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11.
In passive margin salt basins, the distinct kinematic domains of thin‐skinned extension, translation and contraction exert important controls on minibasin evolution. However, the relationship between various salt minibasin geometries and kinematic domain evolution is not clear. In this study, we use a semi‐regional 3D seismic reflection dataset from the Lower Congo Basin, offshore Angola, to investigate the evolution of a network of minibasins and intervening salt walls during thin‐skinned, gravity‐driven salt flow. Widespread thin‐skinned extension occurred during the Cenomanian to Coniacian, accommodated by numerous distributed normal faults that are typically 5–10 km long and spaced 1–4 km across strike within the supra‐salt cover. Subsequently, during the Santonian–Paleocene, multiple, 10–25 km long, 5–7 km wide depocentres progressively grew and linked along strike to form elongate minibasins separated by salt walls of comparable lengths. Simultaneous with the development of the minibasins, thin‐skinned contractional deformation occurred in the southwestern downslope part of the study area, forming folds and thrusts that are up to 20 km long and have a wavelength of 2–4 km. The elongate minibasins evolved into turtle structures during the Eocene to Oligocene. From the Miocene onwards, contraction of the supra‐salt cover caused squeezing and uplift of the salt walls, further confining the minibasin depocentres. We find kinematic domains of extension, translation and contraction control the minibasin initiation and subsequent evolution. However, we also observe variations in minibasin geometries associated with along‐strike growth and linkage of depocentres. Neighbouring minibasins may have different subsidence rates and maturity leading to marked variations in their geometry. Additionally, migration of the contractional domain upslope and multiple phases of thin‐skinned salt tectonics further complicates the spatial variations in minibasin geometry and evolution. This study suggests that minibasin growth is more variable and complex than existing domain‐controlled models would suggest. 相似文献
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安哥拉格埃路地层（Quelo）棕红色砂土形成年代较长,低含水率下地基承载力较高,但遇水后地基承载力降低幅度较大,地基处理方案常有争议。大量的工程实践和专项研究表明,地基土的承载力与含水率有明显的相关性,通过探井取样和原位测试,查明了地基土的物理力学特性,分析不同含水率条件下的静力触探试验(CPT)和平板载荷试验(PLT)数据,建立了适用于该地层的地基承载力与含水率的幂函数关系式以及地基承载力与锥头阻力的线性经验公式。静力触探试验结果表明,地层锥头阻力qc和侧壁摩阻力fs随深度增加而逐渐提高,土质均匀,但摩阻比Rf较小。分析认为,采用不经碾压的砂土作为地基,具有较高的风险;对于多层及9～13层建筑的地基处理,采用换填垫层法较桩基更合理。 相似文献
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The seasonal variations in the advection and mixing of water masses in the northern Benguela were studied in relation to the oxygen minimum zone over the Namibian shelf. The used data set consists of hydrographic and current measurements from an oceanographic mooring 20 nm off Walvis Bay, monthly CTD transects from the Namibian 23°S monitoring line and recent large-scale hydrographic surveys. The current time series showed an intermittent southward continuation of the Angola Current (AC) through the Angola–Benguela frontal zone (ABFZ) into the northern Benguela, commonly known as poleward undercurrent. In austral summer hypoxic, nutrient rich South Atlantic central water (SACW) from the Angola Gyre is transported into the northern Benguela, whereas during the winter season the oxygen rich Eastern SACW (ESACW) spreads northward. The water mass analysis reveals a mixing between both water masses in the northern Benguela between the ABFZ and the Lüderitz upwelling cell (27°S). The oxygen balance over the Namibian shelf depends to a high extent on the water mass composition of the upper central water layer, controlled by the large scale and local circulation. The deviation of the measured oxygen concentration from its mixing concentration, calculated with the source water mass properties, was used to quantify the oxygen consumption. A new local definition SACW was derived to exclude biogeochemical processes, taking place in the Angola Gyre. The oxygen deficit in the northern Benguela central water amounted to about 60–80 μmol l−1 at the shelf edge and increased up to 150 μmol l−1 on the shelf, due to local oxygen consumption. In the austral summer anoxic bottom waters are observed at the central Namibian shelf, which correlate to an SACW fraction >55%. Periods with high SACW fraction in the water mass composition were congruent with hydrogen sulphide events detected by remote sensing. 相似文献
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This paper examines the performance of three climate re-analysis datasets in Angola, namely the ERA-40, NCEP-r1 and JRA-55, by comparing annual and seasonal estimates of precipitation, surface air temperature and evaporation with ground observation measurements. The observational dataset describes a region poorly covered by international climate databases and it is believed that most of its data have not been used in the data assimilation procedures of the climate models. This paper therefore provides a fresh look at the performance of these climate re-analysis datasets in a vast area where distance and civil war have hindered ground monitoring efforts. The re-analysis exercises offer better temperature estimates than precipitation estimates. When offered, the evaporation estimates from all three products are very poor. The three products are able to describe the main features of the spatial distribution of average annual precipitation and temperature, but struggle to reproduce the temporal changes of these variables. The results from a set of performance criteria show that the correlation between the observed ground measurements and re-analysis estimates is poor overall and the NSE values indicate that the average measured value at each location is usually a better estimate than the re-analysis estimate.
EDITOR A. CastellarinASSOCIATE EDITOR S. Kanae 相似文献
16.
Controls on turbidite sand deposition during gravity-driven extension of a passive margin: examples from Miocene sediments in Block 4, Angola 总被引：1，自引：0，他引：1
In recent years, exploration of the Lower Congo Basin in Angola has focused on the Neogene turbidite sand play of the Malembo Formation. Gravity tectonics has played an important role during deposition of the Malembo Formation and has imparted a well-documented structural style to the post-rift sediments. An oceanward transition from thin-skinned extension through mobile salt and eventually to thin-skinned compressional structures characterises the post-rift sediments. There has been little discussion, however, regarding the influence of these structures on the deposition of the Malembo Formation turbidite sands. Block 4 lies at the southern margin of the Lower Congo Basin and is dominated by the thin-skinned extensional structural style. Using a multidisciplinary approach we trace the post-rift structural and stratigraphic evolution of this block to study the structural controls on Neogene turbidite sand deposition.In the Lower Congo Basin the transition from terrestrial rift basin to fully marine passive margin is recorded by late Aptian evaporites of the Loeme Formation. Extension of the overlying post-rift sequences has occurred where the Loeme Formation has been utilised as a detachment surface for extensional faults. Since the late Cretaceous, the passive margin sediments have moved down-slope on the Loeme detachment. This history of gravity-driven extension is recorded in the post-rift sediments of Block 4. Extension commenced in the Albian in the east of the block and migrated westwards with time. In the west, the extension occurred mainly in the Miocene and generated allochthonous fault blocks or “rafts”, separated by deep grabens. The Miocene extension occurred in two main phases with contrasting slip vectors; in the early Miocene the extension vector was to the west, switching to southwest-directed extension in the late Miocene. Early Miocene faults and half-grabens trend north–south whereas late Miocene structures trend northwest–southeast. The contrast in slip vectors between these two phases emphasises the differences in driving mechanisms: the early Miocene faulting was driven by basinward tilting of the passive margin, but gravity loading due to sedimentary progradation is considered the main driver for the late Miocene extension. The geological evolution of the late Miocene grabens is consistent with southwest-directed extension due to southwest progradation of the Congo fan.High-resolution biostratigraphic data identifies the turbidite sands in Block 4 as early Miocene (17.5–15.5 Ma) and late Miocene (10.5–5.5 Ma) in age. Deposition of these sands occurred during the two main phases of gravity-driven extension. Conditions of low sedimentation rates relative to high fault displacement rates were prevalent in the early Miocene. Seafloor depressions were generated in the hangingwalls of the main extensional faults, ultimately leading to capture of the turbidity currents. Lower Miocene turbidite sand bodies therefore trend north–south, parallel to the active faults. Cross-faults and relay ramps created local topographic highs capable of deflecting turbidite flows within the half grabens. Flow-stripping of turbidity currents across these features caused preferential deposition of sands across, and adjacent to, the highs. Turbidite sands deposited in the early part of the late Miocene were influenced by both the old north–south fault trends and by the new northwest–southeast fault trends. By latest Miocene times turbidite channels crosscut the active northwest–southeast-trending faults. These latest Miocene faults had limited potential to capture turbidity currents because the associated hangingwall grabens were rapidly filled as pro-delta sediments of the Congo fan prograded across the area from the northeast. 相似文献
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We analyse tectonic and sedimentary field and subsurface data for the Angola onshore margin together with free-air gravity anomaly data for the offshore margin. This enables us to characterize the mode of syn-rift tectonism inherited from the Precambrian and its impact on the segmentation of the Angola margin. We illustrate that segmentation by the progressive transition from the Benguela transform-rifted margin segment to the oblique-rifted South Kwanza and orthogonal-rifted North Kwanza margin segments. The spatial variation in the intensity of post-rift uplift is demonstrated by the study of a set of geomorphic markers detected in the post-rift succession of the coastal Benguela and Kwanza Basins: Upper Cretaceous to Cenozoic uplifted palaeodeltas, erosional unconformities, palaeovalleys, Quaternary marine terraces and perched Gilbert deltas. The onshore Benguela transform margin has a distinctive, mainly progradational stratigraphic architecture with long-term sedimentary gaps and high-elevation marine terraces resulting from moderate Upper Cretaceous–Cenozoic to major Quaternary uplifting (i.e. 775–1775 mm/ky or m/Ma). By contrast, repeated synchronous episodes of minor Cenozoic to Quaternary uplift occurred along the orthogonal-rifted North Kwanza segment with its Cenozoic aggradational architecture, short-term sedimentary gaps and low-elevation Pleistocene terraces. Margin style likewise governs spatial variations in the volume of offshore sediment dispersed in the associated deep-sea fans. Along the low-lying North Kwanza margin, sedimentation of the broad Cenozoic to Pleistocene Kwanza submarine fan was probably governed by the width of the Kwanza interior palaeodrainage basin combined with the wet tropical Neogene climate. Along the high-rising Benguela margin, the small size of the Benguela deep-sea fan is related to the interplay between moderate continental sediment dispersal from long-lived small catchments and a warm, very arid Neogene climate. However, the driving forces behind the epeirogenic post-rift uplift of the Angola coastal bulge remain a matter of speculation. 相似文献
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This paper provides an overview of the small-scale fisheries sector in countries within the Benguela Current Large Marine Ecosystem (BCLME), one of the most productive large marine ecosystems in the world. The study revealed that Angola, Namibia and South Africa have very different legal and policy frameworks, show different levels of compliance with international and regional agreements to protect the livelihoods and food security of small-scale fishers, as well as of integration of fisheries into food security objectives. Angolan law recognises and protects small-scale fishers through legal and institutional mechanisms. In Namibia, this sector of fishers is not legally recognised, while in South Africa traditional fishers have been largely excluded from the new fisheries management framework. Trends in national and regional fish consumption and in the extent of export orientation in fisheries are explored, as well as the potential threats to small-scale producers and food security in the region posed by ongoing drives to incorporate fisheries in the World Trade Organisation (WTO) agreements. 相似文献
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《African Journal of Marine Science》2013,35(3):319-331
Atlantic humpback dolphins Sousa teuszii are a priority for research due to their restricted geographic range, narrow ecological niche and the paucity of existing information. The distribution and behaviour of S. teuszii off Flamingos, southern Angola, was investigated during summer and winter 2008 using boat- and shore-based surveys. In all, 71 S. teuszii sightings were recorded, ranging from one to eight animals. Sousa teuszii inhabited shallow, nearshore waters throughout the region, with the exception of southern areas adjacent to fishing villages. Small bays, sheltered waters behind reef-breaks and areas off dry river mouths were used for foraging/feeding behaviour, whereas most travelling occurred along exposed coast. Ten individual S. teuszii were photo-identified. Multiple resightings (and absence of unmarked animals) indicate that all individuals present at the time of the surveys were photocaptured, exhibited high site fidelity and had year-round occurrence. Association indices of 0.77-1.0 indicated strong social affiliation between eight individuals, particularly in winter. Off Flamingos, S. teuszii occurs in small numbers and exhibits high site fidelity to a relatively small stretch of nearshore habitat, making the species vulnerable to local extirpation. Wider-scale surveys and development of a management plan are crucial to ensuring the long-term conservation of S. teuszii off Flamingos. 相似文献
20.
Zhiyuan Ge Rob L. Gawthorpe Atle Rotevatn Leo Zijerveld Christopher A.-L. Jackson Ayodeji Oluboyo 《Basin Research》2020,32(5):875-893
Salt tectonics is an important part of the geological evolution of many continental margins, yet the four-dimensional evolution of the minibasins, the fundamental building block of these and many other salt basins, remains poorly understood. Using high-quality 3D seismic data from the Lower Congo Basin, offshore Angola we document the long-term (>70 Myr) dynamics of minibasin subsidence. We show that, during the Albian, a broadly tabular layer of carbonate was deposited prior to substantial salt flow, diapirism, and minibasin formation. We identify four subsequent stages of salt-tectonics and related minibasin evolution: (i) thin-skinned extension (Cenomanian to Coniacian) driven by basinward tilting of the salt layer, resulting in the formation of low-displacement normal faults and related salt rollers. During this stage, local salt welding led to the along-strike migration of fault-bound depocentres; (ii) salt welding below the eastern part of the minibasin (Santonian to Paleocene), causing a westward shift in depocentre location; (iii) welding below the minibasin centre (Eocene to Oligocene), resulting in the formation of a turtle and an abrupt shift of depocentres towards the flanks of the bounding salt walls; and (iv) an eastward shift in depocentre location due to regional tilting, contraction, and diapir squeezing (Miocene to Holocene). Our study shows that salt welding and subsequent contraction are key controls on minibasin geometry, subsidence and stratigraphic patterns. In particular, we show how salt welding is a protracted process, spanning > 70 Myr of the salt-tectonic history of this, and likely other salt-rich basins. The progressive migration of minibasin depocentres, and the associated stratigraphic architecture, record weld dynamics. Our study has implications for the tectono-stratigraphic evolution of minibasins. 相似文献