Himalaya to the Sea: Geology, Geomorphology and the Quaternary
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The Kregnes “moraine” ridge in Gauldalen, a north‐trending valley south of Trondheim, is a Gilbert‐type delta formed at a Younger Dryas glacier terminus. The gravelly delta consists of a north‐dipping foreset, 150 m thick, comprised of turbidites, debrisflow beds and debrisfall deposits. The bottomset consists of turbiditic sand and mud layers. The topset, 2–3 m thick, is a braided‐river alluvium with local beach deposits, matching the marine limit of 175 m a.s.l. The fjord‐wide delta front had an extent of 3 km and prograded over a distance of 1.5 km, in probably less than 100 years, with the delta toe climbing by 50 m against the basin's rapidly aggrading muddy floor. The delta advanced through the alternating episodes of its toe aggradation and progradation, related to the increases and decreases of the delta‐slope gradient. Slope steepening led to intense sediment sloughing by chutes and occasional large‐scale failures. The fjord's wave fetch was low and the wave base no deeper than 1.5‐2 m, but strong storm waves occasionally reworked the delta front to a depth of 6 m. Glacitectonic deformation was limited to the system's upfjord end. Allostratigraphic analysis suggests that the proglacial system commenced its development as an ice‐contact submarine fan that was deformed, quickly aggraded to the sea surface and turned into an ice‐contact delta, which further evolved into the large glaciofluvial delta. The Kregnes ridge represents an episode of the ice‐front re‐advance due to climatic deterioration and is tentatively correlated with the Hoklingen substage.
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Abstract The sediments of a large accumulation landform, interpreted as one of six Pleistocene rock glaciers occurring in the Ślęża Massif, southwest Poland, are described. The sediments have been subject to strong compression and the main deformation mechanism is that of shearing. The similar angle of shear in sediments of extremely different lithology is inferred from the geometry of failure surfaces and indicates that ice was necessary for the debris transport. Because the geomorphological setting excludes a glacigenic origin, the sediments are interpreted to result from the creep of permafrost, i.e. the landform is a relict rock glacier. The movement of the rock glacier, at least in its terminal stage, is associated with a dense pattern of shear planes and would have occurred even in the presence of a small amount of interstitial ice. Copyright © 2002 John Wiley & Sons, Ltd.
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Abstract A pit located near B allyhorsey, 28 km south of D ublin (eastern I reland), 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 B allyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the I rish S ea I ce S tream flow mechanisms.
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The Oak Ridges Moraine in southern Ontario is a poly- genetic moraine constructed of a number of coalesced deposits of gla- cifluvial and glacilacustrine origin. A detailed study of the facies ar- chitecture has been completed on a series of pit sections extending ; 300 m subparallel to the paleoflow direction. Eight major lithofacies and five facies associations have been described. These data have been interpreted to be upper-flow-regime hyperconcentrated-flood-flow de- posits emplaced under a regime of rapid flow expansion and loss of transport capacity within a plane-wall jet with an associated hydraulic jump. Deposition from the plane-wall jet with jump occurred in three zones of flow transformation: zone of flow establishment, transition zone, and zone of established flow. Massive gravels with unconsolidated sand intraclasts and open-work gravel / gravel-sand couplets were de- posited in the zone of flow establishment by hyperconcentrated and supercritical flows, respectively. Immediately downflow low-angle cross-stratified sand incised by steep-walled scours infilled by diffusely graded sand define the transition zone, the zone of maximum vortex erosion, and the distal limit of deposits emplaced under upper-flow- regime conditions. These strata record rapid bed aggradation from sediment-laden supercritical flows that episodically were scoured by large vortices generated within migrating hydraulic jumps. Strati- graphically upward and downflow strata consist only of lower-flow- regime sedimentary structures. Medium-scale, planar cross-strata and small-scale cross-lamination related to migrating 2-D dunes and cur- rent ripples, respectively, characterize the zone of established flow. The facies and sediment architecture suggest that this fan was deposited during a relatively short period of time (days, weeks) by energetic sed- iment-laden floods.
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