Eric S. Cheney

Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia

The Transvaal succession in the northeastern part of the Kaapvaal Province of southern Africa is the erosional remnant of four, 2.7 to 2.1 Ga unconformity-bounded stratigraphic units, or sequences, each of which is 102 to 103 m thick and spans 107 to 108 years. Three of these Phanerozoic-like cratonic sequences occur in the western part of the Kaapvaal Province and in the Pilbara Province of Western Australia; thus, the two provinces once were part of the same continent, Vaalbara. The stratigraphically lowest sequence historically assigned to the Transvaal succession is part of the Ventersdorp Supergroup in Kaapvaal and the upper part of the Fortesque Group in Pilbara. It consists predominantly of arkosic sandstone and basaltic andesite; it probably is ⩽ 2687 ± 2 Ma. The second sequence contains the quartz arenites, dolomites, and banded-iron formations of the Transvaal and Griqualand West successions in the Kaapvaal and the Hamersley Group in the Pilbara. The span of ages (2684 ± 6 to 2432 ± 31 Ma), lithologies, and geographic extent of this sequence imply that the Limpopo orogeny, which welded the Zimbabwe Province to Vaalbara, is ⩽ 2.47 Ga (not 2.67 ± 0.05 Ga). The third sequence (the Pretoria and Postmasburg groups in the Kaapvaal and the lower part of the Wyloo Group in the Pilbara probably is the clastic wedge generated by the Limpopo orogeny. The fourth sequence (dominated by the felsites of the Rooiberg Group) is restricted to northeastern Kaapvaal. Sequence stratigraphy, lithostratigraphy, and lithofacies indicate that the Pilbara rifted from the southern edge of western Kaapvaal. The Penge Iron Formation in northeastern Kaapvaal, the Asbesheuwels Iron Formation in southwestern Kaapvaal and the Brockman Iron Formation in the Pilbara are the same formation, in which cyclothemic units 0.6 to 15.0 m thick extended ⩾ 1200 km.

The conformable emplacement of the Bushveld mafic rocks along a regional unconformity in the Transvaal succession of South Africa

Abstract A previously unrecognized regional unconformity occurs in the Proterozoic Transvaal succession of the Kaapvaal province of southern Africa. The predominantly marine and siliciclastic rocks of the lower two-thirds of the Pretoria Group are unconformably overlain by a terrestrial and largely volcanic sequence, the most voluminous portion of which is the Rooiberg Felsite. If the Bushveld Complex is mentally subtracted from map patterns, the Rooiberg Felsite also locally overlies pre-Pretoria sequences and the Archean crystalline basement. The Bushveld Complex is not lopolithic as commonly supposed. Instead, regionally condordant emplacement of the Complex along the unconformity explains both the seemingly discordant footwall features and the lack of Pretoria and older strata (other than a few xenoliths) in the hanging wall. The lack of any geochemical evidence for major crustal contamination of Bushveld magmas precludes disposing of any such hanging wall strata by wholesale assimilation. The Bushveld Complex, the unconformity, and the enclosing strata are synformally folded, making the Bushveld appear to be lopolithic, Due to this folding and multiple periods of erosion, the original extents of the Bushveld Complex, the unconformity, and the enclosing strata are now much reduced. Other smaller mafic intrusions along or immediately below the same regional unconformity probably are Bushveld-related.

Evidence for the transition to an oxygen-rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern Africa

Abstract Strata > 2.5 to ⩽ 1.8 Ga in southern Africa appear to span the transition from a non-oxidizing atmosphere to one characterized by free oxygen. Red beds in 2.3 to 2.1 Ga unconformity-bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks, ironstones, or banded iron formations, which evidently were sources of iron for the matrices. However, red beds in ⩽ 1.9 Ga unconformity-bounded sequences not only matrix-staining but also hematitic coatings on grains; these beds also have far fewer interstratified ferruginous lithologies. Because red beds with stained clay matrices, banded iron formations, pyritic (Witwatersrand-type) conglomerates, and iron-depleted paleosols all occur in the ∼ 2.2 Ga Pretoria Group, formation of these lithologies may not have been time-dependent until atmospheric oxygen approached 10−2 atm. By 1.9 Ga atmospheric oxygen was ⩾ 10−2 atm, causing lateritic weathering and red beds with hematitic coatings on grains.

Low-angle faulting and the origin of Kettle dome, a metamorphic core complex in northeastern Washington

The low-angle, eastward-dipping Kettle River fault has been mapped for >45 km along the eastern flank of Kettle dome. The fault juxtaposes lower-plate, mylonitic, sillimanite-grade metamorphic rocks against upper-plate, greenschist-grade metamorphic rocks and unmetamorphosed but folded Eocene volcanic and sedimentary rocks. The geometry of high-angle listric(?) faults, which cut upper-plate rocks, suggests that the upper plate moved eastward relative to the lower plate. Cataclasis and chloritization spatially related to the Kettle River fault cut the mylonitic foliation in the lower plate, indicating that the gneisses in the lower plate were not part of a hot, mobile diapir during at least the latest stage of their “emplacement.” The similarity of the Kettle River fault to the folded Newport fault along the northern contact of Spokane dome suggests that these faults postdate the mylonitization of the lower plate but predate regional doming.

Stable carbon isotopic composition of biogenic carbonates

Abstract The geology and sulfur isotopic composition of epigenetic pyrite of the Gas Hills district, Wyoming, indicate a bacterial origin for these sandstone-type uranium deposits. The δC13 value of epigenetic calcite associated with organic materials, however, is heavier than the isotopic composition of calcite of biogenic native sulfur deposits. Stratigraphic and carbon isotopic evidence suggest that contamination by sedimentary carbonate is minimal in the Gas Hills samples. Field and laboratory data show that anaerobes produce isotopically heavier CO2 and lighter methane than the associated organic material which they consume as an energy source. The δC13 values of the salt-dome calcites overlap the methane field, the Gas Hills values overlap the CO2 field, and analyses of Sicilian Sulfur limestones are intermediate. Deposits Gas Hills (calcites with organic material) Gulf Coast Cap rock Sicilian Sulfur limestone (gyngenetic calcites) Number of samples 20 35 27 Mean δC 13 −22.5 −37.9 −28.7 Sample standard deviation 4.0 6.7 8.9 Presumably a spectrum of values could be obtained by the oxidation of methane and contamination by the original bacterial CO2 and sedimentary carbonate. The quantized values of the three deposits are, therefore, evidence of strong geologic control. Methane was stable under the reducing conditions (shown by the presence of uraninite and sulfides) of the Gas Hill, while methane was concentrated and oxidized in the cap rock of salt-domes (mineralogy includes native sulfur), and neither methane nor CO2 was preferentially lost during the syngenetic precipitation of the Sicilian Sulfur limestone.

The Archean-Proterozoic boundary in the Kaapvaal Province of Southern Africa

Abstract The Kaapvaal province has long been considered unusual in that Archean cover sequences developed over granite-greenstone-gneiss terranes ∼ 3060 ± 25 Ga; whereas, in other provinces the oldest cover sequences are generally 2500 Ga. However, two unconformity-bounded sequences ∼ 2.5 Ga and of greater inter-regional extent than the unconformably underlying Archean sequences can now be recognized in the Kaapvaal province. The two basal unconformities of these sequences are candidates for the temporally defined 2.5 Ga Archean-Proterozoic boundary. The older (but geologically more discordant) unconformity occurs at the base of the uppermost group of the Ventersdorp Supergroup (the Pniel Group) and in at least five other erosional remnants in central Transvaal and adjacent Botswana. These other remnants include parts of the Buffelsfontein and Wolkberg Groups, which heretofore have been considered to be conformable protobasinal phases of the younger Transvaal succession. The Pniel and the other remnants are truncated by the locally well known unconformity at the base of the Transvaal succession. On a world-wide basis, cratonization and the evolution of mineralogically mature cover sequences were (and continue to be) diachronous. The Kaapvaal province is unusual only in that it also contains Archean cover sequences. However, with major conformities at about 2.5 Ga and the presence of Proterozoic unconformity-bounded sequences the Kaapvaal province seems little different than most other provinces that contain the Archean-Proterozoic boundary.

Evidence for the transition to an oxygen-rich atmosphere in the Rooiberg Group, South Africa — A note

Abstract The Rooiberg (Felsite) Group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen-rich atmosphere. This conclusion is reached indirectly from a review of the Proterozoic stratigraphy of South Africa, and directly from a consideration of the Rooiberg itself. The atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the Damwal Formation, and the overlying red rhyodacites and intercalated red-bed sedimentary rocks of the Selonsrivier Formation. Radiometric age measurements indicate that this transition occurred after 2224 ± 21 Ma but before 2090 ± 40 Ma.

The Chiwaukum structural low: Cenozoic shortening of the central Cascade Range, Washington State, USA: Reply

The Chiwaukum area on the eastern flank of the Cascade Range of Washington State is a type locality for a clastic basin in what is, broadly speaking, a continental arc province. [Evans (1994)][1] concluded that the Eocene Chumstick Formation was deposited syntectonically in the Chiwaukum graben in a

Evidence for sulfurization and the origin of some sudbury-type ores

The suggestion that Sudbury-type ores may be formed by the introduction of country rock sulfur into still hot intrusions (i.e., sulfurization) suffers from a reputed lack of field evidence. Permissive evidence for sulfurization includes the epigenetic nature of many Sudbury-type ores and that many Sudbury-type ores crystallized from sulfide melts. Visual evidence exists for sulfurization of a gabbro in Zambia. The lead isotopic composition of ore minerals at Sudbury implies that at least some of the metals were derived from the erruptive. Published sulfur isotopic data from several Sudbury-type ores differ from and do not exhibit a common pattern of isotopic enrichment with respect to sulfides within associated intrusions. Evidently the sulfur was derived from the country rocks. Sudbury-type ores exhibiting magmatic textures commonly occur within more siliceous dikes than the host intrusions. Inorganic reduction of sulfate occurs only above 600° C. Reduction of sulfate with resultant sulfurization of ferrous iron and traces of other metals originally present in the still hot parental intrusive rock would make the rock more siliceous. Above 1100° C the silicate-residue and newly formed sulfides would form immiscible magmas. Therefore, ore magmas within and near mafic intrusives can be epigenetic. The processes by which sulfur is introduced into intrusions are still speculative.ZusammenfassungFür die Annahme, daß Erze vom Sudbury-Typus sich durch Zuführung von Nebengesteinsschwefel in noch heiße Intrusionen bilden können (“sulfurization”), fehlt es angeblich an Feldunterlagen. Folgende Tatsachen lassen sich mit einer Schwefelung (“sulfurization”) vereinigen: der epigenetische Charakter vieler Erze vom Sudbury-Typ; auch sind viele Erze von Sudbury-Typus aus sulfidischen Schmelzen kristallisiert. In einem Gabbro in Zambia ist der Beweis für Schwefelung (“sulfurization”) direkt sichtbar. Die Isotopen-Zusammensetzung von Blei in Erzmineralien in Sudbury zeigt, daß mindestens einige der Metalle aus dem Eruptivgestein stammen. Schwefelisotop-Daten, die für mehrere Lagerstätten von Sudbury-Typen veröffentlicht worden sind, haben hinsichtlich der Isotop-Anreicherung keine gemeinsamen Züge. Offensichtlich stammt der Schwefel aus dem Nebengestein. Erze vom Sudbury-Typ mit magmatischem Gefüge finden sich oft in Gängen, die saurer sind als das Wirtsgestein. Anorganische Reduktion von Sulfat findet nur oberhalb 600°C statt. Reduktion von Sulfat und die entstehende Schwefelung (“sulfurization”) von zweiwertigem Eisen und Spuren anderer Metalle, die ursprünglich in dem noch heiß eruptiven Gestein anwesend sind, machen das Gestein noch saurer. Oberhalb 1100°C würden der Silicat-Rest und die neugeformten Sulfide nicht mischbare Magmen bilden. Deshalb können sich sulfidische Schmelzen innerhalb und in der Nähe von “Mafic”-Intrusionen später gebildet haben. Die Prozesse, durch die Schwefel in Intrusionen eingeführt wird, sind noch unbekannt.

The role of iron sulfides in the diagenetic formation of iron-poor manganese nodules

ABSTRACT Iron-poor manganese nodules form diagenetically. Under reducing conditions protonodules dissolve; some iron is fixed in sulfide minerals, but manganese migrates up to the oxidized zone and reprecipitates as iron-poor nodules.