C. P. Snyman

Bio: C. P. Snyman is an academic researcher from University of Pretoria. The author has contributed to research in topics: Sedimentary rock & Clastic rock. The author has an hindex of 8, co-authored 9 publications receiving 200 citations.

Petrography and Geochemistry of Sandstones Interbedded with the Rooiberg Felsite Group (Transvaal Sequence, South Africa): Implications for Provenance and Tectonic Setting

Abstract: The 2150 Ma Rooiberg Felsite Group, Transvaal Sequence, contains thin sandstone interbeds within its 3500-5000 m thick volcanic succession. Whereas uppermost feldspathic arenites and wackes are thought to represent reworked felsitic material, quartz arenites and subordinate lithic sandstones, present throughout most of the Rooiberg sequence, comprise mainly reworked sedimentary detritus, probably belonging to the underlying Pretoria Group. Sandy braided-river systems probably transported clastic material into the basin from relatively stable source areas, subject to intense chemical weathering. Geochemistry, and sandstone petrography indicate that mixing of Pretoria Group detritus with sediment derived from erosion of felsitic material occurred within the basin during late Rooiberg ti es. The inferred hiatuses in volcanism represented by the predominant siliceous sedimentary interbeds appear to have been of relatively short duration and occur throughout much of the Rooiberg stratigraphy. Upper arkosic sandstones indicate longer breaks in volcanism as Rooiberg eruptions came to an end. The sandstones provide evidence compatible with an impact origin for the Rooiberg Felsite Group, and for its successor, the Bushveld Complex.

The early proterozoic Mississippi Valley-type Pb-Zn-F deposits of the Campbellrand and Malmani Subgroups, South Africa

Abstract: Pb-Zn-F deposits occur in the very late Archaean (2.55 Ga) shallow marine dolostone of the relatively undeformed Campbellrand and Malmani Sub-groups, which are overlain unconformably by the lower Proterozoic Postmasburg and Pretoria Group siliciclastics. They consist of stratiform deposits formed by replacement and porosity-filling, as well as pipes, ring-shaped and irregular bodies associated with collapse breccia. In the Transvaal basin the latter were generated during the karst denudation period between the deposition of the Chuniespoort Group (ending at ∼ 2.4 Ga) and of the Pretoria Group (starting at 2.35 Ga). A part of these mineralisations were overprinted by the metamorphism of the Bushveld Complex intrusion at 2.06 Ga. In the Transvaal basin, the age of the mineralisation is constrained between the start of the Pretoria Group deposition and the Bushveld intrusion. It is concluded that, although most of the mineralisations are characteristic of the Mississippi Valley-type, some of the northernmost occurrences, rich in siderite, are less typical. A classic genetic model is proposed. In an environment characterised by tensional tectonics and basin development, brines of basinal origin were heated by circulation into pre-Chuniespoort rocks, leached metals from the rocks they permeated, and rose as hydrothermal plumes. At relatively shallow depth they deposited minerals after mixing with water of surficial origin.

Chemostratigraphy of the Paleoproterozoic Duitschland Formation, South Africa: Implications for Coupled Climate Change and Carbon Cycling

Abstract: The Paleoproterozoic Duitschland Formation lies stratigraphically beneath the Timeball Hill Formation, which contains the only unequivocal glacial unit of this era in the Transvaal Basin, South Africa Lithologic evidence in Paleoproterozoic successions of North America, however, indicates the existence of three discrete and potentially global ice ages within this 300 my interval Carbonates of the Duitschland Formation are significantly enriched in 13C up to +101 permil in the upper part of the succession above a notable sequence boundary In contrast, the lower part of this unit contains carbonates with consistently negative δ13C values Trace and major element compositions of these carbonates as well as carbon-isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly The stratigraphic constraints indicate that 13C-enriched carbonates were deposited prior to Paleoproterozoic glaciation in southern Africa, similar to carbonates stratigraphically beneath Neoproterozoic glacial diamictites worldwide Also mirroring the Neoproterozoic record are strongly negative δ13C values in cap carbonates atop glacial diamictites in Paleoproterozoic strata of Wyoming and Ontario The litho- and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well-preserved carbonates of the lower Duitschland Formation may reflect a second Paleoproterozoic ice age in the Transvaal succession This interpretation is further supported by recently discovered bullet-shaped clasts with striations in diamictite from the basal part of the succession Thus, the emerging temporal pattern of carbon isotope variations and glaciation in the Paleoproterozoic has a close analogue to Neoproterozoic events, suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

Iron Formations: Their Origins and Implications for Ancient Seawater Chemistry

Abstract: Iron formations are economically significant, iron- and silica-rich sedimentary rocks that are restricted to Precambrian successions. There are no known modern or Phanerozoic analogues for these deposits that are comparable in terms of areal extent and thickness. Although many aspects of iron formation origin remain debatable, it is generally accepted that secular changes in the style of deposition are genetically linked to plate tectonic processes, mantle plume events, and evolution of Earth's surface environments. Two types of Precambrian iron formations have been recognized based on depositional and tectonic settings. Iron formations formed proximal to volcanic centers are interlayered with or laterally linked to submarine volcanic rocks and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger sedimentary rock-hosted iron formations are developed in passive-margin settings and typically lack a direct association with volcanic rocks. A full gradation between these two end-members exists in the rock record. Texturally, iron formations are divided into two groups. Banded iron formation (BIF) is predominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is more common in middle to late Paleoproterozoic successions, having been deposited in shallow-marine settings after the rise of atmospheric oxygen at ~2.4 Ga. Secular changes in the style of iron formation deposition have been linked to a diverse array of environmental changes. Geochronologic studies emphasize the periodicity in deposition of giant iron formations, which are coeval with large igneous provinces (LIPs). Giant sedimentary rock-hosted iron formations first appeared ~ 2.6 Ga, possibly when the construction of large continents changed the heat flux across the core–mantle boundary. From ~ 2.6 to ~ 2.4 Ga, global mafic-to-ultramafic magmatism culminated in the deposition of giant sedimentary rock-hosted iron formations in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited immediately before a shift from reducing to oxidizing conditions in the ocean–atmosphere system. Counterintuitively, enhanced magmatism at 2.50–2.45 Ga, which likely delivered large amounts of reductants to shallow-marine environments, may have triggered atmospheric oxidation. After the rise of atmospheric oxygen ~ 2.4 Ga, GIF became more abundant in the rock record than BIF. Iron formations largely disappeared ~ 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global-scale glaciations, during the so-called snowball Earth events. In the Phanerozoic, deeper-water iron formation deposition became restricted to local areas of closed to semi-closed basins, where volcanic and hydrothermal activity was extensive, such as in back-arc basins. In contrast, episodically deposited, basin-scale Phanerozoic oolitic and pisolitic ironstones are linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and at least some Paleozoic ironstones were deposited at the redoxcline, where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to the production of the conditions necessary for deposition of iron formation. Mantle plume events that led to the emplacement of LIPs also enhanced spreading rates of mid-ocean ridges and resulted in higher growth rates of oceanic plateaus; both processes thus contributed to a higher hydrothermal iron flux to the oceans. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the ocean oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the ocean redox state, iron was deposited mainly proximally, generally as oxides or sulfides; manganese was more mobile. It is concluded that occurrences of BIF, GIF, Phanerozoic ironstones, and hydrothermal sedimentary rocks of exhalative origin (exhalites) surrounding VMS systems are a record of a complex interplay among mantle plume events, plate tectonics, and ocean redox conditions throughout Earth's history, in which mantle heat unidirectionally decreased and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter-term fluctuations.