Subglacial deformation and water-pressure cycles as a key for understanding ice stream dynamics: evidence from the Late Ordovician succession of the Djado Basin (Niger).

• Published in: International Journal of Earth Sciences
• Main Authors: Denis, Michaël, Guiraud, Michel, Konaté, Moussa, Buoncristiani, Jean-François
• Other Authors: Biogéosciences UMR 6282 (BGS), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Département de Géologie, Université Abdou Moumouni Niamey, Part of this work funded by research grants provided by the TOTAL ‘‘Projets Nouveaux Afrique'' and research allocation from the Ministère de l'Education Nationale, de la Recherche et de la Technologie.
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2010
• Subjects: Hirnantian; Soft-bed deformation; Subglacial shear zones; Subglacial water pressure; Ice/bed decoupling; Ice stream; [SDU.STU.ST]Sciences of the Universe [physics]/Earth Sciences/Stratigraphy; [SDU.STU.GL]Sciences of the Universe [physics]/Earth Sciences/Glaciology; Ice Sheet
• Online Access: https://hal.archives-ouvertes.fr/hal-00509918; https://doi.org/10.1007/s00531-009-0455-z

27 pages International audience Subglacial deformation is crucial to reconstructing glacier dynamics. Sediments associated with the Late Ordovician ice sheet in the Djado Basin, Niger, exhibit detailed structures of the subglacial shear zone. Three main types of subglacial shear zones (SSZ) are discriminated. The lowermost SSZ, developed on sandstones, displays Riedel macrostructures and cataclastic microstructures. These resulted from brittle deformation associated with strong glacier/bed coupling and low porewater pressure. Where they developed on a clay-rich bed, the overlying SSZ display S–C to S–C' fabrics, sheath folds, and dewatering structures. These features indicate high ductile shear strain and water overpressure. On finegrained sand beds, the SSZ exhibit homogenized sand units with sand stringers, interpreted as fluidized sliding beds. The succession of subglacial deformation processes depends on fluid-pressure behavior in relation to subglacial sediment permeability. Fluid overpressure allows subglacial sediment shear strength and ice/bed coupling to be lowered, leading to ice streaming.