乍得Bongor盆地储层沉积成岩特征及其主控因素

乍得Bongor盆地储层沉积成岩特征具有明显的独特性。结合盆地区域构造演化,利用地震、钻井、岩心、薄片、测试等资料,系统分析了Bongor盆地主力储层沉积展布特征和成岩演化。研究结果表明:Bongor盆地早白垩世强烈拉张裂陷作用形成了一系列NW-SE向深陡凹陷,主力储层局限分布在凹陷内部,以近岸水下扇沉积为主,毗邻控凹断层分布,具有近源、深水、快速沉积特征,结构成熟度和成分成熟度均较低;受多期构造运动及烃类流体改造,储层压实作用不充分,胶结成岩作用减弱,溶蚀现象普遍,目前尚处于中成岩A期阶段;以原生孔隙为主,属中孔、中高渗储层,有效储层埋深下限明显大于现今油层深度,表明盆地深层仍发育有优质储层,具有良好的勘探潜力。     

The influence of transgression and regression on the spatial and temporal distribution of diagenetic kaolin in the Upper Ordovician glaciogenic sandstones within a sequence stratigraphic framework, Murzuq Basin, SW Libya

Linking diagenesis to depositional facies and sequence stratigraphy can provide a better understanding of some of the parameters that control the spatial and temporal distribution of diagenetic alterations and of their impact on reservoir quality. A study of the paralic, glaciogenic sandstones of the Melaz Shuqran and Mamunyiat formations (Late Ordovician) of the Murzuq Basin, SW Libya, reveals that the distribution of diagenetic kaolin can be constrained within depositional facies and sequence stratigraphy. Eogenetic kaolinite was formed by the dissolution of unstable detrital grains as a result of influx of meteoric waters into: (i) glacial, fluvial incised-valley lowstand systems tract (LST), glacial, tide-dominated estuarine transgressive systems tract (TST) during formation of overlying sequence boundary, (ii) paraglacial, tide-dominated deltaìc highstand systems tract (HST), paraglacial, foreshore to shoreface HST during progradation and basinward shift of the shoreline and/or formation of overlying sequence boundary, (iii) postglacial, Gilbert-type deltaic LST sandstones during relative sea level fall. On the other hand, formation of kaolinite immediately below maximum flooding surfaces is attributed to dissolution of unstable detrital grains by organic acids that were presumably derived from thermal alterations of organic matter, possibly during mesodiagenesis. The transformation of eogenetic kaolinite into dickite during mesodiagenesis is probably a consequence of low α_K⁺ / α_H⁺ ratio in the pore waters due to the scarcity of detrital K-feldspars.     

Magellanic penguin mortality in 2008 along the SW Atlantic coast

Magellanic penguins migrate from Patagonia reaching northern Argentina, Uruguay, and southern Brazil on their winter migration, in parallel with the seasonal pulse of anchovy spawning. In 2008, Magellanic penguins went further north than usual. Many died and a few swam nearly to the Equator. Twelve groups surveyed 5000 km of coastline encountering 3371 penguins along the coast. Most penguins arrived in northern Brazil (68.4%) without petroleum (2933, 87%). Almost all penguins without petroleum were juveniles (2915, 99%) and 55% were alive when found. Penguins were dehydrated, anemic, hypothermic, and emaciated. Of the penguins with petroleum, 13% arrived in the southern half of Brazil, showing that petroleum pollution remains a problem along the SW Atlantic coast. The mortality occurred in the winter of 2008 when sea surface temperature were unusually cold perhaps reducing the prey for penguins.     

Using standardized indicators to analyse dry/wet conditions and their application for monitoring drought/floods: a study in the Logone catchment,Lake Chad basin

The standardized precipitation index (SPI) and standardized streamflow index (SSI) were used to analyse dry/wet conditions in the Logone catchment over a 50-year period (1951–2000). The SPI analysis at different time scales showed several meteorological drought events ranging from moderate to extreme; and SSI analysis showed that wetter conditions prevailed in the catchment from 1950 to 1970 interspersed with a few hydrological drought events. Overall, the results indicate that both the Sudano and Sahelian zones are equally prone to droughts and floods. However, the Sudano zone is more sensitive to drier conditions, while the Sahelian zone is sensitive to wetter conditions. Correlation analysis between SPI and SSI at multiple time scales revealed that the catchment has a low response to rainfall at short time scales, though this progressively changed as the time scale increased, with strong correlations (≥0.70) observed after 12 months. Analysis using individual monthly series showed that the response time reduced to 3 months in October.     

A rapid rise in relative sea level ~9–7 cal ka bp along the SW Cumbria coast,NW England

Along the macro-tidal (10.7 m tide range) SW Cumbria coast in situ rooted tree stumps are infrequently exposed below present-day low Spring Tide elevations (≤−4.5 m OD). To date these remnants, we conducted a salvage operation at two sites on in situ rooted tree stumps exposed below the lowest tide levels; these sites are no longer exposed. We obtained four radiocarbon dates on these tree remnants and reference an earlier site recovered in 1972. One wood sample was identified as Quercus robur or Q. petraea, while samples from Haverigg and Kirby-in-Furness are tree Salix spp. The median group age for the rising relative sea level (RSL) that killed the trees was 8602 ± 243 bp cal a bp with an estimated Mean Tide Level (MTL) of ~≤−9 m. Evidence indicates that RSL rose ~10 m along this section of coast at a rate of between of between 18.5 and 8.7 mm a^–1 until ~6800 cal a bp , or earlier, before the rate of global sea level rise fell below that of the local glacial isostatic recovery. The transgression is coeval with rapid collapse and deglaciation of the Laurentide Ice Sheet in Hudson's Bay and slightly lags the global Meltwater Pulse-1C.     

Opx–Cpx exsolution textures in lherzolites of the Cretaceous Purang Ophiolite (S. Tibet,China), and the deep mantle origin of Neotethyan abyssal peridotites

ABSTRACT

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al₂O₃ = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr₂O₃ = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na₂O, with lower TiO₂ and Al₂O₃ contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.     

Remarks on the period variation of HS Herculis and SW Cygni

In order to get a satisfactory understanding of the periodic variation of the orbital period in the binary system HS Herculis, the study of this problem is resumed. Using recently observed primary and secondary minima, it is evident that after 1955 (E > -2000) the corresponding O – C diagram reflects the effect of apsidal motion. Any assumption on the presence of a third body is rejected, at least as long as the current aspect of the O–C diagram is concerned. For the interpretation of the sinusoidal period variation of the semi-detached system SW Cygni, 130 primary minima were compiled form the literature. Though it is considered as very likely that this variation of the period is primarily caused by apsidal motion, the hypothesis of a third body is analysed too. Further precise photometric and spectroscopic observations are recommended.     

Using trace elements of magnetite to constrain the origin of the Pingchuan hydrothermal low-Ti magnetite deposit in the Panxi area,SW China

The Pingchuan iron deposit, located in the Yanyuan region of Sichuan Province, SW China, has an ore reserve of 40 Mt with ~60 wt% Fe. Its genesis is still poorly understood. The Pingchuan iron deposit has a paragenetic sequence of an early Fe-oxide–Pyrite stage(Ⅰ) and a late Fe-oxide–pyrrhotite stage(Ⅱ). Stage Ⅰ magnetite grains are generally fragmented, euhedral–subhedral, largesized crystals accompanying with slightly postdated pyrite.Stage Ⅱ magnetite grains are mostly unfragmented, anhedral, relatively small-sized grains that co-exist with pyrrhotite. Combined with micro-textural features and previously-obtained geochronological data, we consider that these two stages of iron mineralization in the Pingchuan deposit correspond to the Permian ELIP magmatism and Cenozoic fault activity event. Both the Stage Ⅰ and Ⅱ magnetites are characterized with overall lower contents of trace elements(including Cr, Ti, V, and Ni) than the ELIP magmatic magnetite, which suggests a hydrothermal origin for them. ‘‘Skarn-like' enrichment in Sn, Mn, and Zn in the Stage Ⅰ magnetite grains indicate significant material contributions from carbonate wall-rocks due to water–rock interaction in ore-forming processes. Stage Ⅱ magnetite grains contain higher Mn concentrations than Stage Ⅰ magnetite grains, which possibly implies more contribution from carbonate rocks. In multiple-element diagrams, the Stage Ⅰ magnetite shows systematic similarities to Kiruna-type magnetite rather than those from other types of deposits. Combined with geological features and previous studies on oxygen isotopes, we conclude that hydrothermal fluids have played a key role in the generation of the Pingchuan low-Ti iron deposit.     

Comparing quality of estuarine and nearshore intertidal habitats for Carcinus maenas

Estuarine and nearshore marine areas are vital habitats for several fish and benthic invertebrates. The shore crab Carcinus maenas (Crustacea: Brachyura: Portunidae) inhabits a variety of coastal, estuarine and lagoon habitats. At low tide, habitat structural complexity may be most important for crabs in the intertidal, providing refuge from predation and desiccation. The quality of different vegetated and nonvegetated estuarine and rocky shore habitats in SW Portugal and SW England was evaluated for intertidal C. maenas populations. We estimated population density, size–structure, and potential growth (RNA/DNA ratios) to investigate habitat quality. Vegetated estuarine habitats supported higher crab densities, than nonvegetated estuarine and rocky shore habitats. Investigation of population size–structure revealed that all habitats seem important recruitment and nursery areas although estuarine habitats in SW Portugal appeared to support higher densities of new recruits than equivalent habitats in SW England. Significant variation was found in RNA/DNA ratios among habitats. Ratios were highest in the rocky shore suggesting a high quality habitat where growth potential is high. We speculate that competition from other top-predators (Pachygrapsus spp.) rather than low habitat quality may limit the occurrence of C. maenas in intertidal rocky shore habitats in SW Portugal. In estuarine environments RNA/DNA ratios were significantly higher in the vegetated than in the nonvegetated estuarine habitats in SW Portugal but not in SW England, suggesting geographic differences in the extent to which highly structure habitats represent high quality. Our results challenge the current paradigm that structured habitats are necessarily those of higher quality for C. maenas.