OR/16/001 Approach: two-part classification

Dove, D, Bradwell, T*, Carter, G, Cotterill, C, Gafeira, J, Green, S, Krabbendam, M, Mellet, C, Stevenson, A, Stewart, H, Westhead, K.; INFOMAR: Scott, G, Guinan, J, Judge, M, Monteys, X.; MAREANO: Elvenes, S, Baeten, N, Dolan, M, Thorsnes, T, Bjarnadóttir, L, and Ottesen, D . 2016. Seabed geomorphology: a two-part classification system. British Geological Survey Internal Report, OR/16/001. (* now at Stirling University)

Marine geoscientists are highly reliant on remotely-sensed observations due to the common paucity of ground-truthing data, hence we need approaches that most effectively exploit the available data. At the outset of the process, it was important to ensure that the classification system allowed for the communication of useful and specific geological information, while also providing an objective description of the seabed. Instead of collating all features into a single list or catalogue, it was also preferred to present features in a consistent logical framework (i.e. hierarchy) to inform on the physical/environmental processes that led to their formation (e.g. glacial vs. fluvial vs. submarine). However, initial attempts to integrate detailed geological interpretations together with objective morphological descriptions (e.g. ridge, channel, escarpment) within a single classification tree were unsuccessful as this required the scheme to become either excessively complicated (e.g. permitting sufficient variation of form), or excessively prescriptive (e.g. permitting insufficient variation of form). The end result of trying to merge objective and interpreted descriptions together was generally ineffective because of the non-unique relationship between morphology and process origin.

The solution presented here demonstrates that separating the two elements (geomorphology and morphology) provides a more straightforward, logically-consistent, and flexible means of mapping the seabed. Within this revised approach, two independent classification schemes are employed to describe the seabed: 1) Morphology and 2) Geomorphology. ‘Morphology’ features are those characterised by their physical attributes (e.g. shape, size, texture), while ‘Geomorphology’ features are those defined by their process origin, i.e. biogenic features are those that were formed primarily by biological processes, and glacial features are those that were formed primarily by glacial processes. In a general sense, ‘Morphology’ provides the fundamental objective description of the feature(s), whereas ‘Geomorphology’ provides an interpretation of the feature(s). Employing these two independent classification schemes frees the mapper to describe features purely according to their morphology, or the environmental systems and processes that led to their formation.

Within the ‘Morphology’ classification tree, features are described at four hierarchical levels according to their general form (e.g. channel, ridge, etc), and optionally with separate morphological attributes (e.g. sinuous, streamlined, etc) (Figure 5). Morphological forms are organised into two broad groups: ‘Relative bathymetric high’ or ‘Relative bathymetric low’, together with several other features that don’t fit into ‘high’ or ‘low’ categories (i.e. plane, slope, and lineament features as well as provinces/assemblages of features). There are also a series of morphological attributes (e.g. ‘sinuous’) that may optionally be appended to feature designations.

Figure 5    Version 1 of the two-part Seabed Geomorphology system — Morphology and Geomorphology.

Within the ‘Geomorphology’ classification tree, features are also described at four hierarchical levels. Within this hierarchy features are typically described in increasing detail from environmental system (e.g. submarine), to environmental setting (e.g. shelf), to process (e.g. current-induced bedform) to bedform/landform type (e.g. sediment wave). At present, the classification tree incorporates 8 environmental systems/themes: Bedrock/Volcanic/Tectonic, Biogenic, Coastal, Fluid-escape, Fluvial, Glacial, Marine, and Mass-wasting, with the intention that ‘Anthropogenic’ features will be incorporated at a later date (Figure 5). This geomorphological classification was developed with the intention of characterising seabed features found in UK and NW European waters, and almost certainly excludes many features that may be found in other geographic areas (e.g. Mediterranean, tropics). Nonetheless, we believe that the classification system is sufficiently flexible and robust that such features could easily be appended where relevant.

To demonstrate how the two independent classification trees would be used to map a single feature, we give the example of a ‘drumlin’ below:

1. Morphology: (‘Bathymetric high’ >> ‘Mound’ >> ‘Streamlined mound’);
2. Geomorphology: (‘Glacial’ >> ‘Subglacial landform’ >> ‘Streamlined landform’ >> ‘Drumlin’).

Further attributes of the two-part classification approach

• Features may be mapped at different hierarchical levels on both the ‘Morphology’ and ‘Geomorphology’ classification trees. This enables significant flexibility for the mapper to map features at greater or lesser detail according to the intrinsic nature of the feature(s), ensuring mappers can be confident in their interpretations. An example might be hummocky terrain that the geoscientist can confidently attribute to glacial processes (due to local context/surrounding features), but is uncertain whether the features result from subglacial or ice-marginal processes. In this instance, the feature could be mapped: ‘Morphology’ >> ‘Province/Assemblage’ >> ‘Hummocky terrain’’; and ‘Geomorphology’ >> ‘Glacial’;
• Every feature mapped will have a ‘Morphology’ classification, whereas only features in which the mapper has sufficient confidence in their interpretation will have a ‘Geomorphology’ classification. This follows from ensuring a justifiable scientific approach is applied to the mapping: i.e. observations and description are made before interpretation. By definition, any geomorphological feature has an associated morphology, but there are instances where the origin and environmental associations of a ‘Morphology’ feature(s) is unclear, so no ‘Geomorphology’ classification would be given. This also allows features to be assigned a ‘Geomorphology’ classification at a later date without remapping e.g. if additional information arises to support interpretation such as ground-truth data;
• Applying this two-step approach is flexible and robust because it allows the mapper to separate their basic observations and descriptions (Morphology) from their informed interpretation (Geomorphology), in effect better future-proofing their work. This is of use to the mapper and map users for a number of reasons: 1) A map user may disagree with some aspects of the interpretation, but could still make use of the ‘Morphology’ classification; 2) the origin of some seabed features may be ambiguous, despite having similar morphological characteristics; 3) the map user might only want a map of the ‘Morphology’ classes, or components of the ‘Morphology’ (e.g. filter for seabed depressions only; and 4) the map user might want to view and filter for features associated with certain environmental systems only (e.g. fluvial systems), specific processes (e.g. fluid-escape features), or morphologies (e.g. scour). This is particularly useful as the origin of seabed features can be ambiguous, e.g. a bank of sediment can be produced through glacial or marine processes (or both), such that it is not always possible to distinguish between the two in the absence of ground-truth data;
• Each feature has an associated glossary definition (which are as yet incomplete, and will be published with a later version of the scheme);
• Apart from the independent ‘Morphology’ and ‘Geomorphology’ classifications, mapped features will incorporate a number of further quantitative (e.g. height, maximum slope, etc) and qualitative (e.g. active vs. moribund, free-text comments) attributes. This will be implemented through the associated SIGMA GIS tools. The SIGMA tools have been adapted to apply the two-part classification scheme and allow for efficient mapping of morphological and geomorphological features within an ArcGIS environment. The SIGMA tools will calculate several quantitative attributes (as described above) automatically and will allow for direct user input as well;
• Seabed geomorphological map products will be made available digitally. In the UK, this will be managed through the MAREMAP web portal (www.maremap.ac.uk/index.html). Maps will be presented with separate seabed Geomorphology and Morphology cartographic schemes, and map symbologies will reflect the respective hierarchical classification systems. Shapefiles will be downloadable to access further feature attributes.