Esker Sediments Research Articles

Structural and textural response to dynamics of fluvioglacial processes of the Rzymsko esker sediments, Central Poland

The study was conducted in the Rzymsko esker, in the Eastern part of the Turek Upland Plain, within the range of the Warta Glaciation (Saalian II). The form was accumulated in a tunnel valley, which was partially filled with esker sediments. Lithofacies analysis, extended by a detailed analysis of clasts, made it possible to identify a considerable variability of sedimentation conditions. At the initial stage, transport and accumulation of sediments took place in the conditions of high energy hydraulic flows or hyperconcentrated flows. The flow energy rose cyclically, which was recorded as sediments coarsening. Deposits of boulders as well as massive and cross-bedded gravels were identified, a record of catastrophic glacial floods. During the later stage of esker formation, sandy and gravelly sediments were accumulated in an open crevasse. The strong erosional force of subglacial water is confirmed by an extremely high content of local rocks in the gravelly fraction (30–87%) and its very good roundness. Zingg shape analysis (1935) revealed a clear predominance of oblate forms among local clasts and equant fennoscandian erratics. A dependency was also found between the processing method and petrographic characteristics.

The role of sediment supply in esker formation and ice tunnel evolution

Meltwater is an important part of the glacier system as it can directly influence ice sheet dynamics. Although it is important that ice sheet models incorporate accurate information about subglacial meltwater processes, the relative inaccessibility of contemporary ice sheet beds makes direct investigation challenging. Former ice sheet beds contain a wealth of meltwater landforms such as eskers that, if accurately interpreted, can provide detailed insight into the hydrology of former ice sheets. Eskers are the casts of ice-walled channels and are a common landform within the footprint of the last Laurentide and Cordilleran Ice Sheets. In south-western Alberta, esker distribution suggests that both water and sediment supply may have been important controls; the longest esker ridge segments are located within meltwater valleys partially filled by glaciofluvial sediments, whereas the shortest esker ridge segments are located in areas dominated by clast-poor till. Through detailed esker ridge planform and crest-type mapping, and near surface geophysics we reveal morpho-sedimentary relationships that suggest esker sedimentation was dynamic, but that esker distribution and architecture were primarily governed by sediment supply. Through comparison of these data with data from eskers elsewhere, we suggest three formative scenarios: 1) where sediment supply and flow powers were high, coarse sediment loads result in rapid deposition, and rates of thermo-mechanical ice tunnel growth is exceeded by the rate of ice tunnel closure due to sediment infilling. High sedimentation rates reduce ice tunnel cross-sectional area, cause an increase in meltwater flow velocity and force ice tunnel growth. Thus, ice tunnel growth is fastest where sedimentation rate is highest; this positive feedback results in a non-uniform ice tunnel geometry, and favours macroform development and non-uniform ridge geometry. 2) Where sediment supply is limited, but flow power high, the rate of sedimentation is less than the rate of thermo-mechanical ice tunnel growth. Here the ice tunnel enlarges faster than it fills with sediment and its evolution is independent of sedimentation, resulting in more uniform ice tunnel geometry. In these cases esker architecture is dominated by extensive vertical accretion of tabular units and ridge geometry is more uniform. 3) Where sediment is truly supply-limited the sedimentation rate is negligible regardless of water supply and, like scenario 2, ice tunnel growth is independent of sediment deposition, forming a relatively uniform ice tunnel (or eroding the bed). Because meltwater flows transport few gravel clasts the ice tunnel is not completely filled with gravel and, instead, saturated and pressurized diamicton or bedrock (if deformable) from beneath the surrounding ice is “squeezed” into the relatively low pressure ice tunnel during waning flow (or after ice tunnel shutdown), resulting in deformation of limited gravels deposited within the ice tunnel and a landform cored with diamicton or deformed bedrock, and with a relatively uniform ridge geometry. Our data demonstrate that an esker map is a minimum map of ice-walled channel location and that continued detailed investigation of morpho-sedimentary relationships is essential to gaining a complete picture of esker forming processes. Validating the morpho-sedimentary relationships identified in south-western Alberta (and other areas) with a larger data set may allow improved remote predictive esker mapping over larger areas and inferences to be made about spatial and temporal variations in esker depositional environments and ice tunnel evolution.

New methods to reconstruct clast transport history in different glacial sedimentary environments: Case study for Old Red sandstone clasts from polythermal Hrbyebreen and Bertilbreen valley glaciers, Central Svalbard

The objective of this study were Little Ice Age (LIA) to recent subglacial tills, glaciofluvial sediments of proglacial sandur, esker sediments and sediments of moraine-mound complexes of Hrbyebreen polythermal valley glacier and LIA to recent sediments of lateral moraine, frontal moraine, ice-cored moraine and glaciofluvial sediments of proglacial sandur of Bertilbreen polythermal valley glacier. Fossil (probably early Holocene) subglacial and supraglacial tills and sediments of coarse-grained glaciomarginal delta of Bertilbreen have also been studied. The research focused on Old Red sandstone clasts, for which roundness, shape and striation presence have been investigated. The results from this research led to the proposal of new methodological approaches. It is mainly the covariant plot of striations and RA (the share of striated clasts versus the share of very angular and angular clasts), which effectively differentiate subglacial tills from glaciofluvial sediments and allows for identification of the source material of moraine-mound complexes, especially the oldest, more degraded parts near the maximum LIA glacier extent. The second is the covariant plot of distance and RS index (the distance from the starting point versus the share of subangular and subrounded clasts) to present downstream roundness trends in proglacial glaciofluvial sediments. The research confirmed the crucial role of lithological properties of thinly bedded rocks on the shape of these rock clasts in sediment and a minimum impact of passive and active transport on the clast shape modification.

Sequence stratigraphy of a glaciated basin fill, with a focus on esker sedimentation

A large integrated data set of cores, outcrop data, and seismic transects from the mud-buried Vars-Winchester esker in the Champlain Sea basin, Canada, was studied to gain insight into how muddy glaciated basins fill with sediment, and how esker sedimentary systems contribute to this process. Three stratigraphic units—a till sheet over carbonate bedrock, the Vars-Winchester esker , and overlying Champlain Sea mud—are identified in the data set. The till is massive, mud rich, carbonate rich, and drumlinized. The esker is also carbonate rich, and rests erosively on till or bedrock. It consists of two elements, a narrow gravelly central ridge and a broad sandy carapace. Three units comprise the overlying mud package: gray carbonate-rich rhythmites, massive bioturbated mud, and carbonate-poor, red-and-gray rhythmites. A sequence stratigraphic model is proposed to explain these observations. Emphasis is placed on gradual ice-front translation superimposed by rapid meltwater events. The esker is interpreted to have been derived from the underlying till by water that flowed through a subglacial conduit (R-channel), within which the narrow gravelly central ridge was deposited. Most mud and finer sand bypassed the conduit and was deposited proglacially on the floor of the Champlain Sea, first as sandy outwash and, farther basinward, as muddy carbonate-rich rhythmites. Gradual ice-front retreat superposed distal facies over proximal facies, generating the upward-fining succession that starts with the esker gravel and ends with muddy rhythmites. Most esker sediment appears to have been deposited during rapid, jokulhlaup-like floods that punctuated gradual retreat. Discharges are estimated to have been high, possibly on the order of several hundred to, perhaps more commonly, several thousand cubic meters per second. The chaotic and random-looking appearance of the resultant sedimentological signatures in the esker sensu stricto is sharply contrasted with the regularity of the muddy rhythmites. If the rhythmites are indeed correlative to the esker, which seems reasonable given their geochemistry and the fact that their volume scales to the volume of mud in the till, the flood events that deposited the esker must have been seasonally mediated, and the basin water must have attenuated the flood signal, resulting in a rhythmic “on-off” signature in more distal portions of the system. The regularity of the rhythmites does not betray the chaotic nature of the esker sensu stricto, and vice versa. Studying either one in isolation would lead to a very different “end-member” impression of how eskers form and how esker sedimentary systems operate during the infilling of glaciated basins.

Genesis of hummocks in glaciofluvial corridors near the Keewatin Ice Divide, Canada

Glaciofluvial corridor hummocks (GCHs) within the Walker Lake map area, Canada, were examined in order to determine the character and genesis of these geomorphic features and their associated deposits. Located south of the Chantrey Moraine and north of the Keewatin Ice Divide, these corridors occur within a belt extending approximately 120 km east–west and approximately 60 km north–south. They are spaced 5–10 km apart and are hundreds of metres to several kilometres in width. They have undulating longitudinal profiles, abrupt material boundaries with the surrounding till and occur in valleys and over interfluves. Hummocks were investigated using longitudinal and perpendicular ground‐penetrating radar (GPR) surveys in conjunction with pit excavations. From these analyses, the hummocks comprise a single lithofacies consisting of coarsely stratified, matrix‐supported gravely sand to a depth of approximately 10 m. This sediment is similar to that of a ‘sliding bed facies’ observed in esker sediments and hyperconcentrated flow deposits, both of which are attributed to high meltwater discharges. Therefore, we hypothesize that the Walker Lake GCHs formed from sedimentation in cavities at the base of the ice sheet by a rapid influx of meltwater.

Structural controls on englacial esker sedimentation: Skeiðarárjökull, Iceland

AbstractWe have used ground-penetrating radar (GPR) to observe englacial structural control upon the development of an esker formed during a high-magnitude outburst flood (jökulhlaup). The surge-type Skeiðarárjökull, an outlet glacier of the Vatnajökull ice cap, Iceland, is a frequent source of jökulhlaups. The rising-stage waters of the November 1996 jökulhlaup travelled through a dense network of interconnected fractures that perforated the margin of the glacier. Subsequent discharge focused upon a small number of conduit outlets. Recent ice-marginal retreat has exposed a large englacial esker associated with one of these outlets. We investigated structural controls on esker genesis in April 2006, by collecting >2.5km of GPR profiles on the glacier surface up-glacier of where the esker ridge has been exposed by meltout. In lines closest to the exposed esker ridge, we interpret areas of englacial horizons up to ~30m wide and ~10–15m high as an up-glacier continuation of the esker sediments. High-amplitude, dipping horizons define the base of esker materials across many lines. Similar dipping surfaces deeper in the profiles suggest that: (1) the dipping surfaces beneath the esker are englacial tephera bands; (2) floodwaters were initially discharged along structurally controlled englacial surfaces (tephra bands); (3) the rapid increase in discharge resulted in hydrofracturing; (4) establishment of preferential flow paths resulted in conduit development along the tephra bands due to localized excavation of surrounding glacier ice; and (5) sedimentation took place within the new accommodation space to form the englacial structure melting out to produce the esker.

Controls on the sedimentary architecture of a single event englacial esker: Skeiðarárjökull, Iceland

Eskers have been used to infer the dynamics and palaeo-hydrology of large ice sheets yet there are few suitable contemporary analogues for esker sedimentation during Quaternary glaciations. Consequently, models of esker sedimentation have been derived from morpho-sedimentary analyses of Quaternary eskers, but these lack rigorous constraints on their depositional timescales. We identify the controls on the large-scale sedimentary architecture of a large (>700 m long, <30 m high) proto-esker deposited during a single, high-magnitude jökulhlaup at Skeiðarárjökull, Iceland. Over 3.8 km of ground-penetrating radar (GPR) lines were collected (in 2006 and 2007) as grids on all workable areas of the proto-esker and associated supraglacial ice-walled canyon-fill sediment. Six main radar facies were identified: (1) discontinuous, undular reflections, interpreted as trough-cross strata, associated with the development of antidune or dune sequences; (2) sub-horizontal reflections (dip <10°), associated with deposition of upper-stage gravel plane beds; (3) up-flow dipping moderate to high-angle (>10°) inclined reflections, interpreted as backset accretion associated with large-scale bedform development; (4) down-flow dipping high-angle (>15°) inclined reflections, interpreted as foreset accretion associated with macroform progradation; (5) quasi-hyperbolic reflections, interpreted as zones of boulder clustering; and (6) zones of enhanced noise and EM signal attenuation, bounded on the upper surface by a continuous reflection sub-parallel to the ground surface, interpreted as buried ice associated with the conduit/channel base. These data suggest that deposition took place within a non-uniform englacial conduit, which has resulted in major up-flow to down-flow variations in sedimentary architecture. The complexities observed are a direct consequence of variations in sediment supply and the within-event evolution of conduit geometry. This study provides the first detailed insight into the large-scale sedimentary architecture of a single event jökulhlaup proto-esker.

Deglacial meltwater drainage and glaciodynamics: inferences from Laurentide eskers, Canada

This paper evaluates current knowledge of Laurentide eskers in Canada in the light of developments in glacier hydrology and glacial sedimentology. Questions regarding the morpho-sedimentary relations of eskers, the synchroneity and operation of R-channel systems, the role of supraglacial meltwater input and proglacial water bodies, the controls on esker pattern, and the glaciodynamic condition of the ice sheet at the time of esker formation are discussed. A morphologic classification of eskers is proposed. Five types of eskers are identified and investigated. Type I eskers likely formed in extensive, synchronous, dendritic R-channel networks under regionally stagnant ice that terminated in standing water. Type II eskers likely formed in short, subaqueously terminating R-channels or reentrants close to an ice front or grounding line that may have actively retreated during esker sedimentation. Type III eskers plausibly formed in short R-channels that drained either to interior lakes in, or tunnel channels under, regionally stagnant ice. Type IV eskers may have formed as time-transgressive segments in short, subaerially terminating R-channels (or reentrants) that developed close to the ice margin as the ice front underwent stagnation-zone retreat or downwasted and backwasted regionally (stagnant ice); however, formation in synchronous R-channels cannot be discounted on the basis of reported observations. Type V eskers may have formed in H-channels that terminated subaerially. The spatial distribution of these esker types is discussed. The factors that determined Laurentide R-channel pattern and operation were likely a complex combination of (i) supraglacial meltwater discharge, (ii) the number and location of sink holes, (iii) the ice surface slope, thickness and velocity, and (iv) the permeability, topography and rigidity of the bed. These factors cause and respond to changes in ice dynamics and thermal regime over the glacial cycle.

The ice-contact environment

Water plays a key role in glacial sedimentation. Subglacial and englacial hydraulic flow is through a unique form of karstic conduit network which is kept open trough a delicate balance between hydraulic and cryostatic pressures. During periods of low, water flow some conduits are kept open by the sediment which had previously been deposited during periods of maximum meltwater production. Esker sediments are deposited in increments sequentially younging towards the source as they are deposited in tunnels relatively close to the ice margin and only survive in situations where the ice margin is receding. Proglacial lakes seem to have triggered sedimentation by dampening flow in feeding conduits in some palaeogeological situations. Coarse grained alluvial braid plains are common in the ice marginal situation, but chaotic and ephemeral conditions are also characteristic and catastrophic flow events are common. Complex sequences of lacustrine, fluvial, debris flow and direct glacial sediments result. Deformation due to collapse of supporting ice walls or buried ice blocks is common. Such is the volatility of the ice marginal environment that much of the sedimentary record is destroyed almost as soon as it is deposited.

Macroforms, large bedforms and rhythmic sedimentary sequences in subglacial eskers, south-central Ontario: implications for esker genesis and meltwater regime

Eskers of south-central Ontario were deposited in closed, subglacial conduits which were continuous (main conduits). This interpretation is supported by: the intimate association of eskers with an anastomosing network of tunnel channels; relatively continuous esker ridges; minimal post-formational disturbance of esker sediments; intercalation of till and stratified sand and gravel; diapiric folding at an esker core; low variability in palaeocurrent direction; and upslope flow paths. Down-esker trends in clast lithology, roundness and sphericity indicate continuous conduits with sediment supply by subglacial deformation of adjacent material into the conduits, melt-out of sediment from the conduit walls, and fluvial resedimentation.Esker ridge morphology is attributed to synchronous erosion, transportation and deposition along the main conduits, such that ridge discontinuities are primarily explained as zones of non-deposition during esker formation. Composite, pseudoanticlinal, and oblique accretion avalanche bed macroforms are identified. Gravel facies within these macroforms were deposited from fluidal flows or hyperconcentrated dispersions. Progradation of macroforms or migration of constituent large bedforms through the main conduits may have temporarily blocked constricted portions of those conduits and acted as possible internal (autogenic) controls on sediment availability and flow resistance. The location of macroforms within conduits was primarily controlled by conduit geometry and sediment availability, and later by feedback between macroform and conduit geometries.Sand and gravel units alternate rhythmically in vertical section. Rhythmicity is interpreted as a response to episodic flood flows, controlled by seasonal changes in water supply, but not necessarily representing annual events. Fans, beads, anabranched reaches and extended, hummocky deposits are intimately associated with the main esker ridges. They are interpreted in terms of subglacial cavities and localized flotation zones, connected to the main conduits during flood events. In-phase wave structures are the products of hydraulic jumps in hyperconcentrated flood flows at flow expansions into swells within the main conduits, into cavities connected to the main conduits, or at a grounding line. Esker ridges record only the most powerful flood events, whereas fans and beads record flow events in finer detail, being primarily depositional reservoirs. Laterally fining deposits record the waning stages of conduit operation.

Esker formation and glacier dynamics in eastern Skane and adjacent areas, southern Sweden

Eskers were investigated in an area with overall terrestrial deglaciation ‐ the eastern part of the province of Skåne and adjacent areas in southern Sweden. On the basis of the proposed model of esker formation, the dynamics of the receding Weichselian Ice Sheet are discussed. The deglaciation was characterized by the gradual retreat of an active ice sheet, bordered by a zone of thin, stagnant ice. For the most part, the ice sheet was probably at the pressure melting point in a marginal zone, where it was penetrated by surface meltwater which constituted most of the subglacially flowing meltwater. The esker sediments, consisting of glaciofluvially reworked basal debris and basal till, accumulated progressively in an up‐glacier direction. Deposition took place close to the live ice boundary in the zone with stagnant ice that fringed the receding ice sheet. The time‐transgressive formation of the eskers is reflected by repeated sediment sequences (morphosequences), i.e. sedimentary units composed of ridges that merge into extended hummocky deposits in a down‐glacier direction. They represent the momentary deposition of stratified drift in the proximal portion of the zone with stagnant ice.

Sedimentation in an esker system influenced by bedrock topography near Kingston, Ontario

Late Wisconsinan sand and gravel deposits on the Frontenac Arch, Ontario, record a major meltwater drainage system developed subglacially and subaqueously in one or more glacial lake phases of the Lake Ontario basin during retreat of the Laurentide Ice Sheet. Factors influencing channel location, morphology, and sediment deposition are ice flow direction and the Precambrian bedrock topography, in turn dependent upon bedrock structure and composition. Meltwater drainage across the Frontenac Arch is localized within a broad depression oriented approximately parallel to glacial flow. Sediment deposition within the regional depression follows ice-flow direction despite irregular bedrock relief, indicating formation of the meltwater system and associated sediments in three stages: (i) establishment of a continuous meltwater system subglacially under high hydrostatic pressure with minor erosion of underlying Precambrian bedrock; (ii) deposition of poorly sorted, coarse-grained sediment in cavities or channels associated with irregular bedrock topography; and (iii) deposition of several coalescing subaqueous outwash fans at the ice margin as the glacier receded from the area. The discontinuous nature of the deposits and the association of proximal to distal outwash fan facies within a deposit suggest that esker sedimentation occurred during periodic stabilization of the ice front during deglaciation.