Geology and impact features of Vargeão Dome,southern Brazil

Abstract– Vargeão Dome (southern Brazil) is a circular feature formed in lava flows of the Lower Cretaceous Serra Geral Formation and in sandstones of the Paraná Basin. Even though its impact origin was already proposed in the 1980s, little information about its geological and impact features is available in the literature. The structure has a rim‐rim diameter of approximately 12 km and comprises several ring‐like concentric features with multiple concentric lineaments. The presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the Botucatu and Pirambóia formations. We present the morphological/structural characteristics of Vargeão Dome, characterize the different rock types that occur in its interior, mainly brecciated volcanic rocks (BVR) of the Serra Geral Formation, and discuss the deformation and shock features in the volcanic rocks and in sandstones. These features comprise shatter cones in sandstone and basalt, as well as planar microstructures in quartz. A geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the BVRs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies. No traces of meteoritic material have been found so far. The results confirm the impact origin of Vargeão Dome, making it one of the largest among the rare impact craters in basaltic targets known on Earth.     

Atlantic humpback dolphins Sousa teuszii in the Saloum Delta (Senegal): distribution,relative abundance and photo-identification

During October and November 2015, the first systematic survey of Sousa teuszii was carried out in the Saloum Delta (Senegal, West Africa), comprising 1 617.5 km of boat-based survey coverage. Thirty sightings were recorded in the Saloum and Diomboss rivers, and along the southern coastline. Dolphins were also observed entering the Bandiala and Djinack channels, and travelling across the border into Gambia. The initial sighting locations were 0.043–1.192 km from shore, and tracked dolphins did not move more than 2.082 km from shore. Groups comprised 1–29 animals (mean 9.3 animals), and at least three neonate calves were observed during November. The overall relative abundance was 0.018 sight. km^?1 and 0.175 ind. km^?1. Sightings were concentrated in the Diomboss where relative abundance reached 0.037 sight. km^?1 and 0.331 ind. km^?1. Non-intensive photo-identification produced a minimum population size of 103 animals, the highest recorded for S. teuszii anywhere in its range. Photo-identification also confirmed a movement of individuals between different parts of the Saloum Delta. Combined travel–forage dominated the behaviour. Dolphins were photographed capturing mullet (Mugil sp.) on three occasions. The distribution, population size and movements of S. teuszii are discussed in relation to management.     

First results from the arcminute cosmology bolometer array receiver

We review the first science results from the Arcminute Cosmology Bolometer Array Receiver (ACBAR); a multi-frequency millimeter-wave receiver optimized for observations of the Cosmic Microwave Background (CMB) and the Sunyaev–Zel’dovich (SZ) effect in clusters of galaxies. ACBAR was installed on the 2 m Viper telescope at the South Pole in January 2001 and the results presented here incorporate data through July 2002. We present the power spectrum of the CMB at 150 GHz over the range ℓ=150–3000 measured by ACBAR as well as estimates for the values of the cosmological parameters within the context of ΛCDM models. We find that the inclusion of Ω_Λ greatly improves the fit to the power spectrum. We also observe a slight excess of small-scale anisotropy at 150 GHz; if interpreted as power from the SZ effect of unresolved clusters, the measured signal is consistent with CBI and BIMA within the context of the SZ power spectrum models tested.     

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。     

Non-Gaussian foreground residuals of the WMAP first-year maps

We investigate the effect of foreground residuals in the WMAP ( Wilkinson Microwave Anisotropy Probe ) data by adding foreground contamination to Gaussian ensembles of cosmic microwave background (CMB) signal and noise maps. We evaluate a set of non-Gaussian estimators on the contaminated ensembles to determine with what accuracy any residual in the data can be constrained using higher-order statistics. We apply the estimators to the raw and cleaned Q -, V - and W -band first-year maps. The foreground subtraction method applied to clean the data in Bennett et al. appears to have induced a correlation between the power spectra and normalized bispectra of the maps which is absent in Gaussian simulations. It also appears to increase the correlation between the  Δℓ= 1  inter-ℓ bispectrum of the cleaned maps and the foreground templates. In a number of cases the significance of the effect is above the 98 per cent confidence level.     

Insights into the morphology of the Serra da Cangalha impact structure from geophysical modeling

Forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources. In this context, we have developed 2.5‐D gravity models of the Serra da Cangalha impact structure for the purpose of investigating geological bodies/structures underneath the crater. Interpretation of the models was supported by ground magnetic data acquired along profiles, as well as by high resolution aeromagnetic data. Ground magnetic data reveal the presence of short‐wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process. Aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1.9 km, whereas in the region beneath the central uplift it is raised to 0.5–1 km below the current surface. These depths are also supported by 2.5‐D gravity models showing a gentle relief for the basement beneath the central uplift area. Geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater, as well as on shock‐induced modifications of target rocks. The results showed that the morphology is consistent with the current observations of the crater and that Serra da Cangalha was formed by a meteorite of approximately 1.4 km diameter striking at 12 km s^?1.     

The first description and confirmation of the Vista Alegre impact structure in the Paraná flood basalts of southern Brazil

Abstract– The Vista Alegre structure, centered at 25°57′S and 52°41′W, has been recently proposed as a meteorite impact structure. The 9.5 km‐diameter structure is located in the Paraná state of southern Brazil, within the Paraná Basin, which contains one of the largest and most extensive flood basalt provinces on Earth. The Paraná flood basalts belong to the Serra Geral Formation and are temporally related to the opening of the South Atlantic Ocean, having been dated at about 133–132 Ma. Tholeiitic basalts dominate the western portion of Paraná state, with some minor rhyodacites. Morphologically, Vista Alegre has a prominent circular outline, in the form of an incomplete ring of escarpments, and an inner depression. The presence of a central uplift is not obvious, but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure. These sandstones are possibly related to the Triassic Pirambóia Formation and/or to the Cretaceous Botucatu Formation. These units are normally at stratigraphic depths of about 700–800 m below the present surface in this portion of the Paraná Basin. The structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick, extensively used for agriculture. As a result there are limited outcrops in the interior of the structure, all of polymict breccias, some of them melt‐bearing. We report the extensive occurrence of shatter cones, in the form of fine‐grained rock clasts within the polymict breccias. The shatter cone‐bearing breccias occur at different locations within the structure, separated by several kilometers. The nested shatter cones range in size from about 0.5 to 20 cm for individual cones, and up to half a meter for complete assemblages. The shatter cones formed in fine‐grained Parana flood basalt and might be the first examples of shatter cones in such a rock type. In addition, planar deformation features (PDFs) were found in quartz grains within sedimentary rock clasts of the polymict breccia. These findings confirm the impact origin of the Vista Alegre structure.