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31.
Daniel N. Obiora Mirianrita N. Ossai Francisca N. Okeke Andrew I. Oha 《Journal of the Geological Society of India》2016,88(5):654-667
Airborne geophysical (aeromagnetic and gravity) data of Nsukka area was interpreted qualitatively and quantitatively with the aim of determining the susceptibilities of rock types, depth/mass of the anomalous bodies, possible cause of the anomalies and type of mineralization prevalent in the area. The estimated depths from forward and inverse modeling of aeromagnetic data for profiles 1, 2, 3, 4 and 5 were 1200m, 1644m, 1972m, 2193m and 2285m respectively. The respective susceptibility values were 0.0031, 0.0073, 1.4493, 0.0069 and 0.0016. These indicate dominance of iron rich minerals like limonite, hematite, pyrrhotite, and pyrite and forms lateritic caps on sandstones. SPI depth result ranges from 151.6m minimum (shallow magnetic bodies) to 3082.7 m maximum (deep lying magnetic bodies). Euler depths for the four different structural index (SI = 0.5, 1, 2, 3) ranges from 7.99 to 128.93m which are depths of shallow magnetic sources resulting from lateritic bodies in the outcrops in the study area. From the gravity data interpretation, Euler depth estimation reveals that depth to anomalous bodies ranges from 89.13 to 2296.92m. Density of the causative body obtained from modeling results for profile 1 was 1498kg/m3, which is in the range of clay material and the depth was about 923m. From models 2, 4 and 5, the densities of the causative bodies were 3523, 4127 and 3707kg/m3, while depths to the surface were about 604, 815 and 1893m respectively. These density ranges correspond to that of ironstone. From model three, the density of causative body obtained was 2508kg/m3, located at a depth of about 268m below the surface. This work has shown that Nsukka area is underlain by thick strata of shales, sandstones and ironstones, which together are suitable for ceramic production, and sufficiently thick sediments suitable for hydrocarbon accumulation. 相似文献
32.
Daniel N. Obiora John A. Yakubu Francisca N. Okeke Josiah U. Chukudebelu Andrew I. Oha 《Journal of the Geological Society of India》2016,88(1):98-106
Aeromagnetic data of Idah area (sheet 267), north-central Nigeria, has been interpreted by applying source parameter imaging (SPI), Euler deconvolution and forward and inverse modeling methods. Quantitatively, depth estimates obtained by employing SPI have shown minimum to maximum depth to anomalous source at 57.591m to 664.841m. Applying Euler deconvolution for various structural indices (SI), the depth obtained for SI=1, ranges from 5.6m to 197.6m, 22.0m to 204.7m for SI=2 and for SI=3, the depth to magnetic source obtained ranges from 38.0m to 205.5m. The results from forward and inverse modeling for profiles 1, 2, 3, 4 and 5 indicate depths of 95.6m, 103.9m, 477.5m, 1239.8m and 76.8m respectively. From the results of susceptibility values; the result obtained from profile 1 with susceptibility value of -0.0175 suggests that the body is associated with non-magnetic sedimentary deposits. Profile 2 with a susceptibility value of 0.07 is typical of basic igneous rock; gabbro and basalts. The results for profiles 3, 4 and 5 having susceptibility values of 0.01 to 0.013 are typical of intermediate igneous rocks; diorite and andesite. 相似文献
33.
Paul Glombick Douglas R. Schmitt Wei Xie Todd Bown Ben Hathway Christopher Banks 《Meteoritics & planetary science》2014,49(5):872-895
Geological and geophysical evidence is presented for a newly discovered, probable remnant complex impact structure. The structure, located near Bow City, southern Alberta, has no obvious morphological expression at surface. The geometry of the structure in the shallow subsurface, mapped using downhole geophysical well logs, is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region. Detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region. Field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure. Evidence of deformation along the eastern margin of the central uplift includes thrust faulting, folding, and steeply dipping bedding. Normal faults were mapped along the northern margin of the annular region. Isopach maps reveal that structural thickening and thinning were accommodated primarily within the Belly River Group. Evidence from legacy 2‐D seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure, including a series of listric normal faults in the annular region and complex faulting within the central uplift. The absence of any ejecta blanket, breccia, suevite, or melt sheet (based on available data) is consistent with the Bow City structure being the remnant of a deeply eroded, complex impact structure. Accordingly, the Bow City structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks. 相似文献
34.
Celia Ojeda-Martínez Francisca Giménez Casalduero Just T. Bayle-Sempere Carmen Barbera Cebrián Carlos Valle Jose Luis Sanchez-Lizaso Aitor Forcada Pablo Sanchez-Jerez Pablo Martín-Sosa Jesús M. Falcón Fuensanta Salas Mariagrazia Graziano Renato Chemello Ben Stobart Pedro Cartagena Angel Pérez-Ruzafa Fréderic Vandeperre Elisabeth Rochel Serge Planes Alberto Brito 《Ocean & Coastal Management》2009,52(2):89-101
A general conceptual framework for the management of marine protected areas (MPAs) was developed. The driver-pressure-state-impacts-response (DPSIR) framework was used to determine the elements affecting MPAs. The developed evaluation framework helped to select an appropriate suite of indicators to support an ecosystem approach, an assessment of the MPAs functioning and policy decisions. Gaps derived from the management and policy responses in the MPAs were also outlined. It was concluded that the DPSIR framework can help to simplify the complexity of MPA management. This document is a tool for policy makers, scientists and general public on the relevance of indicators to monitor changes and MPAs management. 相似文献
35.
Ian Dodkins Francisca AguiarRui Rivaes António AlbuquerquePatricia Rodríguez-González Maria Teresa Ferreira 《Limnologica》2012,42(2):95-107
A metric was developed for assessing anthropogenic impacts on aquatic macrophyte ecology by scoring macrophyte species along the main gradient of community change. A measure of ecological quality was then calculated by Weighted Averaging (WA) of these species scores at a monitoring site, and comparison to a reference condition score. This metric was used to illustrate the difficulties of developing aquatic macrophyte indices based on indicator species in Mediterranean rivers. The response of the metric to a nutrient gradient was examined within two different river typologies: the national typology designed for the Water Framework Directive and a typology that segregates the environmental variables to produce maximum species similarity within a river type. Both typologies showed the strong north-south climatic divide in Portugal, with southern rivers having long periods without rainfall and often without flowing water in the summer. Overall, the metric responded well to nutrient impacts however it performed poorly in some southern lowland river types. This was thought to be due to low numbers of aquatic macrophytes in temporary rivers. Non-aquatic species that establish in the river channel of temporary rivers may have to be included in indices to improve performance. Also, simple Weighted Averaging (WA) metrics may be insensitive to abundance changes and loss of rarer indicators in lowland Mediterranean rivers. More sophisticated methods of using WA are suggested, as well as further research into developing assessment methods specific to the character of Mediterranean rivers. 相似文献
36.
Antonio González-Ramón Bartolomé Andreo Antonio Ruiz-Bustos David A. Richards José Antonio López-Sáez Francisca Alba-Sánchez 《Quaternary Research》2012,77(2):264-272
Cucú cave is a small cavity, 1600 m above sea level on the southern slope of Sierra de María (Almería Province, SE Spain), where current mean annual precipitation is < 450 mm. Fossils and palynomorphs contained within a sedimentary sequence, up to 9 m in depth, allow us to consider the prevailing climatic conditions, and the timing of cavern development. The lithological sequence is dominated by clast-supported detrital material with no evidence of alluvial transport. These sediments were formed by freeze-cracking during periglacial conditions, causing further cave enlargement after initial solutional development. The clastic sequence formed during cold climates is covered by a flowstone that was deposited during a period of warmer, wetter conditions. This provides a minimum U–Th isochron age of 40.2 ± 4.5 ka for the timing of periglacial action. Micromammal fossil species indicate a chronology between 140 and 80 ka. Paleoecological data based on the structure of the mammal community indicates that cold conditions prevailed at the time of deposit. In the studied sequence the presence of anthropogenic components has not been documented. The pollen assemblages identified are a common feature of Pleistocene cold stages that are in semi-arid regions. 相似文献
37.
Jose Antonio de‐la‐Ossa‐Carretero Yoana Del‐Pilar‐Ruso Francisca Giménez‐Casalduero José Luis Sánchez‐Lizaso 《Marine Ecology》2016,37(3):503-517
The distribution and composition of Amphipoda assemblages were analysed off the coasts of Alicante (Spain, Western Mediterranean), a disturbed area affected by several co‐occurring anthropogenic impacts. Although differences among sampled stations were mainly related to natural parameters, anthropogenic activities were linked with changes in amphipod assemblages. Expansion of the Port of Alicante, a sewage outfall and a high salinity brine discharge could be causing the disappearance of amphipods at stations closer to these disturbances. However, the completion of port enlargement works and mitigatory dilution of the brine discharge has led to the recovery of the amphipod assemblage. Among the natural parameters, depth determines the distribution of some of the species. While Siphonoecetes sabatieri was abundant at shallow stations, Ampelisca spp., Photis longipes, Pseudolirius kroyeri, Apherusa chiereghinii and Phtisica marina were more abundant at deeper stations. Grain size and percentage of organic matter also influenced amphipod distribution, resulting in changes in species composition and in the relative percentages of different trophic groups. Species such as Ampelisca brevicornis, Perioculodes longimanus, Urothoe hesperiae and Urothoe elegans were more abundant at stations with a high content of fine sand. Carnivorous species, mainly of the Oedicerotidae family, were more abundant at those stations with a low organic matter content, while detritivorous species were more abundant at stations with a higher mud content. Among 62 identified species, three were reported for the first time from the Spanish Mediterranean coast, two species were recorded for the second time and a new species of Siphonoecetes was found, Siphonoecetes (Centraloecetes) bulborostrum. These results confirm the need for further data on amphipods from the Mediterranean Spanish coast. 相似文献
38.
David Gallego-Torres Oscar E. Romero Francisca Martínez-Ruiz Jung-Hyun Kim Barbara Donner Miguel Ortega-Huertas 《Quaternary Research》2014
Previous paleoceanographic studies along the NW African margin focused on the dynamics of surface and intermediate waters, whereas little attention has been devoted to deep-water masses. Currently, these deep waters consist mainly of North Atlantic Deep Waters as part of the Atlantic Meridional Overturning Circulation (AMOC). However, this configuration was altered during periods of AMOC collapse. We present a high-resolution reconstruction of bottom-water ventilation and current evolution off Mauritania from the last glacial maximum into the early Holocene. Applying redox proxies (Mo, U and Mn) measured on sediments from off Mauritania, we describe changes in deep-water oxygenation and we infer the evolution of deep-water conditions during millennial-scale climate/oceanographic events in the area. The second half of Heinrich Event 1 and the Younger Dryas were recognized as periods of reduced ventilation, coinciding with events of AMOC reduction. We propose that these weakening circulation events induced deficient deep-water oxygenation in the Mauritanian upwelling region, which together with increased productivity promoted reducing conditions and enhanced organic-matter preservation. This is the first time the effect of AMOC collapse in the area is described at high resolution, broadening the knowledge on basin-wide oceanographic changes associated with rapid climate variability during the last deglaciation. 相似文献