Geology and lithology

The geology and/or the lithology of substrate influences many rock characteristics that are important for wetlands and how they function. It influences the shape or form, the patterns resulting from geomorphological processes such as weathering, erosion and deposition. The lithology of a rock influences its chemical composition, the shape and terrain form, and that of the surrounding area where weathered material from that rock has been deposited. Lithology also provides a history of changes of the biota of the earth and their environment over time, through the fossil record preserved in sedimentary rocks.

Rocks are consolidated materials composed of one or more minerals, of igneous, sedimentary or metamorphic origins. Lithology is defined as the mineral composition of rocks.[9]

Geological characteristics of substrates can be simplified into land zones, based on similar landforms and geomorphic processes based around events in the gological timescale. The Queensland's land zone framework consists of twelve zones, six of which are consolidated, and six unconsolidated (see Substrate Composition). The lithologies of the sea floor and intertidal areas directly correspond to land zones of consolidated landscapes, also reflecting the the three different rock types in the rock cycle[9].

Geological events in the earth's history have influenced the geology and lithology of eastern Australia (see geological timescale diagram to familiarise with the time periods referred to below) and include:[3]

• The breakup of the supercontinents (e.g. Gondwanaland) and continental drift (Permian).
• Movements between the continental plates (collisions that result in either uplift, subduction or both) causing mountain-building (orogeny) or stretching apart (rifting) and subsidence. Folding and faulting may be associated with movements of the earth’s crusts (e.g. uplift of the Great Dividing Range).
• Tectonic (volcanic) activity as the plates travel over[4] ‘hot spots’ in the earth’s crust.
• Erosion and deposition of sediment formed sedimentary basins in subsidence areas such as lakes, under the sea, or in river valleys.
• Chemical soil forming processes, such as deep weathering and precipitation, accumulation of organic material and carbon[9].
• Biomineralisation, where calcium carbonate from dead biota (e.g. animal skeletons) become rock.
• Changes in sea levels associated with glacial cooling and melting over the last 500,000 years[5][8].

Three different types of rocks

There are three different types of rocks - igneous, metamorphic and sedimentary. The rock types were formed in response to geophysical and geochemical processes of heat, pressure, melting and crystallization, weathering, erosion and deposition (see Rock cycle). Rock types can be further subdivided based on their age, using the geological time scale to differentiate older (Mesozoic and older) rocks from the younger rocks of the Cainozoic[9].

Sedimentary rock refers to rock formed by the accumulation and cemetation of fragments of other rocks, minerals and organisms, or as chemical precipitates and are formed in layers. Rocks are weathered into fragments, becoming unconsolidated sediments that are transported and deposited by water, wind or ice, or layers form chemically by precipitation from a solution (indurated), and/ or are secreted by biota. Over millions of years the layers experience heat, pressure and chemical change (diagenesis) that turns them into rock (lithification). Usually the layers of sedimentary rocks (strata) form a planar surface, that is the layering is horizontal. Depending on the uniformity of the particle sizes within the rock, they are further divided into fine grained or coarse grained. Particle size of rocks follows the same classification as grain size of unconsolidated sediments. The spaces between the particles in the rock and the cracks between rocks can potentially hold water, and different layers of sedimentary rocks can be aquifers that can hold water. Yet others are so hard that they form barriers to water, termed aquitards[9]. Many sedimentary rocks in Australia have formed under the sea or in saline coastal lakes, resulting in a high sodium content including salt. Fine grained sedimentary rocks may have a high silt content and weather to form clays and silts.

Biogenic rocks are a form of sedimentary rock that is produced directly by the physiological activities of organisms, either plant or animal e.g. coral reefs, shelly limestone, pelagic ooze, coal, and peat[2].

Metamorphic rocks refers to rock that has undergone metamorphism, that is subjected to heat and pressure resulting in physical and/or chemical change. Examples include gneiss, phyllite, marble, quartzite, schist, and slate. During the movements of the earth’s continental plates, the edges of plates pushed up against each other to form mountains, which is termed orogenesis. These older rocks have experienced deformation (forces associated with folding and faulting) which can be seen in their jumbled, twisted and sometimes broken layers. Hilly terrains where layering of the rock strata is no longer horizontal indicate some folding or faulting has occurred. The degree of deformation experienced during folding and faulting can be determined by the angles between the rock strata of the fold (termed the fold limbs –hence ‘interlimb angle’[9]). Folding and faulting may form cracks in the rock structure, enabling groundwater to permeate and discharge. The very hardest metamorphic rocks form barriers to groundwater movement (aquitards). To understand the degree of metamorphism it is essential to consult a geology map. Usually sedimentary rocks that are older than the Permian (Mesozoic era) have been metamorphosed (metasediments). Older volcanic rocks that are interbedded with these metasedimentary rocks, and of a similar age, are also metamorphosed (metavolcanics). Metavolcanic rocks are rocks that came into contact with an intrusive volcanic rock, becoming metamorphosed by the temperature and pressure. Metasedimentary rocks include phyllites, slates, quartzites marble and schist, and metavolcanics include greenstone, gneiss and serpentinite.

Igneous rocks Igneous rock refers to rock formed by magma or lava cooling and solidifying. Igneous rock includes those rocks that crystallize below the land surface (e.g. diority, gabbro, granite, etc.) and those that cool quickly at the land surface (e.g. andesite, basalt, rhyolite, tuff, etc.) and result from volcanic activity. The grain size of the rock depends how fast the rock cooled, which will determine whether water can pass through it, how big the grain size will be when it weathers into sediments and other properties. The chemical content of the rocks also determines factors such as pH and water-holding properties.

• Finer-grained igneous rocks have cooled close to the surface. Typically Cainozoic era igneous rocks have extruded from lava flows (plains, vents, plugs) and are included in land zone 8. Fine-grained igneous rocks can vary in pH, from acid to intermediate to basic, and includes basalts, andesites, rhyolites, trachites and tuffs, also gabbros and syenites. Landforms resulting from Cainozoic igneous rocks include lava flows as plateaux and cliffs, which may weather into columns and rounded boulders[9].
• Coarse-grained igneous rocks are large-crystalline rocks that have cooled slowly underground, usually buried deep at the base of a volcano. These rocks are included in Land Zone 12, along with older igneous rocks from the Mesozoic to Proterozoic era. Coarse grained igneous rocks are also associated with orogenesis, that is, they are very old mountains that are resistant to weathering (e.g. granite, adamelite, granodiorite, diorite, syenite, monzonite, gabbro and dolerite). Land Zone 12 also includes volcanic ash clouds that have formed into rock such as tuffs and ignimbrites.

Geology is a critical component of wetlands function, influencing essential wetland components and processes such as hydrological, geomorphological and biogeochemical processes and the cycling of water and elements such as nitrogen and minerals. A rock’s geology/lithology influences the degree to which groundwater can pass between pores and cracks in the rock, its ability to act as an aquifer, store water, and the permeability and drainage of soils derived from it. Geology influences the soil water and other soil properties.that are important for freshwater wetland function, including that of groundwater dependent ecosystems. Rocks that are aquitards form impenetrable barriers to water. Layering of aquifers and aquitards influence groundwater systems and the amount of freshwater influence in estuarine and marine systems. For more information on the different rock types and their role in groundwater dependent ecosystems[7][6].

Geology influences the shape and form of the rock itself, its chemical properties, hardness and how it decomposes, which in turn influences other attributes such as terrain morphology, roughness, substrate grain size/shape, and the degree to which water is stored and/or seeps through the rock itself, cracks in the rock, and how the rock weathers.

Sedimentary rock is reconstituted from weathered material and sediment, thus its components and processes are important for wetlands function, notably fine grained sedimentary rocks with their sodium, salt, silt and clay content.

Understanding the geology and lithology is fundamental background knowledge underpinning how wetland components and processes work in the wetland social-ecological system, and is prerequisite information required to apply the Whole-of-system, Values-Based Framework and the Aquatic Ecosystem Rehabilitation Framework. Collecting baseline geology and lithology information is the first critical step in the ‘Walking the Landscape’ process as water will move differently depending on the different geologies. Once this information is known, it is possible to understand wetland processes better as part of the whole system.

Geology/lithology underpins management of coastal, estuarine and marine (intertidal and subtidal) ecosystems – for example determining the properties of headlands and rock bars, coastal sediment transport erosion and deposition that are important considerations for coastal management.

References

1. ^ Allen, JD & Castillo, MM (2007), Stream ecology: structure and function of running waters, Springer, Dordrecht.
2. ^ (23 April 2019). 'biogenic rock'. [online] Available at: https://www.americangeosciences.org/word/biogenic-rock [Accessed 9 October 2023].
3. ^ Geoscience Australia (15 May 2014), Australian Landforms and their History. [online] Available at: https://www.ga.gov.au/scientific-topics/national-location-information/landforms/australian-landforms-and-their-history [Accessed 25 September 2020].
4. ^ Gürer, D (4 January 2021), Pinging in the new year: mapping the tasman and coral seas adding to the tectonic puzzle of the tasman sea. [online] Available at: https://schmidtocean.org/cruise-log-post/adding-to-the-tectonic-puzzle-of-the-tasman-sea/ [Accessed 18 October 2021].
5. ^ Hopley, D, Smithers, SG & Parnell, K (2007), The geomorphology of the Great Barrier Reef: development, diversity and change, Cambridge University Press.
6. ^ Queensland, , AU & o=The State of, Soil properties | Soil management. [online] Available at: https://www.qld.gov.au/environment/land/management/soil/soil-properties [Accessed 12 September 2023].
7. ^ Queensland, , AU & o=The State of, Soil water | Soil properties. [online] Available at: https://www.qld.gov.au/environment/land/management/soil/soil-properties/water [Accessed 12 September 2023].
8. ^ Sherman, CE, Locker, SD, Webster, JM & Weinstein, DK (2019), 'Geology and Geomorphology', in Y Loya, K A Puglise & T C L Bridge (eds), Mesophotic Coral Ecosystems. [online], vol. 12, Springer International Publishing, Cham, pp. 849-878. Available at: http://link.springer.com/10.1007/978-3-319-92735-0_44 [Accessed 1 January 2020].
9. ^ a b c d e f g Wilson, PR & Taylor, PM (2012), Land Zones of Queensland, pp. 79-pp., Queensland Herbarium, Queensland Department of Science, Information Technology, Innovation and the Arts, Brisbane.

Last updated: 18 October 2023