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

2003 ◽  
Vol 73 (5) ◽  
pp. 725-734 ◽  
Author(s):  
N. K. Larsen ◽  
J. A. Piotrowski
Keyword(s):  
Basal Till

Genesis of carbonate till in the lee sides of Precambrian Shield uplands, Hemlo area, Ontario

1987 ◽  
Vol 24 (10) ◽  
pp. 2004-2015 ◽  
Author(s):  
Stephen R. Hicock
Keyword(s):  
Ice Sheet ◽  
Glacial Maximum ◽  
Laurentide Ice Sheet ◽  
The South ◽  
Precambrian Shield ◽  
Basal Zone ◽  
Compressive Flow ◽  
Late Wisconsinan ◽  
Basal Till

Near Hemlo, Ontario, highly calcareous till is confined to areas located downglacier from Precambrian uplands, at least 150 km from the Paleozoic–Precambrian boundary. It comprises subglacial meltout till between lodgment tills, and the calcareous package overlies noncalcareous basal till (not studied) and underlies noncalcareous supraglacial meltout till. The tills can be distinguished by textural, carbonate, and clast compositions. Glaciotectonic deformations, stone fabrics and striae, and stone provenance from the tills, as well as erosional and depositional landforms, indicate that ice advanced to the south–southwest across bedrock contacts and over Precambrian uplands.Deposition of all five tills can be explained with one glacial event. As the Late Wisconsinan margin of the Laurentide ice sheet advanced against uplands about 20 km northeast of Hemlo it experienced compressive flow while depositing the non calcareous basal till. Upshearing of stoss-side local debris high into the ice also occurred as englacial ice overrode the slowed basal zone. Once over the upland, englacial ice assumed extending flow, and downshearing of distal debris, which was deposited as calcareous lodgment till on the lee sides of uplands. After the glacial maximum, the glacier ceased internal movement and subglacial meltout till was laid down. A late reactivation of the ice deposited the upper lodgment till and final stagnation formed the supraglacial meltout till.

scholarly journals Estimation of hydraulic properties of subglacial till from ploughmeter measurements

1998 ◽  
Vol 44 (148) ◽  
pp. 517-522 ◽  
Author(s):  
Urs H. Fischer ◽  
Neal R. Iverson ◽  
Brian Hanson ◽  
Roger LeB. Hooke ◽  
Peter Jansson
Keyword(s):  
Water Level ◽  
Hydraulic Properties ◽  
Water Pressure ◽  
Inverse Correlation ◽  
Borehole Water ◽  
Potential Gradients ◽  
Glacial Tills ◽  
Basal Till ◽  
Range Of Values ◽  
Good Agreement

Abstract Force variations on a "ploughmeter" and fluctuations in subglacial water pressure have been measured in the same borehole at Storglaciaren, Sweden, to investigate hydraulic properties of the basal till layer. A strong inverse correlation of the pressure and force records, in conjunction with a significant lime lag between the two signals, suggests that pore-water pressures directly affect the strength of the till. Variations in sub-glacial water pressure result in potential gradients across the water till interlace at the bottom of the borehole that drive pressure waves downwards through the till layer when the borehole water level is high and back upwards when the water level is low. Analysis of the propagation velocity of this pressure wave indicates that the hydraulic diffusivity of Storglaciaren till is in the range 1.9−3.6 x 10−6m2s−1,in good agreement with estimates obtained in the laboratory. Hydraulic conductivity values associated with these difrusivities are between 10−9 and 10−8ms−1 and thus are well within the range of values for other glacial tills.

scholarly journals In-situ measurement of the strength of deforming subglacial till

1994 ◽  
Vol 40 (136) ◽  
pp. 497-503 ◽  
Author(s):  
Neal R. Iverson ◽  
Peter Jansson ◽  
Roger Leb. Hooke
Keyword(s):  
Pore Pressure ◽  
Residual Strength ◽  
Plastic Material ◽  
Excess Pore Pressure ◽  
Cone Penetration ◽  
New Instrument ◽  
Average Residual ◽  
Basal Till ◽  
Source Of Error

AbstractThe shear strength of deforming till beneath Storglaciären, Sweden, was continuously measured with a new instrument, a “dragometer”. A cylinder with conical ends, roughened with till from the glacier bed, was dragged through the till, and the force on the cylinder was recorded. Consistent with studies of cone penetration in soils, it was assumed that the till behaves as a Coulomb plastic material. This allows the residual strength of the till to be calculated from the measured force. Results from laboratory experiments confirm the validity of the analytical procedure.The average residual strength of the till was ∼55 kPa. A source of error is the potential generation of pore pressure in excess of hydrostatic ahead of the cylinder, which could significantly weaken the till. Calculations indicate that excess pore pressure did not develop during the experiment, but could develop during similar experiments beneath rapidly sliding glaciers with less permeable basal till layers.

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

2002 ◽  
Vol 48 (163) ◽  
pp. 575-586 ◽  
Author(s):  
Thomas S. Hooyer ◽  
Neal R. Iverson
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Shear Stresses ◽  
Flow Mechanism ◽  
Des Moines Lobe ◽  
Des Moines ◽  
Rapid Flow ◽  
Basal Shear ◽  
Basal Till

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.

The Quaternary deposits of western Leicestershire

1980 ◽  
Vol 288 (1028) ◽  
pp. 259-286 ◽  
Keyword(s):  
Quaternary Deposits ◽  
Entire Sequence ◽  
Bedrock Surface ◽  
Proglacial Lake ◽  
Sand And Gravel ◽  
Basal Till

The Quaternary sequence of an area of 120 km 2 , centred on Market Bosworth in Leicestershire, is described. The entire sequence is regarded as the product of the Wolstonian stage. The bedrock surface of the area shows the buried Hinckley valley, which opened southwards into the proto-Soar basin. The first evidence of sedimentation onto this surface in western Leicestershire is the Basal Till which betokens an ice advance prior to the accumulation of the extensive glacio-lacustrine member, the Bosworth Clays and Silts. This proglacial lake is interpreted as being the northern continuation of lake Harrison, which was initiated on the retreat and stagnation of ice, following the deposition of the Basal Till. A readvance of ice led to the deposition of a large sandur represented by the Cadeby Sand and Gravel and followed by a till sequence comprising the Pennine and Chalky Tills of northwestern and northeastern provenance, respectively.

scholarly journals Karst sinkhole mapping using GIS and digital terrain models

1969 ◽  
pp. 25-28
Author(s):  
Peter Brøgger Sørensen ◽  
Holger Lykke-Andersen ◽  
Peter Gravesen ◽  
Bertel Nilsson
Keyword(s):  
Information System ◽  
Late Glacial ◽  
Digital Terrain Models ◽  
Landscape Elements ◽  
Digital Terrain ◽  
Terrain Models ◽  
Front End ◽  
Limestone Bedrock ◽  
Karst Sinkhole ◽  
Basal Till

Danish glacial landscape elements such as basal till plains, hummocky moraine areas and outwash plains contain a variety of small and large depressions. They were probably formed in glacial, late-glacial or Holocene time and may represent dead-ice holes or degraded pingos, or sinkholes formed by interaction between pre-Quaternary chalk or limestone bedrock and the thin glacial co ver. The aim of this study is to map terrain depressions that might potentially be karst sinkholes by analysing digital terrain models in the geographic information system (GIS). The incentive to apply the technique for mapping of sinkholes came from an accidental acquaintance with a farmer, Jens Kirk, whose farmland is located near Thisted. Jens Kirk told us that the front end of his tractor had suddenly sunk into the ground during routine farming work, and this incident was our inspiration to start the project described here.

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