Porewater pressure control on subglacial soft sediment remobilization and tunnel valley formation: a case study from the Alnif tunnel valley (Morocco).
Résumé
In the eastern part of the Moroccan Anti-Atlas Mountains, the Alnif area exposes a buried Ordovician glacial tunnel valley (5 km wide, 180 m deep) cut into preglacial marine sediments. The preglacial sedimentary sequence, deposited in a marine environment, is characterized by a typical "layer-cake" configuration of permeable (sand) and impermeable (clays and early-cemented sandstones) layers. At the base of the tunnel valley, a discontinuous and fan-shaped glacial conglomeratic unit 10 to 15 m thick occurs, erosively deposited over preglacial marine sediments. The conglomeratic unit is composed of preglacial intraclasts embedded within a sandy matrix. Both preglacial and glacial sediments display soft-sediment deformation structures related to fluctuating porewater pressure and strain rates, including ball structures, clastic dykes, fluted surfaces, turbate structures, folds and radial extensional normal faults. Kinematics and relative chronology of these deformation structures allow the role of porewater pressure in the process of tunnel valley genesis on soft beds to be understood. The tunnel valley formed through multi-phased episodes of intense hydrofracturing of the preglacial bed due to overpressure development promoted by ice sheet growth over the study area, and configuration of the substratum. Transport of the resulting conglomerate composed of preglacial intraclasts and fluidized sand occurred through subglacial pipes. The brecciated material is deposited in subglacial cavities, forming fans of massive sandy conglomerate infilling the base of the tunnel valley. The conglomeratic unit is partially reworked by meltwater and exhibits intense softsediment deformations, due to episodes of ice-bed coupling and decoupling.