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) |
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Authors: | Michael Denis M Guiraud M Konat�� J-F Buoncristiani |
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Institution: | 1. UMR CNRS 5561 Biog??osciences, UFR Sciences de la Terre et de l??Environnement, Universit?? de Bourgogne, 6, boulevard Gabriel, 21000, Dijon, France 3. TOTAL CST-JF, 18, avenue Larribau, 64018, Pau Cedex, France 2. D??partement de G??ologie, Universit?? Abdou Moumouni de Niamey, BP 10662, Niamey, Niger
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Abstract: | 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 pore-water 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 fine-grained 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. |
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