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Jean-loup Rubino - One of the best experts on this subject based on the ideXlab platform.

  • Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)
    Earth-Science Reviews, 2015
    Co-Authors: Flavia Girard, XAVIER BERNARD, Jean-françois Ghienne, Jean-loup Rubino
    Abstract:

    Fromthe Proterozoic to the Quaternary, the evolution of the Earthwas characterised by recurrent periods of glaciation. However, the margins of many ancient ice-sheets are poorly defined on palaeogeographic reconstructions. The extent and outlines of ancient ice sheets can be better understood through careful documentation of sediments deposited at the ice-sheet margin. An outstanding example is provided herein based on an end- Ordovician archive in Libya (Tihemboka area, Murzuq Basin). The four sets of structures include: i) subglacial glaciotectonic structures and soft sediment deformations from flowing glacier ice, such as intraformational glacial striae, intraformational deformation (shear planes, sheath folds), normal microfaults, and large-scale glaciotectonic folds-and-thrusts; (ii) structures resulting from overpressured subglacial (meltwater) flows such as clastic dykes and tunnel valleys; (iii) proglacial depositional structures and facies related to highmagnitude meltwater floods such as sandstone intraclasts, large-scale bedforms resulting from supercritical flows, climbing-dune cross-stratification and kettle holes; and (iv) deformation structures resulting from free floating and nonglacier ice such as ice-keel scours and ice-crystal marks. Such a set of structures points to an ice-marginal (essentially continental) depositional setting, and provides an excellent suite of criteria to identify margins of ancient ice sheets in the stratigraphic record. At a regional scale, a reconstruction through time and space of the related depositional wedge is proposed. This corresponded to a seismic-scale (N120 m in thickness, 40 km in length) ice-marginal wedge in front of an essentially warm-based ice-sheet inducing concomitant large-scale glaciotectonic deformation, glacial basin and tunnel valley downcuttings. The related ice-front was associated with high-energy meltwater flows feeding a network of deeply incised proglacial channels downstream and, beyond them, a fluvioglacial deltaic system. Shallow ice-marginal permafrost most likely affected the depositional wedge. At a larger scale, the Tihemboka ice-marginal wedge is interpreted as related to a stillstand period over the Gondwana platform, developed over an estimated interval of a few thousands of years. Based on these data, the conditions that arose in a particularly favourable context for the development, the preservation and the identification of ice-marginal wedges in the geological record are reviewed. Significant meltwater-derived sediment deposition and aggradation in an accommodation space resulting either from preglacial inheritance, glacial downcuttings and/or glacio-isostatic lithospheric flexure, or active tectonic subsidence (>1 Myr) are required for their formation and subsequent preservation.

  • lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidie and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidie (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

Jean-françois Ghienne - One of the best experts on this subject based on the ideXlab platform.

  • Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)
    Earth-Science Reviews, 2015
    Co-Authors: Flavia Girard, XAVIER BERNARD, Jean-françois Ghienne, Jean-loup Rubino
    Abstract:

    Fromthe Proterozoic to the Quaternary, the evolution of the Earthwas characterised by recurrent periods of glaciation. However, the margins of many ancient ice-sheets are poorly defined on palaeogeographic reconstructions. The extent and outlines of ancient ice sheets can be better understood through careful documentation of sediments deposited at the ice-sheet margin. An outstanding example is provided herein based on an end- Ordovician archive in Libya (Tihemboka area, Murzuq Basin). The four sets of structures include: i) subglacial glaciotectonic structures and soft sediment deformations from flowing glacier ice, such as intraformational glacial striae, intraformational deformation (shear planes, sheath folds), normal microfaults, and large-scale glaciotectonic folds-and-thrusts; (ii) structures resulting from overpressured subglacial (meltwater) flows such as clastic dykes and tunnel valleys; (iii) proglacial depositional structures and facies related to highmagnitude meltwater floods such as sandstone intraclasts, large-scale bedforms resulting from supercritical flows, climbing-dune cross-stratification and kettle holes; and (iv) deformation structures resulting from free floating and nonglacier ice such as ice-keel scours and ice-crystal marks. Such a set of structures points to an ice-marginal (essentially continental) depositional setting, and provides an excellent suite of criteria to identify margins of ancient ice sheets in the stratigraphic record. At a regional scale, a reconstruction through time and space of the related depositional wedge is proposed. This corresponded to a seismic-scale (N120 m in thickness, 40 km in length) ice-marginal wedge in front of an essentially warm-based ice-sheet inducing concomitant large-scale glaciotectonic deformation, glacial basin and tunnel valley downcuttings. The related ice-front was associated with high-energy meltwater flows feeding a network of deeply incised proglacial channels downstream and, beyond them, a fluvioglacial deltaic system. Shallow ice-marginal permafrost most likely affected the depositional wedge. At a larger scale, the Tihemboka ice-marginal wedge is interpreted as related to a stillstand period over the Gondwana platform, developed over an estimated interval of a few thousands of years. Based on these data, the conditions that arose in a particularly favourable context for the development, the preservation and the identification of ice-marginal wedges in the geological record are reviewed. Significant meltwater-derived sediment deposition and aggradation in an accommodation space resulting either from preglacial inheritance, glacial downcuttings and/or glacio-isostatic lithospheric flexure, or active tectonic subsidence (>1 Myr) are required for their formation and subsequent preservation.

  • lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidie and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidie (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

Julien Moreau - One of the best experts on this subject based on the ideXlab platform.

  • mid weichselian interstadial in kolari western finnish lapland
    Boreas, 2014
    Co-Authors: Velipekka Salonen, Julien Moreau, Outi Hyttinen, K Eskola
    Abstract:

    Finnish Lapland is known as an area where numerous sites with sediments from Pleistocene glacial and interglacial periods occur. Recent sedimentological observations and dating call for reinterpretation of the record, which shows a complicated Mid-Weichselian ice-sheet evolution within the ice-divide zone. Here, a large, previously unstudied section from a former Hannukainen iron mine was investigated sedimentologically and dated with optically stimulated luminescence (OSL). Ten sedimentary units were identified displaying a variety of depositional environments (glacial, glaciolacustrine, fluvial and aeolian). They are all – except for the lowermost, deeply weathered till – interpreted to be of Mid- or Late Weichselian/Holocene age. Five OSL samples from fluvial sediments give ages ranging from 55 to 35 ka, indicating two MIS 3 ice-free intervals of unknown duration. The Mid-Weichselian interstadial was interrupted by a re-advance event, which occurred later than 35 ka and caused glaciotectonic deformation, folding and stacking of older sediments. This new evidence emphasizes the importance of the Kolari area when unravelling the complex Late Pleistocene glacial history of northern Finland and adjacent regions.

  • lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidie and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidie (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

Lionel Degermann - One of the best experts on this subject based on the ideXlab platform.

  • lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidie and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidie (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

  • Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)
    International Journal of Earth Sciences, 2013
    Co-Authors: Jean-françois Ghienne, Lionel Degermann, Julien Moreau, Jean-loup Rubino
    Abstract:

    The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

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  • Micromorphological evidence for the role of pressurised water in the formation of large-scale thrust-block moraines in Melasveit, western Iceland
    'Cambridge University Press (CUP)', 2020
    Co-Authors: Sigfusdottir Thorbjorg, Phillips Emrys, Benediktsson, Ívar Örn
    Abstract:

    Pressurised meltwater has a major impact on ice dynamics, as well as on sedimentary and deformational processes occurring below/in front of glaciers and ice sheets, but its role in glaciotectonic processes is yet to be fully understood. This study explores micro- and macroscale structures developed within décollements in two thrust-block moraines of Late Weichselian age in Melasveit, western Iceland. The aim is to investigate how pressurised subglacial meltwater can aid the dislocation and transport of large, unfrozen and unlithified sediment blocks by glaciers. A detailed model is constructed for the development of the thrust-block moraines and the microscale processes occurring along their detachments during thrusting. The detachments are characterized by relatively thin zones of crosscutting hydrofractures, which reflect fluctuating water pressures during glaciotectonism. Little evidence of shearing is observed along the leading edges of the thrusts in both moraines. This is supported by high water pressures along the detachments and indicates that the thrust blocks were initially decoupled from the underlying deposits. As the thrust moraines evolved, an increased amount of shear occurred in between events of sediment liquefaction, hydrofracturing, and fluid escape. This was followed by progressive locking up of the detachments and eventual cessation in the accretion of the thrust blocks

  • Past dynamics of a marine-terminating glacier in lower Borgarfjörður, west Iceland - Analyses of glaciotectonic sediments and landforms
    Department of Geology Lund University, 2019
    Co-Authors: Sigfusdottir Thorbjorg
    Abstract:

    Large and complete sections through glacial landforms formed in subaqueous environments are rare, impeding our knowledge of their internal composition and the processes responsible for their formation. Following the last deglaciation of the Icelandic Ice Sheet (IIS), a marine-terminating outlet glacier advanced out of the fjord of Borgarfjörður resulting in large-scale glaciotectonic deformation of glaciomarine sediments. Due to isostatic uplift and erosion, these glaciotectonic formations are now extensively exposed in the region and provide an opportunity to study a glaciotectonised marine sequence on land. The aim of this study is to investigate the inter-relationship between ice-marginal deformation and deposition in a glaciomarine setting, increase the understanding of glaciotectonic processes at the margins of marine-terminating glaciers, and elucidate past glacier dynamics and the regional glacial history.The main focus of the sedimentological and structural work was on the coastal cliffs of Belgsholt, Melabakkar-Ásbakkar and Skipanes. The sediments and glaciotectonic structures were analysed on a range of scales using sedimentological and structural field methods, high resolution LiDAR scans and micromorphological thin sections. Shells of marine molluscs were sampled for radiocarbon dating and interpreted in the context of the stratigraphy and Glaciotectonics to constrain the timeline of the regional glacial history.The study revealed a series of glaciotectonic moraines in the Melabakkar-Ásbakkar and Belgsholt coastal cliffs. The southernmost moraine is the largest and structurally most complex and is interpreted to indicate the maximum extent of the Borgarfjörður glacier. Other moraines in the series record repeated re-advances of the glacier during its active northward retreat. The moraines were mainly formed by large-scale thrusting and folding of glaciomarine sediments and subsequent deposition of ice-marginal sand and gravel. During the active retreat, glaciomarine sediments accumulated in front of the glacier providing source material for the formation of subsequent moraines.Detailed analysis of micro- and macroscale structures developed within décollements show that the detachment and transport of unlithified and unfrozen sediment blocks was enabled by overpressurisation of subglacial/ice marginal porewater. This implies that hydrogeology played a key role in the construction of the moraines.The advances and subsequent active retreat of the Borgarfjörður glacier occurred between c. 13.0 and 11.7 cal. ka BP indicating that it coincided with widespread glacier advances in Iceland and in the North Atlantic region during the Younger Dryas (c. 12.7-11.5 cal. ka BP). In the Early Holocene, after c. 11.3 cal. ka BP, the glacier re-expanded to a position around 5 km inside the Younger Dryas ice limit, indicating more extensive glaciation in the region than previously thought.The results of this thesis highlight the diversity of sedimentological and glaciotectonic processes involved in the construction of large glaciotectonic moraines at the margins of marine-terminating glaciers, and indicate that such glaciers were more dynamic during the deglaciation than previously thought

  • Past dynamics of a marine-terminating glacier in lower Borgarfjörður, west Iceland – Analyses of glaciotectonic sediments and landforms
    University of Iceland School of Engineering and Natural Sciences Faculty of Earth Sciences, 2019
    Co-Authors: Sigfusdottir Thorbjorg
    Abstract:

    Large and complete sections through glacial landforms formed in subaqueous environments are rare, impeding our knowledge of their internal composition and the processes responsible for their formation. Following the last deglaciation of the Icelandic Ice Sheet (IIS), a marine-terminating outlet glacier advanced out of the fjord of Borgarfjörður resulting in large-scale glaciotectonic deformation of glaciomarine sediments. Due to isostatic uplift and erosion, these glaciotectonic formations are now extensively exposed in the region and provide an opportunity to study a glaciotectonised marine sequence on land. The aim of this study is to investigate the inter-relationship between ice-marginal deformation and deposition in a glaciomarine setting, increase the understanding of glaciotectonic processes at the margins of marine-terminating glaciers, and elucidate past glacier dynamics and the regional glacial history. The main focus of the sedimentological and structural work was on the coastal cliffs of Belgsholt, Melabakkar-Ásbakkar and Skipanes. The sediments and glaciotectonic structures were analysed on a range of scales using sedimentological and structural field methods, high resolution LiDAR scans and micromorphological thin sections. Shells of marine molluscs were sampled for radiocarbon dating and interpreted in the context of the stratigraphy and Glaciotectonics to constrain the timeline of the regional glacial history. The study revealed a series of glaciotectonic moraines in the Melabakkar-Ásbakkar and Belgsholt coastal cliffs. The southernmost moraine is the largest and structurally most complex and is interpreted to indicate the maximum extent of the Borgarfjörður glacier. Other moraines in the series record repeated re-advances of the glacier during its active northward retreat. The moraines were mainly formed by large-scale thrusting and folding of glaciomarine sediments and subsequent deposition of ice-marginal sand and gravel. During the active retreat, glaciomarine sediments accumulated in front of the glacier providing source material for the formation of subsequent moraines. Detailed analysis of micro- and macroscale structures developed within décollements show that the detachment and transport of unlithified and unfrozen sediment blocks was enabled by overpressurisation of subglacial/ice-marginal porewater. This implies that hydrogeology played a key role in the construction of the moraines. The advances and subsequent active retreat of the Borgarfjörður glacier occurred between c. 13.0 -11.7 cal. ka BP indicating that it coincided with widespread glacier advances in Iceland and in the North Atlantic region during the Younger Dryas (c. 12.7-11.5 cal. ka BP). In the Early Holocene, after c. 11.3 cal. ka BP, the glacier re-expanded to a position around 5 km inside the Younger Dryas ice limit, indicating more extensive glaciation in the region than previously thought. The results of this thesis highlight the diversity of sedimentological and glaciotectonic processes involved in the construction of large glaciotectonic moraines at the margins of marine-terminating glaciers, and indicate that such glaciers were more dynamic during the deglaciation than previously thought.Við lok síðasta jökulskeiðs gekk jökull í sjó fram í Melasveit í neðri hluta Borgarfjarðar. Vegna landriss í kjölfar afjöklunar er svæðið nú ofan sjávarmáls. Má þar víða sjá jökulræn setlög og landform sem veita innsýn í jöklunarsögu svæðisins og ferli setmyndunar og afmyndunar við jaðar jökla í sjávarumhverfi. Rannsókn þessi beindist að jökulhöggun og setgerð í Melabökkum, Ásbökkum og Belgsholti. Setgerð, setlagaskipan og byggingareinkenni voru kortlögð á vettvangi og bakkarnir mældir með leysiskíki. Einnig var setsýnum safnað til smásjárskoðana og skeljum til aldursgreininga. Í bökkunum má finna a.m.k. sjö stóra jökulgarða, sem grafnir eru undir yngri sjávarsetlögum. Garðarnir eru að mestu úr sjávarseti, sem jökull úr Borgarfirði þrýsti upp við jökulsporðinn. Aukinn vatnsþrýstingur undir og framan við sporðinn ýtti undir afmyndun undirlagsins og auðveldað jöklinum að byggja upp garðana. Syðsti og stærsti garðurinn markar hámarksútbreiðslu jökulsins. Aldursgreiningar sýna að hann myndaðist á Yngra Drías (fyrir u.þ.b 13-11,7 þús. árum). Hinir garðarnir verða almennt yngri til norðurs og marka smærri framrásir á tímabili hörfunar jökulsins inn Borgarfjörð. Myndaðist sá yngsti snemma á nútíma, eða fyrir u.þ.b 11,3 þús. árum. Þessar niðurstöður gefa til kynna að jöklar hafi verið virkari á tímum afjöklunar en áður var talið. Auk þess veitir rannsóknin upplýsingar um innri gerð jökulgarða sem myndast í sjó og þau ferli sem eru að verki við myndun slíkra garða