Pellets, ooids, sepiolite and silica in three calcretes of the southwestern United States

Pellets and ooids are widespread and locally abundant in mature calcrete profiles in the Argus Range, California; near Wickieup, Arizona; and in Kyle Canyon, Nevada. Most concentrations of pellets and ooids either overlie laminar calcrete at various levels in the calcrete profile or fill subhorizontal fractures in the petrocalcic horizon. In all three profiles the petrocalcic horizon has been thickened by the pelletal, chemically deposited fracture fillings. Pellets range from 0.02 to 8.0 mm in diameter and consist principally of micritic calcite and sepiolite. Ooid coatings are chiefly calcite and opal or calcite and sepiolite. The pellets represent small concretions, some of which grew by accretion, either in void space or by displacing adjacent sediment, and the others of which were formed by cementation of pellet-shaped bodies of porous micrite. Ooid coatings with opal or sepiolite may have been deposited as a gel with sufficient strength for surface tension to thin the coatings over angular corners of nuclei so as to increase the roundness and sphericity of the particles. Major problems in calcrete genesis are (1) the cause of subhorizontal fractures and the mechanism for widening a fracture as sediment accumulates in it and (2) what determines the deposition of calcite, sepiolite, and opal as pellets and ooid coatings or as laminar layers.     

Origin of Ca in South Indian calcretes developed on metamorphic rocks

Calcrete profiles developed on Precambrian silicate rocks in south peninsular India are studied and compared in terms of mineralogical, chemical and isotopic composition. The aim of this study is to determine the origin of calcium and the processes governing the development of a minor economic deposit in different geomorphic settings. The investigation integrates observations from an outcrop to a microfabric scale and reveals that the origin of calcium is local and linked with the weathering of the minerals of the surrounding rocks. The CaCO₃ precipitation occurs after vertical and lateral translocation in the slope system.     

The chemistry and mineralogy of some South Australian calcretes and associated soft carbonates and their dolomitisation

The chemistry and. mineralogy of calcretes and associated calcareous fine earth from the lower River Murray Basin and Yorke Peninsula in South Australia suggest that the carbonate material was deposited as a continuous unit and subsequently modified. At each site examined the Ca/Mg ratio progressively decreases with increasing depth, and where data are available, a similar decrease in the Ca/Sr ratio is observed. This sequence is independent of the hardness or physical condition of the carbonate. In some places the carbonate from the lowest horizons is nearly pure dolomite. The change in Ca/Mg ratio with depth is considered to be due to leaching, probably during periods of the past when the climate was less arid and the rainfall penetrated to about 2 m.

In the lower Murray Basin the Ca/Mg ratio of the surface calcrete horizon progressively increases from Tailetn Bend N to Sedan, and the silicate clay fraction changes from sepiolite at Tailem Bend, to palygorskite at Murray Bridge and illite at Sedan. The bedrock also varies, from magnesium‐rich amphibolites at Tailem Bend to granite at Sedan and, further, there is an apparent relation between the chemistry of the non‐carbonate fraction of the calcretes and the underlying rock. The original carbonate deposit probably formed in lakes of different chemistry, and has been modified by solution and precipitation.     

Carboniferous calcretes in the Canadian Arctic

Calcrete palaeosols have been found in the Upper Carboniferous Canyon Fiord Formation of southwestern Ellesmere Island, Canadian Arctic. These calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins, in which the tectono-sedimentary environment led to the deposition of five different sedimentary facies: (i) floodbasin sandstones; (ii) alluvial fan sandstones; (iii) alluvial fan conglomerates; (iv) braided fluvial sandstones; and (v) shallow marine limestones. Nodular/massive palaeosol profiles, consisting of cryptic, nodular, massive and laminar horizons, occur within the floodbasin sandstone and alluvial fan sandstone facies. Plugged palaeosol profiles, consisting of cryptic, plugged and laminar horizons, are restricted to the alluvial fan conglomerate facies. Massive/brecciated palaeosol profiles, consisting of cryptic, massive/brecciated and laminar horizons, occur essentially within the shallow marine limestone facies. The relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material: nodular/massive profiles are restricted to silicate-rich sandstone hosts, plugged profiles are restricted to carbonate-rich conglomerate hosts and massive/brecciated profiles are restricted to limestone hosts. Important relationships also exist between the maturity levels of nodular/massive profiles and sedimentary facies: profiles are generally mature in the floodbasin sandstones, invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones. These different maturity levels were probably controlled mainly by exposure time, vegetation and substrate composition.     

South African geography and post-apartheid reconstruction

South African geography and post-apartheid reconstruction     

Fissure calcretes in the arctic: a paleohydrologic indicator

The effects of modern climate warming are expected to be amplified in northern periglacial regions. Paleohydrological reconstructions in the arctic provide important insights into natural climatic variations during the Pleistocene and Holocene with which to compare predictions of modern impacts. Here the authors describe a new material, calcrete found in fissures in arctic carbonate terrains, which record hydrogeological and climatic conditions during climatic optimums in the north. ²³⁴U and ²³⁰Th activity ratios suggest growth during northern insolation maxima.These fissure calcretes strongly resemble biogenic calcite precipitates (endostromatolites), and grow on the external face within fissures in carbonate bedrock in periglacial environments. In thin section, samples are finely laminated and are composed of diverse fabrics including a uniformly present basal layer with regularly spaced columns from several microns to 3 cm in length. Their surface morphology is often preserved in the relief of the adjacent fissure face in a micro-lapiès texture strongly suggestive of a dissolution-reprecipitation mechanism whereby the fissure widens in lock-step with column growth. Their highly enriched δ¹³C (0 to +8.4‰) composition suggests that methanogenenic bacteria, supported by a substrate of soil-derived organic matter, control growth in a network of groundwater-saturated near-surface fissures extending to a depth of 3 to 5 m. Accordingly, δ¹⁸O holds valuable paleotemperature information.     

Sulphate generation in South African underground and opencast collieries

Deterioration in the water quality results from coal mining, and the significant impact on the environment in South Africa has been reported (Funke in SA Water, Sci Technol 15:115–144; 1983). Pyrite in with South African coal can be oxidised to yield acidic and high salinity water. This study deals with the rate at which sulphate could be produced for collieries under different conditions. Four collieries in the Witbank coalfield were selected for this investigation. The boreholes were investigated with an in-situ multi-parameter probe, and then sampled at different times with a specific depth sampler to determine the chemical composition of the water in the collieries. Water level measurements and volumetric calculations were done to determine influx rates. Daily sulphate generation rate for underground mining was found to be in the range of 0.4–2.7 kg/ha, depending on the availability of oxygen and the residence time of the water (the time available for neutralising agents present to react) in the mining void. The exclusion of oxygen due to flooding and the presence of calcareous carbonates influences the observed sulphate generation.     

Japan's hidden involvement in South African manufacturing

This paper examines the development of Japanese involvement in South African manufacturing. In the extensive literature dealing with the growth and spread of Japanese investment abroad the unique situation of Japanese companies in South Africa has been overlooked. Despite the introduction of a ban on direct investment in South Africa there is a considerable hidden Japanese presence in local manufacturing which resulted from licensing arrangements. In particular, Japanese enterprise is strongly represented in the South African motor-vehicles sector where it is at the forefront in innovating new flexible production systems.     

Diagenetically enhanced bedding in argillaceous platform limestones: stratified cementation and selective compaction

A field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the U.K. Lithologies include grainstones, packstones, wackestones and lime mudstones. The rocks are of Silurian, Carboniferous and Jurassic ages. Additional information was obtained from other limestones in the U.K., the U.S.A., Canada and continental Europe. The beds are parts of sequences composed of couplets of strata, fissile limestones alternating with hard limestones. In the fissile limestones the effects of mechanical compaction and pressure-dissolution have been concentrated, whereas in the hard limestones they are minimal or absent. Bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe. The features produced by pressure-dissolution are dissolution seams and fitted fabric: there are no stylolites. The original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed. The vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent. It is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed. Thenceforth, mechanical compaction and then pressure-dissolution were concentrated in the less cemented strata: these became the fissile limestones. Pressure-dissolution acted late in the diagenetic history and provided only an insignificant part, if any, of the carbonate for cementation. It is concluded that the orientation of beds (couplets) is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously. Similar couplets in platform limestones of the Mississippian and Pennsylvanian of the U.S.A. extend over thousands of square kilometres. The signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected. The presumed order of events was (1) accumulation of carbonate sediment, terrigenous clay and organic matter, (2) hydrodynamic reworking and bioturbation. the latter finally overprinting the former, (3) selective cementation of the more carbonate-rich strata yielding couplets, each consisting of a relatively well-cemented stratum and a poorly cemented stratum, (4) mechanical compaction concentrated in the less cemented strata, (5) pressure-dissolution concentrated in the same strata.     

Pyritized ooids,stringed ooids and reticulate tubular structures from within the black shale at Amjhor,Bihar, India

Carbonate ooids, notionally incompatible with an euxinic environment, and some curious reticulate tubular structures with fibrous carbonate walls are described from intraclasts occurring in Proterozoic black shales containing the pyrite deposit at Amjhor. While some of the tubes are visibly due to coalescence of ooids, others are interpreted as worm burrows on the basis of their strata-transgressive orientation, branching nature and larger diameter than those of the ooids.All stages of pyritization of ooids and tube-cores, displaying truncation of the growth-fabric of calcite grains by growing pyrite crystals, point to a late- or post-diagenetic age of pyritization. The clasts containing ooids and tubes are believed to have been transported from a shallow-bank and/or a nearshore environment into a lagoonal, euxinic basin. An environmental model, consistent with the observed data, is proposed to account for the ooid—black shale association.     

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information. Much of the work performed in this area has focused on either of two types of terrestrial carbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be a gradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes. Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period of sub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the water table, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within a previous host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs in a lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes may form on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short (even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do not form on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the development of thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of these carbonates.

The formation of these carbonates is controlled by palaeoenvironmental factors. Both commonly form in semi-arid climates. Arid climates are also suitable for calcretes, but sub-humid conditions are more suitable for palustrine carbonates. More indications of climatic conditions may be obtained through the analysis of the δ¹⁸O content of both calcretes and palustrine carbonates, and from the depth of the horizon containing carbonate nodules in pedogenic calcretes. Vegetation is also important in the formation of these types of carbonates. Data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the δ¹³C signal of the primary carbonates, which, in pedogenic carbonates, has also been used to estimate atmospheric pCO₂ during the Phanerozoic. These terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or accommodation rates, and therefore as indicators of different tectonic regimes.

Even though the study of these carbonates has notably increased in recent years, much less is known about them than about marine carbonates. Presently, there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins, with a need to include the carbonates analysed in this paper.     

Researching the South African tourism and development nexus

Under apartheid, tourism was essentially anti-developmental in focus and tourism research largely centred on the recreation of white South Africans. Since the 1994 democratic transition tourism has become recognised as an increasingly important sector for South Africa's economy and for achieving the government's goals for reconstruction and development. In this paper a review and critical analysis is presented of the changing directions and foci of tourism scholarship in South Africa. It is shown that South African scholars are beginning to respond to the challenges posed by tourism as a developmental focus. This revised version was published online in August 2006 with corrections to the Cover Date.     

Biogenic laminar calcretes: evidence of calcified root-mat horizons in paleosols

Laminar calcretes are described from the Lower Carboniferous of South Wales, the Upper Jurassic of southern England and the Upper Jurassic-Lower Cretaceous of northern Spain. They are interpreted as calcified root-mats (horizontal root systems) and are compared with other examples in the geological record and with possible modern analogues. All three occurrences consist of virtually identical, centimetre to decimetre-thick, locally organic carbon-rich, laminar micrites containing up to 50% by volume of millimetre-sized typically calcite-filled, tubular fenestrae set in an irregular but very finely laminated matrix. It is suggested that root-mat calcretes are probably very common in the geological record in peritidal, lacustrine margin and floodplain deposits, but owing to their crudely biogenic microstructure, they more closely resemble cryptalgal laminites than do other laminar calcretes. The recognition of such root-mat calcretes in sedimentary sequences not only provides evidence of subaerial exposure and vegetation cover but can also indicate positions of palaeo-water-tables in certain circumstances.     

The African Renaissance and its relation to the geosciences: a South African perspective

Implicit in the African Renaissance is the synergy between government, the private sector, the educated minority and the disadvantaged majority. For this concept to work, belief and commitment must arise first from the African individual, whatever his or her potential contribution may be. The geosciences in South Africa provide a currently vibrant example of such cooperation, which has the potential to contribute significantly to the upliftment of the country and its neighbouring states. Based largely on personal interviews with various role players, from the Presidency of South Africa, through ministerial levels, the corporate sector and down to the individual, we present a spectrum of viewpoints and initiatives which are starting to result in practical implementation of the African revival. An end to conflict and xenophobia, the entrenchment of democratic government and corporate expression of the entrepreneurial spirit are essential to provide the framework within which the individual African can become a “Renaissance Man or Woman”.     

Garnet peridotite xenoliths in South African kimberlite pipes and their petrogenesis

Whole rock analyses are presented for nine garnet peridotite, two garnet pyroxenite, one garnet free peridotite and two highly altered garnet peridotite xenoliths from South African kimberlite pipes. Garnets from eight of these samples have also been chemically analysed, while olivine compositions in the twelve fresh samples have been estimated by X-ray diffraction. The chemistry and mineralogy of these specimens are compared with the results of previous studies, and the controversies concerning the petrogenesis of these xenoliths are discussed in detail.     

Authigenic pyrite and gypsum in South West African continental slope sediments

An unusual association of authigenic pyrite and authigenic gypsum has been found in silty clays recovered from the South West African continental slope. Nannofossil content suggests that the sediments are Upper Miocene-Lower Pliocene in age. Pyrite occurs as (1) granular masses, (2) ‘worm’tubes, (3) foraminiferal infillings, and (4) framboids. Gypsum occurs as euhedral single or twinned crystals of selenite. ‘Worm’tubes and foraminiferal infillings of pyrite are partially or completely enclosed in some gypsum crystals. Electron microprobe analyses show a relatively high concentration of manganese in both the granular masses and tubes. Present-day waters off this coast are dominated by the upwelling of cold, nutrient-enriched waters (the Benguela Current). These rich waters support an enormous population of plankton. Death and decomposition of these plankton consume oxygen, thereby creating a belt of anaerobic sediments close to shore. An Upper Miocene-Lower Pliocene regression (Dingle & Scrutton, 1974) lowered sea level and shifted an older analogue of this upwelling zone seaward, eventually establishing an anaerobic environment in places on the present continental slope. Anaerobic bacteria thrived in these conditions. They reduced SO₄ dissolved in sea water, initiating the formation of H₂S. The H₂S reacted with iron minerals present in the sediment to form FeS. Addition of elemental sulphur produced pyrite. This strongly reducing, low pH, environment became saturated with calcium obtained by the dissolution of planktonic calcareous organisms. Gypsum was precipitated once the product of the concentrations of dissolved calcium and SO₄ exceeded the gypsum solubility product.