A depositional model for the terminal Neoproterozoic–Early Cambrian Ara Group evaporites in south Oman

Abstract Six evaporite–carbonate sequences are recognized in the terminal Neoproterozoic–Early Cambrian Ara Group in the subsurface of Oman. Individual sequences consist of a lower, evaporitic part that formed mainly during a lowstand systems tract. Overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts. Detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment, source of brines and basin evolution. At the beginning of the evaporative phase (prograding succession), a shallow-water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas. Minor amounts of highly soluble salts locally record the last stage of basin desiccation. This gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow-water halite from shallow-water sulphate, and shallow-water sulphate from relatively deeper water carbonate respectively. These surfaces record repeated flooding of the basin, dissolution of evaporites and stepwise reduction in salinity. Final flooding led to submergence of the basin and the establishment of an open-water carbonate ramp. Marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines. The Ara Group sequences represent a time of relatively stable arid climate in a tectonically active basin. Strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres, while tectonic barriers simultaneously provided the required restricted conditions. Subsidence allowed evaporites to blanket basinal and platform areas. The study suggests a deep-basin/shallow-water model for the evaporites.     

Zur Entwicklung von Riftsystemen im Frühstadium (Hebung — Spaltung — Vulkanismus — Spreading) am Beispiel der Afar Senke

A combination of palaeomagnetic, seismological, gravitational, aeromagnetic and geochemical observations, as well as geological and regional considerations are strongly indicative of anticlockwise rotational movements of the Danakil Alps and formation of new oceanic crust in the Northern Afar Triangle. The decreasing amount of spreading in the Southern Red Sea is compensated by en chelon crustal spreading (formation of oceanic crust in a continental environment) in the Danakil-Afar Depression. Here, the geophysical properties are generally intermediate between the more typical continental (Ethiopia) and oceanic (Red Sea, Gulf of Aden) data. Such intermediate type crust is proposed to be caused by “oceanization” of formerly continental crust, i. e. fragmentation and basification through massive dyke injections (mantle diapirism). The structure and evolution of the wider Afar Triangle, East-African Rift System, Red Sea and Gulf of Aden are used to derive a model for possible stages during initial continental break-up and compared with selected, similarly structured parts of the n-Atlantik. The continental break-up probably develops in the following stages: 1. general uplift associated with surface fracturing above an asthenospheric diapir (uplift), 2. development of linear “Scheitel”-Grabensystems along the crest of the uplift or uplift chains (rupture), 3. graben with (contaminated) volcanism stage (volcanism), 4. “oceanization” of the developing depression through fragmentation and basification by massive oceanic and/or contaminated dyke-injections of the former continental crust along several sporadically active lineaments, 5. “crustal spreading” on land or concentration of mantle derived, oceanic crust-injections along one major lineament in a dry, continental environment, 6. “evaporit-stage of sea-floor spreading” with sporadic seawater connections to an open marine basin and 7. “ocean-floor spreading” in the deep-sea environment of advanced oceanic troughs. The derivation of these stages basically involves the addition of “sea-floor spreading” processes (oceanization, crustal-, sea- and ocean-floor spreading) to the well known sequence: Hebung — Spaltung — Vulkanismus (Cloos, 1939) and relate it to mantle-diapirism processes. All the above stages are recognizable along the Afro-Arabian Rifts and seem to have morphological equivalents in the Atlantic.     

Über sprunghafte Änderungen von Funkpeilungen nach Consol-Funkfeuern bei Nacht

Zusammenfassung Wenn man Consol-Funkfeuer unabhängig von ihrer Eigenschaft als Leitstrahlsender mit einem Rahmenpeiler, insbesondere mit Sichtpeilapparatur, beobachtet, ergeben sich bei Nacht während der Umtastperiode häufig verschiedene Peilrichtungen für die beiden Tastzustände der Consol-Aussendung. Dadurch entstehen für den Beobachter am Sichtfunkpeiler sprunghafte Änderungen der Peilanzeige, die im Rhythmus der Umtastung erfolgen. Es wird gezeigt, daß sich dieser Effekt durch die Überlagerung der Bodenwelle und einer anomal polarisierten Raumwelle, die am Ort des Peilers auftritt, erklären läßt.

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On sudden changes in radio direction finding indications of Consol stations by night

Summary Regardless of their character as radio range beacons Consol stations may be observed with a normal radio direction finder. Using a direction finder with loop aerial system it will be noticed — particularly when a cathode ray direction finder is used — that by night during the dot-dash-period of the Consol transmission different bearings for the dots and for the dashes are often observed. Hence, the operator at the visual direction finder perceives sudden changes of the bearing indications occurring in the dot-dash-rhythm. It is shown that this effect can be explained by the superposition of the ground-wave and an anomally polarized sky-wave, both being present at the place of the direction finder.

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Sur des changements subits des relèvements radiogoniométriques observés aux radiophares Consol pendant la nuit

Résumé Indépendamment de leur qualité de radiophares directifs, les radiophares Consol peuvent être observés, comme des transmetteurs normaux, à l'aide d'un radiogoniomètre à cadre. Pendant la période de renversement des courants d'antennes Consol, on remarque souvent de nuit — surtout si l'on se sert d'un radiogoniomètre à lecture visuelle — différents gisements de relèvements dans les deux phases de l'émission Consol. L'observateur devant le goniomètre visuel remarquera, donc, que les indications radiogoniométriques sont soumises aux changements subits suivant le rhythme des renversements de la manipulation Consol. On montre que cet effet est dû à la superposition de l'onde directe par une onde réfléchie et anomalement polarisée se manifestant auprès de la place du goniomètre.

     

Diagenetische Veränderungen in den Rotliegend Sandsteinen im NW-Deutschen Becken

Zusammenfassung Mit seinen auf 3000 m bis 6000 m versenkten Schichten bildet das Rotliegende in Nordwestdeutschland eine Gasprovinz, deren Lagerstätten-Inhalte aus dem Karbon im Liegenden abgeleitet werden. Die Reservoire sind Sandsteine, die mit Tonsteinen wechsellagern. Sie wurden nach einer Phase reger vulkanischer Aktivität des untersten Perm als red bed-Sedimente in einem ariden Klima abgelagert.Das Untersuchungsgebiet ist der Raum zwischen Weser und Elbe. Es wurde versucht, eine grundsätzliche Teufen/Porositäts-Variation der Sandsteine zu erarbeiten, was wegen der komplexen Gefügebedingungen sehr schwierig ist. In diesem Zusammenhang ergab sich die Markierung des Endes der mechanischen Kompaktion bei einer Porosität um 22% für feinkörnige Sandsteine.Die in den Poren (epigenetisch) ausgefällten allochemischen Zementminerale zeigen Merkmale einer synsedimentären bis frühdiagenetischen Kristallisation. Nicht zuletzt kommt das in der regionalen paläogeographischen Anordnung der Zementminerale Karbonat, Sulfat und Halit zum Ausdruck, in dieser Reihenfolge zunehmend evaporitischen Charakters vom Beckenrand im Süden zur Beckenachse im Norden hin bevorzugt verteilt.In den Intergranularen der Sandsteine sind authigene Tonminerale weit verbreitet, besonders als Coating- und Overgrowth-Gefüge. Die Reaktion von Feldspat und die gesteigerte Authigenese von Tonmineralen in einem Gebiet, in dem die gewöhnlich anzutreffenden Effusivgesteine im Liegenden fehlen, werfen ein Licht auf die Herkunft dieser Tonminerale und auf die Migrationsvorgänge.

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With its sequences attaining depths of 3000 to 6000 m, the Rotliegend in NW Germany forms a gas province whose reservoir contents are derived from underlying Carboniferous. The reservoirs consist of sandstones alternating with shales, which were deposited in an arid climate as red bed sediments after a phase of intensive volcanic activity in the lowermost Permian.The investigated area is situated between the rivers Weser and Elbe. It was tried to obtain a basic depth/porosity variation of the sandstones, a rather difficult attempt because of the very complex fabric conditions. In this connection the end of the mechanical compaction could be marked at a porosity of about 22% for fine-grained sandstones.The allochemical cement minerals precipitated (epigenetically) in the pores show features of a synsedimentary to early-diagenetic crystallisation. This is not least reflected in the regional paleogeographic distribution of the cement minerals carbonate, sulfate and halite which, in the order of their listing, become more evaporitic, with increasing proportions from the basin margin in the south towards the basin axis in the north.In the intergranulars of the sandstones, authigenic clay minerals occur frequently, in particular in the form of coating and overgrowth. Reaction of feldspar and advanced authigenesis of clay minerals in an area where the generally occurring effusive rocks are missing in underlying formations give an idea of the genesis of clay minerals and of migration processes.

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Résumé Le Rotliegend constitue au nord-ouest de l'Allemagne une province à gisements de gaz naturel. Les réservoirs consistent en grès alternant avec des argiles, qui ont subi un affaissement de 3000 à 6000 m. Ces couches sont des »sédiments rouges« déposés dans un climat aride après une phase d'activité volcanique intense dotant du permien le plus inférieur. Le gaz provient du carbonifère.La région étudiée est située entre le Weser et l'Elbe. On a eu en vue la variation fondamentale entre la profondeur et la porosité des grès, tâche assez difficile en raison des conditions de texture très complexes. A cette occasion on a pu enregistrer la fin de la compaction mécanique avec une porosité de 22% environ pour les grès fins.Les minéraux allochémiques du ciment préciptés (épigénétiquement) dans les pores montrent les caractéristiques d'une cristallisation synsédimentaire à protodiagénétique. Cela se manifeste aussi dans la distribution paléogéographique régionale des minéraux du ciment, carbonate, sulfate et halite, qui deviennent dans cet ordre, de plus en plus évaporitiques à partir du bord méridional du bassin vers son axe; en d'autres termes les minéraux les plus évaporitiques sont déposés vers le nord.Dans les pores des grès, les minéraux d'argile authigènes sont très répandus, en particulier sous forme d'une pellicule enveloppante et d'enrobement. La réaction de feldspath et l'authigenèse avancée des minéraux d'argile dans une région où les roches effusives normalement présentes sont absentes les couches inférieures, permettent de se faire une idée de la genèse des minéraux d'argile et des processus de la migration.

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- , 3000 6000 , , , , , . , . (red bed-Sediments) . . , . 22 % . , , - . — , — , . , , . , , . .

     

Geology of massive sulphide deposits in the Spanish-Portuguese Pyrite Belt

The Spanish-Portuguese Pyrite Belt covers a large area in the SW part of the Iberian Peninsula from Seville to the westcoast of Portugal. Total reserves of aprox. 1.000 million tons of massive sulphide ores have an average content of 46% S, 42% Fe, and 2–4% Cu+Pb+Zn. The stratiform sulphide deposits and accompanying manganese mineralizations are of synsedimentary-exhalative origin. They occur in a Lower Carboniferous, geosynclinal, volcanic-sedimentary rock sequence, strongly folded during the Hercynian Orogeny. A brief outline of the regional geology of this ore province is given, and the geology of three mining districts is described: Lousal (Portugal), La Zarza and Tharsis (Huelva Province, Spain). A close relationship between sulphide and manganese ores with the submarine, acid alkaline volcanism is emphasized. Solfataric activity is responsible for the formation of sulphides in the final stages of volcanic extrusions. The ore concentration in big deposits (ore-lenses with up to 100 million tons of massive sulphides) has been due to inflows of sulphide muds and/or detrital sulphides into newly formed depressions of a contineously changing seafloor topography due to volcano tectonic movements.     

Ein Beitrag zur Isotopengeochemie des Kohlenstoffs in magmatischen Gesteinen

Several arguments indicate that the mean carbon isotopic composition of the earth's crust and the upper mantle should be around-7‰. This agrees quite well with a balance calculation (Table 7) and with what we know about the carbon isotope composition of carbonatites and diamonds. Since fractionation factors decrease with increasing temperatures, the differences in isotopic compositions found in igneous rocks might be expected to be relatively slight and not to differ very much from the mean δ-value for the earth's crust. This also applies to the elements oxygen and sulfur, and to a lesser extent even for hydrogen, but not for carbon. Hoefs (1965) has shown that all igneous rocks contain carbon in at least two different forms:

1. an oxidized form mainly as carbonate and/or as CO₂ (in fluid and gaseous inclusions) in variable concentrations between <100 ppm and several thousand ppm CO₂, and
2. a reduced form with a relatively constant concentration around 200 ppm C. To 1). If the carbonate were of primary magmatic origin, we should expect, in analogy to carbonatites or to some hydrothermal carbonates, a δ ¹³C-value around-7 and a δ ¹⁸O-value between ?15 to ?25‰ relative to PDB, but on the contrary, the variable δ ¹³C- and the relatively heavy δ ¹⁸O-values make it seem probable that the carbonate is not of pirmary magmatic origin, but of secondary, maybe groundwater origin. This does not exclude the possibility that in some cases there may also be some carbonate which is of primary magmatic origin. To 2). If the reduced carbon found in igneous rocks is indigenous to these specimens, theoretically it may occur as elemental carbon (graphite), as carbides, and as organic compounds or as all three combined together.

This reduced carbon has a very light and fairly constant isotopic composition between ?24 and ?28‰ relative to PDB in all igneous rock types. There are two very different possible explanations for these values. The first and simplest one is that this carbon is also of secondary origin, or in other words of biogenic origin—some kind of assimilation of sedimentary organic material. But since this carbon is very evenly distributed, this means that all igneous rocks with a very small, but not negligible, porosity and permeability are impregnated by surface waters containing biogenic-derived organic substances in a concentration of around 200 ppm C. Since bore samples have also been analyzed, this also means that these waters penetrate into igneous rocks even at greater depths. Due to certain similarities in carbon isotopic composition found in extraterrestrial material, in meteorites and in lunar rocks (Table 9), I favor the second possibility of explaining the rather light δ ¹³C values: Several mechanisms have been postulated for the formation of organic matter in our solar system (Fischer-Tropsch type synthesis, Miller-Urey reactions etc.). Evidence supporting the hypothesis of inorganically formed organic matter on the earth has accumulated since Miller (1957) first demonstrated the synthesis of organic compounds from methane, ammonia and water. It is postulated that photosynthesis is not the only process leading to isotopically light carbon, but that some of these reactions (perhaps Fischer-Tropsch type synthesis) may also yield to isotopically light carbon. In addition to these data, some gaseous CO₂-samples of probably volcanic origin from Germany have been analyzed. The CO₂ discharged in areas of ancient volcanic activity shows δ ¹³C value between ?2 and ?5‰, typical for geothermal areas (e.g. Yellowstone, New Zealand). The CO₂ found in inclusions in evaporites, some of them near basaltic dikes, shows a strikingly different δ ¹³C composition (between ?15 and ?25%.) comparable to CO₂ sampled over liquid Hawaiian lavas. On the basis of the isotope-exchange reaction CH₄+2H₂O ? CO₂+4H₂, temperature seems to be the most important parameter, being responsible for the observed differences in isotopic composition.     

Low-Frequency Variations, Chandler and Annual Wobbles of Polar Motion as Observed Over One Century

Polar motion data are available from the mid-19th century to the present. Basedon time series with a variety of sampling intervals (monthly, 0.05-year, 5-day anddaily), we have separated the low-frequency terms by low-pass filtering and theChandler and annual terms by recursive band-pass filtering of the pole coordinates.Using a simple unweighted least-squares fit to the filtered low-frequency terms, thelinear trends of the rotation pole were estimated. Assessing the estimates based onintercomparisons, the most reliable trend estimate was found. Using a Fast FourierTransform, we have computed the prograde, retrograde and total amplitude spectraof the low-frequency part of polar motion in order to reveal the long-periodic signals.The characteristics and time evolution of the Chandler and annual wobbles aredescribed by changes in their parameters (radii, directions and period lengths) overone century.     

57Fe-Mössbauer spectroscopy on natural eosphorite-childrenite-ernstite samples

Samples of the eosphorite-childrenite [(Mn²⁺, Fe²⁺)AlPO₄(OH)₂H₂O] series from Divino das Laranjeiras and Araçuaí (Minas Gerais State) and Parelhas (Rio Grande do Norte State) pegmatites have been investigated by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy at 295 and 77 K. The Mössbauer spectra of ernstite [(Mn²⁺, Fe³⁺)AlPO₄(OH)_2-xO_x] showed the existence of ferric ions in both A and B sites, whereas ferrous ions seem to be located exclusively in the A site. Nonoxidised samples show ferrous ions located in both sites, and no Fe³⁺ could be detected. The interpretation of the Mössbauer spectra of both, oxidised and nonoxidised samples, is difficult because the hyperfine parameters of these minerals are rather similar, rendering it difficult to make proper site assignments.