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The carbonates of the Transvaal and Griqualand West sequences of the Kaapvaal craton,with special reference to the Lime Acres limestone deposit 总被引：1，自引：0，他引：1
The Campbell Group carbonates were deposited in a basin transgressing from southwest to the east and to northeast. At the southwestern margin of the Kaapvaal craton, in Griqualand West, mainly tidal flat carbonates and shallow marine deposits are found to be followed by subtidal carbonates marking the slow transgression. North of the Griquatown Fault, where the center of the Griqualand West sub-basin developed, continuous subsidence matched by stromatolitic carbonate growth, led to accumulation of the thickest carbonate pile (> 2000 m). There, the largest limestone deposits of South Africa are located. The deposit at Lime Acres and mining operations of the PPC Lime Ltd. are described and discussed in respect to diagenetic processes that permitted limestone preservation. 相似文献
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An overview of the geology of the Transvaal Sequence and Bushveld Complex,South Africa 总被引：1，自引：0，他引：1
The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites. 相似文献
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Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of
Ma and
Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of
Ma for the upper Gamohaan Formation and
for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau. 相似文献
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Mineralogy and Petrology - Petrographic and geochemical characterization of phosphate accessory minerals represents a powerful tool in understanding the mineralization and metasomatic history of... 相似文献
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The oldest filament- and colonial coccoid-containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the Neoarchean, approximately 2600-Ma-old Campbell Group (Ghaap Plateau Dolomite, Lime Acres Member) obtained at Lime Acres, northern Cape Province, South Africa. The assemblage is biologically diverse, including entophysalidacean (Eoentophysalis sp.), probable chroococcacean (unnamed colonial coccoids), and oscillatoriacean cyanobacteria (Eomycetopsis cf. filiformis, and Siphonophycus transvaalensis), as well as filamentous fossil bacteria (Archaeotrichion sp.); filamentous possible microfossils (unnamed hematitic filaments) also occur. The Campbell Group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the Transvaal intracratonic sea. Although only moderately to poorly preserved, they provide new evidence regarding the paleoenvironmental setting of the Campbell Group sediments, extend the known time-range of entophysalidacean cyanobacteria by more than 400 million years, substantiate the antiquity and role in stromatolite formation of Archean oscillatoriacean cyanobacteria, and document the exceedingly slow (hypobradytelic) evolutionary rate characteristic of this early evolving prokaryotic lineage. 相似文献