The Skaergaard intrusion is a layered, ferrobasaltic intrusion emplaced during the Early Eocene into the rifting volcanic margin of East Greenland. The magma chamber crystallised in response to cooling from the roof and margins upwards and inward, forming upper, marginal and bottom series, the latter referred to as the Layered Series. The phase layering in the bottom series suggests an evolved, olivine-normative tholeiitic melt saturated in plagioclase and olivine, followed by augite, and then simultaneously by ilmenite and magnetite forming primocrysts. Pigeonite appears in the lower parts and continues until the centre of the series. Apatite appears in the upper part concurrently with liquid immiscibility. Cryptic variations of the individual primocrysts record a systematic upward increase in iron and decrease in magnesium for the mafic minerals and a systematic increase in sodium and decrease in calcium for plagioclase. The appearance of pigeonite is caused by reactions and crystallisation in the trapped melt and by subsolidus adjustments without this phase reaching liquidus saturation. The high mode of olivine at the base of the upper part with the appearance of apatite is interpreted to mark the onset of liquid immiscibility. This may have led to the separation of conjugate melts with granophyre migrating upward and the basic component largely staying stationary or sinking. Petrologic and geochemical observations indicate differentiation in the lower part of the intrusion, principally controlled by crystal fractionation with the efficiency of fractionation controlled by the evolution and escape of liquid from the solidifying mush. During the final stages of solidification, the onset of liquid immiscibility and termination of melt convection impeded differentiation. Modelling by perfect Rayleigh fractionation shows that major and included trace elements conform reasonably to observations, while excluded elements deviate from model predictions. This decoupling is caused by the mobility of a granophyre component formed in the trapped melt and in the main residual magma chamber. Consequently, the sampled gabbros may not be representative of the final solid-melt mush. By restoring the gabbros to their original mush compositions, it is possible to constrain granophyre migration pathways. We suggest that the granophyre formed in the trapped melt in the lower part of the intrusion mostly migrated laterally through pressure release pathways to form lenses and pockets with only limited upward migration into the main magma reservoir. Near the end stage of differentiation, the residual magma exsolved and formed complex mixtures of ferrobasaltic and granophyric melts. Estimates predict that a substantial amount of the granophyric melt penetrated as sills into the downward crystallising, upper part of the body as well as into the host rocks. The redistribution of granophyric melts within the solidifying crystal mush complicates predictions of trapped-melt
{"title":"Petrology of the Skaergaard Layered Series","authors":"P. Thy, C. Tegner, C. Lesher","doi":"10.34194/geusb.v56.8327","DOIUrl":"https://doi.org/10.34194/geusb.v56.8327","url":null,"abstract":"The Skaergaard intrusion is a layered, ferrobasaltic intrusion emplaced during the Early Eocene into the rifting volcanic margin of East Greenland. The magma chamber crystallised in response to cooling from the roof and margins upwards and inward, forming upper, marginal and bottom series, the latter referred to as the Layered Series. The phase layering in the bottom series suggests an evolved, olivine-normative tholeiitic melt saturated in plagioclase and olivine, followed by augite, and then simultaneously by ilmenite and magnetite forming primocrysts. Pigeonite appears in the lower parts and continues until the centre of the series. Apatite appears in the upper part concurrently with liquid immiscibility. Cryptic variations of the individual primocrysts record a systematic upward increase in iron and decrease in magnesium for the mafic minerals and a systematic increase in sodium and decrease in calcium for plagioclase. The appearance of pigeonite is caused by reactions and crystallisation in the trapped melt and by subsolidus adjustments without this phase reaching liquidus saturation. The high mode of olivine at the base of the upper part with the appearance of apatite is interpreted to mark the onset of liquid immiscibility. This may have led to the separation of conjugate melts with granophyre migrating upward and the basic component largely staying stationary or sinking. Petrologic and geochemical observations indicate differentiation in the lower part of the intrusion, principally controlled by crystal fractionation with the efficiency of fractionation controlled by the evolution and escape of liquid from the solidifying mush. During the final stages of solidification, the onset of liquid immiscibility and termination of melt convection impeded differentiation. Modelling by perfect Rayleigh fractionation shows that major and included trace elements conform reasonably to observations, while excluded elements deviate from model predictions. This decoupling is caused by the mobility of a granophyre component formed in the trapped melt and in the main residual magma chamber. Consequently, the sampled gabbros may not be representative of the final solid-melt mush. By restoring the gabbros to their original mush compositions, it is possible to constrain granophyre migration pathways. We suggest that the granophyre formed in the trapped melt in the lower part of the intrusion mostly migrated laterally through pressure release pathways to form lenses and pockets with only limited upward migration into the main magma reservoir. Near the end stage of differentiation, the residual magma exsolved and formed complex mixtures of ferrobasaltic and granophyric melts. Estimates predict that a substantial amount of the granophyric melt penetrated as sills into the downward crystallising, upper part of the body as well as into the host rocks. The redistribution of granophyric melts within the solidifying crystal mush complicates predictions of trapped-melt","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"77 S42","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-12-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139165934","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Olivarius, A. Kazerouni, R. Weibel, T. Kokfelt, J. Hovikoski
The influence of rifting on the composition of Kimmeridgian to Barremian mudstones from northern Wollaston Forland, North-East Greenland is investigated by petrographic and mineralogical analyses of the Brorson Halvø-1 and Rødryggen-1 cores, and provenance of analysis by zircon U-Pb age dating of nearby sandstones. Mudstone composition varies systematically as a function of the timing of rifting progression and position in the half-graben depositional system. Pyrite primarily precipitated in the early rift to rift climax phases. Euhedral pyrite overgrowths on framboids formed only during the rift climax phase (Lindemans Bugt Formation). Dolomite is the dominant carbonate cement, except for the sediments deposited in the early waning rift phase (Palnatokes Bjerg Formation) where calcite is dominant, and in the late waning rift phase (Stratumbjerg Formation) where siderite dominates. The highest-temperature reactions with precipitation of illite, quartz, ankerite and barite signify sediment burial depths of >2 km prior to exhumation. Uplift-induced fracturing occurred mainly in the early rift to rift acceleration succession (Bernbjerg Formation). Mudstones in the proximal part of the half-graben (Rødryggen-1) include more detrital kaolinite than the distal mudstones (Brorson Halvø-1), which contain more mixed-layer illite-smectite and illite. Vermiculite was deposited only in the proximal part of the basin in the rift climax and waning rift successions. Chlorite was deposited proximally and distally during the waning rift phase, though supply began earlier in the distal part. Fine-grained sediment in the distal part of the half-graben was therefore probably supplied by axial transport from Palaeoproterozoic crystalline rocks and Meso- to Neoproterozoic metamorphic rocks located to the north and north-west. This agrees with the zircon provenance signature from outcropping sand-rich facies, where zircon grains with U-Pb ages of 2.0–1.6 Ga are dominant, in addition to common 1.6–0.9 Ga ages, and fewer 2.8–2.6 Ga and 0.47–0.36 Ga ages.
{"title":"Mudstone diagenesis and sandstone provenance in an Upper Jurassic – Lower Cretaceous evolving half-graben system, Wollaston Forland, North-East Greenland","authors":"M. Olivarius, A. Kazerouni, R. Weibel, T. Kokfelt, J. Hovikoski","doi":"10.34194/geusb.v55.8309","DOIUrl":"https://doi.org/10.34194/geusb.v55.8309","url":null,"abstract":"The influence of rifting on the composition of Kimmeridgian to Barremian mudstones from northern Wollaston Forland, North-East Greenland is investigated by petrographic and mineralogical analyses of the Brorson Halvø-1 and Rødryggen-1 cores, and provenance of analysis by zircon U-Pb age dating of nearby sandstones. Mudstone composition varies systematically as a function of the timing of rifting progression and position in the half-graben depositional system. Pyrite primarily precipitated in the early rift to rift climax phases. Euhedral pyrite overgrowths on framboids formed only during the rift climax phase (Lindemans Bugt Formation). Dolomite is the dominant carbonate cement, except for the sediments deposited in the early waning rift phase (Palnatokes Bjerg Formation) where calcite is dominant, and in the late waning rift phase (Stratumbjerg Formation) where siderite dominates. The highest-temperature reactions with precipitation of illite, quartz, ankerite and barite signify sediment burial depths of >2 km prior to exhumation. Uplift-induced fracturing occurred mainly in the early rift to rift acceleration succession (Bernbjerg Formation). Mudstones in the proximal part of the half-graben (Rødryggen-1) include more detrital kaolinite than the distal mudstones (Brorson Halvø-1), which contain more mixed-layer illite-smectite and illite. Vermiculite was deposited only in the proximal part of the basin in the rift climax and waning rift successions. Chlorite was deposited proximally and distally during the waning rift phase, though supply began earlier in the distal part. Fine-grained sediment in the distal part of the half-graben was therefore probably supplied by axial transport from Palaeoproterozoic crystalline rocks and Meso- to Neoproterozoic metamorphic rocks located to the north and north-west. This agrees with the zircon provenance signature from outcropping sand-rich facies, where zircon grains with U-Pb ages of 2.0–1.6 Ga are dominant, in addition to common 1.6–0.9 Ga ages, and fewer 2.8–2.6 Ga and 0.47–0.36 Ga ages.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"34 1","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138952740","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Hovikoski, J. Ineson, M. Olivarius, J. Bojesen‐Koefoed, S. Piasecki, P. Alsen
Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.
{"title":"Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift","authors":"J. Hovikoski, J. Ineson, M. Olivarius, J. Bojesen‐Koefoed, S. Piasecki, P. Alsen","doi":"10.34194/geusb.v55.8349","DOIUrl":"https://doi.org/10.34194/geusb.v55.8349","url":null,"abstract":"Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"23 5","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138952692","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Bojesen‐Koefoed, P. Alsen, M. Bjerager, J. Hovikoski, P. Johannessen, H. Nøhr-Hansen, Henrik I Petersen, S. Piasecki, H. Vosgerau
The Oxfordian–Ryazanian was a period of widespread deposition of marine organic-rich mudstones in basins formed during the early phases of the rifting that heralded the formation of the present-day North Atlantic. Occasionally, uninterrupted deposition prevailed for 20 million years or more. Today, mudstones of this time interval are found on the shelves bordering the North Atlantic and adjacent areas from Siberia to the Netherlands. Here, we report data on two fully cored boreholes from Wollaston Forland (North-East Greenland, approx. 74° N), which represent an uninterrupted succession from the upper Kimmeridgian to the Hauterivian. The boreholes record basin development at two different positions within an evolving halfgraben, located at the margin of the main rift, and thus partially detached from it. Although the overall depositional environment remained an oxygen-restricted deep-shelf setting, rifting-related changes can be followed through the succession. The Kimmeridgian was a period of eustatic highstand and records the incipient rifting with a transgressive trend straddling the transition to the lower Volgian by a gradual change from deposits with high levels of total organic carbon (TOC) and kerogen rich in allochthonous organic matter to deposits with lower TOC and a higher proportion of autochthonous organic matter. This is followed by a slight regressive trend with lower TOC and increased proportions of allochthonous organic matter until rifting culminated in the middle Volgian–Ryazanian, indicated by increasing autochthonous organic matter and higher TOC, which prevailed until basin ventilation occurred towards the end of the Ryazanian. The properties of the reactive kerogen fraction remained rather stable irrespective of TOC, underlining the effect of terrigenous matter input for TOC. These variations are also captured by biological markers and stable carbon isotopes. The deposits are very similar to equivalent successions elsewhere in the proto-North Atlantic region, albeit the proportion of terrigenous kerogen is greater.
{"title":"Organic geochemistry of an Upper Jurassic – Lower Cretaceous mudstone succession in a narrow graben setting, Wollaston Forland Basin, North-East Greenland","authors":"J. Bojesen‐Koefoed, P. Alsen, M. Bjerager, J. Hovikoski, P. Johannessen, H. Nøhr-Hansen, Henrik I Petersen, S. Piasecki, H. Vosgerau","doi":"10.34194/geusb.v55.8320","DOIUrl":"https://doi.org/10.34194/geusb.v55.8320","url":null,"abstract":"The Oxfordian–Ryazanian was a period of widespread deposition of marine organic-rich mudstones in basins formed during the early phases of the rifting that heralded the formation of the present-day North Atlantic. Occasionally, uninterrupted deposition prevailed for 20 million years or more. Today, mudstones of this time interval are found on the shelves bordering the North Atlantic and adjacent areas from Siberia to the Netherlands. Here, we report data on two fully cored boreholes from Wollaston Forland (North-East Greenland, approx. 74° N), which represent an uninterrupted succession from the upper Kimmeridgian to the Hauterivian. The boreholes record basin development at two different positions within an evolving halfgraben, located at the margin of the main rift, and thus partially detached from it. Although the overall depositional environment remained an oxygen-restricted deep-shelf setting, rifting-related changes can be followed through the succession. The Kimmeridgian was a period of eustatic highstand and records the incipient rifting with a transgressive trend straddling the transition to the lower Volgian by a gradual change from deposits with high levels of total organic carbon (TOC) and kerogen rich in allochthonous organic matter to deposits with lower TOC and a higher proportion of autochthonous organic matter. This is followed by a slight regressive trend with lower TOC and increased proportions of allochthonous organic matter until rifting culminated in the middle Volgian–Ryazanian, indicated by increasing autochthonous organic matter and higher TOC, which prevailed until basin ventilation occurred towards the end of the Ryazanian. The properties of the reactive kerogen fraction remained rather stable irrespective of TOC, underlining the effect of terrigenous matter input for TOC. These variations are also captured by biological markers and stable carbon isotopes. The deposits are very similar to equivalent successions elsewhere in the proto-North Atlantic region, albeit the proportion of terrigenous kerogen is greater.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"54 12","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138952357","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Bojesen‐Koefoed, P. Alsen, M. Bjerager, J. Hovikoski, J. Ineson, P. Johannessen, M. Olivarius, S. Piasecki, H. Vosgerau
Two fully cored boreholes, the Rødryggen-1 and the Brorson Halvø-1, were drilled in Wollaston Forland, North-East Greenland, in 2009 and 2010, respectively. The objective was to test the stratigraphic development of the Upper Jurassic – Lower Cretaceous mud-dominated succession in two different settings within the same fault block of a developing half-graben: centrally (Rødryggen-1 borehole) and near the uplifted crest of the rotating fault block (Brorson Halvø-1 borehole). The drilled deposits are equivalent to the principal petroleum source-rock sequence of the petroliferous basins of North-West Europe, Siberia, and basins off eastern Canada and provide a new record of an important phase of marine deoxygenation in the proto-North Atlantic region.
{"title":"The Rødryggen-1 and Brorson Halvø-1 fully cored boreholes (Upper Jurassic – Lower Cretaceous), Wollaston Forland, North-East Greenland – an introduction","authors":"J. Bojesen‐Koefoed, P. Alsen, M. Bjerager, J. Hovikoski, J. Ineson, P. Johannessen, M. Olivarius, S. Piasecki, H. Vosgerau","doi":"10.34194/geusb.v55.8350","DOIUrl":"https://doi.org/10.34194/geusb.v55.8350","url":null,"abstract":"Two fully cored boreholes, the Rødryggen-1 and the Brorson Halvø-1, were drilled in Wollaston Forland, North-East Greenland, in 2009 and 2010, respectively. The objective was to test the stratigraphic development of the Upper Jurassic – Lower Cretaceous mud-dominated succession in two different settings within the same fault block of a developing half-graben: centrally (Rødryggen-1 borehole) and near the uplifted crest of the rotating fault block (Brorson Halvø-1 borehole). The drilled deposits are equivalent to the principal petroleum source-rock sequence of the petroliferous basins of North-West Europe, Siberia, and basins off eastern Canada and provide a new record of an important phase of marine deoxygenation in the proto-North Atlantic region.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"14 3","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139166215","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Alsen, S. Piasecki, H. Nøhr-Hansen, Sebastian Pauly, E. Sheldon, J. Hovikoski
Two shallow cores drilled in northern Wollaston Forland, North-East Greenland, provide a combined section covering the upper Kimmeridgian (Upper Jurassic) – Barremian (Lower Cretaceous) and comprising the Bernbjerg, Lindemans Bugt, Palnatokes Bjerg and Stratumbjerg Formations. A new lithostratigraphic unit, the Storsletten Member, is defined within the Lindemans Bugt Formation. The black mudstone-dominated intervals are dated primarily by dinoflagellate cysts and ammonites, whereas the calcareous mudstones of the Palnatokes Bjerg Formation – sandwiched between the black mudstones – are dated by calcareous nannofossils. The stratigraphy demonstrates an almost complete succession in the Rødryggen-1 core, representing a deeper position in the basin, where the hiatus at the latest Jurassic rift climax predicted in previous models for the eastern Wollaston Forland Basin is absent. In contrast, the Brorson Halvø-1 core represents a position closer to a block crest where unconformities developed. In combination, the cores provide a key biostratigraphic reference section for the Jurassic–Cretaceous boundary interval in the Arctic.
{"title":"Stratigraphy of the Upper Jurassic to lowermost Cretaceous in the Rødryggen-1 and Brorson Halvø-1 boreholes, Wollaston Forland, North-East Greenland ","authors":"P. Alsen, S. Piasecki, H. Nøhr-Hansen, Sebastian Pauly, E. Sheldon, J. Hovikoski","doi":"10.34194/geusb.v55.8342","DOIUrl":"https://doi.org/10.34194/geusb.v55.8342","url":null,"abstract":"Two shallow cores drilled in northern Wollaston Forland, North-East Greenland, provide a combined section covering the upper Kimmeridgian (Upper Jurassic) – Barremian (Lower Cretaceous) and comprising the Bernbjerg, Lindemans Bugt, Palnatokes Bjerg and Stratumbjerg Formations. A new lithostratigraphic unit, the Storsletten Member, is defined within the Lindemans Bugt Formation. The black mudstone-dominated intervals are dated primarily by dinoflagellate cysts and ammonites, whereas the calcareous mudstones of the Palnatokes Bjerg Formation – sandwiched between the black mudstones – are dated by calcareous nannofossils. The stratigraphy demonstrates an almost complete succession in the Rødryggen-1 core, representing a deeper position in the basin, where the hiatus at the latest Jurassic rift climax predicted in previous models for the eastern Wollaston Forland Basin is absent. In contrast, the Brorson Halvø-1 core represents a position closer to a block crest where unconformities developed. In combination, the cores provide a key biostratigraphic reference section for the Jurassic–Cretaceous boundary interval in the Arctic.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"37 10","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138949950","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Sandersen, Hyojin Kim, Rasmus Jacobsen, Jesper B Pedersen, Birgitte G Hansen
Nitrogen impact on the aquatic environment, including coastal areas, is too high in many countries worldwide, particularly in areas with intensive agriculture. Efficient mitigation initiatives demand that important pathways and the fate of nitrate in the hydrological cycle are known. In this study, we focus on groundwater nitrate contamination in two near-shore catchment areas in north-west Denmark. Groundwater in the area is mainly located in buried tunnel valleys, which are subsurface structures eroded by meltwater during Pleistocene glaciations in former glaciated areas. Groundwater samples from the aquifers inside the buried valleys reveal the presence of up to 120 mg/l nitrate down to 10 m below sea level and about 1 km down from the stream outlet towards the coast. We interpret the complex tunnel-valley infill to be responsible for the spatial heterogeneity of the groundwater geochemistry, where sandy geological windows create localised hydraulic pathways and complex redox structures. Groundwater and stream water chemistry in the study area clearly demonstrate the role of groundwater in nitrate transport within the catchment as well as the direct pathway to the coast bypassing the stream and riverine systems. Our results show that the buried tunnel valleys potentially contribute to submarine groundwater discharge and therefore could be responsible for a hitherto unaccounted input of nitrogen to the marine environment.
{"title":"Transport of nitrate-containing groundwater to coastal areas through buried tunnel valleys, Denmark","authors":"P. Sandersen, Hyojin Kim, Rasmus Jacobsen, Jesper B Pedersen, Birgitte G Hansen","doi":"10.34194/geusb.v53.8351","DOIUrl":"https://doi.org/10.34194/geusb.v53.8351","url":null,"abstract":"Nitrogen impact on the aquatic environment, including coastal areas, is too high in many countries worldwide, particularly in areas with intensive agriculture. Efficient mitigation initiatives demand that important pathways and the fate of nitrate in the hydrological cycle are known. In this study, we focus on groundwater nitrate contamination in two near-shore catchment areas in north-west Denmark. Groundwater in the area is mainly located in buried tunnel valleys, which are subsurface structures eroded by meltwater during Pleistocene glaciations in former glaciated areas. Groundwater samples from the aquifers inside the buried valleys reveal the presence of up to 120 mg/l nitrate down to 10 m below sea level and about 1 km down from the stream outlet towards the coast. We interpret the complex tunnel-valley infill to be responsible for the spatial heterogeneity of the groundwater geochemistry, where sandy geological windows create localised hydraulic pathways and complex redox structures. Groundwater and stream water chemistry in the study area clearly demonstrate the role of groundwater in nitrate transport within the catchment as well as the direct pathway to the coast bypassing the stream and riverine systems. Our results show that the buried tunnel valleys potentially contribute to submarine groundwater discharge and therefore could be responsible for a hitherto unaccounted input of nitrogen to the marine environment.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"252 1","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-11-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139228103","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Estimating a covariance model for kriging purposes is traditionally done using semivariogram analyses, where an empirical semivariogram is calculated, and a chosen semivariogram model, usually defined by a sill and a range, is fitted. We demonstrate that a convolutional neural network can estimate such a semivariogram model with comparable accuracy and precision by training it to recognise the relationship between realisations of Gaussian random fields and the sill and range values that define it, for a Gaussian type semivariance model. We do this by training the network with synthetic data consisting of many such realisations with the sill and range as the target variables. Because training takes time, the method is best suited for cases where many models need to be estimated since the actual estimation itself is about 70 times faster with the neural network than with the traditional approach. We demonstrate the viability of the method in three ways: (1) we test the model’s performance on the validation data, (2) we do a test where we compare the model to the traditional approach and (3) we show an example of an actual application of the method using the Danish national hydrostratigraphic model.
{"title":"Machine learning-based estimation and clustering of statistics within stratigraphic models as exemplified in Denmark","authors":"Frederik Alexander Falk, Rasmus Bødker Madsen","doi":"10.34194/geusb.v53.8353","DOIUrl":"https://doi.org/10.34194/geusb.v53.8353","url":null,"abstract":"Estimating a covariance model for kriging purposes is traditionally done using semivariogram analyses, where an empirical semivariogram is calculated, and a chosen semivariogram model, usually defined by a sill and a range, is fitted. We demonstrate that a convolutional neural network can estimate such a semivariogram model with comparable accuracy and precision by training it to recognise the relationship between realisations of Gaussian random fields and the sill and range values that define it, for a Gaussian type semivariance model. We do this by training the network with synthetic data consisting of many such realisations with the sill and range as the target variables. Because training takes time, the method is best suited for cases where many models need to be estimated since the actual estimation itself is about 70 times faster with the neural network than with the traditional approach. We demonstrate the viability of the method in three ways: (1) we test the model’s performance on the validation data, (2) we do a test where we compare the model to the traditional approach and (3) we show an example of an actual application of the method using the Danish national hydrostratigraphic model.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"38 1","pages":""},"PeriodicalIF":1.7,"publicationDate":"2023-11-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139266627","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
One of the most common ways to assess ice-sheet reconstructions of the past is to evaluate how they impact changes in sea level through glacial isostatic adjustment. PaleoMIST 1.0, a preliminary reconstruction of topography and ice sheets during the past 80 000 years, was created without a rigorous comparison with past sea-level indicators and proxies in Greenland. The basal shear stress values for the Greenland ice sheet were deduced from the present day ice-sheet configuration, which were used for the entire 80 000 years without modification. The margin chronology was based on previous reconstructions and interpolation between them. As a result, it was not known if the Greenland component was representative of its ice-sheet history. In this study, I compile sea–level proxy data into the Global Archive of Paleo Sea Level Indicators and Proxies (GAPSLIP) database and use them to evaluate the PaleoMIST 1.0 reconstruction. The Last Glacial Maximum (c.20 000 years before present) contribution to sea level in PaleoMIST 1.0 is about 3.5 m, intermediate of other reconstructions of the Greenland ice sheet. The results of the data-model comparison show that PaleoMIST requires a larger pre-Holocene ice volume than it currently has to match the sea-level highstands observed around Greenland, especially in southern Greenland. Some of this mismatch is likely because of the crude 2500 year time step used in the margin reconstruction and the limited Last Glacial Maximum extent. Much of the mismatch can also be mitigated if different Earth model structures, particularly a thinner lithosphere, are assumed. Additional ice in Greenland would contribute to increasing the 3–5 m mismatch between the modelled far-field sea level at the Last Glacial Maximum and proxies in PaleoMIST 1.0.
{"title":"Paleo sea-level indicators and proxies from Greenland in the GAPSLIP database and comparison with modelled sea level from the PaleoMIST ice-sheet reconstruction","authors":"Evan J Gowan","doi":"10.34194/geusb.v53.8355","DOIUrl":"https://doi.org/10.34194/geusb.v53.8355","url":null,"abstract":"One of the most common ways to assess ice-sheet reconstructions of the past is to evaluate how they impact changes in sea level through glacial isostatic adjustment. PaleoMIST 1.0, a preliminary reconstruction of topography and ice sheets during the past 80 000 years, was created without a rigorous comparison with past sea-level indicators and proxies in Greenland. The basal shear stress values for the Greenland ice sheet were deduced from the present day ice-sheet configuration, which were used for the entire 80 000 years without modification. The margin chronology was based on previous reconstructions and interpolation between them. As a result, it was not known if the Greenland component was representative of its ice-sheet history. In this study, I compile sea–level proxy data into the Global Archive of Paleo Sea Level Indicators and Proxies (GAPSLIP) database and use them to evaluate the PaleoMIST 1.0 reconstruction. The Last Glacial Maximum (c.20 000 years before present) contribution to sea level in PaleoMIST 1.0 is about 3.5 m, intermediate of other reconstructions of the Greenland ice sheet. The results of the data-model comparison show that PaleoMIST requires a larger pre-Holocene ice volume than it currently has to match the sea-level highstands observed around Greenland, especially in southern Greenland. Some of this mismatch is likely because of the crude 2500 year time step used in the margin reconstruction and the limited Last Glacial Maximum extent. Much of the mismatch can also be mitigated if different Earth model structures, particularly a thinner lithosphere, are assumed. Additional ice in Greenland would contribute to increasing the 3–5 m mismatch between the modelled far-field sea level at the Last Glacial Maximum and proxies in PaleoMIST 1.0.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":"55 11","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136346664","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Results from numerical simulations play a vital role in the decision process of everyday groundwater management. However, these simulations can be time-consuming for large-scale investigations, and it can be necessary to apply approximate methods instead. This study investigates the abilities of a neural network to replicate simulated drawdown from groundwater abstraction in a numerical groundwater model of the Egebjerg catchment, Denmark. We follow a generalised methodology that uses the information within the deterministic numerical model to create a training set for the neural network to learn from and extend the method to work in a 3D Danish groundwater model case. We compare the abilities of the trained neural network with the results of conventional computations in terms of speed and accuracy and argue that this approach has the potential to improve decision support for decision-makers within groundwater management.
{"title":"Neural network predictions of drawdown from groundwater abstraction in the Egebjerg catchment, Denmark","authors":"Mathias Busk Dahl, Troels Norvin Vilhelmsen, Trine Enemark, Thomas Mejer Hansen","doi":"10.34194/geusb.v53.8357","DOIUrl":"https://doi.org/10.34194/geusb.v53.8357","url":null,"abstract":"Results from numerical simulations play a vital role in the decision process of everyday groundwater management. However, these simulations can be time-consuming for large-scale investigations, and it can be necessary to apply approximate methods instead. This study investigates the abilities of a neural network to replicate simulated drawdown from groundwater abstraction in a numerical groundwater model of the Egebjerg catchment, Denmark. We follow a generalised methodology that uses the information within the deterministic numerical model to create a training set for the neural network to learn from and extend the method to work in a 3D Danish groundwater model case. We compare the abilities of the trained neural network with the results of conventional computations in terms of speed and accuracy and argue that this approach has the potential to improve decision support for decision-makers within groundwater management.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":" February","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135186808","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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