Seismic depth imaging of iron-oxide deposits and their host rocks in the Ludvika mining area of central Sweden     

Lena Bräunig  Stefan Buske  Alireza Malehmir  Emma Bäckström  Monica Schön  Paul Marsden 《Geophysical Prospecting》2020,68(1):24-43

The development of cost-effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth-imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal-to-noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron-oxide deposit at its accurate position in depth domain while simultaneously characterizing the near-surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy data set from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth-imaging techniques to obtain a clear and well-resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low-velocity layer within the depth-imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near-surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our data set the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of reflection image spectroscopy to the data, which allows the imaging of wavelength-specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low-frequency band while the internal structure was obtained within the high-frequency bands.  相似文献   

Plant community patterns in a gypsum area of NE Spain. I. Interactions with topographic factors and soil erosion   总被引：1，自引：0，他引：1  

Joaquín Guerrero-Campo  Francisco Alberto  John Hodgson  Jos Ma. García-Ruiz  Gabriel Montserrat-Martí 《Journal of Arid Environments》1999,41(4):401

The aim of this paper is to explain how topographical factors and soil erosion affect plant community patterns in the gypsum hills of the Middle Ebro Basin (NE Spain).Both the altitudinal location on the hill and, secondarily, the aspect determine a regular distribution pattern of the plant communities, which is closely related to soil and erosion features, as well as to some plant attributes. Tall perennial grasses dominate the valley bottoms, and scrub vegetation the hillsides and hilltops. Water availability, soil depth and particle movement down the slope are the main factors that could explain this segregation.  相似文献   

Use of ACRU,a distributed hydrological model,to evaluate how errors from downscaled rainfall are propagated in simulated runoff in uMngeni catchment,South Africa     

Samuel Kusangaya  Michele Warburton  Emma Archer van Garderen 《水文科学杂志》2017,62(12):1995-2011

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Preferential groundwater flow through a sorted net landscape,Arctic Canada     

Robert Hodgson  Kathy L. Young 《地球表面变化过程与地形》2001,26(3):319-328

Preferential suprapermafrost groundwater flow was observed in deepened channels lying between raised frost mounds. Here, saturated hydraulic conductivity, k, ranged from 90 to 1000 m/day but was only 0·1–1·0 m/day in the mound centres. A high proportion of fines occurs in the frost mound centre due to particle sorting, while channels contain gravels. Three approaches of areal weighting of k and groundwater flow, Q _s, across a wetland–upland boundary were explored. When percentage area covered by channels, mounds or gravel was considered, estimates of water flow on a daily and seasonal basis fell by 30 to 50 per cent. This study is of relevance to northern scientists who require reliable estimates of groundwater flow across patterned ground landscapes. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

Intra-clinothem variability in sedimentary texture and process regime recorded down slope profiles     

Grace I. E. Cosgrove  Miquel Poyatos-Moré  David R. Lee  David M. Hodgson  William D. McCaffrey  Nigel P. Mountney 《Sedimentology》2020,67(1):431-456

Shelf-margin clinothem successions can archive process interactions at the shelf to slope transition, and their architecture provides constraints on the interplay of factors that control basin-margin evolution. However, detailed textural analysis and facies distributions from shelf to slope transitions remain poorly documented. This study uses quantitative grain-size and sorting data from coeval shelf and slope deposits of a single clinothem that crops out along a 5 km long, dip-parallel transect of the Eocene Sobrarbe Deltaic Complex (Ainsa Basin, south-central Pyrenees, Spain). Systematic sampling of sandstone beds tied to measured sections has captured vertical and basinward changes in sedimentary texture and facies distributions at an intra-clinothem scale. Two types of hyperpycnal flow-related slope deposits, both rich in mica and terrestrial organic matter, are differentiated according to grain size, sorting and bed geometry: (i) sustained hyperpycnal flow deposits, which are physically linked to coarse channelized sediments in the shelf setting and which deposit sand down the complete slope profile; (ii) episodic hyperpycnal flow deposits, which are disconnected from, and incise into, shelf sands and which are associated with sediment bypass of the proximal slope and coarse-grained sand deposition on the medial and distal slope. Both types of hyperpycnites are interbedded with relatively homogenous, organic-free and mica-free, well-sorted, very fine-grained sandstones, which are interpreted to be remobilized from wave-dominated shelf environments; these wave-dominated deposits are found only on the proximal and medial slope. Coarse-grained sediment bypass into the deeper-water slope settings is therefore dominated by episodic hyperpycnal flows, whilst sustained hyperpycnal flows and turbidity currents remobilizing wave-dominated shelf deposits are responsible for the full range of grain sizes in the proximal and medial slope, thus facilitating clinoform progradation. This novel dataset highlights previously undocumented intra-clinothem variability related to updip changes in the shelf process-regime, which is therefore a key factor controlling downdip architecture and resulting sedimentary texture.  相似文献   

An integrated process-based model of flutes and tool marks in deep-water environments: Implications for palaeohydraulics,the Bouma sequence and hybrid event beds     

Jeff Peakall  Jim Best  Jaco H. Baas  David M. Hodgson  Michael A. Clare  Peter J. Talling  Robert M. Dorrell  David R. Lee 《Sedimentology》2020,67(4):1601-1666

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Upper Cretaceous bottom current deposits,north-east Greenland     

Jussi Hovikoski  Alfred Uchman  Rikke Weibel  Henrik Nøhr-Hansen  Emma Sheldon  Jon Ineson  Morten Bjerager  Jens Therkelsen  Mette Olivarius  Michael Larsen  Peter Alsen  Jørgen Bojesen-Koefoed 《Sedimentology》2020,67(7):3619-3654

Reported ancient bottom current deposits in deep marine settings are scarce and most of them remain contentious. This study describes sedimentological, ichnological and petrographical characteristics of a drill core that covers ca 10 Myr of Upper Cretaceous stratigraphy at Hold with Hope, north-east Greenland. The core is divided into four facies associations, which are interpreted to reflect deposition from bottom currents, turbidity flows and hemipelagic settling in slope and/or near slope environments. The evidence for bottom current influence is three-fold. Firstly, pervasive indications of winnowing such as marine bioclast-rich lags and outsized clasts on ‘mud on mud’ contacts are suggestive of low-sediment concentration flows capable of transporting up to pebble-sized clasts. Common Mn–Fe–Mg rich carbonate matrix cements and various types of hiatal chemogenic lag deposits showing glauconite, apatite and carbonate clasts also point to condensation, prolonged exposure at the sediment–water interface and recurrent phases of sea-floor erosion. Secondly, such deposits can show indicators for tidal processes such as double mud-drapes, tangential bottom sets in dune-scale cross-bedding and cyclic rhythmites. Thirdly, inverse to normal grading at various scales is common in fully marine, commonly seafloor-derived sediments. Ichnological data indicate considerable taxonomic variability in the bottom current deposits, but recurrent fabrics are characteristically dominated by morphologically simple burrows such as Thalassinoides and Planolites, with secondary Phycosiphon, Nereites, Zoophycos and/or Chondrites. In general, opportunistic taxa are common whereas mature composite ichnofabrics are rare. The omission surfaces are locally burrowed with stiffground to firmground trace fossil suites. The results contribute to establishing sedimentological, ichnological and mineralogical criteria for recognition of bottom current deposits as well as to the understanding of the Late Cretaceous palaeoenvironmental evolution of the Arctic region.  相似文献   

Distribution and origin of hybrid beds in sand-rich submarine fans of the Tanqua depocentre, Karoo Basin, South Africa   总被引：2，自引：1，他引：1  

David M. Hodgson   《Marine and Petroleum Geology》2009,26(10):1940

This study documents the stratigraphic and palaeogeographic distribution of hybrid event beds that comprise both debris-flow (cohesive) and turbidity current (non-cohesive) deposits. This is the first study of such beds in a submarine fan system to combine outcrop and research borehole control, and uses a dataset from the Skoorsteenberg Formation of the Tanqua depocentre in the Karoo Basin, South Africa. Three types of 0.1–1.0 m thick hybrid beds are observed, which have a basal weakly graded fine-grained sandstone turbidite division overlain by a division of variable composition that can comprise 1) poorly sorted carbonaceous-rich material supported by a mud-rich and micaceous sand-matrix; 2) poorly sorted mudstone clasts in a mud-rich sand-silt matrix; or 3) gravel-grade, rounded mudstone clasts in a well sorted (mud-poor) sandstone matrix. These upper divisions are interpreted respectively as: 1) the deposit of a debris-flow most likely derived from shelf-edge collapse; 2) the deposit of a debris flow, most likely developed through flow transformation from turbidity current that eroded a muddy substrate; and 3) from a turbidity current with mudstone clasts transported towards the rear of the flow. All three hybrid bed types are found concentrated at the fringes of lobes that were deposited during fan initiation and growth. The basinward stepping of successive lobes means that the hybrid beds are concentrated at the base of stratigraphic successions in medial and distal fan settings. Hybrid beds are absent in proximal fan positions, and rare and thin in landward-stepping lobes deposited during fan retreat. This distribution is interpreted to reflect the enhanced amounts of erosion and availability of mud along the transport route during early lowstands of sea level. Therefore, hybrid beds can be used to indicate a fan fringe setting, infer lobe stacking patterns, and have a sequence stratigraphic significance.  相似文献   

Sequence stratigraphy of an argillaceous,deepwater basin-plain succession: Vischkuil Formation (Permian), Karoo Basin,South Africa     

Willem C. van der Merwe  Stephen S. FlintDavid M. Hodgson 《Marine and Petroleum Geology》2010

The 380 m thick fine-grained Vischkuil Formation comprises laterally extensive hemipelagic mudstones, separated by packages of graded sandstone and siltstone turbidites, and volcanic ash beds, and is an argillaceous precursor to a 1 km thick sand-prone basin floor fan to shelf succession. The Vischkuil Formation provides an insight into the process by which regional sand supply is initiated and for testing sequence stratigraphic principles in a basin plain setting. Regionally mapped 1–2 m thick hemipelagic mudstone units are interpreted as condensed drapes that represent the starved basin plain equivalents of transgressive systems tracts and maximum flooding surface on the coeval shelf (now removed during later uplift). The section above each mudstone drape comprises siltstone turbidites interpreted as highstand systems tract deposits and a surface of regional extent, marked by an abrupt grain size shift to fine sandstone. These surfaces are interpreted as sequence boundaries, related to abrupt increases in flow volume and delivery of sand grade material to the basin-plain. The interpreted lowstand systems tract comprises sandstone-dominated turbidites and is overlain by another hemipelagic mudstone drape. The upper Vischkuil Formation is marked by three 20–45 m thick debrites, with intraformational sandstone clasts up to 20 cm in diameter that can be mapped over 3000 km². In each case, debrite emplacement resulted in widespread deformation of the immediately underlying 3–10 m of silty turbidites. A sequence boundary is interpreted at the base of each deformation/debrite package. Six depositional sequences are recognised and the interfered energy shift across each successive sequence boundary and LSTs include a larger volume of sandstone increases up section. The lower two sequences thin to the NW and show NW-directed palaeocurrents. The four overlying sequences show a polarity switch in palaeocurrent directions and thinning, to the E and SE. Sequence 6 is overlain sharply by the 300 m thick sandstone dominated Fan A of the Laingsburg Formation. The LST debrites may indicate gradual development of major routing conduits that subsequently fed Fan A. The polarity shift from westward flowing turbidity currents to an eastward prograding deepwater to shelf system represents establishment of a long term feeder system from the west. Sand supply to the Karoo basin floor was established in an incremental, stepwise manner. Given the early post-glacial setting in an icehouse climate, glacio-eustatic sea-level changes are considered to have been the main control on sequence development.  相似文献   

Grazing on coral reefs facilitates growth of the excavating sponge Cliona orientalis (Clionaidae,Hadromerida)     

Emma Cebrian 《Marine Ecology》2010,31(4):533-538

Although bioerosion is among the most destructive forces on coral reefs, indirect effects influencing the bioerosion dynamics are understudied. Here, I assess the hypothesis that coral reef grazers indirectly facilitate proliferation of bioeroding sponges by removing epibiotic fleshy seaweeds from the Great Barrier Reef. This study quantifies the degree of spatial correlation between the distribution of bioeroding sponges and the distribution of grazing pressure, as evidenced by the abundance of seaweed and parrotfish bite marks. While the sponge tissue area was negatively correlated with seaweed coverage, the number of parrotfish bite marks was associated with less algae and more sponge tissue. Several factors derived from grazing on seaweeds may facilitate sponge growth: increases in the availability of light may favor primary production by symbiotic zooxanthellae and thereby increase growth of bioeroding sponges; on the other hand, sponge settlement may be facilitated on grazed substrates. All these factors are likely related, and contribute to an increasing erosion of coral reefs. Similar processes have recently been described in Mediterranean ecosystems, suggesting that the interactions I document here, could be widespread.  相似文献