Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

• Published in: Earth-Science Reviews
• Main Authors: Girard, Flavia, Ghienne, Jean-François, Du-Bernard, Xavier, Rubino, Jean-Loup
• Other Authors: Géosciences Montpellier, Université des Antilles et de la Guyane (UAG)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS), Institut de physique du globe de Strasbourg (IPGS), Université de Strasbourg (UNISTRA)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS), TOTAL-Scientific and Technical Center Jean Féger (CSTJF), TOTAL FINA ELF, ANR-12-BS06-0014,SeqStrat-Ice,Les glaciations du passé: leçons pour un modèle de stratigraphie séquentielle dédié aux systèmes glaciaires(2012)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2015
• Subjects: Supercritical flows; Tunnel valleys; Glaciotectonic; Murzuq Basin; Hirnantian glaciation; Ice front; [SDU]Sciences of the Universe [physics]; Ice Sheet
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-01184739; https://doi.org/10.1016/j.earscirev.2015.06.006

International audience 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 ofsediments 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) subglacialglaciotectonic structures and soft sediment deformations from flowing glacier ice, such as intraformational glacialstriae, intraformational deformation (shear planes, sheath folds), normal microfaults, and large-scaleglaciotectonic folds-and-thrusts; (ii) structures resulting from overpressured subglacial (meltwater) flowssuch as clastic dykes and tunnel valleys; (iii) proglacial depositional structures and facies related to highmagnitudemeltwater floods such as sandstone intraclasts, large-scale bedforms resulting from supercriticalflows, climbing-dune cross-stratification and kettle holes; and (iv) deformation structures resulting from freefloating and nonglacier ice such as ice-keel scours and ice-crystal marks. Such a set of structures points to anice-marginal (essentially continental) depositional setting, and provides an excellent suite of criteria to identifymargins of ancient ice sheets in the stratigraphic record. At a regional scale, a reconstruction through time andspace 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 concomitantlarge-scale glaciotectonic deformation, glacial basin and tunnel valley downcuttings. The related ice-front wasassociated with high-energy meltwater flows feeding a network of deeply incised proglacial channels downstreamand, beyond them, a fluvioglacial deltaic system. Shallow ...