Transvaal Dolomite Research Articles

Carbonate Pb[sbnd]Pb ages of the Wittenoom Formation and Carawine Dolomite, Hamersley Basin, Western Australia (with implications for their correlation with the Transvaal Dolomite of South Africa)

Pb isotopic analyses of carbonates from the Hamersley Group, Western Australia, yield the following results. (1) A late Archean (minimum) age of ⩾ 2.5 Ga is definitely established for the deposition of both the deeper-water basinal Wittenoom Formation and the shallow-water Carawine Dolomite. Stratigraphic correlation of the Transvaal Dolomite (South Africa) with these two formations is supported by the present Pb isotope age data. (2) A PbPb isochron age of 2505 ± 37 Ma for the Paraburdoo Member (lower part) of the Wittenoom Formation is best interpreted as a late diagenetic event, which took place ∼ 100 Ma after the initial deposition. (3) A PbPb isochron age of 2346 ± 38 Ma for the Bee Gorge Member most likely represents a still later local event of calcite replacement in fractures and veins. (4) A PbPb isochron age of 2541 ± 32 Ma for Carawine Dolomite could represent a regional diagenesis slightly earlier than the Paraburdoo carbonates if the Carawine carbonates were deposited contemporaneously with the Wittenoom Formation. Alternatively, it may represent the time of early diagenesis which closely followed the initial sedimentation. A study of trace element abundances and stable isotope compositions has revealed a major difference in Fe and Mn contents and δ18O values between the Wittenoom and Carawine dolomite samples. This suggests that the two suites of dolomites have undergone separate diagenetic histories with different chemical and oxygen isotopic modifications.

A classification of the vegetation of the western Transvaal dolomite and chert grassland, South Africa

Relatively little is known about the vegetation of the western Grassland Biome in South Africa. The classification of the dolomite and chert grassland in the western Transvaal (Fa land type) forms part of a research programme on the syntaxonomic and synecological synthesis of the vegetation of the western Grassland Biome. Using a numerical classification technique (T winspan ) as a first approximation, the classification was refined by applying Braun-Blanquet procedures. The result is a phytosociological table from which two new alliances, six new associations, two new sub-associations and two new communities without syntaxonomic rank are recognized. The new syntaxa are ecologically interpreted as well as described. Associated gradients in habitat were identified by using an ordination algorithm (D ecorana ). This study should contribute to the present knowledge and ecological understanding of the vegetation of the western Transvaal.

A new explanation for extreme [formula omitted] disequilibria in a dolomitic aquifer

High 234 U 238 U activity ratios are found in the shallow groundwater of the phreatic Transvaal Dolomite Aquifer. The aquifer is uranium poor, while the waters are oxygen rich and young. Tritium and 14C are used to show that the disequilibrium increases the longer the water resides in contact with the dolomite host. The 228Ra and 222Rn isotopes were used as supporting evidence that ion exchange between the 238U series nuclides in the water and the carbonate wall rock is necessary in assisting in the fractionation process. The Wolkberg cave speleothems preserve a record of the uranium isotopic fractionations that evolved as water flowed through the aquifer. Extremely variable and elevated 234 U 238 U ratios (of 2–12) are characteristic. Individual caverns may exhibit very large 234 U 238 U variability in their drip water and in derivative carbonate precipitates (speleothems). Deeper chambers, where water has spent a longer time in contact with the aquifer, tend to exhibit a greater degree of uranium isotopic fractionation in their speleothems. The recorded fractionation process has been an ongoing one from at least the middle-late Pleistocene to the present. The speleothem study supports the contention that ion exchange processes that produce sufficient enrichment of uranium on carbonate can provide ideal conditions for generating uranium isotopic fractionation in the water of carbonate aquifers (in the absence of redox changes and within a uranium-poor environment).

Direct dating of stromatolitic carbonates from the Schmidtsdrif Formation (Transvaal Dolomite), South Africa, with implications on the age of the Ventersdorp Supergroup

Pb isotopic analyses of stromatolitic limestones from the Schmidtsdrif Formation (Transvaal Dolomite) of South Africa give a late Archean isochron age of 2,557 {plus minus}49 (2 sigma) Ma. The new date significantly changes the position of the Archean-Proterozoic boundary in the Kaapvaal province and suggests that a possible division of the Transvaal Supergroup is necessary. It also resolves the long-standing controversy about the age of the Ventersdorp Supergroup, whose zircon U-Pb data of ca. 2.7 Ga must be more acceptable than the Rb-Sr dates of about 2.3 Ga. The study provides a new example that demonstrates the great potential of employing the U-Pb systems of carbonate rocks for direct dating of sedimentary sequences.

Scanning Electron Microscopy of Quartz in Precambrian Cherts and Dolomites from Southern Africa

AbstractScanning electron microscopic (SEM) examination of the quartz isolated from chert of Transvaal and Swaziland, of 2000 and 3400 million year (MY) ages, respectively, in southern Africa revealed marked differences in quartz morphology. Well-defined individual euhedral quartz crystals, with polyhedral triple-point faces, were clearly evident on freshly fractured Transvaal chert surfaces as well as with the quartz isolates from the chert. The morphology and coarseness suggest crystal growth with little, if any, metamorphism; however, the δ18O values of 23.8–24.1‰ suggest crystallization temperatures of perhaps 40–45°C. In contrast, fracture surfaces of the older, strongly metamorphosed Swaziland cherts revealed a high degree of grain intergrowth which inhibited fracture between quartz grains. Quartz isolates from these showed strongly interlocked quartz crystal clusters and elongated chips of quartz with poorly defined irregular faces. Intercalation of mineral veins in the cherts on a mm scale and the intergrown character of the quartz grain boundaries provide evidence that the latter cherts have been strongly metamorphosed and recrystallized, in keeping with 14.6–15.1‰ δ18O values, corresponding to 80°C fractionation with water. The SEM micrographs of the fine quartz (1–10 µm) isolated from the Otavi dolomite formation of the 700-MY Damara System and from the 2000-MY Transvaal Dolomite Series revealed well-defined subhedral and euhedral quartz crystals of small size which, together with the 26.9–27.8‰ and 23.8‰ δ18O values, respectively, indicate that these dolomites have been affected little, if any, by post-depositional metamorphism; their crystallization temperatures fall in the range of 25–30 and 40–45°C, respectively.

Biostratigraphic usefulness of stromatolitic precambrian microbiotas: A preliminary analysis

Diverse, cellularly preserved microbial communities are now known from stromatolitic sediments of at least twenty-eight Precambrian formations. These fossiliferous deposits, principally cherts and cherty portions of carbonate units, range in age from Early Proterozoic (Transvaal Dolomite, ca. 2250 Ma old) to Vendian (Chichkan Formation, ca. 650 Ma old) and include units from Australia, India, Canada, South Africa, Greenland, the United States and the Soviet Union. More than three-quarters of these microbiotas have been discovered since 1970. Although few, therefore, have as yet been studied in detail, virtually all of the assemblages are known to be dominated by prokaryotic (bacterial and blue-green algal) microorganisms and to contain three major categories of microfossils: spheroidal unicells, cylindrical tube-like sheaths, and cellular trichomic filaments. Analyses of data now available (including measurements of more than 7800 fossil unicells) indicate that each of these three types of microfossils exhibited a gradual, but marked, increase in mean diameter and size range during the Proterozoic and that taxonomic diversity apparently also increased, especially beginning about 1400 Ma ago. Thus, it now seems evident that (i) the microbial components of Proterozoic stomatolitic assemblages have varied systematically as a function of geologic age and that (ii) such communities are both more abundant and more widespread than had previously been recognized. These observations augur well for the future use of such assemblages in Precambrian biostratigraphy. At present, however, data are sufficient to warrant the provisional establishment of only a few microfossil-based subdivisions of the Proterozoic. Such zones, necessarily relatively long-ranging, are here tentatively defined; it is of interest to note that boundaries between certain of these microfossil-based subdivisions appear to coincide, at least approximately, with previously suggested stromatolite-based boundaries. To some extent, therefore, results of this study seem consistent with, and may be supportive of, the concept of stromatolite-based biostratigraphy. At the same time, however, the study seems to indicate that stromatolites of markedly differing age, whether of similar or of dissimilar morphology, were probably formed by distinctly differing microbiotas. Data are as yet insufficient to indicate whether differing types of coetaneous, stratigraphically useful, stromatolites were formed by differing microbial communities and two what extent the “evolution” of stromatolite morphology was a result of the biologic evolution of stromatolite-building microorganisms. There is thus continued need for investigation of the potential biostratigraphic usefulness of stromatolitic microbiotas and, especially, for more effective integration of results of such studies with those available from studies of stromatolites without preserved microbiotas and from studies of the acritarchs preserved in Proterozoic shales.

Limestone Formation and Dolomitization in a Lower Proterozoic Succession from South Africa

ABSTRACT Limestones, dolomitic limestones and dolomites of the Transvaal Dolomite which accumulated in intertidal, high energy and subtidal environments in an epeiric sea have been chemically investigated, with the view to a further understanding of Lower Proterozoic carbonate formation. The carbonates studied are characterized by unusually high Fe and Mn contents. Both metals are present in their lower oxidation states. This suggests a less oxidizing atmosphere at the time of formation of these rocks relative to the present. The limestones in the area studied apparently formed under conditions of saturation of iron, manganese and calcium. The Fe/Mn ratios of these limestones are contrasted with those of ferruginous dolomitic limestones and ferruginous dolomites elsewhere in the sedimentary basin which formed when manganese was undersaturated in solution. Contemporary dolomitization is most intense in upper intertidal to supratidal zones, but in the section considered this process has been most complete in the subtidal region. The dolomitization process is related to the nature of the depository on the Precambrian craton. This process is considered to be an early one, resulting from the action of waters in contact with limestone. A further influence, involving meteoric water with a lower pH has possibly been responsible for the dolomitization and also recrystallization of some carbonates.

Stromatolitic associations and their palaeo-environmental significance: A re-appraisal of a lower proterozoic locality from the northern cape province, South Africa

A basal section of the Lower Proterozoic Transvaal Dolomite from the northern Cape has been re-examined. Numerous associations of stromatolites and other structures have been distinguished in it. Forms of domical stromatolite dominate the structures, but columns, and associations built around spheroidal forms, are also represented. This material accumulated in a telescoped range of environments through the intertidal to a high-energy agitated zone and out into the subtidal. Columnar forms and flat domes from the intertidal zone appear to be remarkably similar to features described from Hamelin Pool at Shark Bay, Western Australia. The subtidal elongate domes and mounds increase in size with depth to reach dimensions of 40 by 10 m, heights of 13 m and relief of 2.5 m. Minor structures on these subtidal forms reflect decreasing energy outwards; included within them are columnar forms in the shallow subtidal zone. It is suggested that deposition within an epeiric sea was away from a headland on a relatively steeply shelving floor affected by currents at right angles to the shore. There are indications that an arid to semi-arid climate existed at that time.