Abstract. Ice streams are corridors of fast-flowing ice that control mass transfers
from continental ice sheets to oceans. Their flow speeds are known to
accelerate and decelerate, their activity can switch on and off, and even
their locations can shift entirely. Our analogue physical experiments reveal
that a life cycle incorporating evolving subglacial meltwater routing and bed
erosion can govern this complex transitory behaviour. The modelled ice
streams switch on and accelerate when subglacial water pockets drain as
marginal outburst floods (basal decoupling). Then they decelerate when the
lubricating water drainage system spontaneously organizes itself into
channels that create tunnel valleys (partial basal recoupling). The ice
streams surge or jump in location when these water drainage systems maintain
low discharge but they ultimately switch off when tunnel valleys have
expanded to develop efficient drainage systems. Beyond reconciling
previously disconnected observations of modern and ancient ice streams into
a single life cycle, the modelling suggests that tunnel valley development
may be crucial in stabilizing portions of ice sheets during periods of
climate change.
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 sorted sediments consist of fine‐grained laminated facies, cross‐laminated sand and channelized gravels, and are interpreted as subglaciofluvial sediments deposited within a subglacial de‐coupled space. The subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds, triggered by fluid overpressure and shear stress during episodes of ice/bed recoupling (clastic dykes and folds). The upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake, forming a thick sedimentary wedge erosively overlying the subglacial deposits. Gravel facies and large‐scale trough bedding sand are observed within this proximal wedge, while normally graded sand beds with developed bedforms are observed further downflow. The building of the prograding ice‐contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading (normal faults). This study facilitates the recognition of subglacial/submarginal depositional environments formed, in part, during localized ice/bed coupling episodes in the sedimentary record. The sedimentary sequence exposed in Ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the Irish Sea Ice Stream flow mechanisms.
Abstract. Conceptual ice stream land systems derived from geomorphological and
sedimentological observations provide constraints on
ice–meltwater–till–bedrock interactions on palaeo-ice stream beds. Within
these land systems, the spatial distribution and formation processes of
ribbed bedforms remain unclear. We explore the conditions under which these
bedforms may develop and their spatial organization with (i) an experimental
model that reproduces the dynamics of ice streams and subglacial land systems
and (ii) an analysis of the distribution of ribbed bedforms on selected
examples of palaeo-ice stream beds of the Laurentide Ice Sheet. We find that
a specific kind of ribbed bedform can develop subglacially through soft-bed
deformation, where the ice flow undergoes lateral or longitudinal velocity
gradients and the ice–bed interface is unlubricated; oblique ribbed bedforms
develop beneath lateral shear margins, whereas transverse ribbed bedforms
develop below frontal lobes. We infer that (i) ribbed bedforms strike
orthogonally to the compressing axis of the horizontal strain ellipse of the
ice surface and (ii) their development reveals distinctive types of
subglacial drainage patterns: linked cavities below lateral shear margins
and efficient meltwater channels below frontal lobes. These ribbed bedforms
may act as convenient geomorphic markers to reconstruct lateral and frontal
margins, constrain ice flow dynamics, and infer meltwater drainage
characteristics of palaeo-ice streams.
The Carboniferous to Permian volcanic-sedimentary succession shown by the LY-F core from the Lucenay-lès-Aix area, in the northern part of the Massif Central, has been studied in order to obtain both landscape reconstructions (sedimentological analyses) and geochronological constraints (U-Pb dating on zircon and apatite). The lowermost part
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