Sharad Master

A cryptic Mesoarchaean terrane in the basement to the Central African Copperbelt

In a study of the geochronology of the Katangan Sequence and its basement in the Central African Copperbelt, detrital and xenocrystic zircons from Muva quartzites and Katangan lapilli tuffs, were dated using the Sensitive High-Resolution Ion Microprobe (SHRIMP). A detrital population (dated between 3007 and 3031 Ma) and a group of xenocrystic zircons aged between 3169 and 3225 Ma provide the first evidence for the existence of a Mesoarchaean basement beneath the Central African Copperbelt.

Neoarchaean (c. 2.58 Ga) halite casts: implications for palaeoceanic chemistry

Possibly the most extensive and best-preserved Archaean halite casts yet discovered occur in the c. 2.58 Ga upper Black Reef and basal Oaktree formations, Transvaal Supergroup, in Mpumalanga Province, South Africa. Halite casts are isolated on bedding planes, range in size from c. 1 mm to 20 mm, and have cubic, dumbbell and triangular shapes, as well as hopper-like pyramidal hollows on cube faces. Some of the casts display distinct hopper shapes characteristic of halite crystals. The halite cast-bearing pavements are developed within silicified mudstone interbedded with siltstone or stromatolitic dolomite. Associated sedimentary structures pointing to subaerial exposure include adhesion ripples and warts, desiccation and prism cracks, rill marks and tepee structures. Halite cast-bearing beds are interpreted as supratidal flat or sabkha deposits. The presence of isolated casts and hopper-shaped crystals suggests that halite resulted from displacive growth within the sediment from supersaturated residual brines after mudstone deposition. Absence of any indication of the former presence of gypsum or anhydrite supports previous contentions that the Neoarchaean ocean was deficient in sulphate or contained a high bicarbonate to calcium ratio such that with progressive evaporation, most calcium was consumed before the gypsum stability field was reached. The association of halite and carbonate in the upper Black Reef and basal Oaktree formations constrains the palaeolatitude of the Transvaal Basin at 2.58 Ga to subequatorial (10–30°).

The Upper Mantle Seismic Velocity Structure of South-Central Africa and the Seismic Architecture of Precambrian Lithosphere Beneath the Congo Basin

The seismic architecture of the lithosphere beneath the Congo Basin is investigated using a new shear wave velocity model of the upper mantle for central and southern Africa derived from an inversion of Rayleigh wave group velocity measurements. The model is similar to other tomographic models derived from Rayleigh wave phase velocities, revealing a region of fast upper mantle velocities in the 50–100 km depth interval beneath the northwestern, central and southern portions of the basin, and slower upper mantle velocities beneath the northeastern part of the basin, as well as beneath Proterozoic mobile belts to the east and south of the basin. The upper mantle velocity pattern indicates that Proterozoic lithosphere may lie beneath the northeastern side of the basin, but it does not support the presence of Proterozoic lithosphere beneath the entire northern portion of the basin. This finding suggests that lithospheric structure beneath the basin is not uniform, as is commonly assumed in geodynamic models explaining how the basin formed. A second geodynamic implication concerns the Neoproterozoic rifting event that may have initiated basin subsidence. The proposed locations of the rifts are in the region of the velocity model where the velocities begin to change from faster to slower going from the center of the basin toward the northeast. Thus, the rifts may have formed along the border between two separate, smaller Archean blocks, as opposed to within the middle of a single, larger Archean block, alleviating the need to explain how a Neoproterozoic rift might form within the interior of a large Archean craton.

Chapter 12 Neoproterozoic glaciogenic diamictites of the Katanga Supergroup, Central Africa

Abstract Glaciogenic sediments of the Katanga Supergroup are represented by two units. The syn-rift Grand Conglomerat Formation (<765±5 Ma to >735±5 Ma) occurs within the Nguba Group, and the Petit Conglomerat Formation defines the base of the Kundelungu Group deposited in the earliest foreland basin of the Lufilian orogenic belt located between the Congo and Kalahari cratons. Their glacial origin is inferred on the basis of the following features: the common and widespread occurrence of thick polymictic conglomerates and diamictites with faceted and striated clasts, massive structure, abundant poorly sorted fine-grained matrix, and the presence of planar-laminated shales (laminites) with dropstones. Glaciomarine facies associations prevail over most of the geographic extent of both units, but at the northern periphery of the depository, continental glacial facies are present. The glaciomarine units are succeeded by carbonates: the Kakontwe Limestone and ‘Calcaire Rose’ respectively. The clasts in the glaciogenic units are of extrabasinal and intrabasinal provenance. Lower boundaries, conformable in the basin centre, evolve to unconformities in the marginal areas to the N and S. The palaeomagnetic evidence suggests deposition in low latitudes.

Carbonate ooids of the Mesoarchaean Pongola Supergroup, South Africa

Ooids from the Mesoarchaean Chobeni Formation, Pongola Supergroup, KwaZulu-Natal, South Africa are older than any ooids reported to date. They are made of dolomite and ankerite and show concentric, radial-concentric, micritic, and radial fabrics. Radial ooids are interpreted to have originated from high-Mg-calcite and probably formed by microbial activity in a low-energy regime, while concentric ooids had an aragonite precursor and formed biotically under agitated/high-energy environmental conditions. Micritic ooids formed via the recrystallization of concentric ooids. Ooids and other allochems, such as intraclasts and peloids, contain carbonaceous matter. The close association of carbonaceous matter within ooid cortices with metabolically important elements, such as N, S and P, as identified by nano-scale secondary ion mass spectrometry analysis, allows us to propose a biologically induced origin for some ooids. By analogy with modern examples, a variety of microbial communities probably played a role in carbonate precipitation and ooid formation. Shale-normalized rare earth element (REE) distribution patterns of ooids and other allochems show positive LaSN , GdSN and YSN anomalies, superchondritic Y/Ho ratios and depleted light rare earth elements (LREEs) relative to the heavy rare earth elements (HREEs), which resemble those of seawater. These anomalies are less pronounced than expected for an open marine setting, which is interpreted as evidence for deposition in restricted shallow marine environments. Non-seawater REE patterns in recrystallized matrix and pore- and vein-filling carbonate likely reflect redistribution of rare earth elements during post-depositional alteration and/or reflect differences in the elemental and REE compositions of diagenetic fluids.

Gaet’ale- a reactivated thermal spring and potential tourist hazard in the Asale salt flats, Danakil Depression, Ethiopia

This paper serves to document a thermal spring, called Gaet’ale, that was reactivated in 2005, during the majorseismo-volcanic crisis in the Danakil Depression of the Afar region of northern Ethiopia. Many dead birds surrounding the spring attest to deadly gas emanations (almost certainly CO2) coming from this spring, reminiscent of those from other volcanic lakes, and the Pamukkale springs in Turkey. Gae’tale currently features among the tourist attractions of the Dallol region of the northern Afar, but it may pose a potentially dangerous, and even deadly, hazard for tourists and their guides. Some suggestions are made to help mitigate the risks, and to allow for sustainable geotourism in this environmentally sensitive region. These include ensuring that tour operators in the area are made aware of the hazards, and are communicating these to their tourist clients (who should also be aware of these hazards through websites, tour guidebooks and open-access scientific journals), and avoiding the areas closest to the lake, and periodic testing, with lit flames, for the presence of excess CO2 in the area, with plans for quick and safe evacuation if needed. Guidelines for proper conduct are given for geotourists who are planning to visit the region, to ensure their health and safety in the vicinity of the thermal springs.

A Note on Imbricated Granite Boulders on NW Penang Island, Malaysia: Tsunami or Storm Origin?

The Batu Feringgi area (N coast, Penang Island), with a high concentration of beach hotels, is critical to the tourist economy of Malaysia. Three large imbricated granite boulders were discovered on the NE end of the beach, at 5°28′51.77″N, 100°15′72″E. These boulders, dipping 45°–70° seaward, are shaped as tabular parallelepipeds with rounded corners, with maximum masses of 1.1–2.4 t, based on a density of 2.71 g/cm3. The boulder shapes were dictated by the presence of joints in the coastal outcrops, which represent an uplifted and exhumed tropically-weathered granite-tor landscape.