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1.
The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan 总被引:1,自引:0,他引:1
This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition
and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black
pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits
of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence
of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical
spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the
pumice clasts are related to the size and distribution of Fe3+-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with
very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in
thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin
section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting
of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred
from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high
water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and
low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption
probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.
Received: 3 February 1997 / Accepted: 28 July 1997 相似文献
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From an area of brittle shearing and ring complexes in Northern Kordofan, Sudan 28 K-Ar ages were calculated to determine the geochronology of tectonic and magmatic events. The age results for deformational episodes are 560, 340 and 255 Ma, whereas alkaline magma was intruded at 280, 220 and 165 Ma. The incompatibility of age marks leads to the conclusion that magmatic and tectonic activity in this area have to be regarded as independent events. This holds true even when all dated intrusions from the entire region are compared with post-Pan-African continental tectonic features. 相似文献
5.
Peter Wycisk 《International Journal of Earth Sciences》1994,83(4):759-772
The mainly continental deposits of northwest Sudan and south-west Egypt have been correlated with coeval shallow marine and marine deposits in northern Egypt along a north-south running cross-section, based on surface and subsurface data. The palaeodepth curve of northern Egypt illustrates the gradual seal-level rise, reaching its maximum during the Late Cretaceous with conspicuous advances during the Aptian and late Cenomanian. A general highstand is also recorded during the Campanian-Maastrichtian in north-west Sudan. A detailed facies correlation is given for the Aptian and late Cenomanian highstand in western Egypt. The correlation of the Cenomanian Bahariya and Maghrabi formations displays short-term relative sealevel fluctuations. The interpretation illustrates the extensiveness of related erosional processes in the hinterland, partly intensified by temporarily uplift of the Uweinat-Aswan High in the south. Regional uplift and constant erosion took place in south-west Egypt during Coniacian and Santonian times. The regional stratigraphic gaps and uncertain interpretation of the Bahariya Uplift are induced by the influence of the Trans-African Lineament, especially during the Late Cretaceous. Low-stand fluvial sheet sandstones characterized by non-cyclic sequence development and high facies stability occur, especially in the Neocomian and early Turonian. During the Barremian and Albian, fluvial architecture changes to more cyclic fluvial sequences and increasing soil formation, due to increasing subsidence, more humid climatic conditions and the generally rising sea level, culminating in the extensive shallow marine Abu Ballas and Maghrabi formations. 相似文献
6.
E. Schrank 《International Journal of Earth Sciences》1994,83(4):773-786
A palynological investigation of 164 samples from 18 water wells in northern Kordofan, Sudan, enabled the recognition of five informal zones based on pollen and spore assemblages ranging in age from Albian to Maastrichtian. The youngest (late Campanian-Maastrichtian) assemblages are restricted to the Bagbag Basin, whereas Albian-Cenomanian (to Turonian) sediments are widespread to the east and west of the Bagbag area. Impressions of Salvinia floating leaves from outcrops of the upper Hamrat el Wuz Formation, western part of the study area, are among the oldest occurrences of this water fern and indicate a Campanian-Maastrichtian age for these sediments.The vertical distribution of hygrophilous (pteridophytic spores) versus xerophilous (ephedroids and possibly small, weakly sculptured tricolporates) elements in the palynofloras suggests widespread moist or even aquatic habitats in the Albian-Cenomanian and Campanian-Maastrichtian. A shift towards drier conditions occurred in the late Cenomanian-Turonian. Throughout the Cretaceous, however, there may have been extensive arid/semiarid areas of non-deposition and seasonally dry periods. Some characteristics of the local palynofloras are attributed to its inner continental position. Rare Albian-Cenomanian and Campanian-Maastrichtian dinoflagellates could be interpreted as lacustrine phytoplankton rather than as evidence for marine influence. 相似文献
7.
Y. E-A. Mohamedzein J. A. Abdalla A. Abdelwahab 《Bulletin of Earthquake Engineering》2006,4(3):277-293
Khartoum, the capital of Sudan, is located at the confluence of White and Blue Niles. The city is heavily populated. Central Khartoum with its high-rise buildings is the center of governmental and business activities and is located on a strip adjacent to the Blue Nile. Geological and geotechnical data indicate that the subsoil conditions at Central Khartoum are characterized by alluvial deposits underlain by Nubian Sandstone at a depth of 25 m. The alluvial deposits, locally known as Gezira formations, consist of clays grading into silt and sand with depth. Macro seismic zonation of Sudan and its vicinities, developed by the authors, gave the ground acceleration at the bedrock surface. The effect of alluvial deposits in Central Khartoum on propagation of seismic motion parameters to the ground surface is investigated in this study. Correlations are proposed for pertinent cyclic soil properties such as shear modulus, damping, and shear wave velocity. The Equivalent-Linear Earthquake Response Analyses (EERA) Model was used to study the effect of local soil conditions on ground-motion parameters. In the absence of strong-motion records in Khartoum, available worldwide strong-motion records are used. Plots showing the time histories of ground motion parameters at the ground surface are obtained. The results indicate amplification of ground motion of up to 4.93. The predicted fundamental period of soils is about 0.5 s which is typical for these types of soils. The maximum spectral acceleration varied from 0.76 to 0.95 g. For design purposes, a response spectrum curve is proposed. 相似文献
8.
Macrofauna along the Sudanese Red Sea coast: potential effect of mangrove clearance on community and trophic structure 下载免费PDF全文
Mangroves along the Sudanese Red Sea coast are under constant anthropogenic pressure. To better understand the influence of mangrove clearance on the intertidal benthic community, we investigated the composition, biodiversity and standing stock of the macrofauna communities at high‐, mid‐ and low‐water levels in three contrasting habitats: a bare sand flat, a cleared mangrove and an intact mangrove. In addition, a community‐wide metric approach based on taxon‐specific carbon and nitrogen isotope values was used to compare the trophic structure between the three habitats. The habitats differed significantly in terms of macrofaunal standing stock, community composition and trophic structure. The high‐ and mid‐water levels of the intact mangroves showed a distinct macrofaunal community characterized by elevated densities and biomass, largely governed by higher decapod and gastropod abundances. Diversity was similar for cleared and intact mangroves, but much lower for the bare sand flat. Community‐wide metrics indicated highest trophic diversity and community niche breadth in the intact mangroves. Differences between the cleared and intact mangroves can be partly attributed to differences in sediment characteristics resulting from mangrove clearance. These results suggest a significant impact of mangrove clearance on the macrofaunal community and trophic structure. This study calls for further investigations and management actions to protect and restore these habitats, and ensure the survival of this ecologically valuable coastal ecosystem. 相似文献
9.
Barremian–Aptian organic-rich shales from Abu Gabra Formation in the Muglad Basin were analysed using geochemical and petrographic analyses. These analyses were used to define the origin, type of organic matters and the influencing factors of diagenesis, including organic matter input and preservation, and their relation to paleoenvironmental and paleoclimate conditions. The bulk geochemical characteristics indicated that the organic-rich shales were deposited in a lacustrine environment with seawater influence under suboxic conditions. Their pyrolysis hydrogen index (HI) data provide evidence for a major contribution by Type I/II kerogen with HI values of >400 mg HC/g TOC and a minor Type II/III contribution with HI values <400 mg HC/g TOC. This is confirmed by kerogen microscopy, whereby the kerogen is characterized by large amounts of structured algae (Botryococcus) and structureless (amorphous) with a minor terrigenous organic matter input. An enhanced biological productivity within the photic zone of the water columns is also detected. The increased biological productivity in the organic-rich shales may be related to enhanced semi-arid/humid to humid-warm climate conditions. Therefore, a high bio-productivity in combination with good organic matter preservation favoured by enhanced algae sizes are suggested as the OM enrichment mechanisms within the studied basin. 相似文献
10.
This article develops a new framework for understanding environment-conflict relations, on both theoretical grounds and through a qualitative historical analysis of the links between water and conflict in the states of Sudan and South Sudan. Theoretically, the article critiques the dominant emphases on ‘scarcity’, ‘state failure’ and ‘under-development’ within discussions of environmental security, and proposes an alternative model of environment-conflict relations centring on resource abundance and globally-embedded processes of state-building and development. Empirically, it examines three claimed (or possible) linkages between water and conflict in the Sudans: over trans-boundary waters of the Nile; over the links between internal resource scarcities and civil conflict; and over the internal conflict impacts of water abundance and development. We find that there exists only limited evidence in support of the first two of these linkages, but plentiful evidence that water abundance, and state-directed processes of economic development and internal colonisation relating to water, have had violent consequences. We conclude that analysts and policymakers should pay more attention to the impacts of resource abundance, militarised state power and global political economic forces in their assessments of the potential conflict impacts of environmental and especially climate change. 相似文献