International audienceLe Briovérien est une entité géologique qui affleure très largement dans le... more International audienceLe Briovérien est une entité géologique qui affleure très largement dans le Massif Armoricain, épaisse de plus de 1000m et composée de faciès variés de conglomérats, de grès, et de siltites, produits depuis des zones côtières jusqu’à des contextes turbiditiques profonds. Elle est étudiée depuis la fin du XIXème siècle et reste pourtant mal datée et considérée, à tort, azoïque. Tout juste osait-on l’attribuer à la transition Ediacarien-Cambrien sans pour autant pouvoir identifier et localiser cette limite géologique fondamentale.L’étude récente et détaillée de nombreux sites du Finistère, du Morbihan, d’Ille-et-Vilaine et de Normandie permet pourtant d’apporter deux gammes d’informations qui en donnent une idée stratigraphiquement plus précise et paléontologiquement moins austère.D’une part, le Briovérien supérieur (584 à 540 Ma) présente localement des dépôts fossilifères, contenant essentiellement des ichnofossiles horizontaux générés par des métazoaires vermiformes (Helminthoidichnites tenuis, Helminthopsis tenuis,Gordia marina) mais aussi plus ponctuelle- ment des fossiles d’algues macroscopiques (Chuaria), de métazoaires discoïdes (Beltanelloides, ?Nimbia), ou d’annélides indéterminés.D’autre part, les datations radiochronologiques sur grains de zircons détritiques issus de dépôts du Briovérien et provenant de six sites en Bretagne et trois sites en Normandie, fossilifères ou non, dévoilent des cortèges d’âges dont les plus récents plaident, selon les sites, pour de l’Ediacarien final (550 Ma) ou du Cambrien (540 Ma, voire 530 Ma).Qu’il s’agisse d’Ediacarien avéré ou de Cambrien inférieur, le Briovérien n’est donc pas à négliger dans la caractérisation du passage Précambrien-Paléozoïque en Europe, et peut apporter de précieuses informations paléontologiques et stratigraphiques
This study analyses the variability of the spontaneous gamma-ray signal linked to heavy minerals ... more This study analyses the variability of the spontaneous gamma-ray signal linked to heavy minerals in various sedimentary facies of shallow-marine placer deposits of the Punta Serpeddì Formation (Upper Ordovician of SE Sardinia, Italy). The heavy minerals assemblages are mainly made up of titaniferous minerals, zircon and monazite. They were deposited in siliciclastic storm-dominated platform environments. This work is based on sedimentological (facies, sequence stratigraphy) and petrographic analyses combined with outcrop gamma-ray logging. Two facies containing heavy minerals were identified: a shoreface facies dominated by titaniferous minerals and a proximal upper offshore facies composed of zircon, monazite and titaniferous minerals. Depending on relative values of the radioactive signals, it is possible to distinguish three gamma-ray facies: high, medium and low. The first two correspond to fine-grained facies of the proximal part of the upper offshore, whereas the low radioactivity values relate to coarse-grained shoreface facies. Gamma ray signal can then be used in heavy mineral prospection, as well as in well-log analyses, to refine indirect facies interpretation and their stacking pattern.
ABSTRACT Resume-La formation de Punta Serpeddi (Caradoc-Asghill), situee da ns le SE de la Sardai... more ABSTRACT Resume-La formation de Punta Serpeddi (Caradoc-Asghill), situee da ns le SE de la Sardaigne, est constituee par des depots de plate-forme domines par !'action des tempetes. L'etude petrographi-que montre que ces depots sont riches en mineraux lourds (jusqu'a 12 %) et qu 'ils proviennent de !'erosion de quatre zones sources principales :-le volcanisme precaradocien des « Porfidi grigi ».-les depots cambro-ordoviciens des < < Arenarie di San Vito »,-des roches basiques d'origine mantellique,-une zone cratonique situee dans le NE de la Sardaigne. Sedimentology of the post-sardic placer deposits of SE Sardinia Abstract-The Punta Serpeddi Formation (Caradoc-Ashgi/1) of Sardinia is composed of pla(/orm facies deposits accumulated in a sedimentary environment dominated by storm processes. Petrographic studies reveal that these sediments are rich in heavy minerals (up to 12% by volume): the clastic supply probably came from the erosion of" four main source
The identification of source materials contributing to mature terrigenous clastic deposits is mad... more The identification of source materials contributing to mature terrigenous clastic deposits is made difficult due to a lack of useful discriminant criteria. SmNd isotope analysis can provide some indication of the present of mantle-derived constituents in clastic rocks. However, since this method is based on whole-rock samples, it is only possible to obtain the averaged composition of the different source
HAL (Le Centre pour la Communication Scientifique Directe), 2006
Crinoid columnals are major macrobenthic components of the Lower Devonian faunas of the Massif ar... more Crinoid columnals are major macrobenthic components of the Lower Devonian faunas of the Massif armoricain particularly in the Seillou section. Three crinoid columnal associations have been delineated based on R and Q-mode cluster analysis. Distribution of these associations is correlated to the sequence stratigraphic architecture, showing the close relationships between crinoid columnal association dynamics and relative sea-level variations. Comparing the distribution of benthic associations, we note a lesser diversity in crinoid columnal associations than in brachiopod ones. Crinoids (high-level suspension feeders) seem to occupy broader specialised niches than brachiopods (reclining and low-level suspension feeders) and are characterised by a different ecological space utilisation.
This work is aimed at acquiring knowledge and understanding of some placer deposits of the Ordovi... more This work is aimed at acquiring knowledge and understanding of some placer deposits of the Ordovician of SE Sardinia (S Italy) and W Armorican Massif (NW France). These shallow water heavy mineral concentrations were deposited in a siliciclastic storm-dominated shelf environment where the stratigraphic evolution shows an apparently random distribution of such deposits in identical facies. Heavy minerals are mainly represented by titaniferous minerals (rutile and anatase), zircon and monazite; the tourmaline is less common. The study is based on high resolution sequence stratigraphy analysis in outcrops and backstripping procedure, supported by a petrophysical characterization and petrographic study. Gamma-ray logging has been performed on the basis of the presence of zircon and monazite minerals which are natural radioactive. Several gamma-ray facies have been identified. A high radioactivity facies is represented by sandy beds enriched in heavy minerals, in which modal analyses show that their concentration can reach 50%. The high radioactive signal is linked to the abundance of Uranium and Thorium in zircon and monazite. Two facies with medium radioactivity have been recognized; their radioactivity is linked to the Potassium in the silty-clayey intercalations. Stratigraphic key surfaces previously identified by the sequence analysis, are well highlighted also by the gamma-ray signal. Total Counts and K mark fourth-order sequences and stratigraphic key surfaces of thirdorder sequences (Maximum Regressive Surface and Maximum Flooding Surface), whereas the U and Th signal point out placer beds. The comparison between gamma-ray facies located within the depositional sequences and the results of the backstripping analysis point out that heavy mineral concentrations occur during major-order base-level rises in high-energy depositional environments (shoreface and upper offshore). In the shallow environments of shoreface and proximal inner shelf, the storm wave action favours high-density minerals deposition, but this process seems to be controlled by allocyclic factors such as base-level variations. We proposed a model based on the sediment volume partitioning and on the superposition of cycles of distinct frequencies. During major-order cycles sea-level rises, the decreasing of terrigenous inputs in the marine environments and the amalgamation of very high frequency sequences lead concentrations of the heavy minerals. On the contrary, during sea-level falls, the high-density minerals are diluted by abundant terrigenous flux. Acknowledgements: This work was financed by the Sardinia Regional Government (LR7-2007, N.7: “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna”).
International audienceLe Briovérien est une entité géologique qui affleure très largement dans le... more International audienceLe Briovérien est une entité géologique qui affleure très largement dans le Massif Armoricain, épaisse de plus de 1000m et composée de faciès variés de conglomérats, de grès, et de siltites, produits depuis des zones côtières jusqu’à des contextes turbiditiques profonds. Elle est étudiée depuis la fin du XIXème siècle et reste pourtant mal datée et considérée, à tort, azoïque. Tout juste osait-on l’attribuer à la transition Ediacarien-Cambrien sans pour autant pouvoir identifier et localiser cette limite géologique fondamentale.L’étude récente et détaillée de nombreux sites du Finistère, du Morbihan, d’Ille-et-Vilaine et de Normandie permet pourtant d’apporter deux gammes d’informations qui en donnent une idée stratigraphiquement plus précise et paléontologiquement moins austère.D’une part, le Briovérien supérieur (584 à 540 Ma) présente localement des dépôts fossilifères, contenant essentiellement des ichnofossiles horizontaux générés par des métazoaires vermiformes (Helminthoidichnites tenuis, Helminthopsis tenuis,Gordia marina) mais aussi plus ponctuelle- ment des fossiles d’algues macroscopiques (Chuaria), de métazoaires discoïdes (Beltanelloides, ?Nimbia), ou d’annélides indéterminés.D’autre part, les datations radiochronologiques sur grains de zircons détritiques issus de dépôts du Briovérien et provenant de six sites en Bretagne et trois sites en Normandie, fossilifères ou non, dévoilent des cortèges d’âges dont les plus récents plaident, selon les sites, pour de l’Ediacarien final (550 Ma) ou du Cambrien (540 Ma, voire 530 Ma).Qu’il s’agisse d’Ediacarien avéré ou de Cambrien inférieur, le Briovérien n’est donc pas à négliger dans la caractérisation du passage Précambrien-Paléozoïque en Europe, et peut apporter de précieuses informations paléontologiques et stratigraphiques
This study analyses the variability of the spontaneous gamma-ray signal linked to heavy minerals ... more This study analyses the variability of the spontaneous gamma-ray signal linked to heavy minerals in various sedimentary facies of shallow-marine placer deposits of the Punta Serpeddì Formation (Upper Ordovician of SE Sardinia, Italy). The heavy minerals assemblages are mainly made up of titaniferous minerals, zircon and monazite. They were deposited in siliciclastic storm-dominated platform environments. This work is based on sedimentological (facies, sequence stratigraphy) and petrographic analyses combined with outcrop gamma-ray logging. Two facies containing heavy minerals were identified: a shoreface facies dominated by titaniferous minerals and a proximal upper offshore facies composed of zircon, monazite and titaniferous minerals. Depending on relative values of the radioactive signals, it is possible to distinguish three gamma-ray facies: high, medium and low. The first two correspond to fine-grained facies of the proximal part of the upper offshore, whereas the low radioactivity values relate to coarse-grained shoreface facies. Gamma ray signal can then be used in heavy mineral prospection, as well as in well-log analyses, to refine indirect facies interpretation and their stacking pattern.
ABSTRACT Resume-La formation de Punta Serpeddi (Caradoc-Asghill), situee da ns le SE de la Sardai... more ABSTRACT Resume-La formation de Punta Serpeddi (Caradoc-Asghill), situee da ns le SE de la Sardaigne, est constituee par des depots de plate-forme domines par !'action des tempetes. L'etude petrographi-que montre que ces depots sont riches en mineraux lourds (jusqu'a 12 %) et qu 'ils proviennent de !'erosion de quatre zones sources principales :-le volcanisme precaradocien des « Porfidi grigi ».-les depots cambro-ordoviciens des < < Arenarie di San Vito »,-des roches basiques d'origine mantellique,-une zone cratonique situee dans le NE de la Sardaigne. Sedimentology of the post-sardic placer deposits of SE Sardinia Abstract-The Punta Serpeddi Formation (Caradoc-Ashgi/1) of Sardinia is composed of pla(/orm facies deposits accumulated in a sedimentary environment dominated by storm processes. Petrographic studies reveal that these sediments are rich in heavy minerals (up to 12% by volume): the clastic supply probably came from the erosion of" four main source
The identification of source materials contributing to mature terrigenous clastic deposits is mad... more The identification of source materials contributing to mature terrigenous clastic deposits is made difficult due to a lack of useful discriminant criteria. SmNd isotope analysis can provide some indication of the present of mantle-derived constituents in clastic rocks. However, since this method is based on whole-rock samples, it is only possible to obtain the averaged composition of the different source
HAL (Le Centre pour la Communication Scientifique Directe), 2006
Crinoid columnals are major macrobenthic components of the Lower Devonian faunas of the Massif ar... more Crinoid columnals are major macrobenthic components of the Lower Devonian faunas of the Massif armoricain particularly in the Seillou section. Three crinoid columnal associations have been delineated based on R and Q-mode cluster analysis. Distribution of these associations is correlated to the sequence stratigraphic architecture, showing the close relationships between crinoid columnal association dynamics and relative sea-level variations. Comparing the distribution of benthic associations, we note a lesser diversity in crinoid columnal associations than in brachiopod ones. Crinoids (high-level suspension feeders) seem to occupy broader specialised niches than brachiopods (reclining and low-level suspension feeders) and are characterised by a different ecological space utilisation.
This work is aimed at acquiring knowledge and understanding of some placer deposits of the Ordovi... more This work is aimed at acquiring knowledge and understanding of some placer deposits of the Ordovician of SE Sardinia (S Italy) and W Armorican Massif (NW France). These shallow water heavy mineral concentrations were deposited in a siliciclastic storm-dominated shelf environment where the stratigraphic evolution shows an apparently random distribution of such deposits in identical facies. Heavy minerals are mainly represented by titaniferous minerals (rutile and anatase), zircon and monazite; the tourmaline is less common. The study is based on high resolution sequence stratigraphy analysis in outcrops and backstripping procedure, supported by a petrophysical characterization and petrographic study. Gamma-ray logging has been performed on the basis of the presence of zircon and monazite minerals which are natural radioactive. Several gamma-ray facies have been identified. A high radioactivity facies is represented by sandy beds enriched in heavy minerals, in which modal analyses show that their concentration can reach 50%. The high radioactive signal is linked to the abundance of Uranium and Thorium in zircon and monazite. Two facies with medium radioactivity have been recognized; their radioactivity is linked to the Potassium in the silty-clayey intercalations. Stratigraphic key surfaces previously identified by the sequence analysis, are well highlighted also by the gamma-ray signal. Total Counts and K mark fourth-order sequences and stratigraphic key surfaces of thirdorder sequences (Maximum Regressive Surface and Maximum Flooding Surface), whereas the U and Th signal point out placer beds. The comparison between gamma-ray facies located within the depositional sequences and the results of the backstripping analysis point out that heavy mineral concentrations occur during major-order base-level rises in high-energy depositional environments (shoreface and upper offshore). In the shallow environments of shoreface and proximal inner shelf, the storm wave action favours high-density minerals deposition, but this process seems to be controlled by allocyclic factors such as base-level variations. We proposed a model based on the sediment volume partitioning and on the superposition of cycles of distinct frequencies. During major-order cycles sea-level rises, the decreasing of terrigenous inputs in the marine environments and the amalgamation of very high frequency sequences lead concentrations of the heavy minerals. On the contrary, during sea-level falls, the high-density minerals are diluted by abundant terrigenous flux. Acknowledgements: This work was financed by the Sardinia Regional Government (LR7-2007, N.7: “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna”).
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