High-frequency palaeoenvironmental changes on a shallow carbonate platform during a marine transgression (Late Oxfordian, Swiss Jura Mountains)

Based on a well-established bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum ammonite zone (Late Oxfordian) is investigated in six Swiss Jura sections representing a shallow-water carbonate platform. From the detailed facies and microfacies analysis of oncoid-rich (Hauptmumienbank Member) and ooid-rich (Steinebach Member) limestones, a microfacies classification is established, depositional environments are interpreted, and a depositional model for the Swiss Jura platform is proposed. The sequence- and cyclostratigraphic interpretation has been performed for the transgressive part of the medium-scale sequence Ox6+, independently for each section, with a very high time resolution at the scale of elementary depositional sequences. The good correlation of the elementary and small-scale sequences between the six studied sections and the similar number of elementary sequences in all sections strongly suggest that allocyclic processes were involved in their formation. The hierarchically stacked depositional sequences (small-scale and elementary sequences) result from orbitally controlled sea-level changes with periodicities of 100 and 20 kyr, respectively. Thickness variations in the correlated small-scale and elementary sequences imply variable sedimentation rates, probably resulting from differential subsidence due to the activity of tectonic blocks. The tectonically controlled platform morphology contributed significantly to the general pattern of depositional environments and, combined with high-frequency sea-level fluctuations, created a complex facies distribution in time and space on the Swiss Jura carbonate platfor

Oncoid growth and distribution controlled by sea-level fluctuations and climate (Late Oxfordian, Swiss Jura Mountains)

Abundant lagoonal oncoids occur in the Late Oxfordian Hauptmumienbank Member of the Swiss Jura Mountains. Four oncoid types are observed in the studied sections and classified according to the oncoid surface morphology, the structure and composition of the cortex, and the texture and fauna of the encasing sediment. Micrite-dominated oncoids (types 1 and 2) have a smooth surface. Type 1 has a rather homogeneous cortex and occurs in moderate-energy environments. Type 2 presents continuous or discontinuous micritic laminae. It is associated with a low-diversity fauna and occurs in high-energy facies. Bacinella and Lithocodium oncoids (types 3 and 4) display a lobate surface. They are dominated by microencrusters (Bacinella irregularis and Lithocodium aggregatum) and are found in low-energy facies. The stratigraphic and spatial distribution of these oncoid types shows a correlation with the sequence-stratigraphic evolution of the studied interval, and thus with relative sea-level fluctuations. It can be shown that these sea-level fluctuations were controlled by orbital cycles with 100- and 20-kyr periodicities. At the scale of 100- and 20-kyr sequences, types 1 and 2 oncoids are preferentially found around sequence boundaries and in transgressive deposits, while types 3 and 4 oncoids are preferentially found around maximum floodings and in highstand deposits. This implies that changes of water energy and water depth were direct controlling factors. Discrepancies in oncoid distribution point to additional controlling factors. Platform morphology defines the distribution and type of the lagoon where the oncoids flourished. A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous influx, which modifies water transparency and trophic level and thus plays a role in the biotic composition and diversity in the oncoid corte

Dating the onset and nature of the Middle Permian Emeishan large igneous province eruptions in SW China using conodont biostratigraphy and its bearing on mantle plume uplift models

The Middle Permian Emeishan large igneous province of SW China has provided the quintessential example of the phenomenon of kilometre-scale pre-eruption domal uplift associated with mantle plume impingement on the base of the lithosphere. One key line of evidence is an interpreted zone of truncation of the platform carbonates belonging to the Maokou Formation that underlies the volcanic pile. Here we test this interpretation by conodont age dating the uppermost beds of the Maokou Formation in sections from Yunnan, Sichuan, Guizhou and Guangxi provinces, which span locations from the inner part of the igneous province to several hundred kilometres beyond its margins. The results show that eruptions began in the Jinogondolella altudaensis Zone ( 263 Ma) of the Middle Capitanian Stage and greatly increased in extent and volume in the J. xuanhanensis Zone ( 262 Ma). Pre-eruption uplift was muted, and most locations within the terrain and at many locations beyond its margins witnessed platform collapse (not uplift) with deep-water facies (radiolarian cherts, submarine fans) developing in the J. altudaensis Zone. The clearest evidence for an emergence surface occurs around the margins of the province in the J. xuanhanensis Zone. This is after the initial onset of eruptions and marks either a eustatic sequence boundary or a brief pulse of tectonic uplift contemporaneous with volcanism. As with recent studies on the basal volcanic successions of the Emeishan LIP, kilometre-scale plume-related domal uplift prior to Emeishan eruptions is not supported by these data; rather a more complex interaction between plume and lithosphere with minor localized uplift and subsidence is inferred

Co-occurrence of the foraminifer Mohlerina basiliensis with Bacinella–Lithocodium oncoids: palaeoenvironmental and palaeoecological implications (Late Oxfordian, Swiss Jura)

The stratigraphical and spatial distribution of the foraminifer Mohlerina basiliensis and Bacinella–Lithocodium oncoids was studied in the lagoonal deposits of the oncoid-rich Hauptmumienbank Member from the Swiss Jura Mountains (Late Oxfordian, NW Switzerland). Mohlerina basiliensis has a double-layered wall structure (thick outer hyaline-radial calcite layer with a lamellar structure and thin inner microgranular calcite layer) and is preferentially found in low-energy facies and associated with normal-marine bioclasts and Bacinella–Lithocodium oncoids. The distribution of Mohlerina basiliensis is relatively patchy. The ‘distal’ platform facies present a higher abundance and a more extensiv stratigraphic occurrence of this foraminifer. The timing of the ecological evolution is given by the sequence- and cyclostratigraphic framework and illustrates the stepwise flooding of the platform. Bacinella–Lithocodium oncoids have diameters of a few millimetres to several centimetres and are composed of an alternation of Bacinella–Lithocodium meshwork and thin micritic laminae. They are found preferentially in low-energy facies (bioclastic wackestones) of open lagoons. The distribution of Bacinella–Lithocodium oncoids through time and space is also patchy; however, they appear more abundantly in ‘distal’ platform facies. This study reveals the co-occurrence of Mohlerina basiliensis and Bacinella–Lithocodium oncoids and thus suggests similar environmental conditions

High-frequency palaeoenvironmental changes in mixed carbonate-siliciclastic sedimentary systems (Late Oxfordian, Switzerland, France, and southern Germany)

Le but principal de cette étude est d’examiner les changements paléoenvironnementaux de haute fréquence survenant durant une transgression marine dans des systèmes sédimentaires mixtes silicocarbonatés. Basée sur un cadre biostratigraphique et séquentiel bien établi, une fenêtre de temps précise dans la zone à Bimammatum de l’Oxfordien supérieur est étudiée. Sept coupes de plate-forme peu profonde (Jura suisse, Lorraine), deux coupes de plate-forme profonde (Haute-Marne, Jura souabe) et une coupe de bassin (SE France) ont été levées et analysées en détail. Les dépôts ont ensuite été interprétés en terme de paléoenvironnments et de stratigraphie séquentielle et cyclostratigraphie avec une haute résolution dans le temps. L’analyse des faciès et microfaciès permet de proposer des modèles de dépôt pour la plate-forme du Jura Suisse et les autres zones d’étude. L’analyse séquentielle et cyclostratigraphique de haute résolution permet de définir l’empilement hiérarchique des séquences de dépôt (séquences à moyen terme, à court terme et élémentaires) formées par les fluctuations du niveau marin avec des périodicités de 400, 100, et 20 ka respectivement, contrôlées par les cycles orbitaux. Cette étude examine les dépôts situés dans la première moitié d’une séquence à moyen terme, correspondant à deux séquences à court terme, chacune composée de cinq séquences élémentaires. Dans les coupes de plate-forme peu profonde, une séquence élémentaire consiste généralement en un à quatre bancs incluant des intervalles marneux plus ou moins développés. Dans les coupes de plate-forme profonde, une séquence élémentaire consiste généralement en un à deux bancs calcaires avec un intervalle marneux plus ou moins développé. Dans la coupe de bassin, une séquence élémentaire est définie par un banc calcaire et un intervalle marneux. La bonne corrélation des séquences de dépôt sur de longues distances entre les sept coupes de plateforme peu profonde ainsi que le nombre similaire de séquences élémentaires dans toutes les coupes sont des arguments valables pour montrer que des processus allocycliques doivent avoir joué un rôle dans la formation de ces séquences de dépôt. Des facteurs additionnels tels que la position sur la plate-forme et la morphologie pré-existante doivent être considérées dans la formation des séquences sur une plate-forme peu profonde. La présence de variations d’épaisseurs à l’échelle des séquences élémentaires et à court terme révèle des taux d’accumulation sédimentaire variables, interprétés comme résultant principalement de la subsidence différentielle due à l’activité tectonique de blocs. Le relief créé par la tectonique contribue aussi significativement à la distribution générale des faciès. De plus, la distribution irrégulière des siliciclastiques peut être expliquée par des dépressions locales, créées par la subsidence différentielle, et servant de depôtcentres. La distribution verticale et horizontale des faciès, des oncoïdes et des foraminifères benthiques est étudiée dans le cadre de temps haute résolution établi dans ce travail (résolution temporelle de 20 ka). Des changements de faciès latéraux sont mis en évidence par comparaison des séquences élémentaires et à court terme entre les coupes du Jura suisse. Ils reflètent la dynamique et complexité des systèmes sédimentaires où des environnements juxtaposés évoluent et se modifient dans le temps et dans l’espace. Quatre types d’oncoïdes sont définis à partir de la morphologie de surface, la configuration et composition du cortex et le sédiment encaissant. Les oncoïdes dont le cortex est dominé par la micrite (types 1 et 2) ont une surface lisse tandis que les oncoïdes riches en Bacinella-Lithocodium (types 3 et 4) ont une surface lobée. La distribution stratigraphique et spatiale de ces types d’oncoïdes montre une corrélation avec l’évolution séquentielle et donc les fluctuations du niveau marin relatif. A l’échelle des séquences élémentaires et à court terme, les oncoïdes de type 1 et 2 sont préférentiellement trouvées autour des limites de séquence et dans les cortèges de dépôt transgressifs tandis que les oncoïdes de type 3 et 4 sont préférentiellement trouvés autour des maxima d’inondation et dans les cortèges de haut niveau marin. Cela implique que des changements d’énergie et de profondeur d’eau ont été des facteurs contrôlant directs. Des disparités dans la distribution des oncoïdes suggèrent des facteurs contrôlant additionnels. La morphologie de la plate-forme définit la distribution et le type de lagon où les oncoïdes se développent. Un faible taux d’accumulation sédimentaire est requis pour la croissance des oncoïdes. Des changements d’humidité dans l’arrière-pays agissent sur l’influx terrigène ce qui modifie la transparence et le niveau trophique de l’eau et joue donc un rôle dans la composition et la diversité biotique dans le cortex des oncoïdes. Cette étude démontre que les oncoïdes sont des indicateurs valables pour des études paléoenvironnementales et paléoécologiques de haute résolution. Les assemblages de foraminifères benthiques des coupes du Jura suisse comprennent des formes agglutinées (Textularidés), dans une plus faible proportion des formes porcelanées (Miliolidés) et localement des foraminifères hyalins (seulement Lenticulina). La distribution des foraminifères porcelanés et hyalins montre une corrélation avec l’évolution séquentielle. Les Miliolidés sont préférentiellement trouvés dans les cortèges de dépôt de haut niveau marin de certaines séquences élémentaires et/ou à court terme. Les foraminifères hyalins, rare dans les coupes étudiées, sont préférentiellement trouvés dans les dépôts transgressifs de certaines séquences élémentaires. Par conséquence, les foraminifères benthiques sont indirectement liés aux changements du niveau marin relatif (changements d’accommodation) ainsi qu’aux changements climatiques (apports sédimentaires et de nutriments). La distribution du foraminifère benthique Mohlerina basiliensis et des oncoïdes riches en Bacinella-Lithocodium montre une forte corrélation. La co-existence de ces deux composants suggère qu’ils requièrent des conditions écologiques similaires. De plus, l’apparition de plus en plus tardive des M. basiliensis des coupes distales aux coupes proximales illustre l’inondation par étape de la plate-forme et implique alors une dépendance aux conditions marines. Les assemblages des minéraux argileux et les isotopes stables donnent des informations paléoenvironnementales à l’échelle des séquences de 400 ka et 100 ka; une plus haute résolution est difficile à obtenir probablement à cause de l’homogénéisation du temps par des processus sédimentologiques et/ ou diagénétiques. Les tendances à moyen terme du contenu en kaolinite et du rapport kaolinite/illite entre toutes les coupes, de la plate-forme peu profonde au bassin, montre une corrélation particulièrement autour de la limite de séquence SB Ox6+ et du maximum d’inondation à moyen terme MF Ox6+. La distribution des minéraux argileux n’était pas seulement contrôlée par des changements climatiques mais aussi par des changements du niveau marin relatif et de la morphologie de la plate-forme. L’analyse des isotopes de l’oxygène et du carbone montre des fluctuations mineures (± 1 ‰) qui reflètent probablement des variations de conditions environnementales locales (par exemple, la salinité, la température et/ou le niveau trophique).The main goal of this study is to monitor the highfrequency palaeoenvironmental changes occurring during a marine transgression in mixed carbonatesiliciclastic sedimentary systems. Based on a wellestablished bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum Zone of the Late Oxfordian is investigated. Seven shallow platform sections (Swiss Jura, Lorraine), two deep platform sections (Haute-Marne, Swabian Jura), and one basin section (SE France) have been logged and analysed in detail. Then, the deposits have been interpreted in terms of palaeoenvironments and sequence- and cyclostratigraphy with a high time resolution. Facies and microfacies analysis allows to propose depositional models for the Swiss Jura platform and the other studied areas. The highresolution sequence- and cyclostratigraphic analysis permits defining hierarchically stacked depositional sequences: medium-scale, small-scale, and elementary sequences, formed through orbitally controlled sealeve changes with periodicities of 400, 100, and 20 kyr, respectively. This study investigates deposits comprised in the first half of a medium-scale sequence, corresponding to two small-scale sequences, each composed of five elementary sequences. In the shallow platform sections, an elementary sequence generally consists of one to four beds including more or less developed marl intervals. In the deep platform sections, an elementary sequence generally consists of one or two limestone beds with a more or less developed marl interval. In the basin section, an elementary sequence is defined by one marl-limestone couplet. The good correlation of depositional sequences over long distances between the seven shallow platform sections and the similar number of elementary sequences in all sections are valuable arguments that allocyclic processes must have been involved in the formation of these depositional sequences. Additional factors such as the position on the platform and the pre-existing morphology have to be considered in the formation of depositional sequences on a shallow platform. The presence of thickness variations at the scale of small-scale and elementary sequences reveals variable sediment accumulation rates, interpreted as resulting mainly from differential subsidence due to the activity of tectonic blocks. The relief created by tectonics therefore contributed significantly to the general facies distribution. Furthermore, the irregular distribution of siliciclastics can be explained by localized depressions, which were created by differential subsidence and served as depocenters. The vertical and lateral distribution of facies, oncoids, and benthic foraminifera is investigated within the narrow high-resolution time framework established (20-kyr time resolution). Significant lateral facies changes are evidenced by comparison of time-equivalent small-scale and elementary sequences between the Swiss Jura sections. They reflect the dynamics and complexity of sedimentary systems where juxtaposed sub-environments evolved and shifted through space and time. Four types of oncoids are defined based on surface morphology, configuration and composition of the cortex, and the encasing sediment. Micrite-dominated oncoids (types 1 and 2) have a smooth surface and Bacinella-Lithocodium oncoids (types 3 and 4) display a lobate surface. The stratigraphic and spatial distribution of these oncoid types shows a correlation with the sequence-stratigraphic evolution, and thus with relative sea-level fluctuations. At the scale of 100-kyr and 20-kyr sequences, type 1 and 2 oncoids arepreferentially found around sequence boundaries and in transgressive deposits, while type 3 and 4 oncoids are preferentially found around maximum floodings and in highstand deposits. This implies that changes of water energy and water depth were direct controlling factors. Discrepancies in oncoid distribution point to additional controlling factors. Platform morphology defines the distribution and type of the lagoon where the oncoids flourished. A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous influx, which modifies water transparency and trophic level and thus plays a role in the biotic composition and diversity in the oncoid cortex. This study demonstrates that oncoids are valuable proxies for high-resolution palaeoenvironmental and palaeoecological studies. The benthic foraminifer assemblages of the Swiss Jura sections include agglutinated forms (Textularids), in a lower amount porcelaneous forms (Miliolids), and locally hyaline foraminifera (only Lenticulina). The distribution of porcelaneous and hyaline foraminifera shows a correlation with the sequence-stratigraphic evolution. Miliolids are preferentially found in the highstand deposits of the elementary and/or small-scale sequences. The hyaline foraminifera, rare in the studied sections, are preferentially found in the transgressive deposits of elementary sequences. Consequently, the benthic foraminifera are indirectly linked to relative sea-level (accommodation changes) and indirectly to climate changes (sediment and nutrient input). The distribution of the benthic foraminifer Mohlerina basiliensis and Bacinella-Lithocodium oncoids shows a strong correlation. The co-occurrence of these two components suggests that they require similar ecological conditions. In addition, the successively later occurrence of M. basiliensis from “distal” to “proximal” sections illustrates the stepwise flooding of the platform and thus implies a dependence on normal-marine conditions. Clay-mineral assemblages and stable isotopes give palaeoenvironmental information on the scale of 400 kyr and 100 kyr but a higher time resolution is difficult to obtain probably due to time averaging by sedimentological and/or diagenetic processes. The mediumscale trends of the kaolinite content and K/I ratio between all sections from shallow platform to basin show a correlation, particularly around the mediumscale SB Ox6+ and MF Ox6+. Clay mineral distribution was not only controlled by climate changes but also by relative sea-level changes and platform morphology. Carbon and oxygen isotope analysis shows minor fluctuations (±1‰) that probably reflect variations of local environmental conditions (e.g., salinity, temperature, trophic level)

Co-occurrence of the foraminifer Mohlerina basiliensis with Bacinella–Lithocodium oncoids: palaeoenvironmental and palaeoecological implications (Late Oxfordian, Swiss Jura)

this work

Controls on facies mosaics of carbonate platforms: a case study from the Oxfordian of the Swiss Jura

Modern shallow-water carbonate systems commonly display a complex pattern of juxtaposed depositional environments with a patchy facies distribution (facies mo saics). On ancient carbonate platforms, the reconstruction of lateral facies distribution is often hampered not only by discontinuous outcrop but also by lack of suffi ciently high time resolution. This case study from the Oxfordian (Late Jurassic) of the Swiss Jura Mountains demonstrates a way to improve the temporal and spatial resolution for the interpretation of carbonate rocks. Sequence-stratigraphic and cyclostratigraphic analyses have been performed that provide a basis for defi ning depositional sequences, which formed through sea-level changes that were induced by the 400-, 100- and 20-kyr orbital cycles. On the 100-kyr scale, sequence boundaries are well developed and can be correlated between sections. However, identifi cation and correlation of sequences related to the 20-kyr cycle may be diffi cult if local processes overprinted the record of orbitally controlled sea-level changes. The reconstruction of facies distribution along selected time lines gives a dynamic picture of platform evolution with time steps of a few ten thousand years and helps to interpret the controlling factors such as differential subsidence, low-amplitude eustatic sea-level fl uctuations, climate and ecology of the carbonate-producing organisms. Reefs and ooid shoals developed preferentially on topographic highs and thus accentuated platform morphology. Siliciclastics were shed onto the platform during sea-level falls and increased rainfall in the hinterland; their distribution was controlled by platform morphology. Siliciclastics and associated nutrients hindered carbonate production and thus indirectly infl uenced platform morphology. In addition to these controls, random processes acted on the smaller-scale facies relationships. Sedimentation rates can be estimated for each facies type over time spans of 10–20 kyr. They can thus be compared more easily with Holocene rates than if averaged over millions of years. This study shows that the Oxfordian platform in the Swiss Jura was as complex and dynamic as Holocene carbonate systems

High-frequency palaeoenvironmental changes on a shallow carbonate platform during a marine transgression (Late Oxfordian, Swiss Jura Mountains)

Based on a well-established bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum ammonite zone (Late Oxfordian) is investigated in six Swiss Jura sections representing a shallow-water carbonate platform. From the detailed facies and microfacies analysis of oncoid-rich (Hauptmumienbank Member) and ooid-rich (Steinebach Member) limestones, a microfacies classification is established, depositional environments are interpreted, and a depositional model for the Swiss Jura platform is proposed. The sequence- and cyclostratigraphic interpretation has been performed for the transgressive part of the medium-scale sequence Ox6+, independently for each section, with a very high time resolution at the scale of elementary depositional sequences. The good correlation of the elementary and small-scale sequences between the six studied sections and the similar number of elementary sequences in all sections strongly suggest that allocyclic processes were involved in their formation. The hierarchically stacked depositional sequences (small-scale and elementary sequences) result from orbitally controlled sea-level changes with periodicities of 100 and 20 kyr, respectively. Thickness variations in the correlated small-scale and elementary sequences imply variable sedimentation rates, probably resulting from differential subsidence due to the activity of tectonic blocks. The tectonically controlled platform morphology contributed significantly to the general pattern of depositional environments and, combined with high-frequency sea-level fluctuations, created a complex facies distribution in time and space on the Swiss Jura carbonate platform