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Luanda, Angola

Allen P.A.,Imperial College London | Leather J.,Tullow Oil | Brasier M.D.,University of Oxford | Rieu R.,TOTAL E and P Nederland | And 4 more authors.
Geological Society Memoir | Year: 2011

The Abu Mahara Group (c. 725-<645 Ma) of the Huqf Supergroup in the Jabal Akhdar of northern Oman hosts two glacial successions in the Ghubrah and Fiq formations, separated by the <50-m-thick volcanogenic Saqlah Member. The >400-m-thick Ghubrah Formation is dominated by distal glaciogenic rainout diamictites, laminites and turbiditic siltstones, whereas the <1.5-km-thick Fiq Formation exhibits a cyclical stratigraphy of proximal and distal marine glaciogenic facies, and non-glacial sediment gravity flow and shallow marine facies. The Fiq Formation is overlain by a transgressive, isotopically light carbonate known as the Hadash Formation. A tuffaceous ash interbedded with glacial diamictites of the Ghubrah Formation in Wadi Mistal has yielded a U-Pb zircon age of 713.7±0.5 Ma. The Fiq Formation contains detrital zircons as young as 645 Ma. The use of the CIA (Chemical Index of Alteration) shows the Fiq Formation to be climatically cyclic, with alternations of high and low chemical weathering of contemporary land surfaces driven by phases of glaciation and deglaciation. The transgression into the post-glacial Masirah Bay Formation is marked by a major increase in chemical weathering. © The Geological Society of London 2011.

Allen P.A.,Imperial College London | Rieu R.,TOTAL E and P Nederland | Etienne J.L.,Neftex Petroleum Consultants Ltd | Matter A.,University of Bern | Cozzi A.,ENI Angola
Geological Society Memoir | Year: 2011

Glacial deposits are found in the Ayn Formation and Shareef Formation of the Mirbat Group close to Mirbat in Dhofar, southern Oman. The Mirbat Group is most likely a correlative of the Abu Mahara Group of the Huqf Supergroup of northern Oman. The Ayn Formation, the main subject of this chapter, comprises <400 m of mainly coarse-grained glaciogenic deposits, ponded in 2- to >8-km-wide N- to NW-oriented palaeovalleys eroded into crystalline basement, with few or no deposits preserved on intervening palaeohighs. The Shareef Formation occurs as thin, lenticular, erosional remnants beneath the unconformably overlying Cretaceous. The Ayn Formation is overlain by a thin (<3 m), discontinuous cap carbonate that passes from carbonate-cemented talus on the basin margin to stromatolitic carbonate on palaeohighs and resedimented gravity flows on palaeovalley flanks. © The Geological Society of London 2011.

Ricci T.,Italian National Institute of Geophysics and Volcanology | Finizola A.,University of Reunion Island | Barde-Cabusson S.,ICTJA CSIC | Delcher E.,University of Reunion Island | And 3 more authors.
Geology | Year: 2015

Detecting volcanic unrest is of primary importance for eruption forecasting, especially on volcanoes characterized by highly dangerous, and often seemingly unpredictable, phreatic or phreatomagmatic eruptions. We present a simple and innovative analysis of shallow vertical temperature profiles to depths of 70 cm. These data were recorded at La Fossa cone of Vulcano (Aeolian Islands, Italy), during an episode of increased hydrothermal and seismic activities that occurred between September and December 2009. This work involves the use of the coefficient of determination (R2) on vertical temperature profiles in order to identify changes in conductive versus convective heat transfer modality. The increase in convective heat transfer can be related to the disruption of the hydrothermal system due to its pressurization and/or variation of ground permeability between the hydrothermal system and the surface. While raw temperature data do not evidence any significant variation during the period investigated and the classic temperature gradient is highly influenced by seasonal variations, the fluctuation of R2 displayed striking spikes that coincided with the seismic swarm inside the volcanic edifice. Such a low-cost device associated with easy real-time data processing could constitute a very promising, yet deceptively simple, technique to monitor hydrothermal systems, in order to assess the hazard posed by high-energy eruptions for populations living close to active volcanoes. © 2015 Geological Society of America.

Le Guerroue E.,Geosciences Rennes | Cozzi A.,ENI Angola
Gondwana Research | Year: 2010

There is widespread interest in the Neoproterozoic period of the Earth's history (1000 to 542 Ma) because of unprecedented δ13C fluctuations to < - 10‰ PDB through thick (> 1000 m) succession of stratigraphically complex sedimentary rocks deposited during tens of millions of years. In contrast, Phanerozoic large negative C-isotope excursions have been interpreted as the result of diagenetic fluid mixing during carbonate stabilization and burial and are less enigmatic due to the excellent biostratigraphic control on their timing and duration. The Ediacaran Nafun Group of Oman (part of the Huqf Supergroup spanning the Cryogenian-Early Cambrian) contains a large δ13C negative excursion (the Shuram excursion) reaching values as negative as - 12‰ at the base of the Shuram Formation. A steady recovery to positive values occurs over the entire Shuram and half through the overlying Buah Formation, suggesting a duration on the order of tens of My. Based on trace metal, chemostratigraphic and sedimentological analyses, the carbon isotope record obtained from the Buah Formation of northern Oman indicates a systematic and reproducible shift of δ13C values from - 6‰ to + 1‰ in 1 - a demonstrably diagenetic altered carbonate-cemented siliciclastic facies, and 2 - a least diagenetically altered stromatolitic facies. The identical reproducible isotopic pattern in these time-equivalent sections combined to the presence of exceptionally preserved δ18O values around - 2 to + 1‰ associated with the most negative δ13C values rules out isotopic resetting by diagenetic fluids as a mechanism to explain these values. It is concluded that it is possible to retain depositional δ13C values in demonstrably diagenetically altered carbonates. This raises the issue of the ability to recognize diagenetic alteration of C-isotopic values in Neoproterozoic rocks where a robust time frame to support reproducibility is not available. The results of this study provide strong support to a non diagenetic origin of the negative Shuram C-isotope excursion, believed to be the most profound (in terms of amplitude and duration) in the Earth's history. © 2009 International Association for Gondwana Research.

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