A Practical Guide to Glacial Sediments, 2nd edition

The first edition of “A Practical Guide to the study of Glacial Sediments” (Edited by David Evans and Doug Benn) was an essential handbook to all students of glacial geology. It has helped countless undergraduate and MSc dissertation students, and my well-thumbed copy has come with me every time I go into the field.

It is therefore a delight to see the second edition, published in 2021, with numerous updates and full colour.

Practical Guide to the Study of Glacial Sediments, 2nd edition

The second edition is published by the QRA and is available to purchase for £20 (inc. p&p) from the QRA website. QRA members get a discount! See the flyer below for details of the chapters and content.

This comprehensive book promises to be absolutely essential to anyone undertaking practical work with glacial sedimentology, ranging from sediment description and logging, particle size, clast form, shape and orientation in glacial sediments, thin-section analysis of glacial sediments, and till geochemistry, particle lithology and mineral properties.

Each chapter contains detailed instructions and recipes, accompanied by full colour images. Recommend it to your library!

Unlocking ice-flow pathways using glacial erratics

This article was written by Dr Jenna Sutherland. All photos and images are credited to Jenna Sutherland.

A long history of scientific debate

Along the east coast of the UK, spectacular coastal sections expose sediments that were deposited by the last British Irish Ice Sheet (BIIS) around 21,000 years ago. For many years, Late Devensian sediments along the Holderness coast in Yorkshire were believed to record two separate ice advances from two different ice-flow directions.

These ice advances resulted in two glacial tills, named the “Skipsea Till” and “Withernsea Till”1. However, despite decades of research, the processes regarding their deposition remain controversial.

It has also been challenging to link these tills to other deposits in the region. Their relationship to the regional ‘stratigraphy’ (the branch of geology concerned with the order and relative position of strata, and their relationship to the geological timescale) remains unclear.

In this project, we used the erratic content (an ‘erratic’ is a far-travelled stone of a different lithology to the local bedrock) of the tills to work out the ice-flow pathways of the last ice sheet to cover this area. These tills are exposed especially well at Tunstall, and this was the focus of the research.

Map of Holderness Coast showing its regional setting. Source.

The focus of this new research was to piece together the sequence of events that led to their preservation.

What the majority of the cliff face looked like at Tunstall. Distinct sediment units can be seen via a change in colour and composition, indicating different depositional environments.

Understanding ice-flow pathways

We discovered that the range of rock types (erratics) within the Skipsea and Withernsea Tills were in fact very similar.

This tells us that the ice which deposited these units probably followed the same flow trajectory and reveals that the tills may not have been deposited as separate, discrete events as previously thought, but rather from multiple, minor abrupt shifts of the ice margin.

A rare sunny day on the north east coast. Note the concrete coastal defences and the cliffs of glacial till behind.

How we did it

In this piece of research, investigations were carried out along the east Yorkshire coast at Tunstall beach, an area better known for being one of Europe’s fastest eroding coastlines. A large amount of time was spent describing features of the cliff sediments that were, in some places, up 15 metres high.

A. Great Britain and study area highlighted. B. The Yorkshire and Durham coastline, with places named in the text. C. Study area, showing limits of Skipsea and Withernsea Tills (from Evans and Thomson, 2010). Published ages and geomorphology from Clark et al. (2018); Bateman et al. (2015, 2018); Evans et al. (2016). D. Detail of study area, showing location of section logs. Imagery from ArcMap Basemap. From Sutherland et al., 2020.

Interpreting the depositional processes

Composition, appearance and overall grain size  gave us an insight into the glacial erosion, transportation and depositional history of the sediment which was easily identifiable as a subglacial till due to its diamictic nature, and scratched and far-travelled pebbles.

Glacially striated Carboniferous Limestone boulder within subglacial till, the lines or scratches represent the direction of ice flow

Interpreting the ice-flow pathways

But what we were more interested in was exactly where the ice had travelled from. When ice moves along the ground from its source area, it rips up or ‘entrains’ bits of the underlying bedrock and incorporates them into the base layer of the ice. The further the glacier travels, the further the sediments also get transported.

Identifying the rock types within the till helped us to trace the pathway of the ice. Luckily, source areas of rock lithologies are well known thanks to handy maps like these from the British Geological Survey.

Once we’d matched our rock identifications to their outcrop locations, we were able to ascertain areas across the UK that the ice likely originated from and flowed over.

Representative photographs of key erratic lithologies in each sample a. Greywacke b. Whin Sill Dolerite c. Old Red Sandstone d. Andesitic porphyry e. Sherwood Sandstone f. Magnesian Limestone g. K-feldspar rich Granite h. Rhyolite i. Carboniferous Limestone j. K-feldspar < Quartz Granodiorite k. Cheviot Granite l. Quartzite. From Sutherland et al., 2020.

The lithologies of over 2000 rocks were identified in total, revealing a mixture of different lithologies within the sediment. We identified a large proportion of chalk stones, but that came as no surprise as cretaceous Chalk is the bedrock geology of Holderness and lies directly beneath Tunstall beach.

Magnesian Limestone, Old Red Sandstone, Greywacke, and Whin Sill Dolerite are examples of other lithologies that were present in abundance but all of which we know aren’t local to the area. We use these far-travelled erratics to calculate the ice-flow pathways of the last British-Irish Ice Sheet at the Last Glacial Maximum.

Overall, the range of rock types within the sediments suggested that ice was sourced from southern Scotland and flowed southwards, incorporating material from north-east England such as The Cheviots, and the western margin of the North Sea basin. The ice-flow pathway was similar for both tills, and we could not statistically discriminate between them.

Revised iceflow pathways inferred from the simplified bedrock geology map of Northern Britain with outcrop occurances of the lithostratigraphical group. a. Grampian Highlands b. Midland Valley c. southern uplands d. Cheviot volcanic complex e. Northumberland f. Lake District volcanic complex g. County Durham h. Cleveland basin i. Yorkshire basin Insert – Detailed map of the solid geology from the Tees estuary to The Wash (adapted from Kent and Gaunt, 1980; Busfield et al., 2015). From Sutherland et al., 2020.

Summary

This new research supports other work2,3 that has shown that the various ice lobes of the last British-Irish Ice Sheet were far more dynamic than initially thought, abruptly advancing, oscillating and retreating within the period 22 – 17 thousand years ago.

The key message from our study shows that lithological and structural subtleties within the sediments along the east Yorkshire coast are more complex that what is currently recognised and supports the idea that we should rethink regional stratigraphy’s and correlations.

The two tills represent an oscillating ice margin, with ice-marginal recession between the deposition of two subglacial tills. The two tills both represent ice flow from southern and central Scotland. The ice then flowed southwards down the eastern coast of England. Both tills therefore have a provenance from northern Britain. The two tills have a very similar clast content, showing no change in provenance.

This work is a small step towards understanding more about the dynamism of the BIIS during the Last Glacial Maximum, used in predicting the likely response of future ice sheet change.

Assessing sedimentary structures and collecting fracture measurements from the tills on Tunstall beach

Funding and Publications

This research was funded by the Quaternary Research Association (QRA) New Research Workers Award. The research was undertaken as part of Jenna Sutherland’s MSc Quaternary Science dissertation research in the Department of Geography, Royal Holloway University of London, supervised by Dr Bethan Davies (RHUL) and Dr Jonathan Lee (British Geological Survey). It was published in the Proceedings of the Geologists’ Association in 2020.

Below is the publication from this work:

Sutherland, J. L., Davies, B. J., and Lee, J. R. 2020. A litho-tectonic event stratigraphy from dynamic Late Devensian ice flow of the North Sea Lobe, Tunstall, east Yorkshire, UK. Proceedings of the Geologists’ Association 131(2), 168-186.

Further reading

About the Author

Jenna completed her PhD in the School of Geography at the University of Leeds in 2020. Her research was focussed on the interaction between proglacial lakes and outlet glacier dynamics during the Last Glacial Maximum in New Zealand. Her broader interests lie in reconstructing palaeo-glacial environments and relating the sediment-landform record to past landscape evolution.

Jenna Sutherland

References

  1. Catt, J. A. 2007. The Pleistocene glaciations of eastern Yorkshire: a review. Proceedings of the Yorkshire Geological Society56(3), 177-207.
  2. Boston, C. M., Evans, D. J., and Ó Cofaigh, C. (2010). Styles of till deposition at the margin of the Last Glacial Maximum North Sea lobe of the British–Irish Ice Sheet: an assessment based on geochemical properties of glacigenic deposits in eastern EnglandQuaternary Science Reviews29(23-24), 3184-3211
  3. Davies, B. J., Livingstone, S. J., Roberts, D. H., Evans, D. J. A., Gheorghiu, D. M., and Ó Cofaigh, C. (2019). Dynamic ice stream retreat in the central sector of the last British-Irish Ice Sheet. Quaternary Science Reviews225, 105989.

Glacial erratics

What is a glacial erratic?

Glacial erratics, often simply called erratics, or erratic boulders, are rocks that have been transported by ice and deposited elsewhere. The type of rock (lithology) that the glacial erratic is made from is different to the lithology of the bedrock where the erratic is deposited.

For example, an erratic could be a boulder of sandstone is picked up by a glacier, transported, and deposited on top of a limestone bedrock. Some erratics are useful to scientists because they are of a distinctive rock type, which means that their source outcrop can be identified and located. Glacial erratics are therefore useful in reconstructing past glacier flow directions, the timing of glacier retreat, and even the type of glacier flow.

Where do glacial erratics come from?

As a glacier or ice sheet moves, it can erode bedrock. The ice can then pick up, or entrain, the eroded rock. As the ice flows, it transports the bedrock debris in the direction of flow. The ice then deposited the entrained sediment once it begins to retreat.

The process of formation of glacial erratics. A. Glacial erosion entrains a boulder of the bedrock. B. Continued glacier flow transports the boulder. C. As the glacier retreats, the boulder is deposited on a different type of bedrock, forming a glacial erratic.

Erratics can range from large boulders to smaller stones and pebbles. All erratics are of a different rock type. Glacial sediments often contain a range of rocks of different kinds, which can be used to reconstruct the ‘provenance’1, or source, of the sediment and therefore the direction of ice flow.

img_8231
A granite boulder on sandstone bedrock on Alexander Island, Antarctic Peninsula. There is no granite of this kind on Alexander Island so it was transported by the glacier.

Rocks that are moved by the glacier but are of the same rock type are called ‘glacially-transported’ rocks. All glacially-transported rocks and erratics tend to show evidence of that glacial transport, with scratches (striations), rounded edges and polished faces.

Carboniferous limestone boulder embedded within glacial till at Whitburn Bay, County Durham. Note the faceted shape and scratches (striations). The boulder is shaped by glacial erosion and scratched by contact with other rocks and ice. See: Davies et al., (2009)2

Glacial erratics and glacially-transported rocks can be sourced from rocks falling onto the glacier, rocks being picked up and transported at the base of the glacier, and rocks plucked from valley sides. Rocks transported on the glacier surface are said to be ‘supraglacial’, whilst rocks transported at the base of the ice are ‘subglacially’ transported.

What do glacial erratics tell us about past ice sheets?

The first thing erratics can tell us about past ice sheets is the direction of ice movement. If you find an erratic with a distinctive lithology, you can trace it back to the location where the distinctive bedrock is found.

A good example of this indicator lithology in England is the Shap Granite from Cumbria. Boulders of Shap Granite are found throughout Cumbria, County Durham, North Yorkshire and as far southeast as Bridlington on the Yorkshire Coast2,3. The example shown in the figure below is from Goldsborough Carr in County Durham, which is 40 km east of the Shap Granite.

Examples of erratics from Goldsborough Carr (left) and Assynt (right). Photographs by A. Emery.

Erratics can tell us when the ice sheet retreated, by cosmogenic dating of the boulder. Assuming the boulder was eroded at the base of the glacier, the exposure age given by the cosmogenic dating will tell us when the boulder was deposited by the retreating glacier4. The boulder of Shap Granite in the figure above was deposited by the retreating Eden-Stainmore Ice Stream approximately 19,750 years ago5.

Sampling for cosmogenic nuclide exposure-age dating on an erratic boulder on a moraine in Patagonia.

We can also learn the style of ice-sheet flow from how glacial erratics are grouped. Long lines of glacial erratics are known as dispersal trains. These dispersal trains can show whether flow was focussed into ice streams or as part of a regional, sustained flow6. Boothia-type dispersal trains show that flow over an indicator lithology was focussed into an ice stream, named after the Boothia Peninsula in Arctic Canada. Dubawnt-type dispersal trains have little change in width, which shows that regional, unconstrained flow was active over the indicator lithology.

Styles of glacial erratic boulder dispersal. Left: Boothia-type dispersal. Right: Dubawnt-type dispersal. After Dyke & Morris (1988)6.

Where can you find glacial erratics in the UK?

There are many famous examples of glacial erratics in the UK. These erratics have captured the imagination of amateur and professional geologists for centuries. In 1928, the Yorkshire Geological Society published the work of Frederic Harmer7. This map collated the studies of the Yorkshire Boulder Committee and many similar groups.

As you can see below, the map shows the huge density of glacial erratics in the UK. The Norber erratics in the Yorkshire Dales, near Austwick, Settle, are famous and scenic examples of erratics. More examples of erratics are the Great Stone of Fourstones on the Lancashire/Yorkshire border, and Cloughmore in County Down, Northern Ireland.

The map of glacial erratics and their sources in England and Wales7.
Reproduced from Harmer 1928, Proceedings of the Yorkshire Geological Society, Vol. 21, 79-150, by permission of the Yorkshire Geological Society.

Scotland is full of glacial erratics thanks to its diverse bedrock geology. We use these erratics to reconstruct the dynamics of the British-Irish Ice Sheet. The figure below shows Dubawnt-style dispersal trains in the Assynt region of Scotland. The outcrops of Torridonian Sandstone and the trains of dispersed erratics show that ice flowed towards the west-northwest. The constant width of the dispersal trains shows that the regional flow of ice was a constant velocity over this area8.

Glacial erratic boulder dispersal trains in Assynt. Based on data from Lawson (1995)8. Elevation data: OS Terrain 5
(https://www.ordnancesurvey.co.uk/business-government/products/terrain-5).

References

1.          Evans, D. J. A. & Benn, D. I. A practical guide to the study of glacial sediments. (Arnold, 2004).

2.          Davies, B. J. et al. Interlobate ice-sheet dynamics during the last glacial maximum at Whitburn Bay, County Durham, England. Boreas 38, 555–578 (2009).

3.          Clark, C. D. et al. Map and GIS database of glacial landforms and features related to the last British Ice Sheet. Boreas 33, 359–375 (2004).

4.          Raistrick, A. THE GLACIATION OF WENSLEYDALE, SWALEDALE, AND ADJOINING PARTS OF THE PENNINES. Proc. Yorksh. Geol. Soc. 20, 366–410 (1926).

5.          Vincent, P. J., Wilson, P., Lord, T. C., Schnabel, C. & Wilcken, K. M. Cosmogenic isotope (36Cl) surface exposure dating of the Norber erratics, Yorkshire Dales: Further constraints on the timing of the LGM deglaciation in Britain. Proc. Geol. Assoc. 121, 24–31 (2010).

6.          Davies, B. J. et al. Dynamic ice stream retreat in the central sector of the last British-Irish Ice Sheet. Quat. Sci. Rev. 225, 105989 (2019).

7.          Dyke, A. S. & Morris, T. F. DRUMLIN FIELDS, DISPERSAL TRAINS, and ICE STREAMS IN ARCTIC CANADA. Can. Geogr. Géographe Can. 32, 86–90 (1988).

8.          Harmer, F. W. THE DISTRIBUTION OF ERRATICS AND DRIFT. Proc. Yorksh. Geol. Soc. 21, 79–150 (1928).

9.          Lawson, T. J. Boulder Trains as indicators of former ice flow in Assynt, N.W. Scotland. Quat. Newsl. 75, 15–21 (1995).

Interpretation of Glacigenic Sediments

This section is taken from Bethan Davies’ PhD thesis.

Multiproxy analysis of glacigenic sediments? | Micromorphological interpretation of glacigenic sediments | References | Comments |

Multiproxy analysis of glacigenic sediments

Lodgement and Deformation tills

Thin-section analysis can be used in conjunction with macroscale sedimentological analysis to identify subglacial processes. Traditionally, tills have been subdivided based on the typical processes assumed to have been dominant in their formation. These were thought principally to be sliding1, lodgement2 and deformation3. Lodgement till has a long history of research, being originally defined by Chamberlin (1895) as,

Sediment deposited by plastering of glacial debris from a sliding glacier sole due to the combined effects of pressure melting and frictional drag4.

This process resulted in massive or fissile tills, with slickensides resulting from shearing (Boulton, 1970). Clasts are lodged into the substrate and have typical bullet-shaped ends and clear stoss-and-lee ends. Alternatively, deformation till5,6 refers to a,

Rock or sediment that has largely been homogenised by shearing in the subglacial layer.

Subglacial deformation of soft sediments is considered to account for much of the forward motion by glaciers3,7. Massive tills are thought to record evidence of high cumulative strains8. Others have argued that massive tills are simply the product of melt-out9. Larsen et al. (2004) argued that a melt-out / deformation continuum was responsible for thick sequences of massive tills, with vertical accretion of subglacial sediments being melted out at the ice-bed interface, and then deformed10. However, if deformation of soft beds is widespread, then deformation tills should be more prevalent (cf.11), and macroscopically massive ‘deformation’ tills often overly undeformed sediments12.

Glacier motion by sliding and lodgement over soft beds (cf.13) was thought to occur by the decoupling of the glacier from its bed due to increased basal water pressures, which would prevent the transmission of stress to the substrate1. This theory evolved into an proposal of ‘slip-stick’ sliding at the ice-bed interface, with areas of high water pressure inducing decoupling14.

Clay-rich tills are less permeable (cf. sandy tills), encouraging the development of high water pressures; stick-slip behaviour and decoupling may therefore be at least partially lithologically controlled15,16. Piotrowski and Kraus (1997) were among the first to propose a mosaic of sliding bed conditions and deforming conditions, where the ice is coupled to the bed. This explains the heterogeneity in tills in Germany17.

A continuum with lodgement and deformation end-members will lead to progressive changes in bed properties at a particular location18-20. This viewpoint highlights the spatial and temporal variability of glacier beds, with ice-bed coupling variability brought about by changes in pore-water pressure. Piotrowski et al. (2004) argued that the spatial variability in sliding intensity resulted in a mosaic with sliding conditions and deforming spots. During sliding, ploughing of clasts may take place21. ‘Glaciotectonite’ (as originally defined by Banham, 197722, and Pedersen, 198823) refers to sheared rocks and sediments, which still retains some of structural characteristics of its parent material6. They can display both brittle and ductile deformation, or a combination of the two processes.

Mass flow diamictons (or flow tills) originate from water and sediment released by ablation from debris-rich basal ice. These deposits are typically macroscopically massive. They may have microscopic near-horizontal laminations. Rotational structures, kinking plasmic fabrics, and tile structures form as primary depositional features on a micromorphological scale24. Debris flows share many characteristics in common with subglacial sediments, such as pressure shadows, folds, laminations, shears, faults, water-escape structures, and rotational structures25. Mass flow diamictons, however, can be distinguished by the presence of ‘tile structures’ in close association with rotational structures26. This long and extensive history of thin-section analysis of glacigenic sediments has given rise to a set of well-defined criteria, summarised in Table 1.

A combination of processes

Recently, several researchers have argued that processes at the ice-bed interface are a result of a continuum of processes, including melt-out, lodgement, deformation, and sliding16,20,27-29. It is therefore difficult to pin an exact genetic name onto a specific outcrop of diamicton. Evans et al. (2006) argued that the subglacial processes of deformation, flow, sliding, lodgement and ploughing all exist contemporaneously at the base of temperate ice.

These processes result in the mobilisation, transportation and deposition of sediment. This results in stratified or folded to texturally homogenous diamictons. Evans et al. (2006) argued that, while specific processes can and should be recognised in the sedimentary record, genetic ‘finger-printing’ of subglacial tills should be less process-specific.

Subglacial tills are polygenetic, and till classification must recognise the range of processes involved by the subglacial till ‘production continuum’16. Evans et al. (2006) proposed the use of the terms, ‘glaciotectonite’, as defined above, and ‘traction till’, which includes sediments deposited by sliding or deforming at the glacier bed, sediment released by pressure melting, and sediment homogenised by shearing. While ‘traction till’ is a generic term for all these processes, they can still be individually recognised in the geological record. Evans et al. (2006) also formally recognise ‘melt-out till’, as a sediment released by melting or sublimation at stagnant or slowly-moving, debris-rich ice, without subsequent transport or deformation.

Laminated diamictons

Glaciotectonic deformation of subglacial sediments can result in tectonic laminations, which are distinct from glaciomarine or subaqueous laminations. Roberts and Hart (2005) identified two types of lamination. Type 1 laminations / stringers typically emanate from soft sediment clasts (e.g. chalk), are discontinuous, subhorizontal, and ungraded. In thin section, Type 1 laminations have sharp, undulatory contacts with silty or sandy stringers. Isoclinal folds are common30. Type 2 laminations are laterally continuous, subhorizontal, and poorly sorted with dropstone-like structures and often exhibit reworked soft sediment clasts. Contact boundaries are sharp and unconformable, and dropstone structures are evident. Microfabric birefringence is low, but there are some areas of high birefringence sub-parallel to silty lamination contacts30. Type 1 laminations are the result of subglacial deformation by ductile, inter-granular, pervasive shear. Hart and Roberts (1994) argued that this type of lamination occurs as a result of high extensional shear, leading to boudinage. Smaller, less competent perturbations, such as chalk clasts, can become stretched out into a lamination under this high shear strain. The laminations can deform and rotate within the deforming layer, producing tails to appear as sedimentary augens31.

Type 2 laminations have a subaqueous signal, despite often containing a number of syntectonic ductile deformation structures30. At West Runton, Norfolk, the subhorizontal lateral continuity and dropstone structures with down-warped lower contacts and draped upper contacts are indicative of primary subaqueous origin followed by secondary subglacial deformation. The planar, bedded nature of the strata, with sharp contact boundaries, are characteristic of sediments deposited by underflows, overflows, suspension fallout, ice-rafted debris processes and subaqueous debris flows, with each lamination representing a clear separate depositional event30. The clear characteristics of these laminations enable easy discrimination of glaciomarine and glaciolacustrine diamictons.

Glaciomarine and Glaciolacustrine diamictons

Several additional criteria can distinguish massive and laminated glaciomarine diamictons from subglacial diamictons (Table 1). Glaciomarine diamictons usually have a coarse, winnowed structure with dropstones; common, in situ marine microfossils; a lack of deformation structures; laminations, banding, or graded bedding structures; and a lack of plasmic fabric development30,32,33. Distal glaciomarine sediments are characterised by their bedding and lamination; a medium to fine matrix; uniform grain shapes; few deformation structures; dropstone features and no plasmic fabric development; and the presence of bioturbation32.

Glaciolacustrine sediments share many characteristics of glaciomarine diamictons; however, they lack in situ marine microfossils and are mostly geographically more limited in extent34. Menzies et al. (2006) argued that plasmic fabrics indicate the presence of orientated clay particles by strong birefringence. The type of plasmic fabric is indicative of a suite of orientations induced by ductile deformation.

Micromorphological interpretation of glacigenic sediments

Lodgement tills, formed in a high-strain environment with considerable shear and deformation, undergo complete homogenisation and demonstrate unistrial plasmic fabrics35. Subglacial traction tills can therefore have ductile, brittle, polyphase or intermediate structures (see Thin-Section Analysis). Ductile deformation structures include soft sediment pebbles (Type II and III), banding and flow of matrix material, rotational structures with associated skelsepic / masepic plasmic fabrics, and strain caps and pressure shadows. Planar features such as grain lineations are commonly associated with rotational structures, and occur in plastically deforming sediments36. Brittle deformation structures include edge-to-edge grain contacts and grain crushing, grain stacking, and brittle faulting and discrete shear zones. Grain stacks form to support stresses developing in a sediment, and form perpendicular to the stress field36.

Glaciomarine deposits are characterised by graded laminations, iceberg-dump and dropstone structures, and in situ marine microfossils. Porewater-induced soft-sediment deformation structures include liquefaction and homogenisation of sands, rafting, and water-escape structures.

Table 1. Macroscopic and microscopic criteria for interpretation of some typical glacigenic
sediments.

Citation

Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.

Bethan Davies Thesis (Zipped PDFs – 70MB)

References


1.            Brown, N.E., Hallet, B. & Booth, D.B. Rapid soft bed sliding of the Puget glacial lobe. Journal of Geophysical Research 92, 8985-8997 (1987).

2.            Dreimanis, A. Tills, their genetic terminology and classification. in Genetic Classification of Glacigenic Deposits (eds. Goldthwait, R.P. & Matsch, C.L.) 17-84 (Balkema, Rotterdam, 1989).

3.            Alley, R.B., Blankenship, D.D., Bentley, C.R. & Rooney, S.T. Deformation of till beneath Ice Stream B, West Antarctica. Nature 322, 57-59 (1986).

4.            Chamberlin, T.C. Recent glacial studies in Greenland. Bulletin of the Geological Society of America 6, 199-220 (1895).

5.            Elson, J.A. The geology of tills. in Proceedings of the 14th Canadian soil mechanics conference, Vol. 69 (eds. Penner, E. & Butler, J.) 5-36 (Commission for Soil and Snow Mechanics, Technical Memoir, vol. 69, 1961).

6.            Benn, D.I. & Evans, D.J.A. Glaciers & Glaciation, 802 (Hodder Education, London, 2010).

7.            Humphrey, N., Kamb, B., Fahnestock, M. & Englehardt, H. Characteristics of the bed of the Lower Columbia Glacier, Alaska. Journal of Geophysical Research 98, 837-846 (1993).

8.            Hart, J.K., Hindmarsh, R.C.A. & Boulton, G.S. Styles of subglacial glaciotectonic deformation within the context of the Anglian ice-sheet. Earth Surface Processes and Landforms 15, 227-241 (1990).

9.            Clayton, L., Mickelson, D.M. & Attig, J.W. Evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern Laurentide Ice Sheet. Sedimentary Geology 62, 203-208 (1989).

10.          Larsen, N.K., Piotrowski, J.A. & Kronborg, C. A multiproxy study of a basal till: a time-transgressive accretion and deformation hypothesis. Journal of Quaternary Science 19, 9-21 (2004).

11.          Piotrowski, J.A., Mickelson, D.M., Tulaczyk, S., Krzyszkowski, D. & Junge, F.W. Were deforming subglacial beds beneath past ice sheets really widespread? Quaternary International 86, 139-150 (2001).

12.          Boulton, G.S. & Hindmarsh, R.C.A. Sediment deformation beneath glaciers: rheology and geological consequences. Journal of Geophysical Research 92, 9059-9082 (1987).

13.          Clark, P.U. & Hansel, A.K. Clast ploughing, lodgement and glacier sliding over a soft glacier bed. Boreas 18, 201-207 (1989).

14.          Fischer, U. & Clarke, G.K.C. Stick-slip sliding behaviour at the base of a glacier. Annals of Glaciology 24, 390-396 (1997).

15.          Boulton, G.S. Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation. Journal of Glaciology 42, 43-62 (1996).

16.          Evans, D.J.A., Phillips, E.R., Hiemstra, J.F. & Auton, C.A. Subglacial till: Formation, sedimentary characteristics and classification. Earth-Science Reviews 78, 115-176 (2006).

17.          Piotrowski, J.A. & Kraus, A.M. Response of sediment to ice sheet loading in north-western Germany: effective stresses and glacier-bed stability. Journal of Glaciology 43, 495-502 (1997).

18.          Lian, O.B. & Hicock, S.R. Thermal conditions beneath parts of the last Cordilleran Ice Sheet near its centre as inferred from subglacial till, associated sediments, and bedrock. Quaternary International 68-71, 147-162 (2000).

19.          Boulton, G.S., Dobbie, K.E. & Zatsepin, S. Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system. Quaternary International 86, 3-28 (2001).

20.          Nelson, A.E., Willis, I.C. & Ó Cofaigh, C. Till genesis and glacier motion inferred from sedimentological evidence associated with the surge-type glacier, Brùarjökill, Iceland. Annals of Glaciology 42, 14-22 (2005).

21.          Piotrowski, J.A., Larsen, N.K. & Junge, F.W.F.W. Reflections on soft subglacial beds as a mosaic of deforming and stable spots. Quaternary Science Reviews 23, 993-1000 (2004).

22.          Banham, P.H. Glaciotectonites in till stratigraphy. Boreas 6, 101-105 (1977).

23.          Pedersen, S.A.S. Glaciotectonite: brecciated sediments and cataclastic sedimentary rocks formed subglacially. in Genetic Classification of Glacigenic Deposits (eds. Goldthwait, R.P. & Matsch, C.L.) 89-91 (Balkema, Rotterdam, 1988).

24.          Lachniet, M.S., Larson, G.J., Lawson, D.E., Evenson, E.B. & Alley, R.B. Microstuctures of sediment flow deposits and subglacial sediments: a comparison. Boreas 30, 254-262 (2001).

25.          Phillips, E. Micromorphology of a debris flow deposit: evidence of basal shearing, hydrofracturing, liquefaction and rotational deformation during emplacement. Quaternary Science Reviews 25, 720-738 (2006).

26.          Menzies, J. & Zaniewski, K. Microstructures within a modern debris flow deposit derived from Quaternary glacial diamicton – a comparative micromorphological study. Sedimentary Geology 157, 31-48 (2003).

27.          van der Meer, J.J.M., Menzies, J. & Rose, J. Subglacial till: the deforming glacier bed. Quaternary Science Reviews 22, 1659-1685 (2003).

28.          Menzies, J., van der Meer, J.J.M. & Rose, J. Till-as a glacial “tectomict”, its internal architecture, and the development of a “typing” method for till differentiation. Geomorphology 75, 172-200 (2006).

29.          Menzies, J. Strain pathways, till internal architecture and microstructures – perspectives on a general kinematic model – a ‘blueprint’ for till development. Quaternary Science Reviews 50, 105-124 (2012).

30.          Roberts, D.H. & Hart, J.K. The deforming bed characteristics of a stratified till assemblage in north East Anglia, UK: investigating controls on sediment rheology and strain signatures. Quaternary Science Reviews 24, 123-140 (2005).

31.          Hart, J.K. & Roberts, D.H. Criteria to distinguish between subglacial glaciotectonic and glaciomarine sedimentation: I – Deformation styles and sedimentology. Sedimentary Geology 91, 191-214 (1994).

32.          Carr, S.J. Micromorphological criteria for distinguishing subglacial and glacimarine sediments: evidence from a contemporary tidewater glacier, Spitsbergen. Quaternary International 86, 71-79 (2001).

33.          Hiemstra, J.F. Microscopic analyses of Quaternary glacigenic sediments of Marguerite Bay, Antarctic Peninsula. Arctic, Antarctic and Alpine Research 33, 258-265 (2001).

34.          Ó Cofaigh, C. & Dowdeswell, J.A. Laminated sediments in glacimarine environments: diagnostic criteria for their interpretation. Quaternary Science Reviews 20, 1411-1436 (2001).

35.          Khatawa, A. & Tulaczyk, S. Microstructural interpretations of modern and Pleistocene subglacially deformed sediments: the relative role of parent material and subglacial process. Journal of Quaternary Science 16, 507-517 (2001).

36.          Hiemstra, J.F. & Rijsdijk, K.F. Observing artificially induced strain: implications for subglacial deformation. Journal of Quaternary Science 18, 373-383 (2003).

Thin-section analysis

This section is taken from Bethan Davies’ PhD thesis.

Introduction to micromorphology | Sample Collection and Preparation | Sample analysis | References | Comments |

Introduction to micromorphology

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Thin section of glaciomarine sediments from the North Sea Basin. From Davies et al., 2011

Micromorphology, or thin-section analysis, is the microscopic examination of the composition and structure of sediments. It was originally developed in soil science, with concepts of plasmic fabric and morphological features and structures dating from the early 1960s1. The undisturbed sediments are examined for a range of recognised microstructures, such as those first described by van der Meer (1993) and Menzies and Maltman (1992)2,3. They introduced key terms and concepts still used today, such as ‘plasmic fabrics’, and identified key structures indicative of subglacial deformation, such as rotational structures, necking structures, and crushed grains4-7. These structures can be used to account for the origins of a sediment, its transport pathways, and the processes of deposition and deformation8,9. Micromorphology now provides detailed information to aid the interpretation of sediments that are often massive at macroscale, and can give valuable information regarding genesis, deformation history and strain rates10.

Recent developments

Subglacial till from north-eastern England. From Davies et al., 2012

Subglacial till from north-eastern England. From Davies et al., 2012

Later developments attempted to quantify micromorphology and introduce guidelines into its application to glacial sediments10,11. Structural geology recently influenced the development of micromorphology, and analysis of structural features in subglacially and proglacially deformed materials can additionally identify different types of characteristic subglacial deformation12-15. Systematic structural analysis gives a deeper understanding of tectonostratigraphic sequences in soft sediments, and the glacier-induced stresses responsible for their development16.

Understanding depositional processes

Glaciofluvial sediments from Warren House Gill, eastern England. From Davies et al., 2012

Glaciofluvial sediments from Warren House Gill, eastern England. From Davies et al., 2012

Now, the combined use of sedimentology and micromorphology is important in determining the processes of deposition, post-depositional deformation, and porewater fluctuations in glacigenic sediments. It can be used carefully to discriminate between macroscopically similar diamictons, such as debris flows, traction tills, and glaciomarine and glaciolacustrine sediments11,17-20. Furthermore, thin-section microfabrics give valuable information regarding genesis and strain directions in the absence of other directional features, such as clast macro-fabrics11. Micromorphology can be used to account for the origins of a sediment, its transport pathways, and the processes of deposition and deformation. These can be combined to create an understanding of sediment-landform associations and landsystems.

Sample Collection and Preparation

Dr Dave Roberts using a kubiena tin to take a sample for micromorphological analysis.

Thin sections were sampled using Kubiena tins. Representative (and replicate samples where pragmatically possible) samples were collected from each lithofacies. These undisturbed samples were then prepared according to standard techniques2,21,22.

Sample analysis

Microscopic examination

Table 1. Approach for investigation of glacigenic sediments. Adapted from Carr, 2004.

The unlithified, undisturbed samples should be analysed at multiple magnifications under petrological microscopes. The optical properties and relative orientations of the particles can determine the genetic stress history of the sediments. Using both plane- and cross-polarised light highlights the textural and structural characteristics of the sample. Thin sections were investigated at low magnifications between x10 and x100, as higher magnifications observe individual grains, which may not be helpful for structural interpretation.

A systematic description

The analysis of thin sections must employ a systematic, standardised description to be used meaningfully11, such as that outlined in Table 1. Presentation of all data in a single table allows easy comparison between samples. A table of symbols used in the presentation of data is given in Figure 1. A glossary of terms used in analysis of thin sections is given in Table 2. Where possible, photomicrographs and scans show distinct features. Where this is not possible due to the magnification of the image, features that cannot be seen have been marked on to show their position and orientation. Arrows on rotational structures have no inferred direction.

Key for symbols used in thin section analysis.

Recent research has expanded greatly on van der Meer’s (1993) classic interpretations, and the development of criteria to identify different depositional environments. Hiemstra and Rijsdijk (2003) used artificially induced strain in potter’s clay to investigate typical features found in subglacial diamictons. They found a close relationship between unistrial plasmic fabrics and rotational structures (Figure 2), and both increased in number with increasing strain. Grain lineations commonly occur in association with rotational structures such as turbates23.

Glossary of common terms

Table 2: Glossary of common terms used in micromorphology (after van der Meer, 1993; Perkins, 1998; Carr, 2001; Carr, 2004a; Menzies et al., 2006; Hiemstra, 2007). Refer also to Figure 2.

Anisotropic The anisotropic skeleton grains and plasmic matrix of the slide transmit plane polarised light, but under cross-polarised light they extinguish (i.e. transmit no light) four times per complete rotation, every 90°. The refractive index therefore varies with direction.
Birefringence Optical property in which interference colours become visible by turning the stage of the microscope; cause by double refraction of light under crossed polarisers and consequent polarising of bundles of light.
Cross polarised light (XPL) When passing plane polarised light through a second filter at 90° to the first (the upper polariser), we see the light through crossed polars. Cross-polarised light is used to determine properties such as dispersion, birefringence, and extinction.
Domain Small zones in which clay particles are orientated parallel to each other, causing them to behave (optically) as a single crystal. Subglacial tills may exhibit multiple domains with banding and stratification.
Edge-to-edge crushing Clast fragments touching at the edges with visible breakage contacts. Breaking and grinding may have occurred in response to high stress levels resulting in significant grain-to-grain contacts along grain edge asperities. More common in areas with low pore water content.
Galaxy / turbate / Rotational structure Circular alignments of grains around cores of consolidated sediment or larger grains; indicative of rotation. Closely associated with planar features. For example, van der Meer (1993) and Hart (2007).
Grain stacks Edge-to-edge grains forming to support developing stresses. Develop perpendicular to the stress field. For example, Menzies (2000) and Menzies et al. (2006).
Grain alignments Preferred long axis of skeleton grains. Numerous grains in a row with aligned long axes. E.g., Hiemstra & Rijsdijk (2003).
Interference colours The colour of anisotropic minerals under crossed polars varies, and the same mineral shows different colours depending on crystallographic orientation. These colours are on Newton’s Scale, divided into several orders:

  • 1st Order: grey, white, yellow, red
  • 2nd Order: violet, blue, green, yellow, orange, red
  • 3rd Order: indigo, green, blue, yellow, red, violet
  • 4th Order and above: pale pinks and greens.
Isotropic Isotropic minerals remain black in all positions when viewed under cross-polarised light. They have random atomic structures, so that structure and refractive index are the same in all directions.
Lineations Lines of skeleton grains with aligned long axes. May indicate shear zone. For example, Hart (2007).
Microfabric Skeleton grains commonly show preferred long axis. The vertical arrangement of skeleton grains. For example, Carr (2001).
Necking structure Squeezing of plasma between skeleton grains. Indicative of matrix flowage.
Plane polarised light (PPL) In normal, unpolarised light, waves vibrate in all directions. Filtering the light beam in the microscope with the lower polariser makes all the light waves vibrate in one direction, parallel to a particular plane.
Plasma (matrix) Grains of colloidal size (< 2 μm); may consist of clay minerals, oxides and hydroxides of Fe, Al and Mn, soluble salts, etc. Often used to refer to matrix – all material smaller than the thickness of the thin section. Individual particles cannot be seen.
Plasmic fabric: Birefringence models of the plasma. Based on optical properties of the particles as well as the optical properties caused by the orientation of particles relative to each other. For example, see Khatawa & Tulaczyk (2001) and Carr (2001).

Deformation structures

Detailed micromorphological study has highlighted the importance of grain size variation in the production of rotational structures24. Individual larger clasts may generate perturbations, allowing characteristic rotational structures to develop. Increases in grain size allow for more perturbations. As a result, a poorly sorted, coarse grained till will be more micromorphologically inhomogeneous than a finer-grained till.

Figure 2: Conceptual diagram illustrating the relationship between plasmic fabric and aligned grains in response to simple shear (A) and the relationship between unidirectional plasmic fabrics, turbates, and skelsepic plasmic fabrics (B). Adapted from Hiemstra and Rijsdijk (2003).

Stringer initiation and deformable clasts with tails also indicate syntectonic rotation in a ductile, shearing medium25. The combination of lateral shear26 and rotational movement results in a variable response to the applied stress field according to grain size. Hart et al. (2004) argued that the rotational process mobilises particles by incorporating grains from the subjacent undeformed bed, as evidenced by van der Meer’s ‘Till Pebbles’ (van der Meer, 1993).

Hart et al. (2004) therefore proposed that subglacial deformation tills contain associations of rotational features, such as skelsepic plasmic fabrics, orientations of smaller skeleton grains around larger ones, and rotated, augen-like features with tails, with intermediate and linear features24. Linear features include inclined clasts, lines of grains (lineations) and fragmentation of clasts. Intermediate features include mini-shear zones with internal rotation, and clay intraclasts with internal plasmic fabric24. They argued that deformation tills contain a juxtaposition of rotational and linear features. This is a result of dynamism within the subglacial deforming till layer at the microscale, and is related to temporal and spatial variations in porewater content and pressure. The overprinting of ductile and brittle deformation within the same area of till is evidence of sudden phase changes related to fluctuating porewater pressure6.

These structures are illustrated in the conceptual diagrams (Figures 2 and 3). Figure 3 was developed from work by Van der Meer (1993), Menzies (2000), Hiemstra and Rijsdijk (2003), and Menzies et al. (2006). The structures are categorised by their genesis. These images only reflect structures observed in glacigenic sediments at Whitburn Bay, Warren House Gill, and in various boreholes in the North Sea18-20,27.

Figure 3: Microstructures and plasmic fabrics observed from glacigenic sediments in this study. After van der Meer (1993), Menzies (2000), Menzies et al. (2006), and Hiemstra and Rijsdijk (2003).

Citation

Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.

Bethan Davies Thesis (Zipped PDFs – 70MB)

References


1.         Bullock, P., Federoff, N., Jongerius, A., Stoops, G., Tursina, T. & Babel, U. Handbook for soil thin section description, 152 (Waine Research Publications, Wolverhampton, 1985).

2.         van der Meer, J.J.M. Microscopic Evidence of Subglacial Deformation. Quaternary Science Reviews 16, 827-831 (1993).

3.         Menzies, J. & Maltman, A.J. Microstructures in diamictons – evidence of subglacial bed conditions. Geomorphology 6, 27-40 (1992).

4.         van der Meer, J.J.M. Particle and aggregate mobility in till: Microscopic evidence of subglacial processes. Quaternary Science Reviews 16, 827-831 (1997).

5.         Menzies, J. Micromorphological analyses of microfabrics and microstructures indicative of deformation processes in glacial sediments. in Deformation of Glacial Materials, Vol. Vol. 176 (eds. Maltman, A.J., Hubbard, A.J. & Hambrey, M.J.) 245-257 (Geological Society of London, Special Publication, London, 2000).

6.         Menzies, J., van der Meer, J.J.M. & Rose, J. Till-as a glacial “tectomict”, its internal architecture, and the development of a “typing” method for till differentiation. Geomorphology 75, 172-200 (2006).

7.         Menzies, J. Strain pathways, till internal architecture and microstructures – perspectives on a general kinematic model – a ‘blueprint’ for till development. Quaternary Science Reviews 50, 105-124 (2012).

8.         Ó Cofaigh, C. & Dowdeswell, J.A. Laminated sediments in glacimarine environments: diagnostic criteria for their interpretation. Quaternary Science Reviews 20, 1411-1436 (2001).

9.         McCarroll, D. & Rijsdijk, K.F. Deformation styles as a key for interpreting glacial depositional environments. Journal of Quaternary Science 18, 473-489 (2003).

10.       Carr, S.J. Micro-scale features and structures. in A practical guide to the study of glacial sediments (eds. Evans, D.J.A. & Benn, D.I.) 115-144 (Arnold, London, 2004).

11.       Carr, S.J. Micromorphological criteria for distinguishing subglacial and glacimarine sediments: evidence from a contemporary tidewater glacier, Spitsbergen. Quaternary International 86, 71-79 (2001).

12.       Phillips, E. Micromorphology of a debris flow deposit: evidence of basal shearing, hydrofracturing, liquefaction and rotational deformation during emplacement. Quaternary Science Reviews 25, 720-738 (2006).

13.       Phillips, E., Merritt, J., Auton, C. & Golledge, N. Microstructures in subglacial and proglacial sediments: understanding faults, folds and fabrics, and the influence of water on the style of deformation. Quaternary Science Reviews 26, 1499-1528 (2007).

14.       Phillips, E. & Merritt, J. Evidence for multiphase water-escape during rafting of shelly marine sediments at Clava, Inverness-shire, NE Scotland. Quaternary Science Reviews 27, 988-1011 (2008).

15.       Phillips, E., van der Meer, J.J.M. & Ferguson, A. A new `microstructural mapping’ methodology for the identification, analysis and interpretation of polyphase deformation within subglacial sediments. Quaternary Science Reviews 30, 2570-2596 (2011).

16.       Phillips, E.R., Evans, D.J.A. & Auton, C.A. Polyphase deformation at an oscillating ice margin following the Loch Lomond Readvance, central Scotland, UK. Sedimentary Geology 149, 157-182 (2002).

17.       Licht, K.J., Dumbar, N.W., Andrews, J.T. & Jennings, A.E. Distinguishing subglacial till and glacial marine diamictons in the western Ross Sea, Antarctica: Implications for a last glacial maximum grounding line. Bulletin of the Geological Society of America 111, 91-103 (1999).

18.       Davies, B.J., Roberts, D.H., Bridgland, D.R., Ó Cofaigh, C., Riding, J.B., Phillips, E.R. & Teasdale, D.A. Interlobate ice sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England. Boreas 38, 555-575 (2009).

19.       Davies, B.J., Roberts, D.H., Bridgland, D.R. & Ó Cofaigh, C. Dynamic Devensian ice flow in NE England: a sedimentological reconstruction. Boreas 41, 337-366 (2012).

20.       Davies, B.J., Roberts, D.H., Bridgland, D.R., Ó Cofaigh, C., Riding, J.B., Demarchi, B., Penkman, K. & Pawley, S.M. Timing and depositional environments of a Middle Pleistocene glaciation of northeast England: New evidence from Warren House Gill, County Durham. Quaternary Science Reviews 44, 180-212 (2012).

21.       Murphy, C.P. Thin section preparation of soils and sediments., 149 (A B Academic Publishers, Berkhampstead, 1986).

22.       Lee, J. & Kemp, R.A. Thin sections of unconsolidated sediments and soils: a recipe., 45 (Royal Holloway, University of London, Egham, 1994).

23.       Hiemstra, J.F. & Rijsdijk, K.F. Observing artificially induced strain: implications for subglacial deformation. Journal of Quaternary Science 18, 373-383 (2003).

24.       Hart, J.K., Khatwa, A. & Sammonds, P. The effect of grain texture on the occurrence of microstructural properties in subglacial till. Quaternary Science Reviews 23, 2501-2512 (2004).

25.       Roberts, D.H. & Hart, J.K. The deforming bed characteristics of a stratified till assemblage in north East Anglia, UK: investigating controls on sediment rheology and strain signatures. Quaternary Science Reviews 24, 123-140 (2005).

26.       Hart, J.K. An investigation of subglacial shear zone processes from Weybourne, Norfolk, UK. Quaternary Science Reviews 26, 2354-2374 (2007).

27.       Davies, B.J., Roberts, D.H., Bridgland, D.R., Ó Cofaigh, C. & Riding, J.B. Provenance and depositional environments of Quaternary sedimentary formations of the western North Sea Basin. Journal of Quaternary Science 26, 59-75 (2011).

Clast Shape, Till Fabrics and Striae

This section is taken from Bethan Davies’ PhD thesis.

Clast Angularity-Roundness | Clast Macro-Fabric Analysis | Striae Orientation | References | Comments |

Clast Angularity-Roundness

Clasts inherit their shapes from the surrounding environment; erosion, transportation and weathering give clasts distinctive geomorphological signatures1. Angularity-roundness is simple to measure in the field when undertaking till-fabric analysis. Descriptive criteria are used to assign clasts to a roundness category (Table 1). A semi-quantitative approach is used, considering the whole shape of the clast. The sharpest edge may not be representative of the whole roundness. Clasts are therefore assigned to categories based on descriptive criteria2.

Table 1: Descriptive clast-roundness categories. From Benn (2007)1.

Very Angular (VA) Edges and faces unworn, sharp, delicate protuberances.
Angular (A) Faces and edges unworn.
Subangular (SA). Faces unworn, edges worn
Subrounded (SR) Faces and edges worn but clearly distinguishable
Rounded (R) Edges and faces worn and barely distinguishable
Well Rounded (WR) No edges or faces distinguishable

Clast Macro-Fabric Analysis

The measurement of the arrangement of clasts within a diamicton can be a powerful tool in the analysis of Quaternary sediments3, and it is traditionally used, in conjunction with striae data, as a standard quantitative tool in the analysis of past ice flow directions. More recently, till fabric data has been used to infer process4. Till fabric data can be used together with striae data to reconstruct ice flow direction, and can be used to help interpret depositional processes. The resulting data are three mutually orthogonal eigenvectors (V1, V2 and V3), with the principal eigenvector, V1, being parallel to the axis of maximum clustering in the data. V3 is normal to the preferred plane of the fabric. The degree of clustering about the eigenvectors is given by the eigenvalues S1, S2, and S3, with their relative magnitudes reflecting the fabric shape5.

A-axis fabric data has a long history of research3,6,7. Coulomb plastic behaviour involves slippage between clasts and the surrounding, faster-flowing matrix. Therefore, more elongate clasts assume a minimum cross-sectional area, orientating the a-axis parallel to main stress direction. This makes strong, consistent till macro-fabrics in tills useful in interpreting palaeo ice-flow directions8.

Jæren, south-west Norway, forms the onshore border of the Norwegian Channel ice stream. The Jæren escarpment, separating Low Jæren from High Jæren, was formed by erosion by the ice stream, which occupied the Norwegian Channel on multiple occasions during Pleistocene glaciations8. Early analysis of the till macrofabrics indicated a strong westerly to south-westerly flow direction, but in Stavnheim, further south in Jæren, till fabrics measured a northwest to west component9. Andersen et al. (1987) argued that the glacier in Low Jæren moved north-westwards in an earlier phase, and then later moved in a westerly direction. Jónsdóttir et al. (1999) analysed till macro-fabrics and striations, aiming to delineate the pattern of regional glacial movements using macro-fabrics and clast lithology. They interpreted the glacigenic sediments as lodgement tills. The upper till had a strong, unimodal clustering of clast axes around the mean axis, resulting in a high significance value. The clast fabric from the lower till had a weak, equatorial, near random orientation of clast axes8. The direction of maximum clustering (V1 157° to 161°) coincided with the direction of the Jæren escarpment axis. Jónsdóttir et al. (1999) interpreted the upper fabric as representing palaeo ice flow direction as towards the northwest. The lower fabric was, however, probably influenced by cobbles and boulders, leading to a local fabric probably unrelated to glacier flow.

Recently, eigenvalues (S values) and vectors (V values) have been used to infer the genesis of glacial materials, indicating factors such as the rheology of the sediment. For example, debris-rich basal ice subjected to high cumulative strains tends to have strongly clustered clast macro-fabrics, whereas tills formed under low strain can have either strongly clustered or highly variable clast macro-fabrics3. Other researchers have found strong fabrics at low strains10,11.

Figure 1: Schematic diagram illustrating (A) Jeffery Rotation and (B) March Rotation. In Jeffery Rotation clasts are continually rotated as a result of vertical velocity gradients, whereas in March Rotation (B), clasts passively trace the deformation of the surrounding medium (from Benn, 2007b)

Hicock et al. (1996) advise caution in using till fabrics to infer genesis of sediments, and suggest that they only be used as a starting point. Eigenvalues cannot be used alone, given the complexity of the subglacial environment7. Some researchers have argued that Jeffery-type rotation (Figure 1A) is incompatible with the deforming bed hypothesis12. March-type rotation (Figure 1B) through plastic deformation has been identified as the dominant mode of clast orientation in deforming tills13. Weak clast macro-fabrics have often been reported as typical of deforming bed tills14, suggesting that particles are here free to rotate in a viscous medium15. Inhomogeneous deformation may produce a range of clast macro-fabric strengths, and localised fabric patterns reflect the deformation history and local strain conditions of the sediment15.

Carr and Rose (2003) concluded that “particle orientations in subglacial diamictons reflect the strain response of the sediment to the applied total stress during subglacial deformation”, and that particles of different size are rarely consistently orientated in relation to ice flow direction. Therefore it is important to limit the size range in the sample4.

To obtain the clast measurements, clasts in the approximate size range 8-32 mm should be excavated and the long axis (a-axis) and dip angle of 50 clasts per exposure recorded, using a compass-clinometer (Benn, 2007b). The data are presented in equal-area stereonets and rose diagrams, according to procedures in Evans and Benn (2004) and Benn (2007b). Clasts should be sampled from a 2 m2 area. Only clasts with elongate a-axes were measured, with ratios of >1.5:13. All three eigenvalues should be given.

Striae Orientation

Striae are used in conjunction with till-fabric analysis to reconstruct past ice-flow directions. Striae on individual in situ clasts and boulders were measured using a compass-clinometer. Up to 50 striae sets were collected per exposure, and at least 10 per clast. If a clast showed several sets of striae, then these were also noted. The data were collated and presented in rose diagrams.

Striae orientation has been often been used successfully in conjunction with clast macro-fabric analysis to determine palaeo ice flow directions16. In addition, striae on bedrock forms can be used to infer palaeo ice flow directions17-19. Striations are important as they provide an independent evidence for ice flow3.

Citation

Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.

Bethan Davies Thesis (Zipped PDFs – 70MB)

References


1.            Benn, D.I. Clast Form Analysis. in Encyclopedia of Quaternary Science (ed. Elias, S.A.) 904-909 (Elsevier, Oxford, 2007).

2.            Benn, D.I. Fabric strength and the interpretation of sedimentary fabric data. Journal of Sedimentary Research A64, 910-915 (1994).

3.            Benn, D.I. Macrofabric. in A practical guide to the study of glacial sediments (eds. Evans, D.J.A. & Benn, D.I.) 93-114 (Arnold, London, 2004).

4.            Carr, S.J. & Rose, J. Till fabric patterns and significance: particle response to subglacial stress. Quaternary Science Reviews 22, 1415-1426 (2003).

5.            Hubbard, B. & Glasser, N.F. Field Techniques in Glaciology and Geomorphology, 412 (Wiley, 2005).

6.            Lawson, D.E. A comparison of the pebble orientations in ice and deposits of the Matamuska Glacier, Alaska. Journal of Geology 87, 629-645 (1979).

7.            Hicock, S.R., Goff, J.R., Lian, O.B. & Little, E.C. On the interpretation of subglacial till fabric. Journal of Sedimentary Research 66, 928-945 (1996).

8.            Jónsdóttir, H.E., Sejrup, H.P., Larsen, E. & Stalsberg, K. Late Weichselian ice-flow direction in Jæren, SW Norway; clast fabric and clast lithology evidence in the uppermost till. Norweigian Journal of Geography 53, 177-189 (1999).

9.            Andersen, B.G., Wangen, O.P. & Østmo, S.R. Quaternary geology of Jæren and adjacent areas, south-western Norway, 55 (Norges Geologiske Undersøkelse Bulletin, 1987).

10.          Iverson, N.R., Jansson, P. & Hooke, R.L. In-situ measurement of the strength of deforming subglacial till. Journal of Glaciology 40, 497-503 (1995).

11.          Hooyer, T.S. & Iverson, N.R. Clast fabric development in a shearing granular material: implications for subglacial till and fault gauge. Geological Society of America Bulletin 112, 683-692 (2000).

12.          Piotrowski, J.A., Mickelson, D.M., Tulaczyk, S., Krzyszkowski, D. & Junge, F.W. Were deforming subglacial beds beneath past ice sheets really widespread? Quaternary International 86, 139-150 (2001).

13.          Benn, D.I. & Evans, D.J.A. Glaciers & Glaciation, 802 (Hodder Education, London, 2010).

14.          Hart, J.K. The relationship between drumlins and other forms of subglacial glaciotectonic deformation. Quaternary Science Reviews 16, 93-107 (1997).

15.          Evans, D.J.A., Phillips, E.R., Hiemstra, J.F. & Auton, C.A. Subglacial till: Formation, sedimentary characteristics and classification. Earth-Science Reviews 78, 115-176 (2006).

16.          Hicock, S.R. & Fuller, E.A. Lobal interactions, rheologic superposition, and implications for a Pleistocene ice stream on the continental shelf of British Columbia. Geomorphology 14, 167-184 (1995).

17.          Haavisto-Hyvärinen, M. Pre-crag ridges in southwestern Finland. Sedimentary Geology 111, 147-159 (1997).

18.          Ballantyne, C.K. Maximum altitude of Late Devensian glaciation on the Isle of Mull and Isle of Jura. Scottish Journal of Geology 35, 97-106 (1999).

19.          Roberts, D.H., Dackombe, R.V. & Thomas, G.S.P. Palaeo-ice streaming in the central sector of the British-Irish Ice Sheet during the Last Glacial Maximum: evidence from the northern Irish Sea Basin. Boreas 36, 115-129 (2007).

Lithostratigraphy

This section is taken from Bethan Davies’ PhD thesis.

What is Lithostratigraphy? | Vertical Profiles | Lithofacies Associations | Landsystems | References | Comments |

What is Lithostratigraphy?

Bethan Davies cleaning a face for sedimentological logging in County Durham.

Lithostratigraphy is the ‘classification of bodies of rock based on the observable lithological properties of the strata and their relative stratigraphic positions’1. Stratigraphy includes information about processes, geographical distributions, and the palaeo-environment of past glaciers and glaciation. It involves an attempt to determine the chronological sequence of geological events over a wide area.

Lithofacies associations, landform-sediment assemblages, depositional processes, syndepositional tectonics, landsystems, and geochronology are combined in a hierarchical structure to form a ‘stratigraphy’, through which the history and patterns of past glaciations and their associated environments can be reconstructed and interpreted1,2.

Sedimentological approaches should be based upon the ‘lithostratigraphic unit’, which has distinctive lithological properties, should be capable of being mapped and is typically tabular3,4. The lithostratigraphic unit has a hierarchical system with the Group, Formation, Member and Bed sub-categories5, and each new mappable lithostratigraphic unit must be formally proposed with a stratotype, and described emphasising lithological properties1,3. A lithostratigraphic scheme therefore:

  1. Has a hierarchical structure with the formation as the central (top) unit;
  2. Has a clear nomenclature;
  3. Describes each facies properly;
  4. Contains mappable units only.

Therefore, for a sedimentological investigation, the overall facies architecture and different lithofacies associations are mapped, logged and described in detail. The lithofacies associations are ultimately interpreted within a sediment-landform association, primarily in order to assess the processes by which glacigenic sediments were deposited and deformed. Through detailed lithological and petrological analyses, correlations between lithofacies associations and to regional stratotypes, based on processes of deposition, lithological and petrological similarity, and chronostratigraphy, are possible. Ultimately, it is possible to make statements about provenance, age, and regional glacial lithostratigraphy.

Vertical Profiles

Vertical profile from a coastal cliff section on the Durham coastline. Published in Davies et al., 2012.

Lithostratigraphy must take a hierarchical approach. The first stage is individual sediment logging. Vertical profiles are a method of recording detailed sedimentological information from a section, and they can be used for the comparison and correlation of different localities. They highlight gradual, particularly vertical, trends, and provide a representative summary of exposures6. Detailed sketches of macro-scale features such as deformation structures can provide information regarding the genesis and depositional history of glacigenic sediments. The colour of a sediment is the most immediately visible property, and can indicate more fundamental differences in composition, such as mineralogy7. Identifying the colour of a sediment is essential if the lithology is to be fully characterised. Facies characteristics are noted using standard facies codes (Table 1).

Therefore, to log an onshore field section, one requires that a GPS, photography and sketches are utilised to accurately map the overall facies architecture and to record spatial relationships between lithofacies. Specific exposures should be sketched and logged according to standard procedures8, noting the sedimentary structures, contacts, deformation structures, Munsell colour, texture, particle size, clast lithology and shape, grading, and sorting of each facies. All sections should be levelled to metres O.D. using standard levelling techniques.

An example of a vertical profile is shown in the figure opposite9. This log was taken from the Durham coastline at Warren House Gill and is published in the journal Boreas. The orange diamicton is the Blackhall Till formation. Yellow sands are the Peterlee Sand and Gravel Formation. Brown diamicton is the Horden Till Formation.

Table 1: Glossary of abbreviations used in section logs8,10.

Diamicton Fine Gravel (2-8 mm)
Dm Diamicton, matrix-supported GRcl Massive with clay laminae
Dmm Diamicton, massive, matrix-supported GRch Massive and infilling channels
Dms Stratified matrix-supported diamicton GRh Horizontally bedded
Dcm Clast-supported diamicton GRm Massive and homogenous
Dmg Matrix-supported, graded GRmb Massive and pseudo-bedded
Dml Matrix supported, laminated GRmc Massive with isolated outsize clasts
— (p) Includes clast pavement GRmi Massive with isolated, imbricated clasts
— (g) Graded diamicton GRmp Massive with clast stringers
— (b/s) Banded / sheared GRo Openwork structure
  GRruc Repeated upward-coarsening cycles
Silts and Clays (<0.063 mm) GRruf repeated upward-fining cycles
Fm Fines, massive GRt Trough cross-bedded gravel
Fl Fines, laminated. GRcu Upward coarsening (inverse grading)
Flv Fine lamination with rhythmites or varves. GRfu Upward fining (normal)
Frg Graded or climbing-ripple cross-lamination GRp Cross-bedded
Fcpl Cycopels GRfo Deltaic foresets
Fp Intraclast or lens
—(d) with dropstones Coarse Gravel (8-256 mm)
— (w) with dewatering Gms Matrix supported, massive gravel
  Gm Clast supported, massive
Sands (0.063 to 2 mm) Gsi Matrix supported, imbricated
Sm Massive sand Gmi Clast supported, massive, imbricated
St (A) Ripple cross laminated (Type A) Gfo Deltaic foresets
St (B) Ripple cross laminated (Type B) Gh Horizontally-stratified gravel
St (S) Ripple cross laminated (Type S) Gt Trough cross-bedded gravel
Scr Climbing ripples Gp Gravel, planar-cross bedded
Ssr Starved ripples Gfu Upward fining (normal grading)
Sr Sand, ripple-cross laminated Gcu Upward coarsening (inverse grading)
Sh Very fine to very coarse and horizontally / planar bedded or low angle cross lamination Go Open framework gravels
Sd Deformed bedding Gd Deformed bedding
St Medium to very coarse trough cross-bedded Glg Palimpsest (marine) or bedload lag
Sp Medium to very coarse planar cross-bedded  
Sl horizontal or draped lamination Boulders (>256 mm)
Sh Sheared sand B Boulders
Sfo Deltaic foresets Bh Horizontally-bedded boulders
Sfl Flasar bedded Bms Matrix supported, massive
Se Erosional scours with intraclasts and crudely cross-bedded Bcg Clast supported, graded
Su Fine to coarse with broad shallow scours and cross-stratification BL Boulder lag or pavement
Sc Steeply dipping planar cross bedding Bfo Deltaic foresets
Suc Upward coarsening Bmg Matrix supported, graded
Suf Upward fining  
Srg Graded cross-lamination Structure
SB Bouma sequence Bo Boudinage
Scps Cycoplasms Be Bedding
— (d) with dropstones Ba Banding
— (w) with dewatering  

Lithofacies Associations

Each facies is characterised by its individual properties in the vertical profile. On the basis of physical similarities, these sedimentary facies are correlated to form ‘lithofacies’8. Lithofacies are sediments with a distinctive combination of properties, classified on the basis of their colour, texture, the lithology of clastic particles, thickness and geometry, presence / absence of fossils, and other sedimentary structures11. Their spatial organisation is logged using an overall facies architecture sketch. It is important to separate detailed field description and labelling from genetic perspectives and terminology. Inadequate field descriptions thwart later sophisticated environmental re-interpretation12, as the interpretation of a genetic facies is subject to revision as ideas and knowledge change and the science develops. Lithofacies therefore are identified only on their physical, biological and chemical characteristics, with no inferred genesis8. This separation of description and interpretation ensures a more objective approach, less prone to bias, error and subjectivity. In this thesis, each chapter is analysed separately and the sediments are assigned to lithofacies associations particular to that specific site.

A hierarchical approach to sedimentology is a powerful tool for describing how sediments, landforms and landscapes fit together, and in determining how the landscape reflects depositional processes and external controls on the environment13. However, sediments are laid down in associations; these assemblages reflect a range of processes active in any one given environment, which can be deposited at a range of scales. ‘Lithofacies Associations’ (LFAs) are distinct vertical successions of genetically related lithofacies11. Through recognising these packages, ancient glacial settings can be recognised and reconstructed.

Landsystems

Lithofacies associations can be analysed in conjunction with landforms to create sediment-land for associations14. Sediment-landform suites are indicative and characteristic of specific styles of glaciation (‘glacial landsystems’), such as surging glaciers, ice streams, plateau ice fields, sub-aquatic landsystems, and active-temperate terrestrial ice margins15. Glacial landsystems are composed of ‘land units’ (geomorphological features such as drumlin fields, moraine belts, etc.) and ‘land elements’ (a tunnel valley, a moraine, an esker, and the associated sediments), which together form a landsystem, a ‘recurrent pattern of genetically linked land units’16. Recent analyses of glacial landsystems stress their complexity and the fact that sediment-landform associations are dictated by the location and style of deposition17-21.

Citation

Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.

Bethan Davies Thesis (Zipped PDFs – 70MB)

References


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3.         Salvador, A. The International Stratigraphic Guide: a guide to stratigraphic classification, terminology, and procedure., 214 (International Subcommission on Stratigraphic Classification. International Union of Geological Sciences, Boulder, 1994).

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12.       Eyles, C.H. & Eyles, N. Sedimentation in a large lake: a reinterpretation of the late Pleistocene stratigraphy at Scarborough Bluffs, Ontario, Canada. Geology 11, 146-152 (1983).

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18.       Evans, D.J.A. & Twigg, D.R. The active temperate glacial landsystem: a model based on Breiethamerkurjokull and Fjallsjokull, Iceland. Quaternary Science Reviews 21, 2143-2177 (2002).

19.       Jennings, C.E. Terrestrial ice streams – a view from the lobe. Geomorphology 75, 100-124 (2006).

20.       Lukas, S. Morphostratigraphic principles in glacier reconstruction – a perspective from the British Younger Dryas. Progress in Physical Geography 30, 719-736 (2006).

21.       Ottesen, D., Dowdeswell, J.A., Benn, D.I., Kristensen, L., Christiansen, H.H., Christensen, O., Hansen, L., Lebesbye, E., Forwick, M. & Vorren, T.O. Submarine landforms characteristic of glacier surges in two Spitsbergen fjords. Quaternary Science Reviews 27, 1583-1599 (2008).