Microstructural evidence of low‐strain,time‐transgressive subglacial deformation

Microstructural analyses were used to investigate the formation of a macroscale‐massive till at Knud Strand in Denmark. More than 100 thin sections were examined and microstructures mapped and counted for quantitative comparison and interpretation. Microstructures indicative of both brittle (grain lineations, edge‐to‐edge crushed grains) and ductile (turbate structures) deformation are evenly distributed in vertical profiles through the till, suggesting that strain contributed to its formation. Discrete shears (grain lineations and plasmic fabric) probably accommodated most deformation, whereas rotational deformation was less prominent. The microshear geometry fits the predicted Coulomb–Mohr failure criterion, indicating that till behaves as a plastic material. Strain estimate of ca. 10¹ from micromorphological proxies is two–three orders of magnitude lower than expected if the till was subjected to pervasive deformation. A hybrid of lodgement and time‐transgressive deformation is envisaged as the till‐forming processes. Our data suggest that even abundant evidence of microscale deformation at continuing high levels of strain may only record the latest process of deposition and deformation and therefore not fully reflect the complexity of till genesis. Copyright © 2007 John Wiley & Sons, Ltd.     

Recognition of micro‐scale deformation structures in glacial sediments – pattern perception,observer bias and the influence of experience

It is a scientist's mission to try to remain unbiased. However, certain factors play a role in scientific analyses that are not controlled by conscious thought. These factors are potentially very important in areas of science where interpretations are based on a scientist's ability to identify patterns or structures. One such area is the micromorphology of glacial sediments. In this paper we investigate the role of an analyst's experience in relation to pattern perception with specific reference to turbate microstructures in glacial diamictons. An experiment was conducted on 52 participants, which demonstrated that, as may be expected, more experienced (glacial) micromorphologists tend to exhibit a higher sensitivity‐to‐signal, but that complete novices, if given clear instructions, can reach levels of sensitivity similar to those of experts. It also showed, perhaps more surprisingly, that response bias does not decrease with experience. We discuss psychological factors, such as the drive for success and consistency, that may have contributed to these results and investigate their possible implications in the micromorphological analysis and interpretation of glacial sediments.     

Significance of soft-sediment clasts in glacial outwash, Puget Sound, USA

Soft-sediment clasts composed of silt and clay are contained within glacial outwash sands in the Puget Sound, Washington State, USA. The outwash was deposited during ice retreat of the Cordilleran ice sheet around 17 cal kyr BP. The soft-sediment clasts have a distinctive and consistent morphology and disposition within the sand beds. The sedimentology, sedimentary structures and presence of soft-sediment clasts suggest sand was deposited as proglacial outwash with silts and clays deposited in meltwater pools. Following drying-out of the pools and subaerial cracking, lumps of silt and clay were excavated by meltwater and transported distally as soft-sediment clasts within high-density flows. The most likely final depositional setting is as a Salisbury-type ‘delta’ in which subaqueous outwash grades distally into deeper water. This interpretation shows the power of soft-sediment clasts to inform on past processes and palaeogeography for which there is often little evidence in the geologic record.     

Loess sedimentation in Tibet: provenance,processes, and link with Quaternary glaciations

Well-preserved loess deposits are found on the foothills of mountains along the middle reaches of the Yarlung Zangbo River in southern Tibet. Optically stimulated luminescence (OSL) dating is used to determine loess ages by applying the single-aliquot regeneration technique. Geochemical, mineralogical, and granulometric measurements were carried out to allow a comparison between loess from Tibet and the Chinese Loess Plateau. Our results demonstrate that (i) the loess deposits have a basal age of 13–11 ka, suggesting they accumulated after the last deglaciation, (ii) loess in southern Tibet has a “glacial” origin, resulting from eolian sorting of glaciofluvial outwash deposits from braided river channels or alluvial fans by local near-surface winds, and (iii) the present loess in the interior of Tibet has accumulated since the last deglaciation when increased monsoonal circulation provided an increased vegetation cover that was sufficient for trapping eolian silt. The lack of full-glacial loess is either due to minimal vegetation cover or possibly due to the erosion of loess as glaciofluvial outwash during the beginning of each interglacial. Such processes would have been repeated during each glacial–interglacial cycle of the Quaternary.     

Till‐forming processes beneath parts of the Cordilleran Ice Sheet,British Columbia,Canada: macroscale and microscale evidence and a new statistical technique for analysing microstructure data

This paper presents the first integrated macroscale and microscale examination of subglacial till associated with the last‐glacial (Fraser Glaciation) Cordilleran Ice Sheet (CIS). A new statistical approach to quantifying till micromorphology (multivariate hierarchical cluster analysis for compositional data) is also described and implemented. Till macrostructures, macrofabrics and microstructures support previous assertions that primary till in this region formed through a combination of lodgement and deformation processes in a temperate subglacial environment. Macroscale observations suggest that subglacial environments below the CIS were probably influenced by topography, whereby poor drainage of the substrate in topographically constricted areas, or on slopes adverse to the ice‐flow direction at glacial maximum, facilitated ductile deformation of the glacier bed. Microscale observations suggest that subglacial till below the CIS experienced both ductile and brittle deformation, including grain rotation and squeeze flow of sediment between grains under moist conditions, and microshearing, grain stacking and grain fracturing under well‐drained conditions. Macroscale observations suggest that ductile deformation events were probably followed by brittle deformation events as the substrate subsequently drained. The prevalence of ductile‐type microstructures in most till exposures investigated in this study suggests that ductile deformation signatures can be preserved at the microscale after brittle deformation events that result in larger‐scale fractures and shear structures. It is likely that microscale ductile deformation can also occur within distributed shear zones during lodgement processes. Cluster analysis of microstructure data and qualitative observations made from thin sections suggest that the relative frequency of countable microstructures in this till is influenced by topography in relation to ice‐flow direction (bed drainage conditions) as well as by the frequency and distribution of voids in the till matrix and skeletal grain shapes.     

The Last Glacial Maximum in the North Sea Basin: micromorphological evidence of extensive glaciation

Despite a long history of investigation, critical issues regarding the last glacial cycle in northwest Europe remain unresolved. One of these refers to the extent, timing and dynamics of Late Devensian/Weichselian glaciation of the North Sea Basin, and whether the British and Scandinavian ice sheets were confluent at any time during this period. This has been the result of the lack of the detailed sedimentological data required to reconstruct processes and environment of sediments recovered through coring. This study presents the results of seismic, sedimentological and micromorphological evidence used to reconstruct the depositional processes of regionally extensive seismic units across the North Sea Basin. Thin section micromorphology is used here to provide an effective means of discriminating between subglacial and glacimarine sediments from cored samples and deriving process‐based interpretations from sediment cores. On the basis of micromorphology, critical formations from the basin have been reinterpreted, with consequent stratigraphic implications. Within the current stratigraphic understanding of the North Sea Basin, a complex reconstruction is suggested, with a minimum of three major glacial episodes inferred. On at least two occasions during the Weichselian/Devensian, the British and Scandinavian ice sheets were confluent in the central North Sea. Whilst micromorphology can provide much greater confidence in the interpretation of Late Quaternary offshore stratigraphic sequences, it is noted that a much better geochronology is required to resolve key stratigraphic issues between the onshore and offshore stratigraphic records. Copyright © 2006 John Wiley & Sons, Ltd.     

Microstructural analyses of a Middle Pliocene till from the James Bay Lowlands,Canada—evidence of “potential” fast ice streaming

Sediment from the Attawapiskat area near James Bay, Northern Ontario was sampled for micromorphological analyses. The sediment is a glacial diamicton (till) of subglacial origin. The till contains entrained and scavenged sediments of proglacial and/or subglacial glaciofluvial/glaciolacustrine origin from a subglacial deforming layer that was emplaced due to both stress reduction and/or porewater dissipation. Evidence of porewater escape, clay translocation and other microstructures all point to emplacement under active subglacial bed deformation. The limited number of edge to edge (ee) grain crushing events, however, point to lower stress levels than might anticipated under a thin fast ice lobe of the James Bay during the Middle Pliocene. Microstructures of Pleistocene tills were quantitatively compared with the Attawapiskat till and the limited number of ee events at Attawapiskat further highlighted that grain to grain contact was curtailed possibly due to high till porosity, high porewater pressures and low strain rates or alternatively due to a high clay matrix component reducing grain crushing contact events. It is suggested that this Middle Pliocene till may be indicative of sediments emplaced under ice lobe surging conditions or fast ice stream subglacial environments. This proposal has significant implications for the glaciodynamics of this part of the Middle Pliocene James Bay lobe. This research highlights a crucial link between subglacial conditions, till microstructural analyses and glaciodynamics.     

Seismotectonic versus man-induced morphological changes in a cave on the Arrábida chain (Portugal)

Distinctions between cave morphologies originating from seismic or active tectonics and those generated by natural clastic breakdown or by human activity must be made using unambiguous interpretative criteria.Easily accessible caves in particular, which may have been visited for centuries or millennia, or caves located near engineering works or quarries using great quantities of explosives, may have broken speleothems, breakdowns or detachment joints unrelated to seismic events or tectonic movements.Zambujal cave lies near neotectonic and seismic structures associated with a Plio-Quaternary 200 m uplift of the Arrábida chain and has suffered impacts resulting from quarrying, followed by possible vandalism. It is thus an example for which it is difficult to decipher morphological agents as there is the possibility that identical forms have been generated by several causes, which may have repeated at different episodes of its evolution. However, a careful morphological interpretation makes it possible to accept the existence of two seismic episodes, an “ancient” one and a “modern” one. The detection of other episodes between these is only possible using absolute dating.     

Recognition and palaeoclimatic implications of late Quaternary niche glaciation in eastern Lesotho

Geomorphic evidence of former glaciation in the high Drakensberg of southern Africa has proven controversial, with conflicting glacial and non‐glacial interpretations suggested for many landforms. This paper presents new geomorphological, sedimentological and micromorphological data, and glacier mass‐balance modelling for a site in the Leqooa Valley, eastern Lesotho, preserving what are considered to be moraines of a former niche glacier that existed during the Last Glacial Maximum (LGM). The geomorphology and macro‐sedimentology of the deposits display characteristics of both active and passive transport by glacial processes. However, micromorphological analyses indicate a more complex history of glacial deposition and subsequent reworking by mass movement processes. The application of a glacier reconstruction technique to determine whether this site could have supported a glacier indicates a reconstructed glacier equilibrium line altitude (ELA) of 3136 m a.s.l. and palaeoglacier mass balance characteristics comparable with modern analogues, reflecting viable, if marginal glaciation. Radiocarbon dates obtained from organic sediment within the moraines indicate that these are of LGM age. The reconstructed palaeoclimatic conditions during the LGM suggest that snow accumulation in the Drakensberg was significantly higher than considered by other studies, and has substantial relevance for tuning regional climate models for southern Africa during the last glacial cycle. Copyright © 2009 John Wiley & Sons, Ltd.     

High‐resolution seismic reflection profiling of neotectonic faults in Lake Timiskaming,Timiskaming Graben,Ontario‐Quebec,Canada

The Timiskaming Graben is a 400 km long, 50 km wide north‐west trending morphotectonic depression within the Canadian Shield of eastern North America and experiences frequent intraplate earthquakes. The graben extends along the border of Ontario and Quebec, connecting southward with the Nipissing and Ottawa‐Bonnechere grabens and the St. Lawrence Rift System which includes a similar structure underlying the Hudson Valley of the eastern USA. Together they form a complex failed rift system related to regional extension of North American crust during the breakup of Rodinia and, later, Pangea. The Timiskaming Graben lies within a belt of heightened seismic activity (Western Quebec Seismic Zone) with frequent moderate magnitude (greater than magnitude 5) earthquakes including a magnitude 6.2 in 1935. These events threaten aging urban infrastructure built on soft glacial sediments; post‐glacial landslides along the Ottawa Valley suggest earthquakes as large as magnitude 7. The inner part of the Timiskaming Graben is filled by Lake Timiskaming, a large 110 km long post‐glacial successor to glacial Lake Barlow that was ponded by the Laurentide Ice Sheet 9500 years ago. The effects of frequent ground shaking on lake floor sediments was assessed by collecting more than 1000 line kilometres of high‐resolution ‘chirp’ seismic profiles. Late glacial Lake Barlow glaciolacustrine and overlying post‐glacial sediments are extensively deformed by extensional faults that define prominent horsts and grabens; multibeam bathymetry data suggest that faults influence the morphology of the modern lake floor, despite high sedimentation rates, and indicate recent neotectonic deformation. The Lake Timiskaming area provides evidence of post‐glacial intracratonic faulting related to recurring earthquake activity along a weak spot within the North American plate.     

On upper mantle seismic heterogeneities beneath Fennoscandia

Three-dimensional seismic mapping of the upper mantle beneath Fennoscandia (Baltic Shield) using an ACH-type of inversion technique in combination with P-wave travel-time residual observations from the local seismograph network gave the following results. The central parts of the Baltic Shield are characterized by relatively high seismic velocities down to approximately 300 km. Those parts of the shield most affected by the Caledonide orogeny exhibit relatively low velocities particularly in the uppermost 100 km depth interval. The lower part of the upper mantle (300–600 km) does not exhibit pronounced seismic velocity anomalies and in this respect is in contrast to results from similar studies in regions subjected to neotectonic processes like parts of central and southeastern Europe. The seismic anomaly pattern in the presumed thickened lithosphere is in quantitative agreement with similar ones derived from surface wave dispersion analysis and inversion of electrical measurements. The general orientation of these anomalies coincides with that of the glacial uplift.     

Pleistocene succession of the central interior United States

The Quaternary of the continental interior of the United States is characterized by deposits from glacial ice, with associated outwash and eolian deposits, and by alluvial deposits produced by the same climatic pulses. Erosional incision of valleys occurred early in the glacial pulse, outwash deposition during the waning phase of the pulse, and soil formation during times of relative stability between the glacial pulses. These features of deposition, erosion, and soil formation are presented in a series of curves. One way the marine record could be correlated with that of the continental interior is to compare and match the physical records of both environments.     

Groundwater quality variations in glacial drift and bedrock aquifers,Barry County,Michigan, U.S.A.

Groundwater samples from 288 domestic wells in Barry County, Michigan, were analyzed for 33 inorganic chemical parameters. Variations in chemical composition were investigated by considering the possible effects of human impact, aquifer type (bedrock vs glacial drift), chemical evolution along groundwater flow paths, and glacial landform type (moraine vs outwash). Approximately 25 percent of the glacial drift wells were classified as degraded by human impact and were excluded from further analysis of chemical variation. Two-sample tests comparing individual concentrations from drift and bedrock aquifers suggest that groundwater in the Marshall Sandstone aquifer is derived from local recharge through the glacial drift. This conclusion is supported by generalized groundwater flow patterns recognized for the two aquifers.Concentrations in both aquifers were examined in relation to generalized flow paths derived from water level data and also by classification of wells as recharge, transition, and discharge. No spatial concentration trends in major ions were detected, although iron concentrations do appear to increase from recharge to discharge areas. Declining redox potential along groundwater flow paths may explain this trend.The possible influence of glacial landform type was investigated by comparing concentrations of wells in moraines with those in outwash deposits. Wells in moraines have significantly higher concentrations of most parameters, perhaps due to higher content of finer, more chemically reactive sediment grains.     

Seismotectonic versus man-induced morphological changes in a cave on the Arrábida chain (Portugal)

Abstract

Distinctions between cave morphologies originating from seismic or active tectonics and those generated by natural clastic breakdown or by human activity must be made using unambiguous interpretative criteria.

Easily accessible caves in particular, which may have been visited for centuries or millennia, or caves located near engineering works or quarries using great quantities of explosives, may have broken speleothems, breakdowns or detachment joints unrelated to seismic events or tectonic movements.

Zambujal cave lies near neotectonic and seismic structures associated with a Plio-Quatemary 200 m uplift of the Arrábida chain and has suffered impacts resulting from quarrying, followed by possible vandalism. It is thus an example for which it is difficult to decipher morphological agents as there is the possibility that identical forms have been generated by several causes, which may have repeated at different episodes of its evolution. However, a careful morphological interpretation makes it possible to accept the existence of two seismic episodes, an “ancient” one and a “modern” one. The detection of other episodes between these is only possible using absolute dating. © Elsevier, Paris     

Geological evidence for neotectonic activity during deglaciation of the southern Sperrin Mountains,Northern Ireland

In the southern Sperrin Mountains, Northern Ireland, stacked glacigenic sequences that accumulated during deglaciation (ca. 17000–13000 yr BP) overlie a basement of jointed and mascerated bedrock. The glacigenic sequences comprise interbedded glaciotectonic shears, diamictic breccias and rock rafts. At one site a normal fault with a metre‐scale vertical displacement cuts through part of the sequence and is overlain by a glacial diamict. Sediments at an adjacent site show that faulting and associated hydrothermal activity was related to neotectonic reactivation of pre‐existing Caledonian lineaments caused by ice unloading. From stratigraphical and directional evidence, fault reactivation occurred early in the deglaciation after north central Ireland ice had retreated southwards into lowland areas, but before Sperrin Mountain ice readvanced from the north. This relationship provides evidence for the relative timing of neotectonic activity in Northern Ireland, and demonstrates the effects of glacio‐isostatic unloading near ice‐sheet centres. Copyright © 1999 John Wiley & Sons, Ltd.     

Turbidite deposition on the glacially influenced,canyon‐dominated Southwest Grand Banks Slope,Canada

The deeply dissected Southwest Grand Banks Slope offshore the Grand Banks of Newfoundland was investigated using multiple data sets in order to determine how canyons and intercanyon ridges developed and what sedimentary processes acted on glacially influenced slopes. The canyons are a product of Quaternary ice‐related processes that operated along the margin, such as ice stream outwash and proglacial plume fallout. Three types of canyon are defined based on their dimensions, axial sedimentary processes and the location of the canyon head. There are canyons formed by glacial outwash with aggradational and erosional floors, and canyons formed on the slope by retrogressive failure. The steep, narrow intercanyon ridges that separate the canyons are composite morphological features formed by a complex history of sediment aggradation and degradation. Ridge aggradation occurred as a result of mid to late Quaternary background sedimentation (proglacial plume fallout and hemipelagic settling) and turbidite deposition. Intercanyon ridge degradation was caused mainly by sediment removal due to local slump failures and erosive sediment gravity flows. Levée‐like deposits are present as little as 15 km from the shelf break. At 30 km from the shelf, turbidity currents spilled over a 400 m high ridge and reconfined in a canyon formed by retrogressive failure, where a thalweg channel was developed. These observations imply that turbidity currents evolved rapidly in this slope‐proximal environment and attained flow depths of hundreds of metres over distances of a few tens of kilometres, suggesting turbulent subglacial outwash from tunnel valleys as the principal turbidity current‐generating mechanism.     

Characteristics of the sinian glaciogenic rocks of the Shennongjia region,Hubei Province,China

Sinian glaciogenic rocks are widespread in Shennongjia, Hubei. They are best preserved in western Shennongjia where the glacial Gucheng and Nantuo Formations are separated by the non-glacial Datanpo Formation, which is chiefly composed of carbonaceous silty shale and siltstone. In eastern Shennongjia most of the Lower Sinian succession is, however, absent and the Nantuo Formation rests with angular unconformity on the pre-Sinian basement.The glacial sedimentary facies mainly comprise three types; namely the lodgement facies, meltwater facies and proglacial subaqueous facies. The lodgement facies is predominant in the eastern area but outwash deposits are more abundant in the western area. Major glacial transport during the Nantuo Ice Age is inferred to have been from the ESE, based on pre-glacial palaeogeography, provenance of clasts and regional variations in glacial sedimentary facies.During the Nantuo Ice Age, the Jingshan Mountain, which is situated to the east of Shennongjia, was a glacial source area. Jianghan glacial erosion and deposition was in the form of an incomplete ring-shape; the Mufu Mountain area, which is situated at the juncture between southeastern Hubei and northwestern Jiangxi provinces, and was far removed from the source areas, received outwash deposits.     

The impact of gravel extraction on groundwater dependent wetlands and lakes in the Boreal Plains, Canada

The impact of gravel excavation on a groundwater dependent ecosystem (GDE) in a glacial outwash plain was determined using a combination of time-series stable isotopic measurements (??²H and ??¹⁸O) and a numerical flow model of lake?Cgroundwater interaction. Isotopic analyses of the lake and groundwater indicated a shift from a dominance of evaporative enrichment to more meteoric conditions, confirming the hypothesis of increased recharge following forest clearing and gravel extraction from an esker on the outwash plain. The effect of these land-use changes on source water for the GDE was quantified by simulating the lake water budget, seepage, and groundwater conditions for a period spanning pre- and post-mining activity. Enhanced cycling of shallow groundwater, driven by increased recharge in the gravel excavation area, was predicted to cause annual groundwater discharge pulses greater than baseline conditions for the groundwater-fed lake. The additional groundwater discharge represents approximately 4% of the annual lake budget, increasing the flushing rate of the lake. The influence of regional groundwater conditions, represented by variation of water table gradient and outwash hydraulic conductivity, and an alternative excavation location were investigated in a sensitivity analysis. Simulation results illustrate that a simple groundwater capture zone analysis for the GDE could be used to determine a location for gravel excavation that would reduce impact on GDE water source.     

Fault-block structures in the eastern margin of the Amur lithospheric plate,their seismicity,and fluid regimes

The results of long-term studies of the neotectonic processes and related phenomena such as the Earth’s interior degassing, the fluid discharge, and the high seismicity in the zone of interaction between the Amur and Okhotsk lithospheric plates at the transition between the Verkhnii and Srednii Amur regions were used for defining the separate structural blocks with different geodynamics and analyzing their relations with the seismic events during the periods of high seismic activity and quiescence. The heliometric and atmochemical studies in the Zeya-Bureya basin revealed zones of high permeability. Long-term observations of the fluid dynamics at the Konstantinovka mineral water deposit demonstrate a correlation between the variations in the water-soluble helium contents and the changes in the stress-deformed state of the blocks located up to 200 km away from regime observation sites.