0000000000905558
AUTHOR
John Menzies
Porewater pressure control on subglacial soft sediment remobilization and tunnel valley formation: A case study from the Alnif tunnel valley (Morocco)
25 pages; International audience; 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 …
Does porewater or meltwater control tunnel valley genesis? Case studies from the Hirnantian of Morocco.
18 pages; International audience; Several Ordovician tunnel valleys are exposed in the Moroccan Anti-Atlas Mountains, including the Alnif and the Foum Larjamme tunnel valleys, located 150 km away from each other. Sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations.The Alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface. Ball-structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces, clastic dykes, dewatering structures, folds and…
Sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice-contact fan deposits: A Pleistocene example (Ireland)
A pit located near Ballyhorsey, 28 km south of Dublin (eastern Ireland), displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice-contact fan deposits. The coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies. The lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space, mainly controlled by the position of an overlying ice roof during ice-bed decoupling. These sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments. The…
Clastic injection dynamics during ice front oscillations: A case example from Sólheimajökull (Iceland).
18 pages; International audience; Soft-sediment deformation structures are being increasingly used as a tool for reconstructing palaeoenvironments and porewater pressure conditions in glacial settings. However, the potential of hydrofractures and clastic injections in the reconstruction of ice dynamics remains poorly constrained. This paper presents the results of a detailed study of a clastic injection network outcropping in the Sólheimajökull forefield (South Iceland). Sedimentological descriptions are combined with microscopic to macroscopic analyses of clastic injection geometries, sediment-fills, and cross-cutting relationships. The 250 m long and 20 m high exposure observed along the …