This section is taken from Bethan Davies’ PhD thesis.
What is Lithostratigraphy?
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:
- Has a hierarchical structure with the formation as the central (top) unit;
- Has a clear nomenclature;
- Describes each facies properly;
- 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.
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|
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.
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.
Davies, B.J., 2009. British and Fennoscandian Ice-Sheet Interactions during the Quaternary, Unpubl. PhD Thesis. Department of Geography, Durham University, Durham, 502 pp.
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