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Substrate grain size

Substrate grain size categorises the size of unconsolidated substrates by their diameter (regardless of composition). In the field, substrate sediments usually occur as a mixture of grain sizes. While substrate grain size is frequently correlated with energy magnitude (e.g. boulders and cobbles on high energy shorelines) it may also be an artefact of previous processes that no longer occur such as relics of different shorelines or the product of erosion of sedimentary rocks with mixed grain sizes (e.g. conglomerates, duricrusts etc.).

Boulder shore at Bagara. Photo by Kirsten Wortel

Quick facts

Sediment texture
describes the mixtures of mud, sand and gravel on the substrate.

The grain size of unconsolidated substrates is important for several reasons including, the ability of water to move through or remain within the substrate (e.g. sea-water, ground-water), the retention of organic matter, and to support burrowing organisms and their food sources. Grain size affects the permeability and drainage of intertidal unconsolidated substrates, for example gravelly and sandy soils drain quicker than the finer-grained muds, clays and silts. The minerals in clay (e.g. kaolinite) enable particles to stick together[6], enabling burrowing animals to create holes that persist long enough to provide sheltering space. Some clay minerals absorb water, swelling and expanding into a more compact matrix. Subtidal muds and silts may be fine enough to become suspended in the water column, if the energy is sufficient at the sea floor to keep them in suspension. These suspended sediments can decrease water clarity resulting in insufficient light for photosynthesis by corals and seagrasses[2]. Sands drain quickly and may act as a groundwater ‘sponge’ (see coastal sand mass conceptual model). Gravels and boulders increase the rugosity or structural complexity of the substrate, providing three-dimensional structure for biota to attach[5]. Spaces beneath gravel and boulders provide shelter and retain seawater for animals dependent on it in the intertidal area[1]. Gravels and boulders are less mobile, but even very large boulders during high-energy storms and cyclones[7].

The substrate grain size attribute applies to unconsolidated non-living substrates of the sea floor (also known as sediments). There are several ways to classify substrate grain size. The Udden-Wentworth Scale[8] classifies substrate grain size into diameters of increasing sizes, listed below in the Attribute Category Table for Substrate Grain size. Equivalent codes to these grain sizes are also provided by the Collaborative and Annotation Tools for Analysis of Marine Imagery and Video (CATAMI) classification scheme and the Marine Nature Conservation Review (MNCR). In the field, sediments seldom occur in a pure form, appearing as mixtures that can be typed or grouped in different ways. The Folk classification is based on the grain size classes of the Udden-Wentworth Scale[8], reclassified and compared in a typology that is further described below.

Queensland Intertidal and Subtidal Classification Scheme

Substrate grain size falls under the 'Substrate' theme of attributes for the Intertidal and Subtidal Classification Scheme. It categorises the size of unconsolidated substrates (regardless of composition). The size classes used are those used in the Udden-Wentworth Scale[8]. The sediment texture typology is often easier to map than each separate grain size.

Attribute Category Table - Substrate Grain Size

Habitat Seascape Subregion Region
Unknown Unknown Unknown (Not applied)
None None None
Other or unspecified Other or unspecified Other or unspecified
Fine-medium clay Mud (clays and silts) Mud (clays and silts)
Coarse clay
Very fine silt
Fine silt
Medium silt
Coarse silt
Mud - undifferentiated (clays and silts)
Very fine sand Sand Sand
Fine sand
Medium sand
Coarse sand
Very coarse sand
Sand - undifferentiated
Very fine pebbles (granules) Pebbles Pebbles
Fine pebbles
Medium pebbles
Coarse pebbles
Very course pebbles
Pebbles - undifferentiated
Cobbles Cobbles Cobbles
Boulders Boulders Boulders
Gravels (undifferentiated pebbles, cobbles and boulders) Gravels (undifferentiated pebbles, cobbles and boulders) Gravels (undifferentiated pebbles, cobbles and boulders)

Sediment Texture Typology (Folk)

Folk classification of sedimentary rocks. Image by R. L. Folk

Recognising that substrates generally occur in mixtures of grain sizes, a practical application to describe sediments in their mixed form is the Folk typology[3]. The Folk typology is frequently applied in other seabed classifications such as the European Nature Information System (EUNIS), and has been used to harmonise and collate seabed substrate data for European maritime areas[4]. Originally designed to classify the grain size of sedimentary rocks, Folk codes summarise sediment textures in terms of percent mud, sand and gravel fractions. ‘Sediment texture’ is the term used to describe the proportions of sand, gravel and mud as defined by the Udden-Wentworth scale[8] (as in the Attribute Category table for Substrate Grain Size, seascape scale column). The Udden-Wentworth’s sand and mud sediment grain sizes are used to define each of two of the three axes of the triangle that defines proportions or mixtures of Folk sediment texture typology (see right). ‘Gravel’ comprises the third axis of the ternary diagram, constituting a grouping of the Uden-Wentworth ‘pebbles’ ‘cobbles’ and ‘boulders’. Sometimes ‘boulders’ are split out separately from the Folk typology and mapped separately. The notation for the Folk figure is that the dominant grain size (primary) is capitalized, the subdominant (fractional) is lower case, and the next subdominant (slight fraction) is in brackets, for example ‘(g)sM’ represents ‘(slightly gravelly) sandy MUD’[3].

The sediment texture typology is often easier to map than each separate grain size, as a dominant grain size may overlap with other subdominant grain sizes and boundaries may differ. For this reason, the Folk typology is frequently applied in other seabed classifications such as the European Nature Information System (EUNIS), where it has been used to harmonise and collate seabed substrate data for European maritime areas. For the substrate grain size attribute, the Sediment texture (Folk) typology was also chosen to harmonise different seabed grain sizes in the Central Queensland intertidal and subtidal ecosystems classification, typology and mapping.

Folk Sediment Text Typology - (Folk Classification) Proportions of Substrate Grain Size - Folk[3]
- Unknown B BOULDER O None M MUD
(g)mS (Slightly gravelly) muddy SAND gmS Gravelly muddy SAND G GRAVEL sG Sandy GRAVEL
(g)sM (Slightly gravelly) sandy MUD gM Gravelly MUD mG Muddy GRAVEL sM Sandy MUD
(g)sM (Slightly gravelly) SAND gS Gravelly SAND mS Muddy SAND S SAND

Additional Information


References

  1. ^ Bennett, Isobel & Dakin, WilliamJ 1992, Australian seashores, Collins/Angus & Robertson, Pymble, N.S.W..
  2. ^ Fabricius, KE, Logan, M, Weeks, SJ, Lewis, SE & Brodie, J 2016, 'Changes in water clarity in response to river discharges on the Great Barrier Reef continental shelf: 2002-2013', Estuarine, Coastal and Shelf Science, vol. 173, pp. A1-A15, Scopus.
  3. ^ a b c Folk, RL 1974, 'Petrography of sedimentary rocks', Univ.Texas, Hemphill, Austin, Tex, vol. 182.
  4. ^ Kaskela, Anu, Kotilainen, Aarno, Alanen, Ulla, Cooper, Rhys, Green, Sophie, Guinan, Janine, van Heteren, Sytze, Kihlman, Susanna, Van Lancker, Vera, Stevenson, Alan & the EMODnet Geology Partners 2019, 'Picking Up the Pieces—Harmonising and Collating Seabed Substrate Data for European Maritime Areas', Geosciences, vol. 9, no. 2, p. 84, viewed 18 April 2019, <http://www.mdpi.com/2076-3263/9/2/84>.
  5. ^ McHenry, Jennifer, Steneck, RobertS & Brady, DamianC March 2017, 'Abiotic proxies for predictive mapping of nearshore benthic assemblages: implications for marine spatial planning', Ecological Applications, vol. 27, no. 2, pp. 603-618, viewed 22 July 2019, <http://doi.wiley.com/10.1002/eap.1469>.
  6. ^ Nichols, Gary 2009, Sedimentology and stratigraphy, John Wiley & Sons.
  7. ^ Vila-Concejo, Ana & Kench, Paul 8 April 2017, 'Storms in Coral Reefs', in P a o l o Ciavola & G i o v a n n i Coco (eds), Coastal Storms, John Wiley & Sons, Ltd, Chichester, UK, pp. 127-149, viewed 22 July 2019, <http://doi.wiley.com/10.1002/9781118937099.ch7>.
  8. ^ a b c d Wentworth 1922, 'A Scale of Grade and Class Terms for Clastic Sediments', The Journal of Geology, vol. 30, no. 5, pp. 377-392.

Last updated: 23 July 2019

This page should be cited as:

Substrate grain size, WetlandInfo 2019, Department of Environment and Science, Queensland, viewed 6 August 2019, <https://wetlandinfo.des.qld.gov.au/wetlands/ecology/aquatic-ecosystems-natural/estuarine-marine/attributes/substrate-grain-size/>.

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