M. Morake, J. N. F. O'Kennedy, M. Knoper, M. de Kock, J. Kramers, G. Grantham, G. Belyanin, M. Elburg
Abstract New 40Ar/39Ar data from dykes intruded into Sverdrupfjella and Ahlmanryggen, Dronning Maud Land, Antarctica, indicate that dyke emplacement commenced at c. 207 Ma and lasted until c. 178 Ma. Whereas the ages ascribed to the Karoo-age magmatism contributing to Gondwana breakup are typically inferred as being c. 182 Ma, the data indicate that ages older than c.192 Ma in the broader Karoo Province are restricted to western Dronning Maud Land, Antarctica, indicating the locality where breakup was initiated. Limited palaeomagnetic data from c. 178–185 Ma dykes combined with published palaeomagnetic data from similar-aged dykes in Vestfjella and the Ferrar Province, suggest that Antarctica had already drifted/rifted significantly away from southern Africa from c. 207 to c. 180 Ma, earlier than previously thought. The data, if correct, require a re-evaluation of the ages ascribed to ocean-floor anomalies used to constrain reconstructions of Gondwana and may provide insight into the history of microcontinental blocks including the Falkland Islands, Haag nunataks, Ellsworth–Whitmore block and Maurice Ewing Bank. Supplementary material: Petrography, Ar isotope data from all samples and summary of ages are available at https://doi.org/10.6084/m9.figshare.c.5612838
{"title":"The age and palaeomagnetism of Jurassic dykes, western Dronning Maud Land: implications for Gondwana breakup","authors":"M. Morake, J. N. F. O'Kennedy, M. Knoper, M. de Kock, J. Kramers, G. Grantham, G. Belyanin, M. Elburg","doi":"10.1144/SP518-2021-44","DOIUrl":"https://doi.org/10.1144/SP518-2021-44","url":null,"abstract":"Abstract New 40Ar/39Ar data from dykes intruded into Sverdrupfjella and Ahlmanryggen, Dronning Maud Land, Antarctica, indicate that dyke emplacement commenced at c. 207 Ma and lasted until c. 178 Ma. Whereas the ages ascribed to the Karoo-age magmatism contributing to Gondwana breakup are typically inferred as being c. 182 Ma, the data indicate that ages older than c.192 Ma in the broader Karoo Province are restricted to western Dronning Maud Land, Antarctica, indicating the locality where breakup was initiated. Limited palaeomagnetic data from c. 178–185 Ma dykes combined with published palaeomagnetic data from similar-aged dykes in Vestfjella and the Ferrar Province, suggest that Antarctica had already drifted/rifted significantly away from southern Africa from c. 207 to c. 180 Ma, earlier than previously thought. The data, if correct, require a re-evaluation of the ages ascribed to ocean-floor anomalies used to constrain reconstructions of Gondwana and may provide insight into the history of microcontinental blocks including the Falkland Islands, Haag nunataks, Ellsworth–Whitmore block and Maurice Ewing Bank. Supplementary material: Petrography, Ar isotope data from all samples and summary of ages are available at https://doi.org/10.6084/m9.figshare.c.5612838","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"22 1","pages":"255 - 284"},"PeriodicalIF":0.0,"publicationDate":"2021-09-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82769259","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
T. Meshram, S. Mahapatro, J. Aravind, M. Dora, S. Baswani, G. Gopalakrishna, R. Meshram, V. V. Sesha Sai, K. Randive, Jitendra Das
Abstract This study reports on two lamprophyre dykes from the Rapuru area along the margin of the Eastern Dharwar Craton (EDC) and the Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along the southern extension of the Prakassam Alkaline Province (PAP). The RL dykes are deformed, yet still preserve a porphyritic–panidiomorphic texture, with mica phenocrysts, and amphibole and feldspars in the groundmass. Geochemically, the RL dykes have a low Mg# (0.28–0.37), and Ni (30–60 ppm) and Cr (119–228 ppm) concentrations that indicate their evolved nature, such as for other reported lamprophyres from the PAP and EDC. This is further supported by Sr–Nd isotopic ratios that show an affinity towards a mid-ocean ridge basalt (MORB)–ocean island basalt (OIB)-like signature and a juvenile magmatic nature. The RL seems to have been affected by two major influences, namely, the primary source region, which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in a back-arc-basin environment. The initial 87Sr/86Sr ratio (c. 0.7012–0.7045) and initial εNd ratios (3.13–7.93) are in line with back-arc basin basalts recorded in other parts of the world. The field observations and bulk-rock Sr–Nd radiogenic isotope values in the present study support the Paleoproterozoic nature of the RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted-slab-triggered metasomatism of the overlying mantle wedge in a subduction-related geodynamic setting. Such intrusive lamprophyre rocks of older ages are limited in India as well as other parts of the world. The 2.1 and 1.8 Ga rocks are widely considered to represent the initial accretion and final break-up of an erstwhile Columbia supercontinent assembly. We argue that the RL were formed in the Paleoproterozoic during the waxing stages of the Columbia supercontinent assembly in a back-arc basin environment, most probably due to the low degree of partial melting of the asthenosphere–lithospheric interaction caused by the introduction of an influx of subduction components into the arc–back-arc basin system.
{"title":"Geochemistry and Sr–Nd isotopic studies of Paleoproterozoic (c. 2.3 Ga) meta-lamprophyre from the Rapuru area, Nellore Schist Belt, southern India: implications for back-arc basin magmatism and its relevance to the Columbia supercontinent assembly","authors":"T. Meshram, S. Mahapatro, J. Aravind, M. Dora, S. Baswani, G. Gopalakrishna, R. Meshram, V. V. Sesha Sai, K. Randive, Jitendra Das","doi":"10.1144/SP513-2021-4","DOIUrl":"https://doi.org/10.1144/SP513-2021-4","url":null,"abstract":"Abstract This study reports on two lamprophyre dykes from the Rapuru area along the margin of the Eastern Dharwar Craton (EDC) and the Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along the southern extension of the Prakassam Alkaline Province (PAP). The RL dykes are deformed, yet still preserve a porphyritic–panidiomorphic texture, with mica phenocrysts, and amphibole and feldspars in the groundmass. Geochemically, the RL dykes have a low Mg# (0.28–0.37), and Ni (30–60 ppm) and Cr (119–228 ppm) concentrations that indicate their evolved nature, such as for other reported lamprophyres from the PAP and EDC. This is further supported by Sr–Nd isotopic ratios that show an affinity towards a mid-ocean ridge basalt (MORB)–ocean island basalt (OIB)-like signature and a juvenile magmatic nature. The RL seems to have been affected by two major influences, namely, the primary source region, which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in a back-arc-basin environment. The initial 87Sr/86Sr ratio (c. 0.7012–0.7045) and initial εNd ratios (3.13–7.93) are in line with back-arc basin basalts recorded in other parts of the world. The field observations and bulk-rock Sr–Nd radiogenic isotope values in the present study support the Paleoproterozoic nature of the RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted-slab-triggered metasomatism of the overlying mantle wedge in a subduction-related geodynamic setting. Such intrusive lamprophyre rocks of older ages are limited in India as well as other parts of the world. The 2.1 and 1.8 Ga rocks are widely considered to represent the initial accretion and final break-up of an erstwhile Columbia supercontinent assembly. We argue that the RL were formed in the Paleoproterozoic during the waxing stages of the Columbia supercontinent assembly in a back-arc basin environment, most probably due to the low degree of partial melting of the asthenosphere–lithospheric interaction caused by the introduction of an influx of subduction components into the arc–back-arc basin system.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"205 1","pages":"103 - 132"},"PeriodicalIF":0.0,"publicationDate":"2021-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80399564","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract The study of past climate changes is pivotal for understanding the complex biogeochemical interactions through time between the geosphere, atmosphere, hydrosphere and biosphere, which are critical for predicting future global changes. The Toarcian Oceanic Anoxic Event, also known as the Jenkyns Event, was a hyperthermal episode that occurred during the early Toarcian (c. 183 Ma; Early Jurassic) and resulted in numerous collateral effects including global warming, enhanced weathering, sea-level change, carbonate crisis, marine anoxia–dysoxia and biotic crisis. The IGCP-655 project of the IUGS–UNESCO has constituted an international network of researchers with different disciplinary skills who have collaborated and shared conceptual advances on uncovering drivers of the environmental changes and ecosystem responses. This volume, Carbon Cycle and Ecosystem Response to the Jenkyns Event in the Early Toarcian (Jurassic), presents 16 works that investigate the early Toarcian environmental changes related to the global warming, sea-level rise, carbon cycle perturbation and second-order mass extinction through biostratigraphy, micropalaeontology, palaeontology, ichnology, palaeoecology, sedimentology, integrated stratigraphy, inorganic, organic and isotopic geochemistry, and cyclostratigraphy.
{"title":"The Toarcian Oceanic Anoxic Event: where do we stand?","authors":"M. Reolid, E. Mattioli, L. V. Duarte, W. Ruebsam","doi":"10.1144/SP514-2021-74","DOIUrl":"https://doi.org/10.1144/SP514-2021-74","url":null,"abstract":"Abstract The study of past climate changes is pivotal for understanding the complex biogeochemical interactions through time between the geosphere, atmosphere, hydrosphere and biosphere, which are critical for predicting future global changes. The Toarcian Oceanic Anoxic Event, also known as the Jenkyns Event, was a hyperthermal episode that occurred during the early Toarcian (c. 183 Ma; Early Jurassic) and resulted in numerous collateral effects including global warming, enhanced weathering, sea-level change, carbonate crisis, marine anoxia–dysoxia and biotic crisis. The IGCP-655 project of the IUGS–UNESCO has constituted an international network of researchers with different disciplinary skills who have collaborated and shared conceptual advances on uncovering drivers of the environmental changes and ecosystem responses. This volume, Carbon Cycle and Ecosystem Response to the Jenkyns Event in the Early Toarcian (Jurassic), presents 16 works that investigate the early Toarcian environmental changes related to the global warming, sea-level rise, carbon cycle perturbation and second-order mass extinction through biostratigraphy, micropalaeontology, palaeontology, ichnology, palaeoecology, sedimentology, integrated stratigraphy, inorganic, organic and isotopic geochemistry, and cyclostratigraphy.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"63 1","pages":"1 - 11"},"PeriodicalIF":0.0,"publicationDate":"2021-08-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89855006","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Schneider, F. Scholze, A. Ross, B. M. Blake, S. Lucas
Abstract For the biostratigraphy of mixed continental-marine and purely continental sections in the palaeotropical belt of Euramerica, 9 insect and 8 conchostracan zones are newly defined or improved. These zones encompass the time interval from the Early Pennsylvanian (middle Bashkirian) up into the early Permian (early Asselian) of the Euramercian biotic province. They are linked as much as possible to the marine Standard Global Chronostrigraphic Scale by common occurrences of insects and/or conchostracans with conodonts in mixed marine-continental sections as well as by the thus far available and reliable radioisotopic ages of associated volcanic rocks. This insect and conchostracan zonation is an alternative tool to the well-established macro-plant biostratigraphy of the Pennsylvanian. In contrast to the latter, only single specimens of insects or conchostracans, even if more rare than plant remains, allow biostratigraphic dating with a similar high temporal resolution.
{"title":"Improved blattoid insect and conchostracan zonation for the Late Carboniferous, Pennsylvanian, of Euramerica","authors":"J. Schneider, F. Scholze, A. Ross, B. M. Blake, S. Lucas","doi":"10.1144/SP512-2021-93","DOIUrl":"https://doi.org/10.1144/SP512-2021-93","url":null,"abstract":"Abstract For the biostratigraphy of mixed continental-marine and purely continental sections in the palaeotropical belt of Euramerica, 9 insect and 8 conchostracan zones are newly defined or improved. These zones encompass the time interval from the Early Pennsylvanian (middle Bashkirian) up into the early Permian (early Asselian) of the Euramercian biotic province. They are linked as much as possible to the marine Standard Global Chronostrigraphic Scale by common occurrences of insects and/or conchostracans with conodonts in mixed marine-continental sections as well as by the thus far available and reliable radioisotopic ages of associated volcanic rocks. This insect and conchostracan zonation is an alternative tool to the well-established macro-plant biostratigraphy of the Pennsylvanian. In contrast to the latter, only single specimens of insects or conchostracans, even if more rare than plant remains, allow biostratigraphic dating with a similar high temporal resolution.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"32 1","pages":"865 - 891"},"PeriodicalIF":0.0,"publicationDate":"2021-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90857051","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract The Archean age granite gneiss basement along the Prydz Bay coastline in East Antarctica hosts north–south-, east–west-, NE–SW- and NW–SE-trending mafic dyke swarms in the Vestfold Hills region that intruded between 2420 and 1250 Ma. The orientations of dykes do not show a direct correlation with the dyke geochemistry. Instead the dykes can be broadly discriminated into high-Mg and Fe-rich tholeiites. The former type is more siliceous, large ion lithophile elements (LILEs), high field strength elements (HFSEs) and light REEs enriched crystallized from a fractionated melt with a notable crustal component or fluid enrichment through the previous subduction process. The Fe-rich tholeiites are less siliceous, have lower abundances of LILEs and REEs, that indicates derivation from an undifferentiated, primitive melt. The geochemical characteristics of both types underline a shallow level and a high degree of melting in the majority of cases, and a broadly island arc basalt (IAB) affinity. Palaeomagnetic analysis of hand samples shows directional groups consistent with geochemical groupings. The Vestfold Hills dykes show a possible linkage with the coeval mafic dykes in the Eastern Dharwar and Bastar cratons of the South Indian Block, based on the similarity in the Paleoproterozoic palaeolatitudes.
{"title":"Geochemical and palaeomagnetic characteristics of the Vestfold Hills mafic dykes in the Prydz Bay region: implications of a Paleoproterozoic connection between East Antarctica and Proto-India","authors":"M. Pandit, A. Pivarunas, J. Meert","doi":"10.1144/SP518-2021-33","DOIUrl":"https://doi.org/10.1144/SP518-2021-33","url":null,"abstract":"Abstract The Archean age granite gneiss basement along the Prydz Bay coastline in East Antarctica hosts north–south-, east–west-, NE–SW- and NW–SE-trending mafic dyke swarms in the Vestfold Hills region that intruded between 2420 and 1250 Ma. The orientations of dykes do not show a direct correlation with the dyke geochemistry. Instead the dykes can be broadly discriminated into high-Mg and Fe-rich tholeiites. The former type is more siliceous, large ion lithophile elements (LILEs), high field strength elements (HFSEs) and light REEs enriched crystallized from a fractionated melt with a notable crustal component or fluid enrichment through the previous subduction process. The Fe-rich tholeiites are less siliceous, have lower abundances of LILEs and REEs, that indicates derivation from an undifferentiated, primitive melt. The geochemical characteristics of both types underline a shallow level and a high degree of melting in the majority of cases, and a broadly island arc basalt (IAB) affinity. Palaeomagnetic analysis of hand samples shows directional groups consistent with geochemical groupings. The Vestfold Hills dykes show a possible linkage with the coeval mafic dykes in the Eastern Dharwar and Bastar cratons of the South Indian Block, based on the similarity in the Paleoproterozoic palaeolatitudes.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"24 1","pages":"149 - 171"},"PeriodicalIF":0.0,"publicationDate":"2021-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88732138","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Steiner, T. Rooney, G. Girard, N. Rogers, C. Ebinger, L. Peterson, R. Phillips
Abstract The initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene continental large igneous province (LIP), centred on southern Ethiopia and northern Kenya. Here we present a geographically well-distributed geochemical dataset comprising flood basalt lavas of the Eocene continental LIP to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly alkaline small-volume Akobo Basalt (49.4–46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric–mantle metasomes; (2) the primitive and spatially restricted Amaro Basalt (45.2–39.8 Ma), representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly; and (3) the spatially extensive Gamo–Makonnen magmatic unit (38–28 Ma), representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere and resulted in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 myr preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia that is associated with the East African Rift.
{"title":"Initial Cenozoic magmatic activity in East Africa: new geochemical constraints on magma distribution within the Eocene continental flood basalt province","authors":"R. Steiner, T. Rooney, G. Girard, N. Rogers, C. Ebinger, L. Peterson, R. Phillips","doi":"10.1144/SP518-2020-262","DOIUrl":"https://doi.org/10.1144/SP518-2020-262","url":null,"abstract":"Abstract The initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene continental large igneous province (LIP), centred on southern Ethiopia and northern Kenya. Here we present a geographically well-distributed geochemical dataset comprising flood basalt lavas of the Eocene continental LIP to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly alkaline small-volume Akobo Basalt (49.4–46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric–mantle metasomes; (2) the primitive and spatially restricted Amaro Basalt (45.2–39.8 Ma), representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly; and (3) the spatially extensive Gamo–Makonnen magmatic unit (38–28 Ma), representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere and resulted in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 myr preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia that is associated with the East African Rift.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"100 1","pages":"435 - 465"},"PeriodicalIF":0.0,"publicationDate":"2021-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89661266","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract This review summarizes research on the biostratigraphic application of Carboniferous marine and non-marine bivalves worldwide, with a focus on the southern margin of Laurussia and the Palaeotethys. Preliminary and established biostratigraphic zonations based on marine and non-marine bivalves are compiled and, if possible, correlated. Bivalve taxa of potential biostratigraphic significance are discussed, and possible limitations of bivalve zonations are outlined. Although marine and non-marine bivalves have not received primary attention for biostratigraphic purposes, a wealth of as yet unused data exists practically worldwide that can assist and complement the more favoured brachiopod, foraminiferid, ammonoid and conodont biozonations.
{"title":"A global review of Carboniferous marine and non-marine bivalve biostratigraphy","authors":"M. Amler, V. Silantiev","doi":"10.1144/SP512-2021-101","DOIUrl":"https://doi.org/10.1144/SP512-2021-101","url":null,"abstract":"Abstract This review summarizes research on the biostratigraphic application of Carboniferous marine and non-marine bivalves worldwide, with a focus on the southern margin of Laurussia and the Palaeotethys. Preliminary and established biostratigraphic zonations based on marine and non-marine bivalves are compiled and, if possible, correlated. Bivalve taxa of potential biostratigraphic significance are discussed, and possible limitations of bivalve zonations are outlined. Although marine and non-marine bivalves have not received primary attention for biostratigraphic purposes, a wealth of as yet unused data exists practically worldwide that can assist and complement the more favoured brachiopod, foraminiferid, ammonoid and conodont biozonations.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"54 1","pages":"893 - 932"},"PeriodicalIF":0.0,"publicationDate":"2021-08-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82229242","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Alessandro Menini, E. Mattioli, S. Hesselbo, M. Ruhl, G. Suan
Abstract The leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; c. 183 Ma) and the associated negative C-isotope excursion is the massive release of 12C favouring greenhouse conditions and continental weathering. The nutrient delivery to shallow basins supported productivity and, because of O2 consumption by organic-matter respiration, anoxia development. However, several studies have shown that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr (Mochras Farm) borehole, Wales, UK, were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep dwellers (Crepidolithus crassus) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE, probably because of prolonged thermohaline seawater stratification.
{"title":"Primary v. carbonate production in the Toarcian, a case study from the Llanbedr (Mochras Farm) borehole, Wales","authors":"Alessandro Menini, E. Mattioli, S. Hesselbo, M. Ruhl, G. Suan","doi":"10.1144/SP514-2021-19","DOIUrl":"https://doi.org/10.1144/SP514-2021-19","url":null,"abstract":"Abstract The leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; c. 183 Ma) and the associated negative C-isotope excursion is the massive release of 12C favouring greenhouse conditions and continental weathering. The nutrient delivery to shallow basins supported productivity and, because of O2 consumption by organic-matter respiration, anoxia development. However, several studies have shown that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr (Mochras Farm) borehole, Wales, UK, were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep dwellers (Crepidolithus crassus) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE, probably because of prolonged thermohaline seawater stratification.","PeriodicalId":22055,"journal":{"name":"Special Publications","volume":"84 1","pages":"59 - 81"},"PeriodicalIF":0.0,"publicationDate":"2021-08-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76036716","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Srivastava, R. Ernst, U. Söderlund, A. Samal, O. Pandey, G. C. Gautam
Abstract We propose a Precambrian megacraton (consisting of two or more ancient cratons), DHABASI in the Indian Shield, which includes the Dharwar, Bastar and Singhbhum cratons. This interpretation is mainly based on seven large igneous provinces (LIPs) that are identified in these three cratons over the age range of c. 3.35–1.77 Ga, a period of at least 1.6 Ga. The absence of any subsequent break-up of DHABASI since 1.77 Ga suggests that this megacraton has existed for the past 3.35 Ga. In addition to their use in recognizing this megacraton, these LIP events may also provide likely targets for Cu–Ni–Cr–Co–platinum group element deposits. We suggest that the megacraton DHABASI was an integral part of supercontinents/supercratons through Earth's history, and that it should be utilized as a distinct building block for palaeocontinental reconstructions rather than using the individual Dharwar, Bastar and Singhbhum cratons.
摘要:本文提出了印度地盾中的前寒武纪巨型克拉通DHABASI(由两个或多个古克拉通组成)，包括Dharwar、Bastar和Singhbhum克拉通。这一解释主要基于在这三个克拉通中发现的7个大火成岩省(LIPs)，年龄在3.35-1.77 Ga之间，至少为1.6 Ga。自1.77 Ga以来没有任何后续的DHABASI分裂表明这个巨型克拉通在过去的3.35 Ga中已经存在。这些LIP事件除了用于识别这个巨中子外，还可能为cu - ni - cr - co -铂族元素矿床提供可能的目标。我们认为DHABASI巨型克拉通是地球历史上超级大陆/超级克拉通的一个组成部分，它应该被用作古大陆重建的一个独特的组成部分，而不是单独使用Dharwar、Bastar和Singhbhum克拉通。
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J. Hendry, P. Burgess, D. Hunt, X. Janson, V. Zampetti
Modern seismic data have become an essential toolkit for studying carbonate platforms and reservoirs in impressive detail. Whilst driven primarily by oil and gas exploration and development, data sharing and collaboration are delivering fundamental geological knowledge on carbonate systems, revealing platform geomorphologies and how their evolution on millennial time scales, as well as kilometric length scales, was forced by long-term eustatic, oceanographic or tectonic factors. Quantitative interrogation of modern seismic attributes in carbonate reservoirs permits flow units and barriers arising from depositional and diagenetic processes to be imaged and extrapolated between wells. This volume reviews the variety of carbonate platform and reservoir characteristics that can be interpreted from modern seismic data, illustrating the benefits of creative interaction between geophysical and carbonate geological experts at all stages of a seismic campaign. Papers cover carbonate exploration, including the uniquely challenging South Atlantic pre-salt reservoirs, seismic modelling of carbonates, and seismic indicators of fluid flow and diagenesis.
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