Melt-out Till Research Articles

Terrestrial or waterlain glacial diamictites? Three case studies from the late Precambrian and late Ordovician glacial drifts in West Africa

Three examples of extensive diamictites within glacial sequences well exposed in the sedimentary cover of the West African cratonic platform are described. They belong to the Late Precambrian and Upper Ordovician glacial periods and the paleogeography is summarized. The Late Precambrian diamictites of the Jbeliat Group in Adrar of Mauritania displays characteristics of tillite deposited as lodgement or melt-out till, Detail studies on structure, clast shape nature and fabric, texture in thin section, clay mineralogy and carbonate content are given and discussed in order to delimitate useful criteria for tillite identification. Compared to this first example, the two other diamictites from the Late Precambrian Bakoye Group in western Mali and from the Upper Ordovician Tichit Group in southeastern Mauritania have obviously been deposited in an aquatic environment (waterlain glacial diamictites) by the action of ice-rafting, bottom current, and flow processes. These examples show that for such ancient deposits sedimentological interpretations are partly solved by broad scale stratigraphical relationships. However, detailed sedimentological analyses remain essential for precise facies identification. Especially structural and petrographic studies must be encouraged in the lights of modern glaciodynamic processes.

The distribution and engineering significance of superficial deposits in the Upper Clydach Valley, South Wales

Summary Abernant Colliery, in the South Wales Coalfield, disposes of 300 000 tonnes of washery waste each year onto the floor of the Upper Clydach valley. Rockhead is deeply buried by 20–60 m of superficial deposits which consist of a Lower silty stony clay, Middle bedded silts, sands and clays, and an Upper silty stony clay. The Lower silty stony clay, up to 30 m thick, is of variable composition and is probably a glacial melt-out till. The Middle bedded deposits, thickest in the middle of the valley, were probably laid down immediately after the ice had melted. The Upper silty stony clay, 5 to 15 m thick, may have been largely laid down in the form of valleyside mudflows at the same time as the Middle bedded deposits were being formed, but there is also some evidence of movement of surface layers by solifluction. The presence of water-bearing Middle bedded deposits has caused engineering problems in spoil-heap construction and a slip which occurred in 1981 was in part caused by high artesian pressures in silts and sand in these beds. Many simple, low-gradient valley floor slopes in South Wales conceal highly complex drift successions. While the near-surface material may be strong, it may overlie weak, water-saturated material at a few metres depth.

Melt-out till in the Edmonton area, Alberta, Canada: Reply

Dr. Edwards' remarks illuminate the difficult task of relating observed till properties to genetic classes of till. The main purpose of my paper was to show how this might be accomplished. This discussion illustrates how much remains to be done. The tills I described from the Edmonton area and those described from Norway by Dr. Edwards are indeed of similar appearance, but are classified differently. Disagreements of this nature are so common that there is cause to examine them in detail. I do not think that the differences can be explained by the position within the tills of clasts derived from unlithified substrata. These clasts in the Edmonton tills occur preferentially towards the bases of units, so there is no real difference. Dr. Edwards makes a critical point in stating that in the Precambrian examples the clasts were deposited beneath active ice whereas for the Pleistocene examples they were deposited in stagnant ice. 1 am not sure the evidence supports either conclusion. The Edmonton tills were probably deposited from stagnant ice, but active ice might well have been shearing over the depositing zone. On the other hand, the evidence given to support active ice in the Precambrian examples applies to stages before till release. These tills may or may not have been deposited with overriding ice. However, the differences between us here are minor, and I could not agree more that subglacial melt-out tills can occur in different contexts. Yet for all our agreement, we still differ on the way evidence is used to support genetic interpretation. The main issue is with angular clasts of unlithified sediments. I would like to expand on this.

Submarine flow tills at Victoria, British Columbia: Discussion

Hicock et al. describe complex submarine outwash cones from Quarternary glacimarine exposures. I do not question their interpretation of the diamicton sediments in the cones as being submarine flows or slumps from glacial debris; indeed, their descriptions are invaluable for tidewater glacier facies modelling. However, I have some comments on the suggested origin of the flows. Several different configurations of tidewater ice fronts may exist, but few allow the formation and preservation of sediment gravity flow deposits that originate subaerially and flow into the sea. Under polar conditions where glaciers cannot maintain themselves as floating ice tongues or ice shelves, they end in very shallow water and a beach is constructed at the base of their cliffs (e.g., Sugden and John 1976, Fig. 11.94. Most debris is slowly melted out, but rarely flowage may occur. However, such material is highly likely to be reworked by waves and, further, the ice margin in Puget Sound that Hicock et al. describe was probably not under polar-type conditions. In my experience of modem temperate tidewater glaciers in Glacier Bay, Alaska, only restricted conditions allow thick accumulation of supraglacial melt-out tills from englacial debris layers. Ablation of tidewater glaciers occurs more by berg calving rather than upper-surface melting, and ice fronts (at least above tide level) are nearly vertical cliffs. Ice is very crevassed back from the face, and the free face with consequent extensive flow commonly creates large transverse crevasses. Englacial debris layers are exposed across an ice face rather than transversely across the upper surface. However, the sides of each layer may intersect the upper surface of a glacier snout to form supraglacial melt-out debris parallel to the valley walls. Other supraglacial debris is primarily from rockfalls off valley walls and is rubble rather than diamicton. Rarely, some debris flows down the face from the upper edge, but the steepness of slope ensures sorting and separation of particles as the flow proceeds. The larger mass of pebbles and coarse-grained sand particles causes them to fall faster

Sedimentation in glacial environments and the identification of tills and tillites in ancient sedimentary sequences

Abstract Tills (and tillites) are defined and subdivided into ‘primary’ tills, comprising lodgement tills, melt-out tills and sublimation tills, and ‘secondary’ tills comprising flow tills and deformation tills. They are defined on the basis of their mode of deposition which endows them with their distinctive properties. Another category of till, that of supra-till, is defined on the basis of its debris source (derived supraglacially from valley sides and nunataks) which gives these tills a coarse-grained character. The processes of till deposition and of outwash sedimentation are associated in several recurrent combinations which allow the definition of three principal glacial sediment associations and land systems which define the character of the sequences within which tills tend to occur. The criteria which allow tills of different genesis to be identified and to be distinguished from non-till diamictons are reviewed. There are several features which are uniquely characteristic of till but, unfortunately, these are few. It has been found generally necessary to employ an array of characteristics which, though not individually unique to till, when taken together can be strong evidence for particular till types. The recognition of several typical and repetitive facies sequences which form in the major glacial sedimentary environments can also be important indicators of till genesis.

Distinguishing characteristics of Diamictons at the Margin of the Matanuska Glacier, Alaska

The origins of diamictons deposited at the Matanuska Glacier are identified in stratigraphic sequences mainly by the presence or absence of a pebble fabric, internal structure, and variation in gravel-size clast distribution. These properties correlate with major differences in depositional mechanisms and source material. Melt-out till mostly inherits fabric, internal structure, and grain-size distribution from its debris-laden basal ice source. Sediment flow deposits and ice-slope colluvium (deposited by ablational slope processes) have properties developed by resedimentation mechanisms. Melt-out till ranges from structureless to stratified with interspersed lenses and discontinuous laminae, and generally possesses a well-defined pebble fabric. Sediment flow deposits show various combinations of six sedimentologic units with distinct sedimentary features and clast dispersion. Pebble fabric is absent or poorly defined, depending upon the unit's origin. Ice-slope colluvium is usually featureless, except for randomly-dispersed laminated lenses and irregularly-shaped gravelly zones in an otherwise disarrayed assemblage of particles, and is without a pebble fabric.

˝Stagnant-Ice˝ Topography and its Relation to Drumlin Genesis, with Reference to South-Central Ulster (Abstract Only)

Previous work on drumlin genesis considers the problem at the ice-rock interface and ignores the environmental significance of inter-drumlin sediments and superimposed landform associations. Present work by the author examines two drumlin fields in south-central Ulster, Northern Ireland, and proposes a mechanism of drumlin formation which is time-dependent and results from changes of thermal regime through time.Drumlin exposures in south-central Ulster reveal a particular facies association in drumlin tills showing considerable lithological variation; striae, roches moutonnées, and basal lodgement till beneath drumlins indicate an event early in the last glaciation characterized by a temperate thermal regime. This association is overlain by a basal melt-out till, characterized by striated clasts and containing numerous pockets of stratified sediment. The melt-out till is related to changes in provenance of erratics in the till, associated with a shift in the centre of ice accumulation during the course of glaciation.The stratigraphy suggests that a considerable amount of drift was formed prior to drumlin formation and that a change of thermal state may have characterized the shift in the ice accumulation zone. This change is also suggested by evidence of high-level melt-water transport in the Sperrin Mountains, indicating a sub-polar or sub-temperate thermal regime existing in the early part of the late Weichselian deglaciation.A model based on thermal convection theory of polar ice sheets (Hughes 1976) is proposed in an attempt to relate the juxtaposition of drumlins and glaciofluvial assemblages in southcentral Ulster. In an ice accumulation zone, as characterized this area during the late Weichselian glacial maximum, advection is generally absent and convection is the dominant flow in the ice sheet. In this environment motive ice may develop in the basal zone, resulting from vertical buoyancy stresses acting downward on a column in the ice sheet. This flow environment may coexist within an ice sheet which is "stagnant" and perhaps subject to supraglacial insolation melting, depending on the overall climatic regime. If convection is dominant, it may create areas of basal compression and tension, acting to establish co-existing zones of "hot" and "cold" ice; hot-ice zones are subject to basal erosion, leading to migration of subglacial sediments to points of englacial storage above the density inversion layer. Cold-ice zones act to protect underlying sediments and result in isolation of drift nuclei, protection being afforded by the yield strength of frozen drift being greater than either debris-laden or clean ice.If the model is tenable, it seems likely that drift nucleation in drumlin fields may have been accomplished before the drumlin formation or streamlining event. Vertical migration of subglacial materials also explains the similarity of sediment sources of glacigenic and glaciofluvial assemblages which occur in juxtaposition, and, without reduction in drumlin density. The sequence of events in south-central Ulster may be summarized as follows: (1)Build-up of ice, associated with sub-drumlin erosion forms and basal lodgement till. Change of thermal regime from temperate to sub-temperate.(2)Shift in the ice-accumulation zone from highland to lowland, indicated by erratic dispersals of glaciogenic sediments. Change of thermal regime from sub-polar to polar at the late Weichselian glacial maximum.(3)High-level insolation melting with concomitant basal freezing. Initiation of polygonal array in basal zone. Change of thermal regime from polar to sub-polar or sub-temperate.(4)Down-wastage of ice and nunatak formation. Formation of high-level melt-water phenomena. Deposition of basal melt-out till. Further development of basal polygonal array. Till nuclei established and englacial sediment isolated. Change of thermal regime from sub-polar to sub-temperate.(5)Change of thermal state to temperate. Uncoupling of ice from bedrock. Streamlining of till nuclei and reworking of englacial sediment by melt water.(6)Final disintegration of ice.

Depositional Landforming Processes at the Snouts of Five Surging Glaciers in Vestspitsbergen (Abstract only)

Recent work by Clapperton (1975) proposes that the rapid rates of advance experienced by glaciers which surge may lead to enhanced debris incorporation, increased compressive flow near the glacier snout at the point of maximum extension, and to the upward translation and vertical stacking of debris near the glacier snout and margin. Five glaciers in Spitsbergen (Battyebreen, Holmströmbreen, Lisbetbreen, Vonbreen, Elnabreen) display morphological features which are widely accepted as being diagnostic of surging glaciers.Results of detailed observations regarding the nature, distribution, melt-out, and reworking of englacial debris at Battyebreen are presented. Basally derived till is brought to the surface of the glacier in narrow lateral and terminal belts, no more than 100 m wide. Within this zone, (i.e. up-valley from the snout and towards the valley centre) the ice is debris-free with the exception of small amounts of en-glacial debris which form the core of lobate medial moraines. Differential ablation of debris-free and debris-rich ice leads to the production of a topographic basin within which melt-out and reworking processes occupy restricted locations, as follows. Immediately inside the encircling melt-out till, a zone of flow tills is found. Melt streams are located at the foot of, the flow till-mantled slope, producing narrow (150 m wide) outwash trains, which merge into deltas. The central area of the topographic basin is occupied by a supraglacial lake.Observations of the remaining four locations confirm that other glaciers in the vicinity, which display similar characteristics associated with surging, are developing a stagnant-ice zone of identical appearance. The pattern of processes observed at Battyebreen is thus repeated at each site.A simple model of depositional landscape development is proposed for surging valley glaciers in a sub-polar environment.

The Tiskilwa Till, a Regional View of its Origin and Depositional Processes

The Tiskilwa Till Member of the Wedron Formation represents deposition by basal melt-outin the marginal area of the Laurentide ice sheetduring the Woodfordian (late-Wisconsinan) in Illinois. Distinctive characteristics include: a very thick, homogeneous till; relatively little ablation till; red color; sandy texture; illite content that is relatively low withrespect to other Woodfordian tills; and the presence of discontinuous basal zones of differing composition.Erosion and entrainment of debris from both distant and local source areas are evident in the Tiskilwa Jill. Basal thermal regime is suggested as a major controlling factor on the location of the zones of entrainment. The debris was homogenized en route to the margin and eventually was deposited as basal melt-out till near the margin. Deposition occurred within an interval of 6 ka or more during the first half of the Woodfordian.

Clast Shape as an Indicator of Transport and Depositional Mechanisms in Glacial Marine Sediments: George V Continental Shelf, Antarctica

Floating ice deposits (paratills) and grounded ice deposits (orthotills) have been identified on the continental shelf bordering the George V. Coast, East Antarctica. They are recognized on the bases of texture, pebble fabric, presence or absence of fauna, and stratification. The shapes of over 200 clasts taken from these glacial-marine tills indicate transport in basal debris zones. Clast shapes imply passive deposition of paratills from floating ice as supported by other lines of evidence. Deposition of orthotills by melt-out processes is also suggested by clast shape analysis. Melt-out till deposition is untenable on theoretical grounds, however, and a passive form of lodgment till deposition is proposed for orthotills in this area.

A Comparison of the Pebble Orientations in Ice and Deposits of the Matanuska Glacier, Alaska

Depositional processes and their sediment source determine the orientation of pebbles in the deposits of the Matanuska Glacier and the relationship of this orientation to the direction of ice flow. Pebble fabrics in ice-derived deposits differ from those in resedimented deposits. The plunge direction of the long axes of prolate-shaped pebbles and the ab-plane dip of blade- and plate-shaped pebbles parallel the local ice flow direction in basal zone ice and in melt-out till derived from that ice. The pebble fabric in deposits from sediment flow, ablation of exposed basal zone ice, and the slumping and spalling of ice-cored slopes does not correspond to the ice flow direction, but is developed by these depositional processes. Pebbles in basal ice and melt-out till show a unimodal distribution of orientations with individual observations only slightly dispersed about the mean axis. Pebble fabrics in other deposits are polymodal with a significantly larger amount of dispersion about the mean axis. The regional pattern of mean axes of basal zone ice and melt-out till pebble fabrics approximates the local and regional trends of ice flow, but pebble imbrication in ice and sediment does not necessarily indicate the direction from which the glacier flowed. A small number of measurements of pebble orientations at many sites and the analysis of these data by the eigenvalue method appear to be suitable techniques for examining the pebble fabric of glacial deposits, but additional sedimentological data are needed to define the origins of these deposits.

Glacier movement, ice structures, and medial moraine form at a glacier confluence, Berendon Glacier, British Columbia, Canada

On Berendon Glacier, Canada, at the asymmetric Y junction of two large valley glaciers, observations of ice velocity, strain rate, and ice structures indicate that the form of a medial moraine is determined by severe lateral compression between the two ice streams. Debris of the moraine is recycled along complex transport paths determined by ice structures. For example, supraglacial debris is precipitated into the confluence zone between the two glaciers forming a subglacial debris reservoir. Debris is evacuated from the reservoir by shearing between the two glaciers and is revealed on the glacier surface as debris dykes and melt-out tills along the contact plane. Important implications regarding the sedimentology of the moraine can be stated. Elsewhere debris is precipitated into transverse crevasses subsequently resulting in transverse till ridges at the glacier surface.Annually-formed wave ogives, bulges in surface ice, and overriding along the glacier contacts result from severe lateral compresssion, and add further diversity to the moraine.Downglacier, confluence ice structures are destroyed by ablation, the pattern of ice flow becomes less complex, unified flow between the two glaciers is established, and a distinct structurally-determined moraine morphology is replaced by one dependent upon the differential ablation of debris covered and clean ice.

Ice-marginal sedimentary, glacitectonic, and morphologic features of Pleistocene drift: An example from Newfoundland

A Pleistocene drift sequence in hummocky terrain along part of the southern Avalon Peninsula of Newfoundland is interpreted to comprise complexly interrelated lodgement till, melt-out till, flow till, supraglacial and proglacial outwash, and supraglacial rhythmites. The gray and tan melt-out tills are stacked in imbricate fashion, giving rise to exceptionally thick stratigraphic sections. Contacts between melt-out tills are interpreted as remnants of shear planes because they are sharp, they dip in the up-ice direction, and they converge toward valley margins. Overlying flow tills interdigitate with supraglacial outwash. The drift sequence was deposited during a single episode of glaciation, rather than by repeated glacier advance, as previously proposed. It is the product of thrusting of englacial debris along ice-marginal shear planes, subsequent melting-out of englacial debris, and formation of supraglacial flow till and outwash. Preservation of this sequence probably is due to high content of englacial debris within the Wisconsinan ice. The sedimentary, glacitectonic, and morphologic features of this sequence are similar to those found at the margins of certain Arctic glaciers of subpolar thermal regime which have recently been the subject of Pleistocene glacial sedimentation models for west-central Canada and Great Britain. Recognition of these distinct elements indicates wisconsinan glacier lobes were of the cold Arctic type in southeastern Newfoundland. Alternative explanations for this sequence, such as deposition by glaciers of temperate thermal regime or by surging glaciers, are discounted. Because the features described here are complex and difficult to recognize, they may be more widespread in Pleistocene drift than has previously been interpreted.

The influence of genetic processes on some geotechnical properties of glacial tills

Summary The principle characteristics of debris in transport by glaciers are described and the way in which this material is subsequently deposited as till. Three principal types of till are described, lodgement till, melt-out till and flow till, which can be related to typical sediment/landform models and which can form a good basis for site investigation planning. A proglacial/sub-glacial sediment association and land system is defined, in which lodgement till and proglacial outwash are the principal sediments, and in which drumlins, fluted moraines and push moraines, and kettle holes and eskers are the principal landforms. A supraglacial sediment association and land system is also defined, in which a suite of supraglacial sediments and landforms are superimposed on the subglacial/proglacial land system. The sediments comprise flow till, melt-out till and supraglacial outwash sediments, and the landforms largely comprise kame moraines. A third land system, the glaciated valley land system comprises chiefly medial and lateral moraines, in which the till is derived supraglacially from flanking valley sides, and kame terraces. Till genesis influences the geotechnical properties of tills through its control on several important characteristics: grain-size distribution and its spatial variation; the stress history of the till; the nature of the sequence in which the till lies, e.g. the presence of stratified horizons; and jointing in the till. Post-depositional processes which change these characteristics are also important. The processes considered are: post-depositional wetting and drying; freezing and thawing; remoulding; and downward percolation of fines.

On the Deposition of Subglacial and Melt-Out tills at the Margins of Certain Svalbard Glaciers

Two types of basal till are described. First, melt-out tills released by the melting of masses of buried, debris-rich stagnant ice. Top melting of these ice masses is of greatest importance at the present day, and this produces tills which retain some of their englacial fabrics although some are changed by the melting process. Bottom melting of these masses could also produce such tills but these are likely to be much less important. Secondly, tills are also formed subglacially when basal debris-rich glacier ice becomes stationary beneath the over-riding active glacier. When these subglacial masses melt, water is expelled from the resultant till which also shows tectonic shear fractures induced by the over-riding ice. The mode of deposition of these latter tills could also be responsible for the production of certain rock-cored drumlins. Subglacial tills in Svalbard are relatively rare and, except under special conditions, are likely to react to ice loading by water expulsion, compaction and shear fracture rather than by fluid flow.If a basal freezing hypothesis is accepted for the origin of most englacial debris, the lateral and vertical variations in erratic content can be predicted for many melt-out tills.

On the Deposition of Subglacial and Melt-Out tills at the Margins of Certain Svalbard Glaciers

Two types of basal till are described. First, melt-out tills released by the melting of masses of buried, debris-rich stagnant ice. Top melting of these ice masses is of greatest importance at the present day, and this produces tills which retain some of their englacial fabrics although some are changed by the melting process. Bottom melting of these masses could also produce such tills but these are likely to be much less important. Secondly, tills are also formed subglacially when basal debris-rich glacier ice becomes stationary beneath the over-riding active glacier. When these subglacial masses melt, water is expelled from the resultant till which also shows tectonic shear fractures induced by the over-riding ice. The mode of deposition of these latter tills could also be responsible for the production of certain rock-cored drumlins. Subglacial tills in Svalbard are relatively rare and, except under special conditions, are likely to react to ice loading by water expulsion, compaction and shear fracture rather than by fluid flow.If a basal freezing hypothesis is accepted for the origin of most englacial debris, the lateral and vertical variations in erratic content can be predicted for many melt-out tills.