Attributes relating to the composition of the substrate include whether it is consolidated or not (Consolidation), the mineral composition of consolidated (rocky) substrates (Lithology), the grain size of unconsolidated substrates (Sediment grain size), and the source characteristics and composition of the non-living substrate (Substrate composition). Other substrate attributes involve complex processes, which are best explored in conceptual models, for example genesis of the substrate.
Genesis of the substrate tells the story of its origin and formation, including processes and drivers.
The rock cycle describes the genesis of terrigenous lithologies (i.e. of land origin). Geological and geomorphological processes that occur mainly underground (e.g. heat and pressure, melting and crystallization) transform a rock type into either igneous or metamorphic rocks. Surface processes of weathering and erosion break down rocky (consolidated) substrates into unconsolidated fragments or sediments, which become available for transport and deposition by hydrodynamic energy (see energy source and magnitude attributes). The sediment layers are buried and become compacted underground, turning into rock (lithification), i.e. sedimentary rocks. Relevant attributes are consolidated /unconsolidated; igneous, sedimentary, metamorphic; terrigenous; all categories
The story of the genesis of the Great Barrier Reef (GBR) and its various reef types are best described through conceptual models and specialist typologies. For example most modern coral reefs grow on the bones of their ancestors (i.e. old carbonate reefs), so it is important to distinguish what is the antecedent substrate within the GBR. Processes involve thousands of years of sea level rise and fall, position on the continental shelf, width of the continental shelf, erosion, chemical dissolution, sediment transport, deposition, calcification, and lithification (into sedimentary limestones). The resulting coral reef is composed both of a consolidated carbonate framework and large areas of unconsolidated (generally carbonate) sediments. The depth of the growing reef and its resultant shape modify energy patterns of sediment transport, erosion and deposition. Relevant attributes are consolidated/unconsolidated; sedimentary-limestones; calcareous- biogenic - coral; boulder, cobble, pebble, sand, mud.
The following characteristics describe a change or variability in the substrate and apply to several substrate attributes (e.g. Lithology, Consolidation, Sediment grain size, Substrate composition). Note that sediments usually occur in mixtures of different grain sizes termed 'sediment texture'. These qualifiers include:
Hardness/compaction of the substrate is important to burrowing fauna or for flora growing on its surface. Compaction or hardness is a result of soil- and rock-forming processes involving primary and secondary chemical and/or physical compaction. For example:
chemical percolation of dissolved iron and aluminium oxides that solidify as an indurated layer, or overburden of material weighing down on the substrate below.
cementation of unconsolidated substrates can be irregular, including nodules or concretions, or form regular bands of rock of intermediate consolidation such as beach rock or coffee rock.
subsurface sediment compaction is important for different infauna taxa which burrow to different depths within the substrate.
Voids or interstitial spaces are available to be occupied by fauna, and substrate permeability may be inferred from substrate grain size and its porosity (gaps between particles). Importantly, porewater/groundwater/seawater filters through permeable substrates, driving groundwater–seawater movements and interactions through a variety of processes including wave, tidal, and hydraulic action
Layering of the substrate (e.g. a thin veneer of sand over clay) may be inferred from percentage composition of the Sediment grain size attribute. Layering and
Colour of the substrate. Darker consolidated substrates reach higher temperatures and retain heat longer than lighter colours (see Lithology attribute for further information). Different colours of unconsolidated substrates are influenced by their chemistry and different mineral content, through processes such as the leaching of minerals through a soil profile, source lithology/lithology (composition) and the wetness of the substrate.
Last updated: 16 March 2021
This page should be cited as:
Department of Environment and Science, Queensland (2021) Substrate Lithology, WetlandInfo website, accessed 13 May 2021. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/ecology/aquatic-ecosystems-natural/estuarine-marine/attributes/substrate-lithology/