IMPACT AND IMPACT-LIKE STRUCTURES IN ALGERIA PART I FOUR BOWL-SHAPED DEPRESSIONS     

Philippe Lambert  John F. McHone  Robert S. Dietz  Messaoud Houfani 《Meteoritics & planetary science》1980,15(2):157-179

From orbital, aviation and geologic documents, four circular depressions on the Sahara sedimentary platform were selected for field investigation because of their possible impact origin. Our results can be summarized as follows: Amguid Crater (26° 05′N; 004° 23.5′E; 450 m diameter, 30 m deep) is perfectly circular, with a steep wall, a raised rim and an ejecta blanket. The strata are uplifted, outward dipping, dislocated and locally overturned at the rim crest. Large blocks are scattered around the rim. There is petrological evidence of shock by planar elements in quartz. Amguid is a well preserved impact crater probably no older than 100,000 years. Talemzane (33° 19′N; 004° 02′E; 1.7 km diameter, 70 m deep) is also perfectly circular and displays a raised rim. The strata are uplifted, outward dipping, and locally highly fractured. Numerous breccia veins are clearly exposed in the crater wall. Consolidated ejected debris form a continuous blanket more than 500 m outward from the rim. Reworked mixed breccias are exposed at the base of the crater wall. Planar elements are observed in quartz clasts in the mixed breccia. Talemzane is an impact crater on the order of 0.5 to 3 million years old. El Mouilah (33° 51′N; 002° 03′E; 4.5 km diameter, 130 m deep) is almost perfectly circular, the walls are steep and there is a central dome. In spite of a promising morphology, there is no field evidence of impact. El Mouilah is possibly a recent collapse structure due to dissolution in the thick underlying limestone and gypsum formations or purely erosional in origin. Aflou (34° 00′N; 002° 03′E) is not circular (3 × 5 km) but was selected because it appears in the literature as a probable impact crater, the main argument being the existence of fused materials in the center (Marks et al., 1972). We found no evidence of impact, but several occurrences of igneous rocks along an E-W direction suggest a structurally controlled volcanic activity. A volcanic activity is also supported by the existence of a local magnetic anomaly centered on the depression. Aflou is neither an impact structure nor a crater. Located on a probable structural dome, at the intersection of several structural trends, the formation of the depression can be due to erosion and/or dissolution in the thick underlying limestone and gypsum formations.  相似文献   

KAABA STONE: NOT A METEORITE,PROBABLY AN AGATE     

Robert S. Dietz  John McHone 《Meteoritics & planetary science》1974,9(2):173-179

The Black Stone of the Kaaba apparently is not a meteorite, although listed as a probable example in the Prior-Hey Catalogue of Meteorites. Diffusion banding and other physical attributes suggest that it is an agate.  相似文献   

Temporal and life history related trends of perfluorochemicals in harbor porpoises from the Danish North Sea     

Galatius A  Dietz R  Rigét FF  Sonne C  Kinze CC  Lockyer C  Bossi R 《Marine pollution bulletin》2011,62(7):1476-1483

Eighty-five stranded or bycaught harbor porpoises collected from the Danish North Sea between 1980 and 2005 were analyzed for perfluorochemicals in the liver. PFOS was the predominant compound, making up on average 88.9% of the ∑PFC, followed by PFOSA (7.8%). PFUnA (1.9%) and PFDA (1.2%) were detected in most samples. PFHxS, PFNA and PFOA were only found in a minority of the samples. We found substantial differences in PFC concentrations among life history stages, the highest concentrations were found in neonates, suckling juveniles and lactating females. Such differences should be considered when PFC levels in wildlife are evaluated. The high concentrations found in young porpoises are of concern as PFCs have known toxic effects on the development of the central nervous system and reproductive organs. Despite efforts to reduce PFC emissions, a decreasing temporal trend of concentrations was not detected for any compound. PFCA concentrations were found to be increasing.  相似文献   

The Peerless structure,Daniels County,northeastern Montana: A probable late Ordovician impact structure     

J. M. Comstock  R. D. Dietz  J. R. Morrow 《Meteoritics & planetary science》2004,39(5):683-692

Abstract— The Peerless structure is an ?6 km‐diameter sub‐surface anomaly located in Daniels County, northeastern Montana. The disruption of sedimentary rock in the structure lies between 2624 to 2818 m below the topographic surface. Seismic mapping shows a typical complex crater composed of a central uplift ?2 km across, which shows structural uplift of up to 90 m, an annular ring ?4 km across, and an outer rim ?6 km in diameter. The youngest disrupted rock unit is the upper Ordovician Red River formation, which indicates that the structure was formed about 430–450 Ma ago.  相似文献   

Occurrence of Sn-Bearing colusite in the ore-body “T” of the Bor copper deposit, Serbia     

Ljubomir Cvetković  Aleksandar Pačevski  Trajče Tončić 《Geology of Ore Deposits》2013,55(4):298-304

The ore body “T” is the newly discovered massive-pyrite type one which is located in the central part of the Bor copper mine. The main copper minerals are chalcocite-digenite, covellite and enargite. Small amounts of colusite are frequently present in the ore-body. It mostly occurs as the distinct exsolutions in digenite and, associating with enargite and covellite. Composition of the studied colusite shows enriched Sn content, giving an empirical formula from Cu_24.7V_1.8Fe_0.2As_5.1Sb_0.2Sn_0.8S₃₂ to Cu_26.7V_2.0Fe_0.3A_s3.0Sb_0.3Sn_3.5S₃₂. This colusite represents a solid solution between colusite and nekrasovite within a range of 14–54 mol % nekrasovite. Most of the analyses show content of <50 mol % nekrasovite corresponding to the Sn-bearing colusite variety, while one analysis shows content of 54 mol % nekrasovite corresponding to the As-bearing nekrasovite.  相似文献   

Circumstellar molecular line absorption and emission in the optical spectra of post-AGB stars     

Eric J. Bakker  Henny J. G. L. M. Lamers  L. B. F. M. Waters  Ton Schoenmaker 《Astrophysics and Space Science》1995,231(1-2):335-338

We present preliminary results obtained with the European Observation Network. This network consists of 9 observatories in the Czech Republic, Germany and Bulgaria and has been involved in the BACODINE activities since April 1, 1994. We also discuss related problems such as the background of unknown variable stars and suggest a strategy for work in this area.  相似文献   

Crystal chemistry of nephelines from ijolites and nepheline-rich pegmatites: influence of composition and genesis on the crystal structure investigated by X-ray diffraction     

Predrag Vuli?  Ton?i Bali?-?uni?  Louise Josefine Belmonte  Volker Kahlenberg 《Mineralogy and Petrology》2011,101(3-4):185-194

Ten nepheline single crystals from five different localities representing rocks from nepheline-syenite pegmatites to urtite, ijolite and cancrinite-ijolite were investigated chemically and structurally. The chemical compositions were determined by electron microprobe, whereas the crystal structures were refined against X-ray diffraction data to R values of 0.022?C0.031. In all cases the crystal structures conformed to space group P6₃, but at the same time all crystals produced some satellite reflections testifying to the modulation effects in their structure. This work confirms the presence of satellite reflections as a universal property of nepheline and resolves some controversy about this issue present in older works. The samples show a range of compositions with variable excess of Si over Al combined with vacancies in the alkaline sites. Vacancies are present exclusively in the symmetric ditrigonal channels occupied preferentially by K atoms in natural nepheline. The influence of the variable amounts of K, Na and vacancies to the increase of volume at this site is shown to be in the order ??>K>Na. It has been confirmed that natural nepheline can partly depart from Loewenstein??s rule and show some degree of disorder in the Si/Al distribution. Most of the samples revealed a practically full Si-Al-ordering, but the crystals from one of the nepheline-syenite pegmatites in Langesundsfjord, Norway and from the type locality of ijolite, Iivaara, Finland, showed a disorder of 10?C20% suggesting a different cooling history for these rocks.  相似文献   

Fighting Over Forest: Toward a Shared Analysis of Livelihood Conflicts and Conflict Management in Ghana     

Mercy Derkyi  Mirjam A. F. Ros-Tonen  Boateng Kyereh  Ton Dietz 《社会与自然资源》2014,27(3):281-298

Conflicts undermine forest-based livelihoods for the rural poor. Conflict management is key to preventing such conflicts. This article analyzes actor perceptions of forest- and tree-related conflicts and conflict management in Ghana's high forest zone. It also assesses a phased methodology that promotes shared problem definition and ownership of recommendations on conflict resolution strategies through the presentation and discussion of findings from document analysis, surveys, interviews, and focus-group discussions at a workshop with forest professionals held in Kumasi, Ghana. The study found that conflicts are inherent in forest-based livelihoods due to policy and legislative failures and institutional deficiencies, perceived goal incompatibility, opportunities for interfering with the attainment of one another's goals, and environmental scarcity. Ongoing forest governance reforms in Ghana should consider the stepwise conflict management model developed by the workshop participants involved in this study, but expand it to include the views of other stakeholder groups.  相似文献   

Structure, agency and environment: Toward an integrated perspective on vulnerability   总被引：6，自引：1，他引：5  

Paul McLaughlin  Thomas Dietz   《Global Environmental Change》2008,18(1):99-111

We review five perspectives on human vulnerability to environmental change—biophysical, human ecological, political economy, constructivist and political ecology—and assess their respective strengths and weaknesses. While each of these perspectives offers important insights, and some theoretical convergence is evident, the field remains divided along a number theoretical fracture lines. Two deeply rooted metatheoretical assumptions—essentialism and nominalism—are hindering the construction of a more integrated perspective on vulnerability, one capable of addressing the interrelated dynamics of social structure, human agency and the environment. We conclude by suggesting that an evolutionary perspective on social change, grounded in a critical realist epistemology, provides the best prospect for avoiding the above pitfalls and advancing our understanding of vulnerability.  相似文献   

Impact and explosion cratering—planetary and terrestrial implications: edited by D. J. Roddy,R. O. Pepin and R. B. Merrill. Pergamon Press,U.S.$137.50. 1301 pp., 1977     

Robert S. Dietz 《Geochimica et cosmochimica acta》1979,43(2):277

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