Identification of early Llandovery (Silurian) anoxic palaeo-depressions at the western margin of the Murzuq Basin (southwest Libya), based on gamma-ray spectrometry in surface exposures

ABSTRACT Following the melting of the Gondwanan icecap and the resulting postglacial sea-level rise, organic-rich shales were deposited in shelfal palaeo-depressions across North Africa and Arabia during the latest Ordovician to earliest Silurian. The unit is absent on palaeohighs that were flooded only later when the anoxic event had already ended. The regional distribution of the Silurian black shale is now well-known for the subsurface of the central parts of the Murzuq Basin, in Libya, where many exploration wells have been drilled and where the shale represents the main hydrocarbon source rock. On well logs, the Silurian black shale is easily recognisable due to increased uranium concentrations and, therefore, elevated gamma-ray values. The uranium in the shales “precipitated” under oxygen-reduced conditions and generally a linear relationship between uranium and organic content is developed. The distribution of the Silurian organic-rich shales in the outcrop belts surrounding the Murzuq Basin has been long unknown because Saharan surface weathering has commonly destroyed the organic matter and black colour of the shales, making it complicated to identify the previously organic-rich unit in the field. In an attempt to distinguish (previously) organic-rich from organically lean shales at outcrop, seven sections that straddle the Ordovician-Silurian boundary were measured by portable gamma-ray spectrometer along the outcrops of the western margin of the Murzuq Basin. It was found that the uranium content of the shales remained largely unaltered by the weathering processes and could therefore be used as a valid proxy parameter to distinguish between pre-weathering organically rich and lean shales. It is now possible to identify and map-out the thickness and approximate organic richness of the black shale using measurement of uranium radiation. Five of the newly measured sections are characterised by uranium-enriched intervals, representing areas of earliest Silurian palaeo-depressions. Major uranium peaks are absent in the spectral gamma-ray curves of two other sections, which are interpreted to mark earliest Silurian palaeo-highs. The new data on the distribution of Silurian black shales from the outcrop belt was integrated with subsurface data from the Murzuq Basin. The resulting map of the distribution of black shales may help with predictions of the occurrence of this unit in less well-explored areas of the basin. Graptolite biostratigraphic data suggests that the anoxic event centred on the middle Rhuddanian, with more oxygenated conditions and onset of deposition of organically leaner shales having commenced sometime during the late Rhuddanian. The presence of anoxic palaeo-depressions during the earliest Silurian within the Ghat outcrop belt indicates that the Tihemboka High at the western margin of the Murzuq Basin could not have been a positive structure during this time.

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Late Jurassic – earliest Cretaceous prolonged shelf dysoxic–anoxic event and its possible causes

Geological Magazine ◽

10.1017/s001675682000076x ◽

2020 ◽

Vol 157 (10) ◽

pp. 1622-1642

Author(s):

MA Rogov ◽

EV Shchepetova ◽

VA Zakharov

Keyword(s):

Organic Carbon ◽

Black Shale ◽

High Latitude ◽

Late Jurassic ◽

Black Shales ◽

Time Interval ◽

Oceanic Circulation ◽

Anoxic Event ◽

Anoxic Events

AbstractThe Late Jurassic – earliest Cretaceous time interval was characterized by a widespread distribution of dysoxiс–anoxiс environments in temperate- and high-latitude epicontinental seas, which could be defined as a shelf dysoxic–anoxic event (SDAE). In contrast to black shales related to oceanic anoxic events, deposits generated by the SDAE were especially common in shelf sites in the Northern Hemisphere. The onset and termination of the SDAE was strongly diachronous across different regions. The SDAE was not associated with significant disturbances of the carbon cycle. Deposition of organic-carbon-rich sediment and the existence of dysoxic–anoxic conditions during the SDAE lasted up to c. 20 Ma, but this event did not cause any remarkable biotic extinction. Temperate- and high-latitude black shale occurrences across the Jurassic–Cretaceous boundary have been reviewed. Two patterns of black shale deposition during the SDAE are recognized: (1) Subboreal type, with numerous thin black shale beds, bounded by sediments with very low total organic carbon (TOC) values; and (2) Boreal type, distinguished by predominantly thick black shale successions showing high TOC values and prolonged anoxic–dysoxic conditions. These types appear to be unrelated to differences in accommodation space, and can be clearly recognized irrespective of the thickness of shale-bearing units. Black shales in high-latitude areas in the Southern Hemisphere strongly resemble Boreal types of black shale by their mode of occurrence. The causes of this SDAE are linked to long-term warming and changes in oceanic circulation. Additionally, the long-term disturbance of planktonic communities may have triggered overall increased productivity in anoxia-prone environments.

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Extent and duration of marine anoxia during the Frasnian–Famennian (Late Devonian) mass extinction in Poland, Germany, Austria and France

Geological Magazine ◽

10.1017/s0016756804008866 ◽

2004 ◽

Vol 141 (2) ◽

pp. 173-193 ◽

Cited By ~ 151

Author(s):

DAVID BOND ◽

PAUL B. WIGNALL ◽

GRZEGORZ RACKI

Keyword(s):

Gamma Ray ◽

Gamma Ray Spectrometry ◽

Late Devonian ◽

Oceanic Anoxic Event ◽

Fabric Analysis ◽

Anoxic Event ◽

Anoxic Events ◽

Frasnian Stage ◽

Early Late ◽

Sediment Fabric

The intensity and extent of anoxia during the two Kellwasser anoxic events has been investigated in a range of European localities using a multidisciplinary approach (pyrite framboid assay, gamma-ray spectrometry and sediment fabric analysis). The results reveal that the development of the Lower Kellwasser Horizon in the early Late rhenana Zone (Frasnian Stage) in German type sections does not always coincide with anoxic events elsewhere in Europe and, in some locations, seafloor oxygenation improves during this interval. Thus, this anoxic event is not universally developed. In contrast, the Upper Kellwasser Horizon, developed in the Late linguiformis Zone (Frasnian Stage) in Germany correlates with a European-wide anoxic event that is manifest as an intensification of anoxia in basinal locations to the point that stable euxinic conditions were developed (for example, in the basins of the Holy Cross Mountains, Poland). The interval also saw the spread of dysoxic waters into very shallow water (for instance, reefal) locations, and it seems reasonable to link the contemporaneous demise of many marine taxa to this phase of intense and widespread anoxia. In basinal locations, euxinic conditions persisted into the earliest Famennian with little change of depositional conditions. Only in the continental margin location of Austria was anoxia not developed at any time in the Late Devonian. Consequently it appears that the Upper Kellwasser anoxic event was an epicontinental seaway phenomenon, caused by the upward expansion of anoxia from deep basinal locales rather than an ‘oceanic’ anoxic event that has spilled laterally into epicontinental settings.

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Reconstruction of the original organic richness in weathered Silurian shale outcrops (Murzuq and Kufra basins, southern Libya)

GeoArabia ◽

10.2113/geoarabia0802299 ◽

2003 ◽

Vol 8 (2) ◽

pp. 299-308 ◽

Cited By ~ 4

Author(s):

Sebastian Lüning ◽

Sadat Kolonic ◽

David K. Loydell ◽

Jonathan Craig

Keyword(s):

Organic Matter ◽

North Africa ◽

Bottom Water ◽

Gamma Ray ◽

Uranium Content ◽

Well Logs ◽

Close Correspondence ◽

Hydrocarbon Source Rock ◽

Natural Radiation ◽

Distribution Maps

ABSTRACT The early Silurian in North Africa and Arabia was characterised by widespread deposition of organic-rich shales in palaeo-depressions. The unit represents an important hydrocarbon source rock in the region and can be detected easily in well logs because of strong uranium-related natural radiation. In exposures, however, organic matter is commonly heavily oxidised through weathering so that identification of the unit in the field is difficult. Uranium and pyrite framboids appear to be less vulnerable to weathering and may be used to identify intervals of originally organic-rich shales in exposures. Framboids are discrete spheroidal aggregates of pyrite microcrystallites and their size distribution is thought to be controlled by palaeo-depositional bottom-water redox-conditions. Analyses of fresh Silurian organic-rich shales from a core reveal a close correspondence, for the most part, between total organic carbon, total gamma-ray response, uranium content (as determined by spectral gamma-ray) and framboid parameters. Feasibility tests of the concept have been carried out at two exposures in southern Libya and may form the basis for improved Silurian organic-rich shale distribution maps and more precise age models for Silurian organic-rich depositional phases in northern Gondwana.

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Assessment of radiation exposure around abandoned uranium mining area of Stara planina Mt., Serbia

Nuclear Technology and Radiation Protection ◽

10.2298/ntrp1401058t ◽

2014 ◽

Vol 29 (1) ◽

pp. 58-66 ◽

Cited By ~ 2

Author(s):

Milan Tanic ◽

Milan Momcilovic ◽

Jovan Kovacevic ◽

Snezana Dragovic ◽

Goran Bacic

Keyword(s):

Gamma Ray ◽

Radiation Risk ◽

Uranium Content ◽

Uranium Mining ◽

Soil Samples ◽

Radiation Hazard ◽

Gamma Ray Spectrometry ◽

External Irradiation ◽

Uranium Ore ◽

Risk Parameters

The aim of this work was to estimate the health and radiation hazard due to external irradiation from terrestrial radionuclides in the Stara planina Mt. region, which is important because of past uranium mining activities on the mountain. Soil samples were collected inside the flotation processing facilities, their surroundings and more distant locations, i.e. from areas considered certainly affected, potentially affected, and unaffected by former mining and uranium ore processing activities. The radiological and health risk assessments were done by calculating the six main parameters, based on the activity concentration of 238U, 232Th, and 40K in soil samples as determined by gamma-ray spectrometry. Increased values of the risk parameters were observed only for sites where uranium ore was processed, while the location surrounding these compounds showed values that are usual for this mountain or slightly above them. Calculations of the risk parameters for the background area showed no radiation risk for the local and seasonal population. The presence of U and Th was detected in all water samples from creeks surrounding the facilities, but only in the water from the facility drainage pipe did their concentration exceed the limits given for the uranium content in drinking water. In conclusion, the results obtained in this study fall within the range of values in similar studies conducted worldwide and are below the values which can cause a significant radiation hazard.

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Geochemical and radioctivity characterization of rocks from the Rio Preto (GO) project

Brazilian Journal of Geophysics ◽

10.22564/rbgf.v30i4.228 ◽

2012 ◽

Vol 30 (4) ◽

pp. 399

Author(s):

Cynthia Romariz Duarte ◽

Daniel Marcos Bonotto ◽

Marco Aurélio Farias de Oliveira

Keyword(s):

Gamma Ray ◽

Organic Matter Content ◽

Uranium Content ◽

Matter Content ◽

Gamma Ray Spectrometry ◽

Weathering Processes ◽

Lower Member ◽

Ratio Data ◽

238U Activity

A extinta Nuclebrás, no final da década de 70 e início dos anos 80, conduziu o Projeto Rio Preto, por intermédio do qual efetuou o mapeamento geológico básico e a caracterização radiométrica por aerogamaespectrometria, sem discriminação de canais, da região nordeste do estado de Goiás, Brasil, a oeste da área do Parque Nacional da Chapada dos Veadeiros, perfazendo um total de 650 km2 e englobando a confluência dos Rios Claro e Preto. Os radioelementos naturais U, Th e 40K do Projeto Rio Preto (GO) foram posteriormente caracterizados por espectrometria gama aplicada aproximadamente a 300 amostras, a qual foi conduzida no LABIDRO-Laboratório de Isótopos e Hidroquímica do Departamento de Petrologia e Metalogenia do Instituto de Geociências e Ciências Exatas da UNESP-Campus de Rio Claro. Este trabalho descreve para todas as amostras os resultados obtidos na caracterizaçãoao petrográfica e análise química dos principais óxidos (SiO2, TiO2, Al2O3, Fe2O3, MgO, MnO, K2O, Na2O, CaO e P2O5), os quais foram utilizados na avaliação da ocorrência dos radioelementos naturais naquela área. Também são apresentados resultados do teor de matéria orgânica obtido por colorimetria em amostras selecionadas de diferentes litotipos para melhor investigar a possibilidade de relação entre a grafita e os radioelementos urânio e tório. Finalmente, dados da concentração de urânio e razão de atividade 234U/238U em amostras selecionadas de xistos e gneisss da Formação Ticunzal – Membro Inferior sugeriram a influência de processos intempéricos na área.ABSTRACT: The Rio Preto Project, developed by the extinct Brazilian nuclear state company, Nuclebrás, during the late 70s and early 80s, consisted of basic geologicalmapping and radiometric characterization by aerogeophysical gamma-ray spectrometry, without channel discrimination, of a surface area of 650 km2 located to the westof the Chapada dos Veadeiros National Park on the northeastern of Goiás State, Brazil, including the confluence area of Claro and Preto Rivers. Additionaly, the naturalradioelements U, Th and 40K were determined by gamma-ray spectrometry in 300 rock samples from cores of the Rio Preto Project area. The tests were conducted at LABIDRO-Isotopes and Hydrochemistry Laboratory of the Departamento de Petrologia e Metalogenia of the Instituto de Geociências e Ciências Exatas, UNESP, in RioClaro, SP, Brazil. This paper reports the results of petrographic characterization and chemical analyses of major oxides (SiO2, TiO2, Al2O3, Fe2O3, MgO, MnO, K2O,Na2O, CaO and P2O5) for all samples used to determine the natural radioelements present in the region. The organic matter content results obtained by colorimetryare also reported for selected cores of different lithotypes in order to investigate the possible relationship between graphite and the radioelements uranium and thorium.Finally, uranium content and 234U/238U activity ratio data for selected samples of schists and gneisses of the Lower Member of the Ticunzal Formation suggest theinfluence of weathering processes in the area.Keywords: Rio Preto (GO) Project, natural radioelements, gamma and alpha spectrometry.

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Gamma-ray spectrometry and magnetometry of the alkaline rock area in Ampipal-Bhulbhule, Nepal Lesser Himalaya

Journal of Nepal Geological Society ◽

10.3126/jngs.v18i0.32200 ◽

1998 ◽

Vol 18 ◽

Author(s):

P. Gautam ◽

M. P. Sharma ◽

Subodh Dhakal

Keyword(s):

Gamma Ray ◽

Thick Layer ◽

Alkaline Rock ◽

Uranium Content ◽

Magnetic Effect ◽

Magnetic Method ◽

K Channel ◽

Gamma Ray Spectrometry ◽

Lesser Himalaya ◽

Host Rocks

Gamma-ray spectrometric and magnetometric survey conducted in the Ampipal-Bhulbhule area characterised by the distribution of syenitic gneisses hosted by the metasediments of the Kuncha Formation revealed that such a geophysical complex is effective in mapping the contact zones of syenitic bodies with the host rocks. Altogether 84 sites within the syenitic bodies and 32 sites in the host rocks were probed by the spectrometer for responses in U, Th and 40K., and total counts windows. The syenites always give readings distinctly higher than the host rocks. Within the main syenitic body, the maximum radiometric response is obtained at the central part. Several sites within the syenitic gneisses yield anomalies in equivalent Uranium content (eUmax = 303.5 ppm) and Thorium content (eTHmax=213.6 ppm) and also in K-channel counts. Across the zone of contact between the syenites and the host metasandstones, both the radiometric counts (especially K-counts) and the magnetic effect vary significantly. Prominent magnetic anomaly has been detected at Bhulbhule Khar and it is interpreted to be the effect of either a thick layer of basic or ultrabasic rock enclosed within the nepheline syenite, or an economic Fe-mineralisation residing in magnetite. Thus, magnetic method also is clearly effective for mapping the subsurface distribution of dyke rocks and also Fe-mineralised areas.

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Gamma-Ray Spectrometric Analysis of Some Trace Elements in the Human Articular Disc of the Mandible

Nuklearmedizin ◽

10.1055/s-0037-1621214 ◽

1963 ◽

Vol 03 (02) ◽

pp. 175-182 ◽

Cited By ~ 1

Author(s):

Bo Bergman ◽

Rune Söremark

Keyword(s):

Trace Elements ◽

Neutron Activation ◽

Quantitative Data ◽

Thermal Neutron Flux ◽

Gamma Ray ◽

Dry Weight ◽

Gamma Ray Spectrometry ◽

Spectrometric Analysis ◽

Articular Disc ◽

Group Separation

SummaryBy means of neutron activation and gamma-ray spectrometry the concentrations in the human mandibular articular disc of the following elements have been determined: Na, Mn, Cu, Zn, Rb, Sr, Cd, W, and Au. The discs were obtained at necropsy from seven men and nine women, ranging in age from 56 to 71 years.The activation was carried out in a thermal neutron flux of about 1.7 XlO12 neutrons × cm−2 × sec.−1 for about 20 hours. A chemical group separationwas performed before the gamma-ray spectrometry. Quantitative data based on the dry weight of the cartilage samples were obtained by comparing the photo-peak area of the identified elements with those of appropriate standards.

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