Late-Precambrian correlation and the relationship between the Damara and Nama Systems of South West Africa

Recent findings about the late-Precambrian stratigraphic succession in the areas between the Orange River and the Klinghardt Mountains and south of Windhoek throw new light on the relationship between the Damara and Nama Systems.In the Orange River area the formations of the Gariep Group are unconformably or pseudo-conformably overlain by the Nama System. In central and northern South West Africa a similar relationship is exhibited by the Nosib Group and overlying Damara System.Pronounced similarities between the sedimentation and deformational history of both Nosib and Gariep indicate a probable correlation of the two groups and this result is also supported by radiometric ages.It follows that Damara and Nama are most likely either completely or partly coeval. The latter may represent a shelf facies of the former geosynclinal succession.The Gariep and Nosib deformation apparently started with the subsidence of their respective troughs and may have lasted from approximately 850 m. y. to 600 m. y. This tectonic event is older than the Damara Orogeny (550 to 450 m. y.) and may be time-equivalent to the Katanga Episode.Two late-Precambrian glaciations were widespread in South West Africa and their sedimentary deposits are important for the correlation of Gariep/Nama with Nosib/Damara.

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Zusammenfassung Neue Ergebnisse über die spätpräkambrische Gesteinsabfolge im Gebiet des Orange-Flusses und im Rehoboth-Distrikt südlich von Windhoek sind von großer Bedeutung für das Altersverhältnis zwischen Damara- und Nama-System.Im Orange-Gebiet werden die Formationen der Gariep-Gruppe diskordant oder pseudo-konkordant vom Nama-System überlagert, während in Zentral- und Nord-Südwest-Afrika das Damara-System ebenfalls diskordant oder pseudokonkordant auf die Nosib-Gruppe folgt. Sedimentations- und deformationsgeschichtliche Ähnlichkeiten deuten auf eine Korrelation zwischen Nosib und Gariep hin, und diese Interpretation wird durch radiometrische Altersbestimmungen unterstützt. Daraus folgt, daß Damara und Nama wahrscheinlich einander entsprechen, wobei die Nama-Sedimente als Plattform-Fazies der geosynklinalen Damara-Serien anzusehen sind.Die Gariep-Deformation begann schon mit dem Einsinken des Gariep-Ablagerungstroges und dürfte von ca 850-600 M. J. angedauert haben, wobei es stellenweise zur Granitisation geeigneter Sedimente und Laven kam. Diese Orogenese ist älter als die von ca. 550-450 M. J. dauernde Damara-Phase und scheint zeitlich der auch im Kaokoveld nachgewiesenen Katanga-Episode zu entsprechen.Zwei spätpräkambrische Vereisungsperioden sind weit verbreitet in Südwest-Afrika, und ihre Glazialablagerungen sind ein wichtiges Hilfsmittel für die Korrelation der Gariep-Nama-Abfolge mit der Nosib-Damara-Stratigraphie.

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Résumé De nouveaux résultats concernant la série de roches du Précambrien récent de la région de l'Orange et du district de Rehoboth au sud de Windhoek se sont révélés de haute importance pour l'étude de relation d'âge entre les systèmes Damara et Nama.Dans la région de l'Orange, le système Nama repose de façon discordante ou pseudo-concordante sur les formations du groupe Gariep; de même, en Afrique Sud-Ouest, centrale et le système Damara prend une position analogue par rapport au groupe Nosib. Des analogies dans l'histoire de la sédimentation et de la déformation permettent de réaliser une corrélation entre le Nosib et le Garieb; des déterminations de l'âge par méthodes radio-électriques appuient cette interprétation. Par conséquent, le Damara et la Nama sont vraisemblablement en rapport de conformité et les sédiments du Nama sont à considérer comme les faciès des séries géosynclinales du Damara.La déformation du groupe Gariep commença déjà avec l'affaissement des auges de dépôt et a probablement duré de 850-600 m. a.; à cette occasion se produisit, par endroit, la granitisation des sédiments et des laves appropriés. Cette orogénèse est plus ancienne que la phase du Damara qui dura de 550-450 m. a. et semble correspondre dans le temps à l'épisode Katanga également dans le Kaokoveld.Deux périodes de glaciation du précambrien récent sont très largement répandues en Afrique du Sud-Ouest et leurs dépôts glacières sont des moyens très importants pour la corrélation de la série Gariep-Nama avec la stratigraphie du Nosib-Damara.

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Damara Nama Rehoboth Windhoek. Gariep , - , , - Damara , Nosib. Nosib Gariep . , Damara Nama, , , Nama , Damara. Gariep Gariep , , 850 600 . . Damara, 550-450 , , , Katanga, Kaokoveld. - Gariep-Nama — Nosib-Damara.

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Expanded and slightly modified English version of a paper read for the author by Prof. H.Martin at the 61st Annual Congress of the Geologische Vereinigung and entitled: Spätpräkambrische Orogenesen und das Verhältnis zwischen Damara und Nama System in Südwest Afrika.     

Der Schwefelgehalt in Knochen und Zähnen rezenter und fossiler Organismen

The sulfur content of teeth and bones has been determined by a microprobe. Recent samples only contain about 0.05 weight percent of sulfur, independent of a marine or terrestrial origin of the organisms. The total sulfur of these samples is partly related to the organic matter and partly to SO ₄ ^2– -ions replacing PO ₄ ^3– -ions in the crystalline matter of the skeletons. In the living organisms this matter consists of octacalciumphosphatecarbonate (OCPC) (Hayek, 1967), which is transformed only to a small degree into apatite. During diagenesis the process of apatite-crystallisation proceeds, and more sulfate enters the lattice of the phosphate, which leads to an enrichment of sulfur up to about one percent by weight. There is no relation observed between the concentration of sulfur and the age of the organisms. Influences of the environment of diagenesis on different minor compounds entering the phosphates of bones and teeth in different state of composition have been evaluated. As a result the possibility of dating fossils by minor element analysis (F, Mn, SO ₄ ^2– etc.) must be questioned. Differences between freshwater and marine invironment of deposition cannot be observed in the total sulfur of diagenetic phosphates.

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Ich danke Herrn Prof. Dr. K. H. Wedepohl für sein Interesse an der Durchführung dieser Arbeit und für seine aktive Unterstützung. Für die Überlassung von Proben danke ich ganz besonders Herrn Prof. Dr. O. H. Walliser, Herrn Dr. Frank Westphal, Mrs. L. S. StevenSon, Herrn Prof. Dr. H. W. Flügel, Herrn Prof. Dr. W. G. Kühne und Herrn Prof. Dr. K. Krömmelbein; für klärende Diskussionen Herrn Prof. Dr. O. H. Walliser, Herrn Prof. Dr. Dr. h. c. C. W. Correns, Herrn Prof. Dr. K. H. Wedepohl, Herrn Priv.-Doz. Dr. med. Kämmerer und Herrn Priv.-Doz. Dr. M. LieflÄnder; für die Übermittlung von Literatur und Hinweisen Herrn Priv.-Doz. Dr. H. Newesely; und für ihre Unterstützung und Diskussionsbereitschaft allen Mitarbeitern des Geochemischen Institutes der Universität Göttingen.     

Zur Genese des elementaren Schwefels im Gips von Weenzen (Hils)

Zusammenfassung In der Hilsmulde tritt bei Weenzen ein Salzstock durch Kreide- und Tertiärsedimente zutage. Dieser Gips führt an einigen Stellen elementaren Schwefel. Durch Isotopenanalysen wurde festgestellt, daß der elementare Schwefel gegenüber dem Ausgangssulfat an³⁴S verarmt und der Schwefelm Restsulfat entsprechend angereichert wurde. Schwefelfreie Bereiche des Gipses zeigen den für Zechsteinevaporite üblichen ³⁴S-Wert. Der Isotopenunterschied von durch-schnittlich 26% deutet auf bakteriellen Ursprung des Schwefels, weil eine anorganische Reduktion des Sulfates unter den vorauszusetzenden Temperaturen nicht möglich ist. Es wird angenommen, daß während des Aufstieges des Salzstockes ein geringfügiger Zutritt von Erdöl Lebensbedingungen für anaerobe Bakterien schuf, was zur Reduktion des Sulfats zu H₂S führte. Der dabei freiwerdende Schwefelwasserstoff wurde durch Reaktion mit SO ₄ ^2– in elementaren Schwefel umgewandelt. Da sich die Reduktion in den einzelnen Bereichen des Gipses sicher in Abhängigkeit von der Intensität der Bakterientätigkeit vollzog, hat sie primär wohl nicht zu homogener Verteilung der Schwefelisotope im Schwefelwasserstoff bzw. im Restsulfat geführt. Während für den elementaren Schwefel die nachträgliche Homogenisierung wegen der Beweglichkeit des Schwefelwasserstoffes verständlich ist, kann sie im Restsulfat so erklärt werden, daß dieses während der Neokomtransgression von oben her stärker der Lösung ausgesetzt war und umkristallisierte, wodurch sich die anfangs wahrscheinlich stärkeren Unterschiede in der Isotopenzusammensetzung des Gipses verwischten.

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Native sulphur is encountered occasionally within the gypsiferous cap-rock of a Permian salt dome in the Hils syncline, in the environs of Weenzen (see map, Fig. 1). A number of sulphur isotope analysis demonstrated that the native sulphur originated out of the primary sulphates, had been depleted in³⁴S, causing a relative enrichment of the remaining sulphate in³⁴S. Those parts of gypsum which are entirely devoid of native sulphur, exhibit a³⁴S value typical of Zechstein (upper Permian). The average enrichment of³²S in native sulphur of about 26% suggests its bacteriological origin, since under the temperatures usually prevailing during the origin of a salt dome, a sulphate cannot be reduced by petroleum. During the rise of the salt dome, the sulphates became contaminated with a small amount of petroleum as a result of which a congenial ecological condition for the development of the sulphate reducing bacterias was produced. The³⁴S depleted H₂S, produced by these bacterial, reacted with the sulphate, thus giving rise to native sulphur (Feely andKulp, 1957).The activity of the sulphate-reducing bacterial leads to different stages of fractionation in the different biochemical environments owing to the varying intensity of the bacterial growth, as well as to the variable ratios of reduced sulphate to primary sulphate. The highly mobile H₂S was homogenized already before the precipitation of native sulphur, whereas the homogenisation of the sulphate is viewed in the solution and recrystallisation processes taking place just below the Neocomian transgression horizon.

     

Observations of Infrasound and Subsonic Disturbances Related to Severe Weather

Summary During the past 10 years the Geoacoustics Group of NOAA's Wave Propagation Laboratory studied travelling low-frequency pressure variations related to thunderstorms and severe weather. Two general categories of waves were associated with severe weather conditions: 'subsonic' pressure disturbances and infrasonic waves with acoustic velocities. The low-frequency pressure variations were measured at the Earth's surface using microphone arrays located at times thousands of kilometres from the severe-weather disturbance. The radiated infra- sound was related to thunderstorms penetrating the tropopause and spectral analyses were performed on several signals. Possible practical applications to storm warning and classification are discussed for both infrasound and 'subsonic' pressure disturbances. Past measurements of these signals are reviewed.     

African, southern Indian and South American cratons were not part of the Rodinia supercontinent: evidence from field relationships and geochronology

We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ˜1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ˜1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa.Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goíás Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil.Furthermore, there is evidence for extensive granitoid magmatism in the period ˜840 to <600 Ma whose predominant calc-alkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern hemisphere, such as the São Francisco/Congo, Kalahari and Rio de La Plata cratons, as well as the Borborema/Trans-Saharan, Central Goiás Massif and Paraná blocks. Moreover, many extensional tectonic events detected in the southern hemisphere continental masses, but also many radiometric ages of granitois that are already associated with the process of amalgamation of Gondwana, are comprised within the 800–1000 age interval. This seems incompatible with current views on the time of disintegration of Rodinia, assumed to have occurred at around 750 Ma.     

Seismic coupling and structure of the Hellenic subduction zone in the Ionian Islands region

The western Hellenic arc has been commonly considered as a largely aseismic subduction zone, from the comparison of a small rate of shortening derived from the seismic moment release, with a large rate of convergence inferred from geology. Complete seismic coupling would instead be expected from models that consider a control by plate tectonic forces, because of the trenchward velocity of the Hellenic–Aegean upper plate now confirmed with GPS measurements. In the region of the Ionian Islands, a subduction interplate boundary has been recently imaged and its seismogenic downdip width suggested to be moderate, from reflection seismic profiling and local earthquake tomography. In the appropriate model for such an earthquake source region, which considers a single interplate fault and takes into account these features, the moderate seismic moment release is found consistent with complete seismic coupling of this subduction. The shallow downdip limit of the seismogenic zone can be interpreted as due to the interplate boundary being overlain there by the ductile deeper crust of the orogenically thickened Hellenides.     

Structure, Composition and Evolution of the South Indian and Sri Lankan Granulite Terrains from Deep Seismic Profiling and Other Geophysical and Geological Investigations: A LEGENDS Initiative

We here argue the need for a multidisciplinary, integrated geophysical/geological survey across southern India and Sri Lanka, part of the largest exposed Neoproterozoic high-grade metamorphic terrain in the world. The purpose is to stimulate deep seismic profiling and other geophysical and geological investigations in order to better understand the structure, composition and evolution of a keystone of Precambrian continental crust and the role it played in the amalgamation and subsequent break-up of the Gondwana supercontinent. Of particular importance is that this lithospheric fragment records a 2 billion year long history of magmatism, sedimentation, deformation and metamorphism and now exposes at the surface rocks that once formed at depths of 30 km or more and that record unusually high metamorphic temperatures of up to 1000°C. Modern geophysical data should help to determine what lies underneath these high-T surface rocks and whether the terrain consists of structural distinct subunits separated by (now hidden) sutures. It is likely that the crustal-scale structures generated during the formation of the SGT may extend to other, now dispersed, segments of the Gondwana landmass and their identification in geophysical data will provide a fresh basis for reconstructing the process of supercontinent assemblage and break-up.     

Shear Wave Anisotropy of the Upper Mantle Beneath the Region from Tingri of Tibet to Golmud of Qinghai

Measurements of shear wave splitting at 43 three-component seismic stationsshow very big difference in anisotropy on both sides of the Indus-Yarlung Zangbo suture(ITS), but little difference on both sides of the older Bangong-Nujiang suture (BNS) and theJinsha River suture (JS) to its north. Obvious discrepancy exists between the anisotropy direc-tion and the superficial tectonic trends, which is not explicable directly by the coherent uppermantle deformation usually supposed to occur in consistency with the trend of a collisional belt.On the other hand, strong spatial relationships are observed from the anisotropy results, such asthe orthogonal directions of anisotropy on both sides of ITS and the good correlation betweenthe region of larger magnitude of anisotropy and the zone of inefficient Sn propagation ofQiangtang as well as the systematic rotation of the directions of anisotropy, which should testifysome much more complicated aspects of the continental convergence mechanism. To the best ofour data, we tend to suppose that the Qinghai-Tibet plateau might result from a mechanismcomplicated by the coexistence of Argand's underthrusting and Dewey's diffuse deformation.     

Late Pliocene to Early Pleistocene productivity in the Arabian Sea

The stable isotopic compositions of organic C and N and the organic carbon (OC) content of the Oman Margin (ODP Site 724) sediments, ranging from Pliocene to Early Pleistocene in age, are used to infer palaeoproductivity in the area. At the boundary between the Pliocene and the Pleistocene, the amount of organic matter preserved is low compared to other periods before and after this transition period. This low OC content is associated with depleted N and C isotope values. This may indicate low primary productivity as a result of a diminished supply of nutrients caused by weak coastal upwelling. Higher amounts of OC, in conjunction with enrichment in the isotopic compositions of both organic C and N before and after the Pliocene-Pleistocene transition period, are likely to be a consequence of an increase in primary productivity, which resulted from a higher nutrient supply. However, a higher amount of OC may have resulted from enhanced preservation. The enhanced preservation may have resulted from an intrusion of highly saline waters from the Res Sea and Persian Gulf and/or restricted circulation.     

On the application of geophysics in the exploration for copper and chrome ores in Albania1

Some generalized results of geophysical exploration for copper sulphide and chromite ores in Albania are presented. The most important geophysical methods used are electrical prospecting, gravity, magnetics and electromagnetics. Physical properties of the ores, genesis and geological problems to be solved have determined the proper choice of any of these methods in the complex exploration.