OR/16/046 Appendix 1 - Geological attribution principles
| Armstrong, R, Daley, D, Lawley, R, Myers, A and Smith, A. 2016. User guide for the BGS geology: 50k dataset (v8). British Geological Survey Internal Report, OR/16/046. |
Lithology
Rocks may be described in a number of different ways. Their lithology, for example, may be defined in terms of their general characteristics of appearance: colour, texture and composition. Some lithologies may require microscope or chemical analysis for the latter to be fully determined.
The BGS Rock Classification Scheme (RCS), which is available in four volumes for download at: www.bgs.ac.uk/bgsrcs/home.html, provides hierarchies that can be used to describe rocks.
The igneous rocks are described in Volume 1, the metamorphic rocks are described in Volume 2 and the sedimentary rocks (and sediment) are described in Volume 3. These three volumes form the basis for the RCS codes used in the Bedrock theme in BGS Geology: 50k.
Volume 4 of the rock classification scheme, classifies man-made and natural superficial deposits according to their genesis (mode of origin) and overall form (shape) or gross composition. This volume forms the basis for identification in the Artificial, Mass Movement and Superficial themes. Note that the use of genesis and form as an identifier means that the descriptions are NOT wholly lithological.
For the purpose of making digital maps each rock unit is labelled with a lithological code based upon the Rock Classification Scheme. For example MDST is the code for 'MUDSTONE'. Many rock units comprise more than one lithology; for example, a formation of interbedded mudstone and limestone may be attributed with the composite code MDLM. Individual components in the mixed lithology are listed separately in the RCS_X field.
For the superficial deposits, the unlithified deposits are encoded to reflect the presence of six key components (clay, C; silt, Z; sand, S; gravel, V; cobbles, L; boulders, B) and peat, P. The codes are extended by the use of an ‘X’ prefix in order to include ‘composite’ lithologies. For example, the code ‘VSL’ describes an admixed lithology of ‘cobbly, sandy gravel’; whilst the code ‘XVSL’ describes an interbedded sequence of Gravel, Sand and Cobbles (of unknown proportions).
Lithostratigraphy
Many rocks are deposited in layers or strata, and the sequence of these strata can be correlated from place to place. These sequences of different rocks are used to establish the changing geological conditions or geological history of the area through time. The description, definition and naming of these layered or stratified rock sequences is termed lithostratigraphy (rock stratigraphy). The strata can also be described in other ways depending on the types of information available: for example in biostratigraphy (life stratigraphy) fossils are used.
Lithostratigraphy is fundamental to most geological studies. Rock units are described using their gross compositional or lithological characteristics and named according to their perceived rank in a formal hierarchy. The main lithostratigraphic ranks in this hierarchy are: Bed (lowest), Member, Formation, Subgroup, Group and Supergroup (highest). The units are usually named after a geographical locality, typically the place where exposures were first described.
| Rank 6 | Rank 5 | Rank 4 | Rank 3 | Rank 2 | Rank 1 | |
| Litho- stratigraphic units |
Formation J | |||||
| Formation K | Group C | Supergroup B | ||||
| Formation L | Subgroup F | |||||
| Formation M | ||||||
| Bed W | Member U | Formation N | ||||
| Bed X | ||||||
| Member V | ||||||
| Formation O | Subgroup G | Group D | ||||
| Formation P | ||||||
| Formation R | Subgroup H | |||||
| Formation S | ||||||
| Bed Z | Formation T | |||||
| BGS geology fields |
BED_EQ | MB_EQ | FM_EQ | SUBGP_EQ | GP_EQ | SUPGP_EQ |
These formal ranks are often appended to names in the BGS Lexicon of Named Rock Units. Formations are the fundamental rock units for mapping purposes at 1:50 000 scale and can stand-alone; they do not have to belong to a group and need not be split into smaller units. A Group is an assemblage of related and adjacent Formations and may be subdivided into Subgroups. A Super-Group is an assemblage of Groups. A Member is a sub-division of a Formation and a Bed is the smallest formal unit. Some possible lithostratigraphic relationships are shown schematically in Table 5.
In this hierarchical scheme, each unit may have parent and child relationships with other units of greater and lesser rank respectively. For example, Formation J does not belong to a group, nor is it subdivided. Part of Group C is recognised as Subgroup F comprising two formations (L and M). Formation N is divided into Members U and V; with Member U comprising beds (W and X). Bed Z forms part of Formation T.
The names of the relevant BGS Geology fields at each of these ranks are also shown.
Lithodemic terminology
Where rocks are not laid down in stratified sequences they are given names using a lithodemic scheme, as shown in Table 6. In the lithodemic hierarchy applied to intrusive igneous rocks, developed for BGS (Gillespie, Stephenson and Millward, 2008; Gillespie, Campbell and Stephenson, 2011) units are placed into one of six ranks (the same number as in the lithostratigraphic scheme, although there is not necessarily any direct correlation in the rank).
In this hierarchical scheme each lithodemic unit may be part of a ‘parent’ unit of greater rank or may be composed of ‘child’ units of lesser rank. Thus within the intrusive units a pluton may be part of a suite or subsuite, and may itself comprise several intrusions. (see Table 6 below). These can be applied to igneous intrusive, highly deformed and/or highly metamorphosed and genetically mixed assemblages of rocks.
| Rank 6 | Rank 5 | Rank 4 | Rank 3 | Rank 2 | Rank 1 | |
| Intrusive units | intrusion | pluton | centre cluster | subsuite | suite | supersuite |
| ring-intrusion | ||||||
| lopolith | ||||||
| intrusion-swarm | ||||||
| laccolith | laccolith-swarm | |||||
| plug | plug-swarm | |||||
| vent | vent-swarm | |||||
| pipe | pipe-swarm | |||||
| neck | neck-swarm | |||||
| diatreme | diatreme-swarm | |||||
| sheet | sheet-swarm | |||||
| dyke | dyke-swarm | |||||
| sill | sill-swarm | |||||
| ring-dyke | ring-swarm | |||||
| cone-sheet | cone-sheet-swarm | |||||
| vein | vein-swarm | |||||
| Tectono-metamorphic units | lens, block | train, swarm | set | sub-assemblage | assemblage | super-assemblage |
| layer, mass | unit | package | sub-succession | succession | super-succession | |
| Mixed class units | sheet-complex sill-complex vein-complex ring-complex |
subcomplex ophiolite-complex central complex volcano-complex |
complex | super complex | ||
| BGS geology fields | BED_EQ_D | MB_EQ_D | FM_EQ_D | SUBGP_EQ_D | GP_EQ_D | SUPGP_EQ_D |
A similar scheme has been developed for the metamorphic and tectono-metamorphic units (Leslie, Krabbendam and Gillespie, 2012). Here an assemblage may comprise several sets (if dispersed) or packages (if contiguous) and within these there may be lenses and blocks, for example. In addition to these, where there are mixtures of rocks such as igneous intrusive and sedimentary or igneous intrusive and metamorphic, then a hierarchy based on the ‘complex’ has been developed.
Rock units are described using their gross compositional or lithological characteristics and named according to their perceived rank in a formal hierarchy. These formal ranks are often appended to names in the BGS Lexicon of Named Rock Units. The name of the relevant BGS Geology field at each rank is also shown.
For expediency the lithodemic hierarchy uses the same field names as the lithostratigraphic hierarchy; the ‘EQ’ suffix (for ‘Equivalent’), does not imply exact geological equivalence of rank between lithostratigraphic and lithodemic units, it is a convenience facilitating the supply of data.
Time and chronostratigraphy
There are a number of ways of describing geological time. Most are ‘relative’ in which the Earth’s geology is subdivided into named units based on their stratigraphical relationships or relative ages, with younger strata typically overlying older strata (in undeformed sedimentary sequences). Some methods are ‘absolute’ and typically measure time units in millions of years (before present). Chronostratigraphy, deals with ‘time & rock’ units and refers to the sequence of rocks deposited in a particular time span. There is an established formal hierarchy of chronostratigraphical terms, shown in Table 7, in which the principal ranks range from stage (small subdivisions) to eonothem (large subdivisions).
| Chronostratigraphical [time-rock] divisions | Stage | Series | System | Erathem | Eonothem |
| Geochronological [time] divisions | Age | Epoch | Period | Era | Eon |
| Example | Gorstian | Wenlock | Silurian | Palaeozoic | Phanerozoic |
Geochronology, as used in BGS Geology: 50k, deals with ‘time’ units and refers directly to the time spans. The corresponding principal formal geochronological terms range from Age to Eon. The same name can be used in both schemes; thus rocks of the Jurassic ‘System’ were deposited during the Jurassic ‘Period’ of time.
The BGS timechart (www.bgs.ac.uk/discoveringGeology/time/timechart/home.html) and the latest version of the ICS time chart (www.stratigraphy.org/index.php/ics-chart- timescale) can be used to discover further information about chronostratigraphy and Geochronology.
Structure
Faults
Geological faults are the most common feature in the Linear theme of the BGS Geology data but uncertainties often affect their mapped position at the surface (or at rockhead). A fault is a fracture or zone of fractures along which the materials on opposing sides of the fracture have been displaced relative to one another, by movements along the surface of the fault.
A fault may split (‘splay’) and the separate surfaces effectively become a fault ‘zone’ rather than a single fault; fault zones may be tens to hundreds of metres wide. Movements along faults may crush the rocks adjacent to the fault plane(s), creating a ‘fault breccia’.
A fault is typically portrayed in BGS Geology: 50k as a single line. Therefore, users should be aware that this linear representation does not imply any specific dimensions or characteristics to the fault/fault zone, the line merely represent the apparent location of a faulted-feature.
Faulting in BGS Geology: 50k has been extracted from previously published paper maps or 3D modelling work. Evidence for the existence of faulting can be based upon observed exposures (above and below ground) or by inference linear depressions, the truncation or displacement of topographical features, or the sudden change in geology proved by boreholes (i.e the fault is inferred). If there are superficial deposits at surface then the position, nature and maybe even the existence of a fault recorded within the underlying bedrock will be conjectural.
Faulting is a response to structural evolution of the landscape, and faulting can be more common in some areas than others. However, because faults are easier to identify and map in areas with large amounts of supporting evidence, some parts of Great Britain appear to be more faulted than others. Users should be aware that BGS Geology: 50k faults are a representation of survey evidence and inference and may not represent the complete distribution of faults in an area.
Traditionally, on paper maps, where one side of a normal fault is downthrown relative to the other, the downthrown side is indicated by a small ‘tick’ on the fault line (representing the ‘hanging wall’ side). Similarly for thrust faults the up thrown side is marked with a triangle (again this is the hanging wall of the fault).
It has not been possible to provide a consistent digital representation of faults and their hanging wall orientation due to processing and conversion to other formats. BGS is currently redeveloping its fault database to provide more robust orientation attribution in future versions. In the latest BGS Geology: 50k data (v8), a new attribute has been added to the linear theme that provides a compass rose indicator of which side of a fault represents the hanging wall. This new field is calculated by comparison of geological ages of materials either side of the fault, and is being trialled for a subset of the faults to see if the data can remain consistent in the normal processes of publication and translation that can occur in digital files. Information on fault throw (hanging wall) remains incomplete and subject to change in BGS Geology: 50k. Users are advised to use the hanging wall indicators with caution, and seek further advice from BGS where necessary.
Folds
Many of the rocks forming the earth’s crust have been deformed by structural evolution and the resulting strata tilted or inclined to form folds. They are best seen in layered sedimentary rocks where the bedding was originally planar. In the simplest examples these folds may have a rounded hinge zone with planar limbs to either side of the hinge; dipping outwards (in an upward arched anticline) or inwards (in a downwards or concave syncline). Simple folds have an ‘axial plane’ about which the folding appears to have taken place. The trace of this axial plane on the Earth’s surface may be shown in BGS Geology: 50k depending upon on the scale of the fold feature (e.g. micro folds may not be shown at the 1:50 000 scale).
As for fault-related features, evidence for fold axes is based on observation or inference. Some uncertainty therefore attaches to their mapped position; their linear representation in the data should be regarded as zones of folding.