The aims of this review are to: (a) describe and interpret structures in valley glaciers in relation to strain history; and (b) to explore how these structures inform our understanding of the ...kinematics of large ice masses, and a wide range of other aspects of glaciology. Structures in glaciers give insight as to how ice deforms at the macroscopic and larger scale. Structures also provide information concerning the deformation history of ice masses over centuries and millennia. From a geological perspective, glaciers can be considered to be models of rock deformation, but with rates of change that are measurable on a human time‐scale. However, structural assemblages in glaciers are commonly complex, and unraveling them to determine the deformation history is challenging; it thus requires the approach of the structural geologist. A wide range of structures are present in valley glaciers: (a) primary structures include sedimentary stratification and various veins; (b) secondary structures that are the result of brittle and ductile deformation include crevasses, faults, crevasse traces, foliation, folds, and boudinage structures. Some of these structures, notably crevasses, relate well to measured strain‐rates, but to explain ductile structures analysis of cumulative strain is required. Some structures occur in all glaciers irrespective of size, and they are therefore recognizable in ice streams and ice shelves. Structural approaches have wide (but as yet under‐developed potential) application to other sub‐disciplines of glaciology, notably glacier hydrology, debris entrainment and transfer, landform development, microbiological investigations, and in the interpretation of glacier‐like features on Mars.
Plain Language Summary
Glacier ice covers about 10% of Earth's land surface and has a profound influence on human activity. As ice continues to melt around the world in response to anthropogenic and natural global heating, the dynamics of glaciers and ice sheets are changing. As this review emphasizes, understanding glacier dynamics is achieved by investigating the brittle and ductile structures that characterize glaciers. The importance of this aspect of glaciology is increasingly being recognized, and draws on its close affinity with structural geology, although interaction between the disciplines to date has been limited. Brittle structures include crevasses, crevasse traces, and a wide variety of faults. Ductile structures include foliation (a layered structure that results from deformation), folds on many scales, and less universally known structures such as ogives and boudins. This review describes, illustrates, and explains the origin of these structures in terms of stress and strain. It also considers areas of controversy, unresolved issues, and offers pointers to future research. A more comprehensive understanding of glacier and ice sheet dynamics is needed to better predict their response to climate change, and ultimately their contribution to sea‐level rise. Structural glaciology can play a large part in this process.
Key Points
Glaciers and ice sheets display a wide range of structures that inform us about past and present ice dynamics
Glaciers reveal structures that are analogous to those in rocks that have deformed close to their melting point in the Earth's crust
Glacier structures influence hydrology, glacial sedimentology and geomorphology, ice microbiology, and help the interpretation of landforms
This paper presents the sediment, landform and dynamic context of four avalanche-fed valley glaciers (Khumbu, Imja, Lhotse and Chukhung) in the Mount Everest (Sagarmatha) region of Nepal. All four ...glaciers have a mantle of debris dominated by sandy boulder-gravel that suppresses melting to an increasing degree towards the snout, leading to a progressive reduction in the overall slope of their longitudinal profile. Prominent lateral–terminal moraine complexes, also comprising sandy boulder-gravel, enclose the glaciers. These terminal moraines originally grew by accretion of multiple sedimentary facies of basal glacial and supraglacial origin, probably by folding and thrusting when the glaciers were more dynamic during the Little Ice Age. The four glaciers are in various stages of recession, and demonstrate a range of scenarios from down-wasting of the glacier tongue, through moraine-dammed lake development, to post-moraine-dam breaching. Khumbu Glacier is at the earliest stage of supraglacial pond formation and shows no sign yet of developing a major lake, although one is likely to develop behind its >250
m high composite terminal moraine. Imja Glacier terminates in a substantial body of water behind a partially ice-cored moraine dam (as determined from geophysical surveys), but morphologically appears unlikely to be an immediate threat. Chukhung Glacier already has a breached moraine and a connected debris fan, and therefore no longer poses a threat. Lhotse Glacier has an inclined, free-draining tongue that precludes hazardous lake development. From the data assembled, a conceptual model, applicable to other Himalayan glaciers, is proposed to explain the development of large, lateral-terminal moraine complexes and associated potentially hazardous moraine dams.
Using satellite remote sensing, this study aims to assess the validity of upscaling ground‐based structural observations of small valley glaciers, to larger‐scale ice masses that are too vast or ...inaccessible for field‐study or ground‐truthing. Focusing on four adjacent valley glaciers on Bylot Island, Nunavut, Arctic Canada, we establish that ground‐based structural observations from two smaller (Stagnation and Fountain Glaciers) can be used to interpret the structures visible in optical satellite imagery in two much larger glaciers (Aktineq and Sermilik Glaciers). All the glaciers investigated have prominent longitudinal lineations, which are interpreted from ground observations to be longitudinal foliation. Other structures that were identified include primary stratification, crevasses, crevasse traces, and thrust‐faults. Strong longitudinal foliation is concentrated at flow‐unit boundaries, with differential ablation of ice facies commonly resulting in a ridge‐and‐furrow supraglacial topography that controls supraglacial streams and debris concentrations. Consequently, areas of strong foliation appear darker than areas of weak foliation in satellite imagery. As coarser resolution imagery is utilized to map large‐scale ice masses, sub‐pixel structural information is lost. Individual lineations mapped in coarser resolution imagery therefore probably comprise groups of clustered foliation at the sub‐pixel scale. Lateral narrowing measurements and calculated one‐dimensional strain across zones of longitudinal foliation are assessed as a tool for identifying large‐scale surface strain patterns, in particular large‐scale pure shear regimes. These one‐dimensional strain measurements suggest that flow‐unit boundaries are areas that undergo considerable cumulative strains. The upscaling approach used here can be applied to the largest ice masses, notably the Antarctic Ice Sheet.
In this study, the validity of upscaling ground‐based structural glaciological observations to inaccessible larger‐scale ice masses through satellite remote sensing is assessed. Coarser resolution imagery, commonly used for mapping large spatial areas, clusters information at the sub‐pixel scale, resulting in visible features that comprise groups of structures with individual dimensions less than the image resolution. Nevertheless, remote sensing‐based studies remain valid as wide‐scale dynamic information is shown to be preserved in the clustered structures.
The Marinoan panglaciation (ca 650 to 635 Ma) is represented in north‐east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn‐glacial carbonates in its upper 100 m. These ...sediments are now known to have been deposited under a CO2‐rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite‐cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave‐rippled or aeolian siliciclastic facies); and Calcareous Lake (slump‐folded and locally re‐sedimented rhythmic/stromatolitic limestones and dolomites associated with ice‐rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (−12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of −6 to −7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (−8 to −15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica.
•Robust Sr- and C-isotope stratigraphy despite greenschist facies metamorphism.•Islay carbonates have molar tooth structure and low positive δ13C.•Cryogenian base at 9m below diamictite, above ...negative δ13C (Garvellach) anomaly.•Ice-rafted deposits overlap with shoaling, evaporative carbonates on the Garvellach Islands.•Simple late Tonian Neoproterozoic section, 1.5km thick, on seabed off the Garvellachs.
The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact.
The Appin Group forms a regionally extensive, >4km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in δ13CVPDB from +5 to +2‰ upwards.
In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re-described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with δ13C of −4 to −7‰ (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1‰) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper-water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft-sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where D1 is absent).
Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian 87Sr/86Sr values of 0.7066–0.7069 on Islay, decreasing to 0.7064–0.7066 in the younger GEF limestones on the Garvellachs, with 1700–2700ppmSr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the Tonian record offshore of the Garvellachs.
A candidate Tonian-Cryogenian GSSP is proposed on Garbh Eileach within the smooth δ13C profile at the cross-over to positive δ13C signatures, 4m below the first occurrence of ice-rafted sediment and 9m below the first diamictite. Although lacking radiometric constraints or stratigraphically significant biotas or biomarkers, the Scottish succession has a thick and relatively complete sedimentary record of glaciation, coherent carbon and strontium chemostratigraphy, lateral continuity of outcrops and 100% exposure at the proposed boundary interval.
•The Port Askaig Formation contains 47 diamictites and shows a pattern of gradual evolution upsection of lithologies of the diamictites, of the interbeds and of clast types. The diamictites are ...interpreted as tillites and their basal surfaces are almost always sharp, recording the change from non-glacial to glacial environments.•The tops of diamictites often preserve a detailed record of glacial to periglacial to non-glacial environments (sandstone wedges, frost shattered clasts, cryoturbations).•The formation records 28 glacial, 25 periglacial and 23 non-glacial episodes. Parts of Members 1 and 2 on the east of Garbh Eileach are more complete than elsewhere. The Argyll Group, from the PAF to the Jura Quartzite, shows huge thickness (7 km) in the Garvellachs area: accommodation space was created continually and at a fast rate.•A comparison of the PAF with other thick, relatively complete Phanerozoic and Cryogenian glacial successions suggests that the PAF is exceptional in its combination of formation thickness (ca. 1100 m), the number of climatically-related stratigraphic episodes (76) and the huge thickness of the Neoproterozoic succession within which it lies.•The completeness of the PAF stratal record is supporting evidence that the base of the PAF in the Garvellach Islands is a succession without a major break and supports it being a candidate for the Cryogenian GSSP.
The Port Askaig Formation (PAF) is a diamictite-bearing succession in the Dalradian Supergroup of Scotland that provides an excellent archive of a Cryogenian glaciation in the Garvellach Islands and Islay, Argyll. The formation is ∼1100 m thick, comprises 5 members and includes 47 diamictite beds, interbedded with siltstones, dolostones and sandstones. Here we document seven features of the PAF that indicate its relative stratigraphic completeness. There are gradual, progressive changes up-section in the lithologies of the diamictites, their interbeds, and clast lithologies. The sharp basal surfaces of the diamictites each show the same, repeated pattern of environmental change, from non-glacial to glacial. Many of the top surfaces of the diamictites show evidence of periglacial conditions. The succession in the PAF records a total of 76 climatically-related stratigraphic episodes: 28 glacial episodes, 25 periglacial episodes and 23 non-glacial episodes. Parts of Member 1 (Diamictites 1–12 and Diamictites 16–18) and Member 2 (Diamictite 31 to the base of Member 3) are most compete on the east coast of Garbh Eileach. The PAF in the Garvellach Islands occurs within a succession that is several kilometres thick, as newly revealed by sea-floor mapping. Compared with other Cryogenian and Phanerozoic glacial successions, the PAF is exceptional in its combination of formation thickness, the number of climatically-related stratigraphic episodes, and the considerable thickness of its host supergroup. Furthermore, these indicators of relative stratigraphic completeness provide evidence that the base of the PAF on the east coast of Garbh Eileach is a succession without a major break in deposition, supporting the account of the strata at and below the base of the PAF in the companion article by Fairchild et al. (2018).
This paper provides a description and evaluation of the sedimentary facies and environments associated with a range of glacier thermal and dynamic regimes, with additional consideration given to the ...tectonic context. New and previously published data are evaluated together, and are presented from modern terrestrial and marine glacial sedimentary environments in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record. Sedimentological data are presented from a total of 28 glaciers in 11 geographical areas that represent a wide range of contemporary thermal, dynamic and topographic regimes. In the context of “landsystems”, representatives from terrestrial environments include temperate glaciers in the European Alps, Patagonia, New Zealand, the Cordillera Blanca (Peru), cold glaciers in the Dry Valleys of Antarctica and the Antarctic Peninsula region, and polythermal valley glaciers in Svalbard, northern Sweden, the Yukon and the Khumbu Himal (Nepal). The glaciomarine environment is illustrated by data from cold and polythermal glacier margins on the East Antarctic continental shelf, and from a polythermal tidewater glacier in Svalbard, along with general observations from temperate glaciers in Alaska. These data show that temperate glacial systems, particularly in high-relief areas, are dominated by rockfall and avalanche processes, although sediments are largely reworked by glaciofluvial processes. Debris in polythermal glaciers is both thermally and topographically influenced. In areas of moderate relief, debris is mainly of basal glacial origin, and the resulting facies association is dominated by diamicton. In high-relief areas such as the Himalaya, the debris load in polythermal glaciers is dominated by rockfall and avalanche inputs, resulting in extensive accumulations of sandy boulder-gravel. Cold glaciers are dominated by basal debris-entrainment, but sediments are little modified from the source materials, which are typically sandy boulder-gravel from older till, and sand (from glaciofluvial, glaciolacustrine and aeolian sources). Similar facies associations, but with different facies geometry and thickness occur in equivalent glaciomarine settings. Application of these concepts can aid the interpretation of glacier thermal regime (and hence palaeoclimate) in Quaternary and ancient glacial systems.
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