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41.
Mass occurrence of benthic coccoid cyanobacteria and their role in the production of Neoarchean carbonates of South Africa 总被引:1,自引:0,他引:1
Jzef Kazmierczak Wladyslaw Altermann Barbara Kremer Stephan Kempe Patrick G. Eriksson 《Precambrian Research》2009,173(1-4):79
The sparse Archean fossil record is based almost entirely on carbonaceous remnants of microorganisms cellularly preserved due to their early post-mortem silicification. Hitherto as an exception, sedimentary carbonate rocks from the Neoarchean Nauga Formation of South Africa contain calcified microbial mats composed of microbiota closely resembling modern benthic colonial cyanobacteria (Chroococcales and Pleurocapsales). Their remains, visible under the scanning electron microscope (SEM) after etching of polished rock samples, comprise capsular envelopes, mucilage sheaths, and groups of cells mineralized by calcium carbonate with an admixture of Al–K–Mg–Fe silicates. The capsular organization of the mucilaginous sheaths surrounding individual cells and cell clusters forming colonies and the mode of mineralization are the characteristic common features of the Neoarchean microbiota described and their modern analogues. The new findings indicate massive production of calcium carbonates by benthic coccoid cyanobacteria in the Neoarchean, and offer a solution to the problem of the origin of Archean carbonate platforms, stromatolites and microbial reefs. 相似文献
42.
Fine grained sericite deposits occur at the interface between Archean Mewar Gneiss Complex and the Proterozoic Aravalli Supergroup
independent of shearing. They show a gradational contact with the basement granites and gneisses and a sharp contact with
the overlying quartz pebble conglomeratic quartzites. Rip-up clasts of these sericite schists are found in the overlying conglomerates.
The sericite schists are rich in sericite towards the top and contain chlorite towards the base. The sericite in these schists
was formed by metasomatic alteration of kyanite and not from the feldspars of the basement granitoids and gneisses. Uni-directional
variations of SiO2 and Al2O3, high Al2O3 content (>30%), positive correlation between Al2O3 and TiO2, Ti/Al and Ti/Zr ratios, high pre-metasomatic chemical indices of alteration (> 90), and enrichment of heavy rare earth elements
relative to the parent granites and gneisses—all these chemical characteristics combined with field evidence suggest that
the sericite schists are formed from a paleosol protolith, which developed on Archean basement between 2.5 and ~2.1 Ga in
the Precambrian of Rajasthan. The superimposed metasomatic alteration restricts the use of Fe2+/Ti and Fe3+/Ti ratios of these paleosols for interpretation of PO2 conditions in the atmosphere. 相似文献
43.
The closure of the Palaeozoic witnessed the greatest biotic crisis in earth history. Surprisingly little is known about the effects and timing of the terrestrial counterpart of the well-described End-Permian mass extinction from known marine successions worldwide. In the present study, reliable paleomagnetic results were obtained from a PT boundary section in the terrestrial Karoo Basin of South Africa. Permo-Triassic aged mudstones from a locality in the Eastern Cape Province yielded two magnetic chrons, reverse followed by normal (with the boundary possibly close to the reversal). This extends to results from a previous study: thereby jointly identifying a R/N/R polarity pattern for this boundary interval. The PTB interval is constrained below the red mudstones of the Beaufort Group at the present locality and within reverse-magnetised green mudstone, implying a diachronic relation between the marine and terrestrial End-Permian mass extinction events. 相似文献
44.
Provenance ages of the Neoproterozoic Katanga Supergroup (Central African Copperbelt), with implications for basin evolution 总被引:2,自引:0,他引:2
S. Master C. Rainaud R.A. Armstrong D. Phillips L.J. Robb 《Journal of African Earth Sciences》2005,42(1-5):41
New age data on detrital zircons and micas are presented from key units within the Neoproterozoic Katanga Supergroup, which hosts the major stratiform Cu–Co deposits of the Central African Copperbelt. Detrital zircon ages indicate a mainly Palaeoproterozoic (between 2081 ± 28 and 1836 ± 26 Ma) provenance for the Katanga basin, derived from the Lufubu Metamorphic Complex of the Kafue Anticline and the Bangweulu Block to the north of the outcrop belt. Detrital zircons and clasts from the Grand Conglomerat glacial diamictite indicate a source from the Palaeoproterozoic metavolcanic porphyries and granitoids of Luina Dome region, which was a basement high during Nguba Group deposition. Minor zircons of Mesoproterozoic age may have been derived from the Kibaran belt. Finally, 40Ar/39Ar age data from detrital muscovites from Biano Group siltstones give a maximum age of sedimentation of 573 Ma, strongly supporting previous models that the Biano Group was deposited in a foreland basin of the Lufilian Orogen. 相似文献
45.
Jyotiranjan S Ray 《Journal of Earth System Science》2006,115(1):149-160
The Vindhyan Supergroup of India is one of the largest and thickest sedimentary successions of the world. Deposited in an
intra-cratonic basin, it is composed mostly of shallow marine deposits. It is believed to have recorded a substantial portion
of Proterozoic time and therefore, likely to contain valuable information on the evolution of the atmosphere, climate, and
life on our planet. It also contains some of the most disputed fossils of earliest animal life. Despite their importance,
the absolute age of these rocks had remained unknown until recently. In this work I evaluate all the recent chronological
information and discuss their implications. From the present findings it appears that the issues surrounding the age of the
Lower Vindhyans in the Son valley are now resolved, whereas problems with the age of the Upper Vindhyans and that with the
stratigraphic correlations remain to be answered. 相似文献
46.
Suryendu Dutta Michael Steiner Santanu Banerjee Bernd-Dietrich Erdtmann Silambuchelvan Jeevankumar Ulrich Mann 《Journal of Earth System Science》2006,115(1):99-112
Chuaria circularis (Walcott 1899) from the Suket Shale of the Vindhyan Supergroup (central India) has been reinvestigated for its morphology
and chemical composition using biostatistics, electron microscopy and pyrolysis-gas chromatography. Morphology and microscopic
investigations provide little clues on the specific biological affinity ofChuaria as numerous preservational artifacts seem to be incorporated. On the contrary, the predominance of η aliphatic pyrolysates
of presently studiedChuaria from India rather supports an algal affinity. Moreover, the reflectance ofC circularis can be used to obtain a comparative maturity parameter of the Precambrian sediments. The review of the age and geographical
distribution ofC circularis constrains that this species cannot be considered as an index fossil for the Proterozoic time. 相似文献
47.
The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleoproterozoic to the Neoproterozoic
period. Chronostratigraphic significance of stable carbon and oxygen isotope ratios of the carbonate phase in Vindhyan sediments
has been discussed in some recent studies. However, the subtle controls of facies variation, depositional setting and post-depositional
diagenesis on stable isotope compositions are not yet clearly understood. The Vindhyan Supergroup hosts four carbonate units,
exhibiting a wide variability in depositional processes and paleogeography. A detailed facies-specific carbon and oxygen isotope
study of the carbonate units was undertaken by us to investigate the effect of these processes and to identify the least altered
isotope values. It is seen that both carbon and oxygen isotope compositions have been affected by early meteoric water diagenesis.
The effect of diagenetic alteration is, however, more pronounced in case of oxygen isotopes than carbon isotopes. Stable isotope
compositions remained insensitive to facies only when sediments accumulated in a shallow shelf setting without being exposed.
Major alteration of original isotope ratios was observed in case of shallow marine carbonates, which became exposed to meteoric
fluids during early diagenetic stage. Duration of exposure possibly determined the magnitude of alteration and shift from
the original values. Moreover, dolomitization is found to be accompanied by appreciable alteration of isotope compositions
in some of the carbonates. The present study suggests that variations in sediment depositional settings, in particular the
possibility of subaerial exposure, need to be considered while extracting chronostratigraphic significance from δ13C data. 相似文献
48.
The Palaeoproterozoic Aravalli Supergroup in Salumber region includes a basal unit of metabasic volcanic rocks (Salumber volcanic
rocks) overlain by a volcaniclastic/conglomerate one. Although these volcanic rocks have been metamorphosed to green-schist
facies, some primary volcanic features are still preserved. This metabasic volcanic sequence can be further differentiated
on the basis of textural variations, and the mineral assemblages are: (a) oligoclase + actinolite + chlorite + epidote; and
(b) oligoclase + hornblende+ chlorite + biotite + Fe-Ti oxides. The SiO2 content ranges from ∼47.7 to 55.8% and MgO from ∼4.2 to 12.8%. Geochemical characteristics allow their subdivision into high
Mg and Fe tholeiites. Inverse relationship of MgO with silica, alkalis and Zr is generally consistent with fractionation mechanism,
also suggested by a change in colour of the rocks from dark greenish to light greenish towards the upper parts of the sequence.
These metabasic volcanic rocks are enriched in incompatible trace elements and LREE (La = 30 − 40 × chondrite, Lu = 2 − 5
× chondrite), and demonstrate affinity mainly with MORB and within plate settings in geochemical tectonic discrimination schemes.
The geochemical characteristics suggest a complex evolutionary history envisaging derivation of the melt from an enriched
heterogeneous lithospheric source. 相似文献
49.
Ravi Prakash Srivastava Nimisha Vedanti V. P. Dimri 《Pure and Applied Geophysics》2007,164(10):2009-2022
Fractal dimension analysis was carried out for optimal designing of 2-D gravity survey network and to determine an optimum
range of gridding interval to generate least aliased Bouguer anomaly maps. As a test case, this method has been successfully
applied to the Jabera-Damoh region of the Vindhyan Basin, which is considered as a potential hydrocarbon bearing area. In
particular, we aim to delineate accurately the lateral extent of a possible hydrocarbon bearing structure. To achieve this
aim, fractal dimension of survey network was computed using 2-D distributions of observation points in the planning phase
of the survey so that the optimum station spacing for gravity survey can be obtained. A range of optimum gridding interval
for the gravity data set was suggested using the box-counting method of fractal dimension determination. Bouguer anomaly maps
of the region are prepared utilizing the optimum gridding interval. For the first time, these anomaly maps clearly outline
the gravity evidence of an anomalous rifted structure, which is bounded by parallel faults on either side. This structure
is interpreted as a favorable basin for the occurrence of hydrocarbons. Another finding of this study has been the delineation
of an apparently small ridge-like structure running east-west, dividing this basin in two parts. A subsurface geological model
along a profile across the Jabera structure has also been presented. 相似文献
50.
The Monadhliath and Leven Schists are separated north of Glen Roy by some 500m of psammitic rock. Junctions are gradational and conformable, and evidence from sedimentary structures indicates that all three face upwards. The two schists should not be equated, nor should the Monadhliath Schist be included in the Dalradian Supergroup unless the entire Grampian Division is also included. 相似文献