Lower Till Research Articles

Sedimentological investigations in the forefield of an Icelandic surge-type glacier: implications for the surge mechanism

A sedimentological study in the forefield of Hagafellsjökull Vestari, Iceland has been employed to distinguish between pervasive sediment deformation and sliding as mechanisms of basal motion in surge. Ice retreat following a surge in 1979/80 has exposed a two-layered quasi-continuous till cover and a suite of streamlined bedforms, including flutes and drumlins. An integrated approach using particle size data, clast morphology, scanning electron microscopy and micromorphology suggests that comminution processes were confined to the uppermost 16–25 cm of the till. The lower till layer remained relatively undisturbed during the surge advance in both drumlinised and non-drumlinised areas. Micromorphological evidence of a clay depositing ice-sediment water layer suggests that decoupling was important during the surge phase. We suggest that the till did not weaken sufficiently for pervasive deformation to occur at depth. Rather the surge mechanism at Hagafellsjökull Vestari is thought to involve the transition of the quiescent phase basal water system, which consists of a few large subglacial conduits emerging at the ice margin, to a distributed or linked cavity system.

Sedimentary and stratigraphic evidence for subglacial flooding, south-central Michigan, USA

The glacial landscape in south-central Michigan is a dissected surface with streamlined, residual, and upland blocks surrounded by lowland valleys dominated by outwash. The upland surface has ridges, hills, and hummocks composed of boulder gravel, which are transitional to drumlins in a down-ice direction, and is dissected by tunnel channels with eskers within them. Sedimentary descriptions from 14 gravel pits indicate a consistent stratigraphy across a width of 15 km. The upland consists of bedrock covered by a lower till with subglacial melt-out or lodgment characteristics. Above the lower till is a sand-and-gravel facies that passes upward into a boulder-gravel facies. Nestled within depressions on the boulder-gravel surface is a discontinuous facies of laminated sand and silt. Above the gravel is an upper till with supraglacial melt-out and flow till characteristics. Analyses of the till and gravel indicate an upward change in clast lithology from locally derived carbonates to clastics and finally to crystallines from the lower till through the gravel facies into the upper till. The transition mimics the bedrock lithology up-ice of the study area, and is explained by meltwater erosion of bedrock, previously deposited subglacial sediment, and overlying debris-rich basal ice. The systematic change in clast lithology and the stratigraphic position of the gravel facies between a subglacial and supraglacial till strongly suggests that the gravel facies sediment was deposited subglacially and that the entire till–gravel–till sequence resulted from the same glacial advance.

一様流中の自由‐固定弾性片の静的空力発散と横挫屈

The static instability of a slender front-free-aft-fixed elastic strip in the nuiform air stream the free end of which is the leading edge is studied analytically. The elastic strip is held vertically. The analysis is performed using the slender wing theory and beam theory and the effect of gravity force are considered that is the effect of the lateral buckling due to the gravity force. The approximate analysis is done by using energy method, and the static aeroelastic divergence speed is obtained. When the flexural rigidity approaches to the lateral buckling condition, the aeroelastic divergence speed becomes lower till zero wind speed.

Stratotype for the Mérida Glaciation at Pueblo Llano in the northern Venezuelan Andes

The Mérida Glaciation (cf. Wisconsinan, Weichselian) as proposed by Schubert (1974b) culminated at about 18ka during the last glacial maximum (LGM) and ended at about 13ka as indicated by 14C dating and correlation with the Cordillera Oriental of Colombia. Moraines of an early stade of Mérida Glaciation reached to 2800m a.s.l. and were largely overrun or eradicated by the maximum Wisconsinan advance (LGM); where they outcrop, the older moraines are characterized by eroded, weathered glacial diamictons and outwash fans.At Pueblo Llano in the central Mérida Andes (Cordillera de Trujillo), older to younger beds of contorted glacitectonized diamict, overlying beds of bouldery till and indurated outwash, all belong to the early Mérida stade. Overlying the early Mérida stade, deposits of rhythmically bedded glaciolacustrine sediments are in turn overlain with contorted sand and silt beds capped with outwash. Above the outwash terrace a loop moraine of LGM age completely encircles the margins of the basin. A stream cut exposed by catastrophic (tectonic or surge?) release of meltwater displays a lithostratigraphic succession that is bereft of organic material for radiocarbon dating. Five optically-stimulated luminescence (OSL) dates place the maximum age of the lowest till at 81ka.Particle size distributions allow clear distinctions between major lithic units. Heavy mineral analysis of the middle and lower coarse units in the section provide information on sediment sourcing and on major lithostratigraphic divisions. Trace element concentrations provide information on the relative homogeneity of the deposits. The HREE (heavy rare earth element) concentrations allow discrimination of the lower till from the rest of the section; the LREE (light rare earth element) concentrations highlight differences between the lower till, LGM till, and the rest of the section.

Early middle Pleistocene glacial sediments (780000‐620000 BP) near Kansas City, northeastern Kansas and northwestern Missouri, USA

Two lithologically distinct tills are present near the pre‐Illinoian glacial boundary in northeastern Kansas and northwestern Missouri. The tills are probably early middle Pleistocene (780 000–620 000 BP) based on normal polarity and their stratigraphic relationship to dated terraces. The upper till is sandier than the lower till and contains more expandable clay minerals and less illite than the lower till. Glaciofluvial and supraglacial sediments are preserved between the tills, yet palaeosols, or other evidence of an interglacial, are not present, suggesting that the tills were deposited during a single glaciation. Southeasterly and southwesterly trending striae and till fabric suggest that the glacial succession was deposited by at least two fluctuations (phases) of a lobe, which probably advanced down the axis of the present Missouri River lowland. These tills probably correlate to early middle Pleistocene tills in southwestern Iowa and eastern Nebraska, although it is possible that the lower till may correlate to even older tills in these areas.

Glaciotectonized quaternary sediments at Dinas Dinlle, Gwynedd, North Wales, and their bearing on the style of deglaciation in the Eastern Irish Sea

The sedimentology and structure of Late Quaternary sediments exposed in a coastal section at Dinas Dinlle, North Wales, are described. The deposits from two small hills, the northern one being more prominent and containing northward dipping units of gravel and diamicton bounded by clay-lined thrust faults. The southern hill comprises complex folded sands and gravels overlying a basal till surface. High-resolution seismic surveys along the beach, 50 m west of the cliffs, show a roughly horizontal rockhead lying some 35 m below beach level, and demonstrate that thrust faults penetrate the entire Quaternary sequence to a detachment immediately above rockhead. Thrust units in the exposed cliff section show excellent correlation with the seismic sequence 50 m to the west, indicating a strike of 080°/260°. Balanced cross-section techniques were applied to an initial simple layered sequence comprising a lower (Irish Sea) till overlain by a coarsening upwards sand to gravel unit, followed by upper till (mixed lithology, including Welsh and northern provenance), and finally an upper gravel unit. Shortening of approximately 54% is estimated within the thrust sequence. It is concluded that the two isolated hills at Dinas Dinlle are erosional remnants of a formerly more extensive push moraine extending westwards into Caernarfon Bay which has now been truncated by coastal retreat. Deglaciation of the area was interrupted by ice sheet readvance towards 170° across a proglacial surface.

Fabric signature of subglacial till deformation, Breidamerkurjökull, Iceland

ABSTRACTThe foreland of Breidamerkurjökull, Iceland, is the only locality where tills known to have undergone subglacial deformation are exposed. Till on the foreland has a two‐tiered structure, consisting of a dilatant upper horizon c 0.5 m thick and a compact lower till; these horizons correspond to the ductile deforming A horizon and the brittle‐ductile B horizon observed below the glacier by G. S. Boulton and co‐workers. The relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till. The upper horizon exhibits variable a‐axis fabrics and abundant evidence for clast re‐alignment, reflecting ductile flow and rapid clast response to transient strains. In contrast, the lower horizon has consistently well organized a‐axis fabrics with a narrow range of dip values, recording clast rotation into parallel with strain axes during brittle or brittle‐ductile shear. The data indicate that till strain history imparts identifiable macrofabric signatures, providing important analogues to guide the interpretation of Pleistocene tills.

Seismic reflection, borehole and outcrop geometry of Late Wisconsin tills at a proposed landfill near Toronto, Ontario

The search for new landfill sites in the Greater Toronto area of southern Ontario, Canada, is producing a wealth of data regarding the subsurface stratigraphy and geometry of Late Wisconsin (<25 ka) till deposits. Till strata are favoured as landfill substrates because of their wide surface extent, thickness (maximum ~60 m), high degree of overconsolidation, apparently massive character, and low permeability. However, problems are emerging where surface contaminants have migrated through till deposits into underlying aquifers along poorly understood transport paths. This paper reports the results of a detailed shallow seismic reflection investigation of a proposed 275 ha landfill site 40 km northeast of Toronto near Whitevale, where previous hydrochemical analysis and hydrogeological monitoring identified rapid vertical recharge of contaminated surface waters through Late Wisconsin tills up to 60 m thick. Seismic reflection data are ground truthed by drilling (36 holes; total drilled 3157 m), coring (1600 m), downhole geophysical logging, and outcrop data. The site stratigraphy at Whitevale consists of an uppermost Late Wisconsin till (Halton Till) separated from a lower till (informally named Northern till) by a silt, sand, and gravel complex. Seismic reflection profiles identify the presence of well-defined reflectors within the Northern till, which are correlated in outcrop with laterally extensive erosion surfaces overlain by sheet-like sands and gravels, up to 1 m thick, and boulder concentrations. Erosion surfaces and associated sediments record episodic scouring by subglacial meltwaters and provide potential "hydraulic windows" for the movement of surface contaminants through the till into underlying aquifers.

Function of the ice streams in the Scandinavian ice sheet: analyses of glacial geological data from southwestern Finland

AbstractMapping of striae trends, macro-scale erosion forms, drumlins, morainic ridges, eskers, till fabric and boulder fans has facilitated reconstruction of glacial dynamics in terms of ice streams, marginal ice lobes and interlobate zones. Data were recorded in a computerised geographical information system (GIS).Data on oriented glaciogenic elements are compared with the evolving patterns of glacial flow. The oldest flow occurred at a distance of several hundred kilometres inside the ice margin, while the later flows were dependent on the dynamics of the ice streams and fan-shaped ice lobes. A model is developed for the zonation of subglacial processes such as erosion, deposition and till deformation beneath the ice sheet. Most of the glacial forms, as well as lower till, were generated in a zone of basal melting and fast ice flow which existed some hundred kilometres from the receding margin and was associated with the formation of ice streams. These results are consistent with recent reconstructions of basal hydrology using mathematical models.Ice streams were important for deglaciation dynamics. In the course of deglaciation, decreased shear stress on the water-saturated substratum resulted in ice-bed uncoupling which lowered the profile and accelerated flow in the ice streams. This did not happen in interstream areas as reflected by the glacial geomorphology typical of inactive ice.

Pleistocene stratigraphy of the Saskatoon area, Saskatchewan, Canada: an update

Pleistocene deposits in the Saskatoon area are divided into the Sutherland and Saskatoon groups. The Sutherland Group is divided into the Mennon, Dundurn, and Warman formations, and the Saskatoon Group is divided into the Floral and Battleford formations. The Floral Formation is subdivided into the lower till, the Riddell Member, and the upper till. Each formation is composed of a sequence of deposits, including basal proglacial sediment, till, upper proglacial sediment, and nonglacial sediment. Each formation is defined primarily on the basis of carbonate content and stratigraphic position of its till, and commonly it is bounded by weathered zones and (or) nonglacial sediments. The Sutherland Group is pre-Illinoian in age, and the lower till of the Floral Formation is Illinoian in age. The Riddell Member, between the lower and upper tills of the Floral Formation, is late Rancholabrean, probably Sangamon, in age. Radiocarbon ages of sediments lying between the upper till of the Floral Formation (Early Wisconsin) and the Battleford Formation (Late Wisconsin) range from more than 38 000 to 18 000 BP. The dissipating ice sheet margin stood in the vicinity of Saskatoon about 12 000 BP.

Late Quaternary Glacial Fluctuations in the Langtang Valley, Nepal Himalaya, Reconstructed by Relative Dating Methods

Glacial fluctuations in the Langtang Himal are discussed on the basis of multiple relative dating (RD) methods in addition to the '4C method. The RD methods used for the classification of the moraine deposits include the Schmidt hammer rebound values, weathering rind thickness, percentage of clasts with oxidation stain, height of mineral projection, pit depth, percentage of weathered clasts, soil development, and soil color index. By using the RD data, we were able to divide the moraines into five stages (Gora Tabela, Langtang, Lirung, and Yala I and II); an older stage (Lama) is recognized only by the glacialtrough configuration extending down to an altitude of approximately 2600 m. The Gora Tabela Stage is defined by the deeply weathered Lower Till extending down to 3200 m a.s.l. The Langtang Stage (3650-3000 14C yr BP) probably corresponds to the greatest advance in the Holocene, followed by a series of smaller advances in the Lirung Stage (2800-550 '4C yr BP), and the two smaller Little Ice Age advances (Yala I and II Stages).

A reconstruction of glacial events in southeastern New Brunswick

Sedimentological and petrographical data obtained from five sections located north and south of the Caledonian Highlands in southeastern New Brunswick demonstrate the existence of three main till units and one glaciofluvial unit, which have been grouped in four distinct lithostratigraphic units. The lower till was deposited by a glacier that overrode the Caledonian Highlands from northwest to southeast and advanced as far as Nova Scotia during Middle(?) to Late Wisconsinan times. The overlying middle till from the north provides evidence that ice continued to advance across the Highlands from northwest toward southeast and then was partially overwhelmed by another glacier that was advancing southwest along the southern border of the Highlands: this glacier deposited a coeval middle till. During Late Wisconsinan deglaciation, ice separated into two masses: a residual ice cap with radial outflow from the Highlands; and a lobe in the Chignecto Bay, retreating toward the northeast. The existence of a plateau ice cap is demonstrated by the presence of till and glaciofluvial deposits in the upper part of all surveyed sections, and is supported by the sequence of ice flow patterns recorded by striae and the centrifugal distribution of meltwater flow indicators. The weak development of soils, the fresh appearance of till and morainic landforms, and the lack of periglacial features throughout the area, especially on the Highlands, all favour the interpretation that the Caledonian Highlands were not a nunatak during the glacial maximum of the Late Wisconsinan Substage.

Preconsolidation pressures in the Battleford Formation, southern Saskatchewan, Canada

Soft, surficial, unoxidized till of the Battleford Formation overlies hard, oxidized and unoxidized till of the Floral Formation. The difference in hardness between these two formations is the result of preconsolidation pressures that range from 1500 to 2200 kPa in the Floral Formation and from 350 to 750 kPa in the lower till unit of the Battleford Formation. The upper till and clay unit of the Battleford Formation, which represents a conformable contact between this formation and the overlying glaciolacustrine clay, is normally consolidated. Preconsolidation of the Floral Formation apparently took place upglacier where the potentiometric surface for subglacial beds was 170–240 m below the ice surface. The lower till unit of the Battleford Formation was deposited from and consolidated under stagnant ice. The upper normally consolidated till and clay unit of the Battleford Formation was let down upon the lower till unit by ablation. The calculated minimum stagnant ice thickness required for consolidation of the lower till unit of the Battleford Formation ranges from 51 to 81 m. The calculated time required for consolidation is less than 100 years.

Elongated hills near Schonhorst, Schleswig ­Holstein: Drumlins or terminal push-moraines?

Examination of bore-holes, test pitting and surficial mapping of one hill belonging to the group of smooth, elongated hills by Sch6nhorst, Schleswig-Holstein reveals three geological units: the lower, fine-grained, massive and compact till; the glaciofluvial sand; and the upper, coarse-grained, compact till with minute stringers and lenses of sand and silt. The sequence is strongly glaciotectonically disturbed. A detailed analysis of thin sections of the till micro-fabric, and of radiographs from undisturbed, oriented cores shows a relatively strong NE-SW and NW-SE particle orientation in the lower till and a weakly clustered to random orientation in the upper till. It is suggested that the field represents either drumlins (the more favourable hypothesis) or terminal push-moraines, formed during the first three ice advances of the Weichselian Glaciation.

Pleistocene stratigraphy of the Boston Harbor drumlins, Massachusetts

Evidence from the Boston Harbor drumlins indicates that two superposed tills were deposited during glacier advances which were separated by a long nonglacial interval. At Long and Peddocks Islands, argillans and truncated clay-filled fractures, along with discontinuities in clay-mineral composition, define the till contacts. Physical indicators separating the tills are not apparent at other exposures, where till boundaries were defined solely by discontinuities in clay-mineral composition. The weathering profile in the upper part of the lower till indicates extensive weathering under a climate similar to that of today, and probably similar to that of the Sangamon Interglaciation. The depth of the weathering profile, the sequence of clay-mineral alteration products, and the presence of pedogenic features in the upper part of the lower till are comparable to Sangamonian weathering profiles in the midwestern United States, implying that the lower till is Illinoian or older.

Single occasion means of height and weight of Sri Lankan Tamil children from Jaffna

Little published material is available on physical growth in any Sri Lankan population. This paper reports attained mean heights and weights of middle class Tamil children from Jaffna, based on single occasion measurements of 358 boys and 517 girls ranging in age between 6 and 17 years. The cross-sectional height and weight data of both boys and girls show a typical pattern. Boys are taller and heavier up to 9 years, then girls lead in both the measurements due to their pubertal phase of rapid growth. Boys supersede them again when they enter into their rapid growth phase, i.e. in height at 13 years but their mean weights remain lower till 16 years. Thus at 17 years boys on average are 5 cm taller than girls. With reference to British standards the attained means of both boys' and girls' height remain around the 10th centile, but later shift to the 3rd centile. In weight, however, they remain below the 3rd centile and indeed much below the range during the later part of the pubertal phase. Apart from nutrition and living conditions, how much of this pattern is influenced by ethnic and tropical climatic factors is still unclear.

Till sampling in the Casa-Berardi gold area, Quebec: A case history in orientation and discovery

Sauerbrei, J.A., Pattison, E.F. and Averill, S.A.,, 1987. Till sampling in the Casa-Berardi gold area, Quebec: A case history in orientation and discovery. In: R.G. Garett (Editor), Geochemical Exploration 1985. J. Geochem. Explor., 28: 297-314. Inco Limited discovered gold in northwestern Casa-Berardi township, Quebec, in April 1981. The discovery, named the Golden Pond Zone, is located under approximately 45 m of overburden.Diamond drilling at Golden Pond up to August 1983 indicated two main zones of Au mineralization on either side of a major east-west-trending graphitic fault zone. The mineralization consists of pyrite-arsenopyrite-native Au-bearing quartz-carbonate veins in carbonatized Archaen sediments and volcanics. A reverse-circulation overburden drilling program was undertaken in the vicinity of the Golden Pond zone in December 1983. The program, under the management of Overburden Drilling Management of Ottawa, was designed to test for glacial dispersal of the known mineralization and thereby to evaluate the effectiveness of till sampling as an Au exploration method in the Casa-Berardi area. Gold and As anomalies related to the Golden Pond zone are found mainly in heavy-mineral concentrates of the lower till that lines the bedrock depression which is spatially associated with bedrock mineralization. These anomalies occur south of the known mineralization and constitute an example of a glacial dispersal train. Most of the till gold grains are very fine (50-150 microns). The gold grains in the till 100 m south of the Golden Pond zone are delicate while the majority of those encountered 400 m to the south are either irregular or abraded, indicating transport over greater distances. Following the success of orientation survey, till sampling was used as a screening tool to evaluate geophysical and/or stratigraphic targets east and west of the Golden Pond zone prior to diamond drilling. During the initial sampling program in early 1984, targets were commonly tested at intervals of either 300 m or 400 m along strike and 25-100 m down ice. Closer-spaced sampling was undertaken to further define anomalous dispersal trains prior to diamond drilling. Two reversecirculation holes drilled 300 m apart in April 1984 to test the continuation of the Golden Pond geophysics encountered an anomalous gold-arsenopyrite dispersal train 2.5 km to the east. Followup till sampling in June 1984 further defined the anomalous train. The heavy-mineral concentrate from the bottom till sample of a hole 100 m east of the earlier anomalous hole contained approximately 700 very delicate gold grains, mostly in the 50-100 micron size range, and was concluded to be at source. Diamond drilling in July 1984 confirmed the existence of the Golden Pond East zone. 0375-6742/87/$03.50 © 1987 Elsevier Science Publishers B.V.

Freshwater organic deposits and stratified sediments between Early and Late Midlandian (Devensian) till sheets, at Aghnadarragh, County Antrim, Northern Ireland

AbstractThe following events have been identified from a complex sequence of Midlandian (Devensian) sediments recently exposed at Aghnadarragh, County Antrim: (1) A lower till interpreted as Early Midlandian was deposited by a major lowland ice sheet which moved south‐eastwards from central Ulster across the Lough Neagh depression. (2) Deglacial conditions were followed by a periglacial phase characterised by ice‐wedge growth and sedimentation by gravelly debris flows. The latter contain the oldest known remains of Mammuthus primigenius in Ireland. (3) An Early Midlandian interstadial is represented by a woody detritus peat, with evidence of Betula, Pinuos and Picea woodlands and a rich beetle fauna. Wood from this horizon has been shown to be beyond the range of radiocarbon dating (>48 180 BP). (4) Interstadial conditions were succeeded by a cold, non‐glacial phase dominated by in‐channel gravelly flows and deposition of organic muds which contain plant and insect fossils. These horizons are older than 46 850 BP. (5) An upper, non‐drumlinised till was deposited during the Late‐Midlandian by a major lowland ice sheet which moved generally eastwards across the Lough Neagh Lowlands from central Ulster. This glaciation probably reached its maximum at ca 20–24 ka. (6) Drumlin formation occurred in the Lough Neagh Lowlands towards the end of the Late‐Midlandian. The limiting moraines are dated to 17 ka. The lower till at Aghnadarragh is the first positive record of a major lowland ice sheet in Ireland during the early part of the last cold stage. The Early Midlandian interstadial peats have not been documented elsewhere in Ireland and correlate broadly with the Chelford Interstadial complex of the English Midlands. Related exposures elsewhere in Ulster confirm that the middle part of the last cold stage was free of major lowland ice masses but deposits of this period are absent from Aghnadarragh.

Glazialtektonik im Weichselglazial Schleswig ­Holsteins und ihre Beziehungen zur Morphologie

Detailed glaciotectonic studies have been conducted in the Weichselian glaciated area of Schleswig-Hol­stein in order to obtain a better understanding about the stratigraphy, the morphogenetic development and thus about the Pleistocene events towards the end of the last glaciation. In general, an upper till discordantly overlies a lower till, which is partly cove'red with meltwater depos­its. The thin upper till cover is probably derived from the last ice readvance in the Oldest Tundra Time, the so-called »Fehmam-advance•, because artifacts found under this till belong to the Younger Hamburg Cul­ture. Various ice tectonic structures such as block-faulted zones, overthrusted folds and folds were observed in these overridden strata. The different directions of the last glacial advance have been determined. The relationship between them and the interpretation of the morphology after Gripp (1952, 1954) is correlated and illustrated with field examples: It is possible to determine the direction of the ice flow from the mor­phology only in regions with strong and definite topography. However, in the gentler rolling regions with weaker and indistinct morphology the overridden strata may either be disturbed or undisturbed without perceptible changes in their palaeorelief. Therefore in these regions, the present day morphology is not the result of the last ice advance alone, but is due to a combination of the penultimate ice advance, the fol­lowing meltwater deposition, the last ice readvance and subsequent melting of dead ice.

Regional significance of pre-Wisconsinan till from Nantucket Island, Massachusetts

A major pre-Wisconsinan glacial event is the only possible source of the lower till on Nantucket Island, Massachusetts. The till occurs near the late Wisconsinan drift border and below fossiliferous marine beds of oxygen-isotope stage 5 (Sangamonian) age. It is considered to be Illinoian in age, but the evidence is tenuous. The till is correlated with the lower till of New England, and its presence supports the view that the New England upper and lower tills represent two glaciations. The pre-Wisconsinan (lower) till in New England may correlate with older tills elsewhere in the northeastern United States and in southeastern Canada that are considered to be early Wisconsinan or older in age.