas Group was joined for this conference by participants of the HERMES EU-funded project and by members of an industry-funded project. Volume 30 (3/4) of Geo-Marine Letters is a double issue containing 25 selected papers from the 9th conference in Bremen, guest edited by G. Bohrmann and B.B. Jørgensen. The papers represent the broad spectrum of oral and poster contributions from the conference, covering a wide range of aspects of gas in marine sediments from many parts of the world. The next conference of the Shallow Gas Group is planned to be held in Listvyanka at Lake Baikal, Russia, in September 2010.  相似文献   

Hybrid sediment gravity flow deposits – Classification, origin and significance   总被引：1，自引：1，他引：0  

Peter Haughton  Christopher Davis  William McCaffrey  Simon Barker 《Marine and Petroleum Geology》2009,26(10):1900-1918

The deposits of subaqueous sediment gravity flows can show evidence for abrupt and/or progressive changes in flow behaviour making them hard to ascribe to a single flow type (e.g. turbidity currents, debris flows). Those showing evidence for transformation from poorly cohesive and essentially turbulent flows to increasingly cohesive deposition with suppressed turbulence ‘at a point’ are particularly common. They are here grouped as hybrid sediment gravity flow deposits and are recognised as key components in the lateral and distal reaches of many deep-water fan and basin plain sheet systems. Hybrid event beds contain up to five internal divisions: argillaceous and commonly mud clast-bearing sandstones (linked debrite, H3) overlie either banded sandstones (transitional flow deposits, H2) and/or structureless sandstones (high-density turbidity currents, H1), recording longitudinal and/or lateral heterogeneity in flow structure and the development of turbulent, transitional and laminar flow behaviour in different parts of the same flow. Many hybrid event beds are capped by a relatively thin, well-structured and graded sand–mud couplet (trailing low-density turbulent cloud H4 and mud suspension fallout H5). Progressive bed aggradation results in the deposits of the different flow components stacked vertically in the final bed. Variable vertical bed character is related to the style of up-dip flow transformations, the distance over which the flows can evolve and partition into rheological distinct sections, the extent to which different flow components mutually interact, and the rate at which the flows decelerate, reflecting position (lateral versus distal) and gradient changes. Hybrid beds may inherit their structure from the original failure, with turbidity currents outpacing debris flows from which they formed via partial flow transformation. Alternatively, they may form where sand-bearing turbidity currents erode sufficient substrate to force transformation of a section of the current to form a linked debris flow. The incorporation of mud clasts, their segregation in near-bed layers and their disintegration to produce clays that can dampen turbulence are inferred to be key steps in the generation of many hybrid flow deposits. The occurrence of such beds may therefore identify the presence of non-equilibrium slopes up-dip that were steep enough to promote significant flow incision. Where hybrid event beds dominate the entire distal fan stratigraphy, this implies either the system was continually out of grade in order to freight the flows with mud clasts and clays, or the failure mechanism and transport path repeatedly allowed transmission of components of the initial slumps distally. Where hybrid beds are restricted to sections representing fan initiation, or occur more sporadically within the fan deposits, this could indicate shorter episodes of disequilibrium, due to an initial phase of slope re-adjustment, or intermittent tectonically or gravity-driven surface deformation or supply variations. Alternatively, changes between conventional and hybrid event beds may record changes in the flow generation mechanism through time. Thus the vertical distribution of hybrid event beds may be diagnostic of the wider evolution of the fan systems that host them.  相似文献   

Radiocarbon dating of charcoal from tropical sequences: results from the Niah Great Cave,Sarawak, and their broader implications     

Thomas F. G. Higham  Huw Barton  Chris S. M. Turney  Graeme Barker  Christopher Bronk Ramsey  Fiona Brock 《第四纪科学杂志》2009,24(2):189-197

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Crack-fill porphyroblastesis     

Andrew J. Barker 《Journal of Metamorphic Geology》2002,20(2):283-294

Garnet‐mica schists from the Scottish Highlands provide new insight into an important mechanism of phyllosilicate growth, termed ‘crack‐fill porphyroblastesis’. It is shown that grain boundary dilatancy, microcracking and porphyroblast‐matrix decoupling all play a significant role in facilitating growth in regimes of noncoaxial shear. With respect to chlorite porphyroblasts, there are three growth stages. Following nucleation, the initial phase of growth is by progressive matrix replacement, to preserve inclusion trails of fine carbonaceous material. The second growth stage produced new optically continuous inclusion‐free chlorite on the {001} margins of those crystals at a high angle to the schistosity. This growth results from decoupling at the porphyroblast–matrix contact on those margins at a high angle to the principal axis of extension. The development of dilatant cracks at porphyroblast margins provides a sink for material migrating down P_f and chemical potential gradients. This causes precipitation of new optically continuous ‘clear’ chlorite on the pre‐existing, heavily included core. The porphyroblast–matrix boundary continues to dilate after porphyroblast growth had terminated, producing plano‐convex quartz‐rich strain shadows. Similar growth behaviour is recognised in biotite porphyroblasts, indicating that ‘crack‐fill porphyroblastesis’ is an important growth mechanism for phyllosilicates in actively deforming metamorphic rocks. It also indicates that decoupling and crack‐fill development at porphyroblast margins could be important in controlling the pattern of material transfer, and may have significant implications for matrix permeability and fluid‐flow characteristics.  相似文献   

Adaptation, innovation and domestic food production in Jamaica: Some examples of survival strategies of small-scale farmers     

Clinton Beckford  David Barker  Steve Bailey 《Singapore journal of tropical geography》2007,28(3):273-286

Small-scale farmers' experimental innovations have not generally been considered for on-farm research trials as those in the traditional sector have been perceived as recipients, rather than originators, of technical knowledge and sustainable and viable practices. Yet there is abundant evidence throughout the tropics that small-scale farmers are adaptive and experimental problem solvers, and experts at devising innovative survival strategies. While literature on the topic is rich with accounts from Africa, Asia and Latin America, there is a general dearth of examples from the Caribbean. This paper highlights some examples of farm- and village-level problem solving and survival practices among small-scale domestic food producers in the challenging agroecological environment of Jamaica.  相似文献   

World Magnetic Charts for 1980 — spherical harmonic models of the geomagnetic field and its secular variation     

F. S. Barker  D. R. Barraclough  S. R. C. Malin 《Geophysical Journal International》1981,65(2):525-533

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Scale‐dependency of hydraulic conductivity: an example from Thorne Moor,a raised mire in South Yorkshire,UK     

J. Bromley  M. Robinson  J. A. Barker 《水文研究》2004,18(5):973-985

The hydraulic conductivity (K) of many low permeability materials is strongly scale‐dependent. In raised mires and other types of peat deposit the effects of features such as abandoned infilled ditches, root holes and localized woody material, cause K to be heterogeneous and scale‐dependent. Despite this, field measurements are routinely made using auger hole (slug) tests at the scale of only a few tens of centimetres. Such measurements are locally valid, but where the regional subsurface movement of water through peat bogs is simulated using groundwater models, typically at the scale of hundreds of metres, they give rise to a systematic underestimate of flows and an overprediction of water table elevations. Until now, techniques to obtain values at a scale sufficiently large to include the effects of localized features of higher permeability have not been applied routinely. Research at Thorne Moor, a large raised mire, demonstrates that the K of peat varies over several orders of magnitude when measured at different scales, using a variety of techniques. Laboratory and auger hole tests cannot be relied upon to provide results that represent the hydraulic conductivity of large expanses of peatland. This has significant implications for the management and long‐term restoration of peatlands where both regional and local control of water levels is crucial. For groundwater models to be used successfully to plan such schemes, it is essential to apply the K values relevant to the scale of the simulation. This paper describes and tests novel techniques, using ditches, for the derivation of K at large scales which overcome many of the problems that have been identified with conventional techniques and are capable of producing estimates that are appropriate to the application of physically based regional flow models. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Geochronology of archean gneisses in the Lake Helen area,Southwestern Big Horn Mountains,Wyoming     

J.G Arth  F Barker  T.W Stern 《Precambrian Research》1980,11(1):11-22

The RbSr and UPb methods were used to study gneisses in the $\text{7}\text{1}\text{2}\text{-}\text{minute}$ Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite.A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial ⁸⁷Sr/⁸⁶Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source.A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The ⁸⁷Sr/⁸⁶Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.  相似文献   

Signal contribution sections and their use in resistivity studies   总被引：1，自引：0，他引：1  

R. D. Barker 《Geophysical Journal International》1979,59(1):123-129

Summary. The electrostatic analogue to direct current resistivity prospecting is employed to construct vertical sections through lines of four electrodes. These show the relative contributions made by individual volume elements of earth to the total potential difference measured between the two potential electrodes. Signal contribution sections enable a comparison of the response of various electrode configurations to subsurface conditions and help to explain, for example, why the dipole—dipole configuration is so suited to horizontal profiling and the Wenner and Schlumberger configurations to vertical sounding.
The spurious response of apparent resistivity curves to near surface lateral resistivity variations is also explained with particular attention being given to the Wenner tripotential configurations.  相似文献