摘要:

While Part I was devoted to the study of bowl‐shaped depressions in Algeria, the present article focuses on multi‐ringed structures of possible impact origin on the Algerian Sahara platform. Four structures were selected from orbital, aviation and geologic documents but only three were visited. TIN BIDER (27°36′N; 005°07′E) is a concentric multiple ring structure at least 6 km in diameter. Upper‐Cretaceous sedimentary beds outside the structure dip inward a few degrees at the periphery and become extremely folded nearer the center, yet a general circular symmetry is always retained. A clearly exposed contact between upper deformed beds and underlying on‐deformed beds is remarkably flat, dipping less than 10° inward. The upper beds display strong centrifugal folding. In the center of the structure Lower‐Cretaceous sandstones about 0.5 km above their normal stratigraphic position are exposed. Although no shatter cones, intensive brecciation, or fracturing were observed, there is definite petrographic evidence of shock metamorphism (planar elements) in the quartz grains of the central sandstones. Tin Bider is a probable impact structure. It is the only astrobleme known with such prominent ductile deformations, a characteristic which may be due to the nature of the target materials. Detailed studies are now required to understand the mechanism of deformation of this multi‐ringed structure. Its formation may be early Tertiary in age. FOUM TEGUENTOUR (26°14.5′N; 002°25′E) is an 8 km diameter bull's eye ring pattern. Although the high circularity and the morphology are consistent with an impact origin, the prominence of ductile deformation, the nature of the formations (clay‐gypsum with sandstone intercalcations), the type of folds, the relationships between the structure and a surrounding plateau, and the lack of any evidence of shock effects better support a diapiric origin. MAZOULA (28°24′N; 007°49′E) is an 800 m diameter multi‐ring feature with a 300 m wide anticlinal dome which rises some 30–35 m above the surrounding horizontal strata. The dome is capped by a flat‐lying massive carbonate layer dipping on the flanks in accordance to the surface topography. There are no special disturbances, no fractures or breccias. Mazoula is not impact in origin but may be a Cretace

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00546.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Tierra Blanca is an achondrite (sensu stricto) composed chiefly of orthopyroxene, olivine, plagioclase and clinopyroxene. The mineralogy, major element chemistry and texture are similar to Winona and also Mt. Morris, Acapulco and Antarctic meteorite ALHA 77081 to some degree. The designation of this group as “Winonaites,” as suggested by Prinz et al. (1980), seems justified and use

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00547.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

We have analyzed Oktibbeha County, the most Ni‐rich iron meteorite, for Ni, Co, Cu, Ga, Ge, As, Sb, Ir, and Au. Cu and Sb are higher than in any other iron, but other trace elements are within the ranges typically found in iron meteorites. Extrapolation of trace element trends in group IAB indicates that Oktibbeha County is a member of this group. This sheds light on the origin of groups IAB and IIICD, which are thought to be derived from impact melts on parent bodies of chondritic composition.Lafayette (iron), another sample reported in the literature to have a similarly high Ni content, is probably a pseudometeorit

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00548.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

Pieces of high‐silica, natural glass (Libyan Desert Glass), found on the desert surface of western Egypt, have been treated as an enigma for 50 years although it is virtually certain they are similar to tektites in being impact‐derived products. New major element analyses of four Libyan Desert Glass specimens agree extremely well with the only other recent analysis and demonstrate that the original bulk analyses reported by Spencer (1939) are in error. The five modern analyses define a very tight chemical range for SiO2(97.38–98.25 wt %), Al2O3(1.16–2.26 wt %), total Fe (0.15–0.60 wt % as Fe2O3) and TiO2(0.13–0.19 wt %). Measurable MgO (0.04–0.20 wt %) was found in one specimen. No other elements are present in greater than trace amounts. Microprobe analyses show that Al, Fe and Ti are all positively correlated with one another and are almost ubiquitously distributed throughout the glass. They must also have been so distributed in the LDG precursor material as mechanical mixing and elemental diffusion in the short‐lived melt were limited. In contrast, Mg is sharply restricted in occurrence and correlates only with Fe, strongly suggesting a precursor Mg‐Fe oxide or silicate mineral present as rare, discrete grains. Aside from rare accessory minerals, the parent material was a sand or sandstone composed of quartz grains coated with a mixture of kaolinite, hematite and anatase. This conclusion is based solely on the elemental distribution in the glass but is buttressed by the occurrence of both sand and sandstone, in southwestern Egypt, with the requisite mineralogy. However, mineralogic identity need not, in general, translate to a chemical match and it is entirely possible that the specific sand or sandstone facies involved in the glass formation no longer exists after 28 million years. Consequently, it may well be that evidence other than chemical comparisons will be needed to identify the presently unkn

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00549.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

The Haig (IIIA) iron meteorite material (BM 1968,280) in the Collection of the British Museum (Natural History) displays pre‐terrestrial shear deformation which transects a small troilite‐daubréelite‐cohenite nodule. Five globular areas (<1 mm) with dendritic structures indicating rapid solidification from melts occur within and around the larger part of the transected nodule. All shock deformation structures and shock‐heating effects exhibited by Haig. are of pre‐terrestrial origin and at least four successive alterations to the original structure are evident.Partial bulk analyses of the melt globules indicate complex mixtures of approximately 55 wt % troilite, 24 wt % metal, 14 wt % daubréelite and 7 wt % cohenite. It is suggested that a compressive environment maintained the melts as discrete pools of liquid.The diameters of the globules (<0.5 mm) and spacing of the dendrite arms (106°C sec−1which is the fastest cooling rate yet recorde

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00550.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

The statistics of meteorite falls and finds are presented. Histograms give the distribution of falls as a function of year, month and time of day. The distributions of the retrieved masses of fallen and found meteorites are given, as is also their distribution over the Earth's surface. The data for this analysis have been taken from the British Museum's Catalogue of Meteorites (1966) and Appendix (1977).

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00551.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00552.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00553.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1981.tb00554.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY