Basal Till Research Articles

Fabric, paleomagnetism, and interpretation of pre-illinoian diamictons and paleosols on Cloudy Ridge and Milk River Ridge, Alberta and Montana

Pebble fabrics and sedimentological properties indicate that pre-Wisconsinan diamictons (Kennedy Drift) on Cloudy Ridge (Alberta) and Milk River Ridge (Montana) are of glacial rather than colluvial origin. S1 and S3 eigenvalues of the upper units on the two ridges are typical of undeformed lodgement till whereas those of the lower unit on Milk River Ridge are typical of glacigenic sediment flow. Other properties, including compact matrices, striations on stones, mean pebble dip angles, and Schmidt equal-area stereonet patterns, suggest each unit is lodgement or basal till. Pedogenic features indicate weathering zones capping the tills are paleosols. Degree of rubification, clay, iron and aluminum buildup, and clay mineral alteration resembles those of very strongly developed soils formed in warmer and moister environments. The argument that “soil-like features” of the Cloudy Ridge unit resulted from post-burial diagenesis is disproven because nearly identical paleosols occur at the surface on Milk River Ridge and other interfluves to the south. Each unit examined has normal polarity. Based on comparisons with similar till/paleosol sequences exposed in Kennedy Drift sections on nearby interfluves, the Cloudy Ridge till and the upper till on Milk River Ridge were probably deposited during the early to middle Bruhnes Normal Chron (780 ka to present) whereas the lower till on Milk River Ridge is of earlier Brunhes age or dates back to the Olduvai (1.98 to 1.76 Ma) Normal Subchron or the Gauss Normal Chron (3.6 to 2.6 Ma).

Luminescence chronology of non‐glacial sediments in Changeable Lake, Russian High Arctic, and implications for limited Eurasian ice‐sheet extent during the LGM

AbstractA 10.5 m core from Changeable Lake in the Severnaya Zemlya Archipelago just north of the Taymyr Peninsula intersects ca. 30 cm of diamicton at its base, interpreted as a basal till. Because the upper 10.13 m of this core consists of non‐glacial sediments, a maximum numeric age for these non‐glacial sediments would provide a clear lower limit to the timing of the last glaciation in the area of Changeable Lake. Radiocarbon (14C) dating of several materials from this core yielded widely scattered results. Consequently we applied photonic dating to sediments above the diamicton. The experimental single‐aliquot‐regenerative (SAR) dose fine‐grain method was applied to two samples, using the ‘double SAR’ approach. With one exception, these fine‐grain SAR results and the results of application of the SAR method to sand‐sized quartz grains from two samples, at ca. 9.95 m and ca. 10.05 m depth, are discrepant with age estimates from the multi‐aliquot infrared‐photon‐stimulated luminescence (IR‐PSL) method applied to fine grains. Multi‐aliquot IR‐PSL dating of 10 samples produces ages increasing monotonically from ca. 4 ka at 2 m to 53 ± 4 ka at 9.97 m. These self‐consistent multi‐aliquot IR‐PSL ages, along with limiting 14C ages of >47 ka at ca. 10 m, provide direct evidence that glacial ice did not advance over this lake basin during the Last Glacial Maximum, and thus delimit the northeastern margin of the Barents–Kara Sea ice‐sheet to somewhere west of this archipelago. The last regional glaciation probably occurred during marine isotope stage (MIS) 4 or earlier. Copyright © 2004 John Wiley & Sons, Ltd.

The Grayling Fingers region of Michigan: soils, sedimentology, stratigraphy and geomorphic development

This paper provides data on the landforms, soils, and sediments within a unique northern Michigan landscape known as the Grayling Fingers, and evaluates these data to develop various scenarios for the geomorphic development of this region. Composed of several large, flat-topped ridges that trend N–S, the physiography of the “Fingers” resembles a hand. Previously interpreted as “remnant moraines”, the Grayling Fingers are actually a Pleistocene constructional landscape that was later deeply incised by glacial meltwater. The sediments that comprise the Fingers form a generally planar assemblage, with thick (>100 m), sandy glacial outwash forming the lowest unit. Above the outwash are several meters of till that is remarkably similar in texture to the outwash below; thus, the region is best described as an incised ground moraine. Finally, a thin silty “cap” is preserved on the flattest, most stable uplands. This sediment package and the physiography of the Fingers are suggestive of geomorphic processes not previously envisioned for Michigan. Although precise dates are lacking, we nonetheless present possible sequences of geomorphic/sedimentologic processes for the Fingers. This area was probably a topographic high prior to the advance of marine isotope stage 2 (Woodfordian) ice. Much of the glacial outwash in the Fingers is probably associated with a stagnant, early Woodfordian ice margin, implying that this interlobate area remained ice-free and ice-marginal for long periods during stage 2. Woodfordian ice eventually covered the region and deposited 5–10 m of sandy basal till over the proglacial outwash plain. Small stream valleys on the outwash surface were palimpsested onto the till surface as the ice retreated, as kettle chains and as dry, upland valleys. The larger of these valleys were so deeply incised by meltwater that they formed the large, through-flowing Finger valleys. The silt cap that occupies stable uplands was probably imported into the region, while still glaciated. The Fingers region, a col on the ice surface, could have acted as a collection basin for silts brought in as loess or in superglacial meltwater. This sediment was let down as the ice melted and preserved only on certain geomorphically stable and fluvially isolated locations. This study demonstrates that the impact of Woodfordian ice in this region was mostly erosional, and suggests that Mississippi Valley loess may have indirectly impacted this region.

A multiproxy study of a basal till: a time‐transgressive accretion and deformation hypothesis

AbstractA two‐part basal till at Knud Strand, Denmark reveals a uniform fabric pattern and strength, petrographical composition and clay mineralogy. The nature of the contact with the underlying sediments, ductile deformation structures, partly intact soft sediment clasts, small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water. A time‐transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds. It is suggested that, although strain indicators occur throughout the entire till thickness, the deformation at any point of time encompassed the uppermost part of the till only, allowing preservation of fragile clasts below. The substantial thickness of the till (up to 6 m) coupled with a much smaller (by more than one order of magnitude) inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition. The lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path. Copyright © 2004 John Wiley & Sons, Ltd.

Fabric Pattern in a Basal Till Succession and Its Significance for Reconstructing Subglacial Processes

Fabric Pattern in a Basal Till Succession and Its Significance for Reconstructing Subglacial Processes

Proglacial sediment—landform associations of a polythermal glacier: storglaciären, northern sweden

Mapping and laboratory analysis of the sediment—landform associations in the proglacial area of polythermal Storglaciären, Tarfala, northern Sweden, reveal six distinct lithofacies. Sandy gravel, silty gravel, massive sand and silty sand are interpreted as glaciofluvial in origin. A variable, pervasively deformed to massive clast‐rich sandy diamicton is interpreted as the product of an actively deforming subglacial till layer. Massive block gravels, comprising two distinctive moraine ridges, reflect supraglacial sedimentation and ice‐marginal and subglacial reworking of heterogeneous proglacial sediments during the Little Ice Age and an earlier more extensive advance. Visual estimation of the relative abundance of these lithofacies suggests that the sandy gravel lithofacies is of the most volumetric importance, followed by the diamicton and block gravels. Sedimentological analysis suggests that the role of a deforming basal till layer has been the dominant factor controlling glacier flow throughout the Little Ice Age, punctuated by shorter (warmer and wetter climatic) periods where high water pressures may have played a more important role. These results contribute to the database that facilitates discrimination of past glacier thermal regimes and dynamics in areas that are no longer glacierized, as well as older glaciations in the geological record.

Boulder-gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south-central Michigan, USA

Boulder-gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south-central Michigan, USA

Boulder‐gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south‐central Michigan, USA

Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south‐central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt‐out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in‐phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble‐rich, sandy subglacial melt‐out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3‐5m wavelength. Bedding within the stratified till, sandy bedforms and melt‐out till are mostly in‐phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.

An extensive late Holocene glacier advance of Kötlujökull, central south Iceland

This paper presents data on an extensive late Holocene advance of Kötlujökull, a southeastern outlet glacier from the ice cap Mýrdalsjökull, central south Iceland. During the advance, basal till was deposited on top of a volcanic ash layer resting on lava hummocks. Remnants of this landscape are now seen 3.5–5.5 km in front of Kötlujökull as a fragment surrounded by the Mýrdalssandur. Electron microprobe analysis of the volcanic ash shows that it is silicic and originates from the volcano Katla underneath Mýrdalsjökull. A 14C AMS date on a twig fragment sampled in the upper part of the volcanic ash layer provides a maximum age of the till of 1610±40 yr BP, and this is supported by tephrochronological and geochemical analyses of the volcanic ash. It is concluded that Kötlujökull must have advanced more than 5.5 km from its present position after 1.6 kyr BP. This advance is much more extensive than the Little Ice Age (LIA) advance and took place in a relatively cold period also identified in the Greenland ice cores and the diatom record in the seas surrounding Iceland. In the proglacial environment in front of the Icelandic ice caps, the preservation potential of older end-moraines is strongly restricted due to intense glaciofluvial erosion. Therefore, the LIA end-moraines have in many cases been interpreted as marking the outermost Holocene glacier front position. This study as well as other recent results stresses the fact that the “LIA glacial maximum” in Iceland may not be identical to the Holocene glacial maximum, and that far more case studies with well-constrained dating control are needed.

Modeling GPR radiation and reflection characteristics for a complex temperate glacier bed

We demonstrate that ground penetrating radar (GPR) reflection data from a temperate glacier are accurately modeled using a Helmholtz‐Kirchhoff diffraction integration technique that incorporates the radiation characteristics of point dipoles on a half‐space interface. This is accomplished by comparing field data to simulated data. Our 40‐MHz field data are from a 100 × 340 m (x‐ and y‐dimensions, respectively) survey grid containing 51 parallel survey lines. The field data were collected with the dipole oriented perpendicular to the survey line (x‐dipole). The synthetic data used isotropic, x‐dipole, and y‐dipole antennas, and reflections were calculated using a bed topography previously defined by 3D Kirchhoff migration. The comparisons between the real and synthetic waveforms show excellent agreement, including reflection arrival times, amplitude trends, interference patterns, and false layering from out‐of‐plane reflections. The location of reflectors determined from exploding reflector rays explains that bed reflections rapidly sink below background noise levels when reflections originate in the antenna's E‐plane. This occurs in both the simulated data and field data. Our results are of general importance for radio‐glaciology because they demonstrate that inappropriate dipole orientation with respect to the specular reflection point can lead to more than 12‐dB reduction in bottom reflection strength. Furthermore, a complicated bottom topography readily generates secondary, out‐of‐plane reflections that are easily confused with basal till layers.

A combined seismic reflection and refraction study of a landfill and its host sediments

In an attempt to delineate the base of a landfill and map the geometries of the host sediments, we have recorded a high-resolution seismic profile. To obtain sufficient resolution in the heterogeneous landfill environment, common midpoint (CMP) spacing was set to 0.125 m and subsurface coverage (i.e. fold) was maintained at ≥120 in the central region of the survey. Despite the high density and high redundancy of the data, severe source-generated noise (i.e. direct, refracted, guided and surface waves) and strong lateral velocity variations made it difficult to identify reflections on processed shot and CMP gathers. However, a quasi-continuous sequence of reflections R1–R3 was eventually traced along the length of the profile. After time-to-depth converting the stacked seismic reflection section using poorly resolved initial stacking velocities, no consistent correlations with boundaries identified in nearby boreholes and on three-dimensional georadar data were apparent. In a first attempt to obtain more reliable velocities, ∼183,000 first-arrival times were tomographically inverted. Unfortunately, the resultant velocity model was found to be incompatible with knowledge supplied by the borehole and georadar data and the seismic reflection section. By including the known depths to a key geological horizon and the R1–R3 traveltimes as constraints, a second suite of tomographic inversions produced a satisfactory model. This model included a thin capping layer of humus and sandy clay (velocities of 400–1000 m/s) overlying a distinctly lower velocity landfill (200–600 m/s) along the northern half of the profile and a southward thickening sequence of fluvial deposits (600–900 m/s) along the southern half. A southward thinning layer of compact lacustrine sediments and basal till (2000–3800 m/s) and a nearly horizontal bedrock interface (4000–5400 m/s) was mapped beneath the entire profile. Although independent applications of the seismic reflection and refraction techniques were not successful in meeting the survey objectives, a combination of the two approaches suitably constrained by borehole information finally provided the required details on the landfill and surrounding sediments. Nevertheless, our study has highlighted the limitations of employing 2-D seismic refraction and reflection methods for resolving problems in highly heterogeneous 3-D media.

Veikimorener i Finnmark, Nord-Norge

In the county of Finnmark, northern Norway, Veiki moraines have been observed by air photo interpretation in the Gaissane mountain area ( Figs. 1 and 2 ). Veiki moraines in Fennoscandia have earlier been described mainly from northern Sweden and some sites in northern Finland. In the areas where Veikimoraines are found in Finnmark, northern Norway, some special erosion landforms also exist. At several places, the cover of basal till has been removed within well-defined patches, often with a size of 400-500 r m in cross-section. The material seems to have been stripped off and transported away, probably as a frozen layer incorporated in the glacier, and then deposited by ablation in the present positions of the Veiki moraines.

Late Wisconsinan stratigraphy and chronology of southern St. George's Bay, Newfoundland: a re-appraisal: Discussion

A sedimentological model has been presented by Bell et al. (2001) to explain the stratigraphy and chronology of glacigenic sediments at localities in southern St. George’s Bay, west Newfoundland. These authors claim that this model succeeds earlier interpretations of the sediments at these localities made by MacClintock and Twenhofel (1940) and Brookes (1969, 1974, 1977 a ), and, by implication, find fault with the history of late-glacial relative sea level for these localities and the wider area presented in Brookes (1974, 1977 a ) and, with some revision, in Forbes et al. (1993). This note draws attention to two points of difference I have with Bell et al. (2001) at the local and regional scales, namely the applicability of the adopted sedimentological model and the relation of the sediments concerned to late-glacial sea level. MacClintock and Twenhofel (1940) presented an inventory and interpretation of glacial and associated sediments exposed in tens of kilometres of coastal cliffs around St. George’s Bay (well portrayed in Bell et al. 2001, fig. 4). They divided the sediments into ( i ) a compact basal till, low in the cliffs, overlain in places by kame gravels — named “St. George’s River Drift,” ( ii ) an overlying marine sequence, comprising lower muds and higher deltaic sands and gravels, in places underlying a delta surface — named “Bay St. George Delta,” and ( iii ) in a few places, an upper unit of till and kame gravel, with morainic topography — named “Robinsons Head Drift.” The sequence was interpreted to have resulted from an ice advance beyond the present coast of the bay, followed by ice retreat and local stagnation at the present coast, then by marine submergence, and, finally, an ice readvance in several lobes across the present coastline into the sea and sediments previously deposited in it. Brookes (1969, 1974, 1977 …

Inferences on glacial flow from till clast dispersal, Waterford area, New Brunswick

Dispersal patterns for till clasts from the Waterford area, New Brunswick, are compared to source outcrops and used to confirm dominant ice-flow directions in a region reported to show multiple and conflicting striae directions. The results demonstrate that the last glaciation produced elongated south and eastward trending dispersal patterns, indicative of the dominant ice-flow directions. Clasts have been derived locally. Train lengths generally vary from 4 km to about 10 km for material in basal till, but can achieve distances up to 26 km because of transport in englacial positions. Felsic and intermediate metavolcanic and intrusive clasts occur in till at locations north of outcropping plutons on the Central Plateau. The till overlies part of the Carboniferous Basin and has been derived in part, from underlying conglomerate bedrock. Since these conglomerate units contain fragments from the surrounding areas including the Central Plateau, they provided a secondary source for some lithologies during glaciation. Glacial erosion of underlying conglomerate units may account for occurrences of distinctive till clasts found at other areas of the New Brunswick lowlands, previously thought to imply northward glacial transport.

Sedimentation associated with Antarctic Peninsula ice shelves: implications for palaeoenvironmental reconstructions of glacimarine sediments

Abstract: Recent disintegration of a number of Antarctic Peninsula ice shelves has given us a unique opportunity to investigate sub-ice-shelf sediments. We characterize three sediment facies associations of two Antarctic Peninsula ice shelves (Larsen-A and Larsen Inlet). Subglacial facies consist mainly of basal till (diamicton) with high shear strengths deposited under a grounded ice sheet. Progressive upward decrease in shear strength reflects a gradual decrease in the confining vertical effective pressure of grounded ice during till deposition. Proximal ice-shelf glacimarine facies (diamicton, gravel-rich and sand-rich facies, gravelly mud, dropstone mud and sandy muds) were deposited by sub-ice-shelf rain out, bottom current activity and sediment gravity flows following decoupling of grounded ice from the sea-floor. Distal, ice-shelf glacimarine and/or open marine facies comprise terrigenous and diatom-bearing bioturbated muds and gravelly muds that contain limited ice-rafted debris; these accumulated after recession of the grounding line to the coast, with coarse-grained surface sediments possibly documenting most recent ice-shelf break-up. Antarctic Peninsula ice-shelf sediments are more heterogeneous than ice-shelf facies deposited elsewhere in Antarctica. Cold, polar Antarctic ice shelves can be differentiated from temperate and sub-polar marine-terminated glacial sedimentary systems by the dominance of coarse-grained proximal, ice-shelf glacimarine facies and an absence of subaqueous outwash/meltwater sediment facies.

Baselines of certain bioavailable and total heavy metal concentrations in Finland

“Total” (hot aqua regia extraction) and “bioavailable” (acid ammonium acetate EDTA extraction) heavy metal concentrations were measured for 90 basal till samples in Finland. The study covers the whole country. The “total” natural concentrations of several heavy metals in many places exceed the Finnish guideline or limit values designated for contaminated land. The “bioavailable” concentrations are usually only 1–5% of the “total” concentrations. The correlations between “total” and “bioavailable” concentrations were significant for Cr (Pearson's correlation coefficient 0.455 ∗∗, N=87), Cu (0.452 ∗∗, N=89), Ni (0.216 ∗, N=89), Pb (0.325 ∗∗, N=89), V (0.613 ∗∗, N=90) and Zn (0.569 ∗∗, N=89). ∗∗ Signifies that the correlation is significant at the 0.01 level, and ∗ signifies that it is significant at the 0.05 level.

Flow mechanism of the Des Moines lobe of the Laurentide ice sheet

AbstractRapid flow of the Des Moines lobe of the Laurentide ice sheet may have been related to its unlithified substrate. New reconstructions of the lobe, based on moraine elevations, sediment subsidence during moraine deposition, and flow-direction indicators, indicate that the lobe may have been ∼3 times thicker than in previous reconstructions. Nevertheless, implied basal shear stresses are <15 kPa, so internal ice deformation was not significant. Instead, the lobe likely moved by a combination of sliding, plowing of particles through the bed surface, and bed shear. Consolidation tests on basal till yield preconsolidation stresses of 125–300 kPa, so effective normal stresses on the bed were small. A model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed, resulting in too small a shear traction on the bed to shear it at depth. Consistent with this prediction, measurements of orientations of clasts in basal till yield a weak fabric, implying pervasive bed shear strain less than ∼2, although some stronger fabrics have been reported by others. We infer, tentatively, that movement was principally at the bed surface by plowing.

ESR dating of the sediments of the Last Glaciation at the source area of the Urumqi River, Tian Shan Mountains, China

Electron spin resonance (ESR) dates were obtained from glacial quartz sands formed during the Last Glaciation at the source area of the Urumqi River, Tian Shan Mountains, northwestern China. ESR dating of a basal till, interpreted as a subglacial melt-out till deposited in an end moraine which was produced in the late stage of the Last Glaciation, is consistent with ages determined through the AMS 14C method. However, dating of the overlying sediment, interpreted as supraglacial melt-out till associated with the same moraine, produced a result that differs from that determined using the AMS 14C method. Subglacial comminution could reset the ESR signal, thus making ESR dating of basal till possible. The Lower Wangfeng Group, formerly of controversial age, was formed ca. 50–70 ka, during the early stage of the Last Glaciation. It was deposited throughout the main valley as low as 2300 m a.s.l., over a length in excess of 19 km. Fluvial sandy gravel was deposited in the interstadial between the early and late stages of the Last Glaciation.

Hydrologic Pathways during Snowmelt in First-order Stream Basins at the Turkey Lakes Watershed

The chemical composition of stream and soil water collected from two first-order stream basins in the Turkey Lakes Watershed (TLW) during the spring melt periods of 1992-1996 was examined to determine the flowpaths of snowmelt to the stream channel. Soil water was intensively sampled from within the soil organic layers as well as above (shallow soil water) and within (deep soil water) a compact basal till. Stream SiO 2 concentrations of the high-elevation basin 47 were the same as the levels found in shallow soil water, and forest-floor percolate SiO 2 concentrations were elevated to these levels during intense melting periods. The SiO 2 concentrations from the stream and the shallow and deep soil water were similar at the low-elevation basin 31. With the exception of deep soil water, water collected from the soil and stream at basin 47 had higher H + and Al and lower base cation concentrations than basin 31. Stream Al concentrations were significantly correlated with forest-floor percolate Al concentrations at the high-elevation basin, whereas stream Al concentrations were correlated with mineral soil water Al concentrations at the low-elevation site. There were significant positive correlations between stream and shallow soil water H + at both basins. Shallow soil water pathways, therefore, were an important contributor to streamflow, and influenced stream chemical response during the spring snowmelt at TLW.

Late Wisconsinan stratigraphy and chronology of southern St. George's Bay, Newfoundland: a re-appraisal

Almost sixty years after the initial mapping of extensive Quaternary sediments along the coast of St. George's Bay, Newfoundland, a revised stratigraphic section is presented which identifies five main sediment types (diamicton, mud, sand, gravelly sand, gravel) and their stratigraphical position along 39 km of coastline from Highlands to Flat Bay. Most of the sediments overlying a basal till (St. George's River Drift) are interpreted to have been deposited in an ice-proximal to ice-distal glaciomarine environment by debris flow, underflow, current flow, or suspension settling, and capped by glaciofluvial outwash (Bay St. George Delta). For the most part, sections display a relatively simple deglacial sequence in a shallowing marine to fluvial environment. Where hummocky ridges intersect the coast, however, the sedimentary sequences are more complex and the variability in depositional style is typical of grounding line fans at a tidewater glacier margin. The occurrence of an upper diamicton or coarse gravels (Robinsons Head Drift) was generally observed only in grounding-line fan sequences and is interpreted here in the context of marginal fluctuations of a quasi-stable tidewater glacier, and not necessarily a climatically-induced readvance of a land-based ice margin (Robinsons Head readvance). Radiocarbon dates on marine shells from varied stratigraphic positions and sedimentary units suggest rapid deposition occurred in a glaciomarine environment between 14 and 13 ka BP.