jeanflop/post_ocr_correction2 huggingface dataset:
roches non métamorphiques; elle est datée du Dévonien moyen au Carbonifère inférieur [1] (fig.). Au sud, l’U-B est faillée contre l’unité du Lyonnais qui contient des roches métamorphiques de faciès amphibolite à granulite, dont le protolithe est interprété comme Cambrien et Ordovicien ([2], [3]) (fig.). L’U-B est composée de roches ignées, épaisses, et des sédiments métamorphisés en faciès schiste vert et amphibolite [6]. Géochimiquement, l’U-B est liée à une marge continentale active [4] et/ou un bassin d’arrière-arc ensialique [5]. La foliation pénétrative (si) est plissée autour d’axes NE-SW à vergence SE. La distribution de la déformation et le degré du métamorphisme augmentent vers le SE [6]. Autrefois, la classification stratigraphique de l’U-B était basée sur des corrélations lithologiques (Dévono-Dinantien) ([7], [8]) ou sur l'arrangement structural et sédimentaire (préDévonien moyen) ([9], [1]). La localité de l’échantillonnage fossilifère que nous décrivons ici se trouve sur la RN7 entre l'Arbresle et Lentilly, à Fleurieux-sur-l’Arbresle (coordonnées Lambert x = 780,65; j = 95,35) (fig.). Trois échantillons différents de phyllites ont été étudiés. Les phyllites, épaisses d’environ 300 m, sont intercalées avec d’épais schistes verts. Elles contiennent du mica blanc, du quartz et des porphyroblastes de biotite brune qui définissent la foliation (si). Les paragenèses métamorphiques des schistes verts autour des phyllites indiquent un métamorphisme du faciès schiste vert élevé [6]. À cause de l’absence de fossiles, la position stratigraphique de ces roches restait incertaine. Cependant, des recherches récentes ont montré que des microfossiles peuvent survivre aux transformations métamorphiques ([10], [11]). Les matériaux des membranes organiques sont alors graphitisés et/ou pyritisés. Un examen microscopique avec combinaison de lumière transmise et réfléchie rend possible l’étude de la morphologie des fossiles dont la préservation est suffisante pour l’identification. Note présentée par Jean AUBOUIN. 0249-6305/88/03071717 S 2.00 © Académie des Sciences C. R. Acad. Sci. Paris, t. 307, Série II, p. 1717-1721, 1988 Les fossiles les plus fréquents dans les phyllites de l’U-B sont représentés par des petites vésicules (15-25 um) satinées, granulaires et sphéroïdales attribuables à Protosphaeridium Timofeev. De plus grands sphéromorphes peuvent être attribués à Trachysphaeridium sp. Timofeev. Ces assemblages sont comparables à des gisements du
métamorphiques des schistes verts autour des phyllites indiquent un métamorphisme du faciès schiste vert élevé [6]. À cause de l’absence de fossiles, la position stratigraphique de ces roches restait incertaine. Cependant, des recherches récentes ont montré que des microfossiles peuvent survivre aux transformations métamorphiques ([10], [11]). Les matériaux des membranes organiques sont alors graphitisés et/ou pyritisés. Un examen microscopique avec combinaison de lumière transmise et réfléchie rend possible l’étude de la morphologie des fossiles dont la préservation est suffisante pour l’identification. Note présentée par Jean AUBOUIN. 0249-6305/88/03071717 S 2.00 © Académie des Sciences C. R. Acad. Sci. Paris, t. 307, Série II, p. 1717-1721, 1988 Les fossiles les plus fréquents dans les phyllites de l’U-B sont représentés par des petites vésicules (15-25 um) satinées, granulaires et sphéroïdales attribuables à Protosphaeridium Timofeev. De plus grands sphéromorphes peuvent être attribués à Trachysphaeridium sp. Timofeev. Ces assemblages sont comparables à des gisements du Protérozoïque supérieur décrits en abondance partout dans le monde ([12], [13], [14]). L’intervalle stratigraphique de ces sphéromorphes comprend le Protérozoïque moyen jusqu’au Cambrien. Cependant, les microfossiles morphologiquement caractérisés par une variation transitionnelle à des formes planctoniques qui signent l’avènement du Cambrien, n’ont pas été observés dans les phyllites de l’U-B. Nous en concluons que celles-ci sont préCambriennes. The discovery of Upper Proterozoic microfossils in low-grade phyllites of the Brévenne Unit (B-U) documents the existence of Brioverian in the northern Massif Central for the first time. GEOLOGY. — In the Beaujolais-Lyonnais area the B-U is limited to the north by the non-metamorphic middle Devonian — lower Carboniferous Beaujolais-Unit [1] (Fig.). To the south the B-U is in fault contact with medium-to high-grade metamorphic rocks of the Lyonnais-Unit whose protolith is interpreted as felsic magmatic rocks is interpreted as Cambrian to Ordovician ([2], [3]) (Fig.). The B-U is a thick sequence of low-to medium-grade meta-igneous rocks (several 1,000 m thick) and metasediments (no more than several 100 m thick). Tectonic slivers of gneissic rocks are frequently distributed in the B-U. Coarse-grained metagreywackes and thin layers of phyllites contrast with thick volcanics. Facies and thickness of sediments indicate a near-shore depositional environment. Geochemically,
Nous en concluons que celles-ci sont préCambriennes. The discovery of Upper Proterozoic microfossils in low-grade phyllites of the Brévenne Unit (B-U) documents the existence of Brioverian in the northern Massif Central for the first time. GEOLOGY. — In the Beaujolais-Lyonnais area the B-U is limited to the north by the non-metamorphic middle Devonian — lower Carboniferous Beaujolais-Unit [1] (Fig.). To the south the B-U is in fault contact with medium-to high-grade metamorphic rocks of the Lyonnais-Unit whose protolith is interpreted as felsic magmatic rocks is interpreted as Cambrian to Ordovician ([2], [3]) (Fig.). The B-U is a thick sequence of low-to medium-grade meta-igneous rocks (several 1,000 m thick) and metasediments (no more than several 100 m thick). Tectonic slivers of gneissic rocks are frequently distributed in the B-U. Coarse-grained metagreywackes and thin layers of phyllites contrast with thick volcanics. Facies and thickness of sediments indicate a near-shore depositional environment. Geochemically, the B-U appears to be related to an active continental margin or island arc [4] and/or to an ensialic marginal basin [5]. Primary bedding features are not preserved in rocks of the B-U. Penetrative foliation (si) with stratal disruption is folded into NE-SW striking open to similar folds with SE-vergence. Strain distribution and metamorphic grade increase toward the SE in direction of the contact with the Lyonnais-Unit [6]. Previous stratigraphic classifications for the B-U were based on lithological correlations (Devonian-Dinantian) ([7], [8]) or on the structural and sedimentary setting (pre-middle Devonian) ([9], [1]). However, our discovery of Upper Proterozoic microfossils in low-grade phyllites of the B-U assigns the correct stratigraphic position of the B-U for the first time. This verifies the existence of Brioverian deposits in the Massif Central. The sample locality of the microfossils is located along the RN7 between l’Arbresle and Lentilly at Fleurieux-sur-l’Arbresle (Lambert coordinates: x = 780.65; j = 95.35) (Fig.). Three different samples of phyllites were collected. The phyllites, up to 300 m thick, are intercalated with thick greenschists. They consist of white mica, quartz-grains and porphyroblasts of brown biotite arranged parallel to the foliation. The phyllites display a penetrative foliation (si) which is subparallel
Strain distribution and metamorphic grade increase toward the SE in direction of the contact with the Lyonnais-Unit [6]. Previous stratigraphic classifications for the B-U were based on lithological correlations (Devonian-Dinantian) ([7], [8]) or on the structural and sedimentary setting (pre-middle Devonian) ([9], [1]). However, our discovery of Upper Proterozoic microfossils in low-grade phyllites of the B-U assigns the correct stratigraphic position of the B-U for the first time. This verifies the existence of Brioverian deposits in the Massif Central. The sample locality of the microfossils is located along the RN7 between l’Arbresle and Lentilly at Fleurieux-sur-l’Arbresle (Lambert coordinates: x = 780.65; j = 95.35) (Fig.). Three different samples of phyllites were collected. The phyllites, up to 300 m thick, are intercalated with thick greenschists. They consist of white mica, quartz-grains and porphyroblasts of brown biotite arranged parallel to the foliation. The phyllites display a penetrative foliation (si) which is subparallel with the contact and the foliation of the enclosing greenschists. An assessment of the metamorphism of the surrounding greenschists suggests already elevated low-grade conditions for the phyllites [6]. PALEONTOLOGY. — Low-to medium-grade metamorphic rocks are commonly believed to be unfossiliferous. Consequently, the stratigraphic position of such rocks remains doubtful in most cases. Our studies have shown that organic microfossils such as acritarchs can survive metamorphism under certain conditions ([10], [11]). However, the examination of microfossils in metamorphic rocks is difficult because their organic wall materials are graphitized and/or pyritized as a result of metamorphic heating. Under the microscope, the remaining fossil appears as an opaque mass lacking observable structures when analyzed under transmitted light. We have found that addition of reflected light can reveal the morphology of the fossil. Often the preservation is good enough to allow its identification. The most abundant fossils detected in the phyllites are small (15-25 μm) smooth-walled, granular, spheroidal vesicles assignable to Protosphaeridium sp. Timofeev. Larger thick-walled sphaeromorphs also occur in these samples, as do granular vesicles assignable to Trachysphaeridium sp. Timofeev. Microfossil assemblages similar to those from the phyllites of the B-U are recorded from Upper Proterozoic occurrences all over the world ([12], [13]).
most cases. Our studies have shown that organic microfossils such as acritarchs can survive metamorphism under certain conditions ([10], [11]). However, the examination of microfossils in metamorphic rocks is difficult because their organic wall materials are graphitized and/or pyritized as a result of metamorphic heating. Under the microscope, the remaining fossil appears as an opaque mass lacking observable structures when analyzed under transmitted light. We have found that addition of reflected light can reveal the morphology of the fossil. Often the preservation is good enough to allow its identification. The most abundant fossils detected in the phyllites are small (15-25 μm) smooth-walled, granular, spheroidal vesicles assignable to Protosphaeridium sp. Timofeev. Larger thick-walled sphaeromorphs also occur in these samples, as do granular vesicles assignable to Trachysphaeridium sp. Timofeev. Microfossil assemblages similar to those from the phyllites of the B-U are recorded from Upper Proterozoic occurrences all over the world ([12], [13]). Vidal [14] for example notes an absolute predominance of Protosphaeridium and Trachysphaeridium in the late Precambrian Visingsö Beds in Southern Sweden. These two species are clearly facies independent and occur in almost every sample. The stratigraphic range of these two thick-walled sphaeromorphs is middle Proterozoic to Cambrian. The advent of the Cambrian, however, caused the appearance of whole new classes of microfossil architecture. It is evident that there was a gradual transitional variation in the morphology of planktons between Late Precambrian and Earliest Paleozoic. These are not represented in phyllites of the B-U. CONCLUSIONS. — The existence of late Proterozoic microfossils in phyllites associated with metavolcanics that formerly belonged to a subduction environment documents that a major convergent zone existed in the northern Massif Central during late Proterozoic time. ACKNOWLEDGEMENTS. — The Profs. G. H. Eisbacher and W. Franke supported our research. Dr. J. P. Burg improved the French, Dr. M. Strecker the English text version. L. Schifano prepared the Figure. Financial support was given by the DFG (Ei 210/2-2, Fr 668/1-1). Note reçue le 15 septembre 1988, acceptée le 19 septembre 1988. RÉFÉRENCES [1] F. WICKERT, Geol. Resch., 77/2, 1988, pp. 467-482. [2] J. L. DUTHOU, M. PIBOULE, M.
the Cambrian, however, caused the appearance of whole new classes of microfossil architecture. It is evident that there was a gradual transitional variation in the morphology of planktons between Late Precambrian and Earliest Paleozoic. These are not represented in phyllites of the B-U. CONCLUSIONS. — The existence of late Proterozoic microfossils in phyllites associated with metavolcanics that formerly belonged to a subduction environment documents that a major convergent zone existed in the northern Massif Central during late Proterozoic time. ACKNOWLEDGEMENTS. — The Profs. G. H. Eisbacher and W. Franke supported our research. Dr. J. P. Burg improved the French, Dr. M. Strecker the English text version. L. Schifano prepared the Figure. Financial support was given by the DFG (Ei 210/2-2, Fr 668/1-1). Note reçue le 15 septembre 1988, acceptée le 19 septembre 1988. RÉFÉRENCES [1] F. WICKERT, Geol. Resch., 77/2, 1988, pp. 467-482. [2] J. L. DUTHOU, M. PIBOULE, M. GAY and E. DUFOUR, C. R. Acad. Sci. Paris, 292, Série II, 1981, pp. 749-752. [3] E. DUFOUR, Thèse 3e cycle, Univ. Lyon, 1982, 241 p. [4] C. PIN, C. DUPUY and J. M. PETERLONGO, Bull Soc. géol. Fr., 7, XXIV, 4, 1982, pp. 669-676. [5] M. PIBOULE, B. BRIAND and M. BEURIER, N. Jb. Miner. Abh., 143, 3, 1982, pp. 279-308. [6] F. WICKERT, N. Jb. Geol. Pal. Mk, 10, 1986, pp. 626-640. [7] A. MICHEL-LEVY, Bull. Serv. Carte géol. Fr., 18, 120, 1908, pp. 193-489. [8] J. M. PETERLONGO, Ann. Fac. Sci. Univ. Clermont, 4, 1960, 187 p. [9] F. WICKERT, Thèse doc, Univ. Freiburg/Br., 1985, 123 p. [10] H. D. PFLUG et E. REITZ, Naturwissenschaften, 74/8, 1987, pp. 386-387. [11] E. REITZ and R. HOELL, Jahrb. Geol. B.-A, 131/2 (in press). [12] B. V. TIMOFEEV, Acad. Sci. U.R.S.S., Nauka, 1969, 146 p. [13] H. J. HOFMANN, Geoscience Canada, 14/3, 1987, pp. 135-154. [14] G. VIDAL, Fossils and Strata, 9/7, 1976, 57 p. E. R. : Institut für Geowissenschaften und Lithosphärenforschung, Justus-Liebig-Universität, Senckenbergstraße 3, D 6300 Gießen, Allemagne fédérale; F. W. : Geologisches Institut, Universität Karlsruhe, Kaiserstraße 12, D 7500 Karlsruhe 1, Allemagne fédérale. Volcanologie / Volcanology Application
and M. BEURIER, N. Jb. Miner. Abh., 143, 3, 1982, pp. 279-308. [6] F. WICKERT, N. Jb. Geol. Pal. Mk, 10, 1986, pp. 626-640. [7] A. MICHEL-LEVY, Bull. Serv. Carte géol. Fr., 18, 120, 1908, pp. 193-489. [8] J. M. PETERLONGO, Ann. Fac. Sci. Univ. Clermont, 4, 1960, 187 p. [9] F. WICKERT, Thèse doc, Univ. Freiburg/Br., 1985, 123 p. [10] H. D. PFLUG et E. REITZ, Naturwissenschaften, 74/8, 1987, pp. 386-387. [11] E. REITZ and R. HOELL, Jahrb. Geol. B.-A, 131/2 (in press). [12] B. V. TIMOFEEV, Acad. Sci. U.R.S.S., Nauka, 1969, 146 p. [13] H. J. HOFMANN, Geoscience Canada, 14/3, 1987, pp. 135-154. [14] G. VIDAL, Fossils and Strata, 9/7, 1976, 57 p. E. R. : Institut für Geowissenschaften und Lithosphärenforschung, Justus-Liebig-Universität, Senckenbergstraße 3, D 6300 Gießen, Allemagne fédérale; F. W. : Geologisches Institut, Universität Karlsruhe, Kaiserstraße 12, D 7500 Karlsruhe 1, Allemagne fédérale. Volcanologie / Volcanology Application d'une analyse fractale à l'étude des cycles éruptifs du Piton de la Fournaise (La Réunion) : modèle d'une poussière de Cantor Jacques DUBOIS et Jean-Louis CHEMINÉE Résumé — De 1930 à 1987, 64 éruptions du Piton de la Fournaise ont été observées, séparées par des périodes calmes variant de quelques jours à six ans. Pour quantifier les séquences de repos, nous utilisons un modèle de poussière de Cantor (Smalley et coll., 1987). Cette analyse révèle deux valeurs caractéristiques de la dimension fractale : (1) D = 0,45 pour des périodes de repos allant de 1 à 10 mois et (2) D = 0,67 pour des périodes allant de 12 à 48 mois. On pourrait interpréter ces deux comportements comme traduisant le remplissage et la vidange d'une (ou plusieurs) chambre magmatique avec des débits différents. La méthode de la poussière de Cantor permet de calculer une probabilité de la longueur des périodes de repos plus sûre que la probabilité inconditionnelle de Aki (1981). On peut améliorer la prévision en incluant des précurseurs. Fractal analysis applied to the sequence of volcanic eruptions of the Piton de la Fournaise (Réunion Island): Cantor dust model Abstract — During the 1930-1987 period the Piton de
jours à six ans. Pour quantifier les séquences de repos, nous utilisons un modèle de poussière de Cantor (Smalley et coll., 1987). Cette analyse révèle deux valeurs caractéristiques de la dimension fractale : (1) D = 0,45 pour des périodes de repos allant de 1 à 10 mois et (2) D = 0,67 pour des périodes allant de 12 à 48 mois. On pourrait interpréter ces deux comportements comme traduisant le remplissage et la vidange d'une (ou plusieurs) chambre magmatique avec des débits différents. La méthode de la poussière de Cantor permet de calculer une probabilité de la longueur des périodes de repos plus sûre que la probabilité inconditionnelle de Aki (1981). On peut améliorer la prévision en incluant des précurseurs. Fractal analysis applied to the sequence of volcanic eruptions of the Piton de la Fournaise (Réunion Island): Cantor dust model Abstract — During the 1930-1987 period the Piton de la Fournaise erupted 64 times which were separated by repose periods of variable lengths (days to six years). In order to quantify the sequence of repose periods we use fractal analysis in the temporal domain (Smalley et al., 1987) with a Cantor dust model of the periods. This analysis reveals two characteristic values in the fractal dimension: (1) D = 0.45 for repose intervals of 1-10 months; and (2) D = 0.67 for repose intervals of 12 to 48 months. We could interpret these two behaviors as indicating the filling and emptying of one (or several) magmatic chamber(s) with different discharges. The method of the Cantor dust allows calculating a probability of the length of repose periods more securely than the unconditional probability of Aki (1981). One can improve the prediction by including precursors. 67 for intervals of 12-48 months. The two behaviors may correlate with feeding and draining of different magma chambers. The Cantor dust method leads to a probability trend of the repose periods which allows forecasting under certain circumstances. The method improves on the "Unconditional probability" of Aki (1981) and may include precursor observations as conditional variables. Abridged English Version — The sequence of volcanic eruptions for
two characteristic values in the fractal dimension: (1) D = 0.45 for repose intervals of 1-10 months; and (2) D = 0.67 for repose intervals of 12 to 48 months. We could interpret these two behaviors as indicating the filling and emptying of one (or several) magmatic chamber(s) with different discharges. The method of the Cantor dust allows calculating a probability of the length of repose periods more securely than the unconditional probability of Aki (1981). One can improve the prediction by including precursors. 67 for intervals of 12-48 months. The two behaviors may correlate with feeding and draining of different magma chambers. The Cantor dust method leads to a probability trend of the repose periods which allows forecasting under certain circumstances. The method improves on the "Unconditional probability" of Aki (1981) and may include precursor observations as conditional variables. Abridged English Version — The sequence of volcanic eruptions for a given volcano generally follows a complex temporal pattern. Quantitative statistical analysis of event recurrence is necessary for accurate forecasting. Wickman (1966, 1976) and Carta et al. (1981) have proposed that for certain volcanoes repose intervals follow a single normal distribution whereas for others, several normal distributions correspond to each style of activity. In detail, however, more complex statistics seem to be applicable. In this work, we use fractal analysis of the lengths of repose periods in the same way as Smalley et al. (1987) did for sequences of seismic events. To be significant, these methods must be applied to long time series. For the Piton de la Fournaise, there exists such a data base with detailed observations on many eruptions (Bénard and Krafft, 1987). Our analysis uses the Cantor dust model in the time domain: given Nt, the number of periods of length ut, we have N; = (1/M,)D. An estimate of D may be obtained considering the fraction xt of intervals of length ut that contain eruptive events. Smalley et al. (1987) have shown that Ln K,/Lnx; = D. In Figures 1 and 2, we give some examples of fragmentation characterized by fractal dimensions (D) between 0 and
style of activity. In detail, however, more complex statistics seem to be applicable. In this work, we use fractal analysis of the lengths of repose periods in the same way as Smalley et al. (1987) did for sequences of seismic events. To be significant, these methods must be applied to long time series. For the Piton de la Fournaise, there exists such a data base with detailed observations on many eruptions (Bénard and Krafft, 1987). Our analysis uses the Cantor dust model in the time domain: given Nt, the number of periods of length ut, we have N; = (1/M,)D. An estimate of D may be obtained considering the fraction xt of intervals of length ut that contain eruptive events. Smalley et al. (1987) have shown that Ln K,/Lnx; = D. In Figures 1 and 2, we give some examples of fragmentation characterized by fractal dimensions (D) between 0 and 1; when only one segment is considered, D is equal to 0, the Euclidean dimension of a point, whereas considering all segments, D is equal to 1, the dimension of a straight line. When a finite approximation of the Cantor dust is used (Mandelbrot, 1963) and when repose intervals are randomized, they become statistically independent. It has also been verified that for most volcanoes, including the Piton de la Fournaise, the length of the repose intervals (U) follows a hyperbolic distribution Pr(U>u) = TuD. We have applied this method to the Piton de la Fournaise volcano over two time intervals: 1930-1987 and 1950-1987. Ln x, - Ln x plots show that data points are aligned on two discrete straight lines for the 1-10 months and 10-48 months intervals respectively (Fig. 3, Table I). The corresponding D values are 0.45 and 0.67 indicating (Smalley et al., 1987) that for the 1-10 months intervals, repose period clusters are more isolated than for the longer intervals. The D values may also be used to estimate the probability that an interval of length U be larger or equal to ut (Mandelbrot, 1982). The cumulative number of repose periods Nu is given by N =