Ablation: The removal of ice and snow by melting, evaporation, wind erosion, sublimation (solid to vapour phase without an intervening liquid water phase), calving (glacial). Melting occurs in more temperate climates. Sublimation in cold, arid climates. Any rocky material dispersed in the ice/snow will concentrate on an ablation surface.
Abrasion: The mechanical wear and tear on sedimentary particles, commonly developed during transport where grain-to-grain impacts are common. Abrasion reduces particle grain size. It is an important mechanism that produces new and smaller sedimentary particles.
Actualistic models: Models based on the principle that natural processes and laws we witness today have acted in the past. This does not mean that the products of such processes, for example some environmental condition, will be the same today and in the distant past, but that the laws governing such processes will be the same. cf. Uniformitarianism
Aeolianite: Dune sands cemented by calcite are an example of shallow meteoric-vadose zone diagenesis. Dune sand mineralogy may be siliciclastic or bioclastic, or a mix of both. Most common in subtropical to tropical coastal dunes.
Aerobic conditions: Reactions that directly utilise available oxygen, the most obvious being respiration in life forms, where oxygen is used in metabolic reactions to generate energy (e.g., from food). In sediments this generally is associated with the metabolic activity of microbes – a distinction is made between these types of reaction and oxidation reactions that do not require intermediary metabolic activity in life forms. Cf. Anaerobic conditions.
Allochem: Framework components of granular or rudaceous limestones that show some evidence of transport or movement; i.e. they have not formed in situ. Common examples are ooids, oncoids, pellets, fossils, and intraclasts.
Allodapic limestone: Slope and deeper basin limestones deposited by turbidity currents.
Alluvial fan: Coarse-grained sediment bodies that are linked to elevated terrain where the rate of sediment supply and aggradation are controlled by tectonics, climate, and the size of the drainage basin, have broadly radial geometry with longitudinal and lateral extents measured in 100s of metres to a few kilometres, have high depositional slopes (several degrees), where sediment is delivered via a single, commonly canyon-like channel at the fan apex, and where sediment supply is episodic.
Alluvium: Sediment (clay to boulder size particles) deposited or reworked by water in a terrestrial setting; the most common forms are fluvial, alluvial fan, and lacustrine environment. Cf. Colluvium.
Anaerobic conditions: conditions where metabolic reactions in life forms do not require molecular oxygen. In sediments, such reactions are commonly generated by microbes that reduce oxygen-bearing compounds like sulphate (to sulphide), nitrate (to nitrite or ammonia), and carbon dioxide to methane. Sediment where these conditions persist tend to be green-black, and may have mineral sulphides (e.g., iron, manganese). One example is the sediment beneath wetlands, including marginal marine mangrove wetlands. Cf. aerobic conditions.
Anastomosing river: A river in which the channels are confined by heavily vegetated banks and floodplains, and within-channel islands also vegetated. The river may contain 2 or 3 sinuous channels but the overall sinuosity of the river is low. Bedload is commonly sandy, forming bars of tabular crossbeds and ripples. Cf. Meandering, braided rivers.
Angle of repose: The natural slope of loose, cohesionless sedimentary particles (sand, gravel) under static conditions, as a function of gravity and friction forces. In dry sand the angle is 34°. In water saturated sand where friction is reduced, the angle is 15° to 30°. It is analogous to the Angle of Internal Friction, a rock or material property that refers to its ability to resist deformation, and is measured as the angle between the normal stress and a resultant stress at the point where shear begins.
Angular velocity: For a rotating body, a measure of the rate of angular change. It is usually stated in radians per unit time, for example the angular velocity for Earth’s rotation about a north-south axis is 1.99 x 10-7 radians/second. The angular velocity at the equator is the same as that for the poles. cf. linear or tangential velocity.
Anisotropic HCS: Applies to hummocky cross-stratification where the geometry and dip of laminae change for profiles viewed at different orientations of the same hummock. Cf. isotropic HCS.
Anoxic conditions: Usually applied to aqueous environments (water masses as well as connate water) where there is none, or insufficient dissolved oxygen for respiration; usually measured at less than 0.5 ml/L. Under these conditions, the sources of oxygen via bacterial reduction are from nitrates and sulphates. Once these sources are depleted carbon dioxide becomes an important source during reduction to methane. Deep waters in lakes where there is no turnover of the water mass, can become anoxic. Anoxia is also implicated in some of Earth’s major extinctions, such as the Late Permian – Triassic event. Early Precambrian oceans and lakes were probably anoxic. e.g., März and Brumsack, 2015.
Anthropogenic: Processes and products produced by human activity that impact natural conditions and environments. There is frequently an emphasis on negative impacts, such as environmental degradation, loss of biodiversity, reduction of the gene-pool, and pollutants.
Antidunes: Bedforms that develop in Upper Flow Regime, Froude supercritical flow. The corresponding stationary (surface) waves are in-phase with the bedforms. Unlike ripples, the accreting bedform face grows upstream – antidunes migrate upstream in concert with deposition on the stoss face. When flow conditions wane, they become unstable and wash out or surge downstream. Their preservation potential is low.
Autotrophs: Organisms that derive energy from light or chemical reactions. Predominantly in the plant domain where the principal mechanism is photosynthesis. In the absence of light, chemotrophic organisms will obtain their energy and carbon for growth from chemical reactions with compounds such as sulphur and ammonia, or carbon dioxide. cf. heterotroph.
Avulsion: (fluvial geomorphology) The rapid abandoning of a channel at one location and formation of a new channel at another location. Avulsion may be forced by geomorphic factors like gradient advantage, floods, seismic events, or abrupt changes in baselevel. Cf. gradual channel migration.
Backwash: Water that completes its run-up across a beach (swash) and returns to the wave-surf zone. Flow velocities are determined primarily by the gravity component imposed by the beach gradient.
Bank-full conditions The point at which the water level in a river channel reaches the top of the bank, beyond which water spills over the floodplain.
Barrier island: Long, skinny, emergent sand bars that separate wave-dominated seas from a lagoon or estuary. Sand bars are commonly aligned in a linear or arcuate chain, each bar separated by a tidal channel that allows regular exchange of seawater between open seas and the enclosed bay. The channels and their ebb-flood tide deltas also help regulate sand supply. Barrier islands are commonly capped by coastal sand dunes. Wave set-up usually induces strong along-shore coastal currents. Barrier island retrogradation or progradation is strongly dependent on relative sea level change, accommodation space, and sand supply.
Beach: Obvious to most what this looks like – the narrow strip of land between mean high and low tides in marine settings, and the wave wash zone along lake shores. But from a sedimentological perspective it is the part of the coast, marine or lacustrine, that delineates the transition between land and water, marine and terrestrial. It is the zone where wave wash and backwash sorts sand and gravel according to the hydraulic potential of the waves, and where invertebrate and vertebrates have adapted to saline conditions and regular periodic exposure. It provides a stratigraphic datum for sea level change and shoreline excursions over geological time frames. It marks the fundamental boundary between marine or freshwater bodies and terrestrial environments.
Bedform: Sedimentary structures produced by bedload transport of loose, non-cohesive sediment. Typically manifested as ripple and dune-like structures.
Bedload: Loose or non-cohesive sediment particles (silt, sand, gravel – sizes) at the sediment-water or sediment-air interface, that will move along the bed if fluid flow velocities exceed the threshold velocity. The bedload consists of a traction carpet, and a suspension load.
Benthic: (adjective) An ecological term applied to organisms that live on a sediment-water interface, or within sediment. It includes invertebrates, vertebrates, and plants (particularly algae and cyanobacteria). The most prolific benthic zones are located within the photic zone that constrains the limits of photosynthesis.
Benthos: (noun) An assemblage of benthic organisms.
Bindstone: Consists of organically bound frameworks (not transported), such as encrusting algae or bryozoa, that bind some pre-existing substrate.
This term was introduced by Embry and Klovan (1971) as a modification of Dunham’s (1962) limestone classification scheme; see review and modification by Lockier and Junaibi (2016).
Bioimurration: The process where the skeletal or encrusting material (commonly calcium carbonate) overgrows another organism. The process has the potential to preserve fine details of the substrate structure – this is important where the substrate is easily biodegraded (e.g., plants).
Bioturbation: The general term for the activity of organisms that live on and within sediment. During the course of scavenging, grazing and burrowing for food, constructing a home, travelling from one place to another, or escaping predation or burial, these critters produce traces that reflect the type of sediment and the behavioural activity of the organisms. Intense bioturbation may destroy primary sedimentary structures like and bedforms.
Bouma sequence: Named after Arnold Bouma, one of the first to recognise the repetitive sedimentological organisation of turbidites. Bouma sequences represent individual turbidity current flow units, whether the sequence is complete or truncated. A complete sequence contains 5 divisions, becoming progressively finer-grained towards the top; some divisions may not develop:
- Massive muddy sandstone, with or without a scoured base.
- Graded and laminated muddy sandstone.
- Laminated with ripples and climbing ripples, commonly convoluted by soft sediment deformation.
- Graded, laminated siltstone-mudstone.
- A mix of turbidity current mud and hemipelagic mud, that are deposited from suspension.
Boundary layer (granular): Also called a no slip or zero shear stress boundary. The contact between a flowing fluid and a solid surface is defined by a boundary layer where friction forces reduce flow velocity to zero. A velocity profile through the boundary layer shows a gradual increase in velocity to the point where free stream flow prevails. Flow along boundary layers is either laminar or turbulent depending on the Reynolds number.
Boundstone: A kind of fall-back term for limestone description where the mode of binding is not readily identifiable. This term replaces Embry and Klovan’s Bafflestone in which the mode of binding and identification of the organisms responsible was equivocal. This term is introduced by Lockier and Junaibi (2016). in their review and modification of Dunham’s (1962) limestone classification.
Brackish conditions: Typical of environments where fresh water and seawater mix, such that the salinity is less than that of seawater. Commonly found in estuaries, particularly their more landward extents, and in the segments of coastal deltas prone to fresh water flushing (e.g., mouth bars, interdistributary bays). They are home to low-salinity tolerant plants (e.g., mangroves, Salicornia), and invertebrates like the air-breathing gastropod Amphibola.
Braided river: Low sinuosity braided rivers contain mostly sand and gravel bedload, and have multiple channels and bars that present a braided pattern. The bars contain a mix of tabular and trough crossbeds from beforms that migrate downstream during flood stages. The bar tops become dissected by chutes and rills during falling stage and low water.
Buoyancy: Buoyancy is the result of fluid forces acting on a body immersed in a fluid. If the resultant force is greater than the gravitational force acting on the body (that itself is a function of its density), then the body will rise (positive buoyancy – negative buoyancy is the opposite). Buoyancy plays an important role in many processes – the rise of mantle plumes and magmas, diapirism, density and temperature stratification in the oceans, the support of clasts in sediment gravity flows and pyroclastic flows.
Carbonate factory: A concept based on the recognition of geologically and geographically recurring facies and associated biotic and abiotic production systems. Definition of a factory is based on the kind of carbonate production. Four primary factories are: Tropical, where photosynthetic autotrophs are a critical energy source for heterotrophic frameworks (such as reefs); Cool-water dominated by hydrodynamically distributed heterotrophs; mud mounds dominated by biotic and abiotic precipitation of carbonate mud, either directly or indirectly by algae, bacteria, and cyanobacteria; and planktic where the primary producers are phytoplankton and zooplankton.
Carbonate mudstone: Dunham’s (1962) limestone classification, reviewed and modified by Lockier and Junaibi (2016). >90% mud-supported framework; <10% clasts larger than 2 mm (i.e. granule and larger). The equivalent Folk designation is micrite.
Carbonates: The most diverse group of sediments and sedimentary rocks, usually presented as limestones and dolostones. Carbonate precipitation (and dissolution) is based on the chemical equilibria involving CO2, HCO3–, CO32-, and H2CO3. Their primary mineralogy includes calcite and aragonite polymorphs (CaCO3), and dolomite (Ca.Mg [CO3]2). Carbonate formation at Earth’s surface is intimately associated with biological production where precipitation is either induced directly by organisms, or indirectly promoted by the activity and metabolism of organisms. Organisms involved in carbonate production range from microbial to large invertebrates.
Carbonate platform: Also called carbonate shelf. Thick successions of carbonate rock, that occupy shelf-like structures attached to continental landmasses, or as stand alone, isolated platforms surrounded by relatively deep ocean basins; also called carbonate banks. Heterotrophs and autotrophs contribute to carbonate production. Evaporites may form part of the stratigraphic succession in arid climates. The proximity to landmasses will determine the degree of mixing with siliciclastic sediment. Islands, banks and bars, and reefs generate significant relief across a platform. Platform-margin reefs mark the transition to slope and deep ocean basins.
Carbonate ramp: A platform-like region of carbonate accumulation that slopes gently seaward to a relatively deep basin. There are no significant margin builds such as reefs or mud mounds.
Cement: Precipitation of pore-filling minerals, such as quartz, calcite, aragonite, high-magnesium calcite, dolomite, clays, and gypsum, is an important process during sediment lithification. Crystal growth begins at grain boundaries, gradually filling the available pore space. Cementation can begin at the sea floor, particularly by aragonite and calcite, and continue during burial. Cementation gradually occludes effective porosity.
Chalk: Very fine-grained, white, porous carbonate rock composed almost entirely of phytoplankton tests, particularly coccoliths. Chalks tend to be pure carbonate but may also have other components such as chert nodules (flint). Iconic examples are the Cretaceous chalks of Europe and Britain (White Cliffs of Dover).
Chemical weathering: The breakdown of sediment and rock usually resulting from reactions with surface and ground water. The reactions can involve aqueous dissolution (carbonates are prone to this when exposed to slightly acidic rainwater), or by REDOX reactions involving metal ions like Fe and Mn. Dissolved material may be removed from the site by surface of groundwater flow, or it can precipitate as new compounds. Intense chemical weathering of igneous rocks commonly produces soils with deep red-brown colours. Karst landscapes are the product of surface or near-surface chemical weathering of limestone.
Chenier plain: The seaward part of a coastal plain or strand plain that consists of a series of beach ridges separated by mud flats or salt marshes. They form on prograding coasts. Ridges commonly consist of shells, sand, and small pebbles that accumulate under modest wave conditions and longshore drift currents. Chenier plains can be many kilometres wide, extending along shore for 10s of kilometres. The older, landward beach ridges may become vegetated.
Chute cutoff: Erosion through the inner or accretionary part of a river bend, that eventually forms a new channel. In meandering river systems the chute develops across the point bar. The former meander bend is abandoned and may eventually form an oxbow lake.
Chute and pool Chute and pool conditions usually develop at flow velocities higher than those responsible for unstable antidunes. Chute and pool morphology is centred on a hydraulic jump – upstream flow in the chute is supercritical, and immediately downstream flow is subcritical (the pool). Chutes and pools can also migrate upstream which means the hydraulic jump moves in tandem.
Clast-supported framework: This term applies to granular rocks where clasts are mostly in contact with one another. It usually refers to lithologies containing clasts that are sand sized and larger; it does not apply to mudstones or siltstones because it is difficult or impossible to distinguish framework from matrix. This textural property applies to siliciclastics and carbonates. Cf. matrix-supported framework.
Clay: This term has two meanings: (1) as a layered or sheet-like silicate mineral such as kaolinite and illite, and (2) as sediment with grain size less than 4 microns. See also Mud which consists of a clay-silt mix.
Coastal plain: A relatively flat, low relief coastal region commonly featuring barrier islands, lagoons, and estuarine drainage, coastal marshes and wetlands, drowned valleys, and chenier plains. Coastal plains exist because there is net, long-term progradation and shoreward migration of the shoreline, interrupted by transgressions.
Coastal setup: The increased elevation of sea level at the coast, where water masses pile up because of wind shear, and Ekman Veering of currents that flow at right angles to the wind direction (deflecting to the right in the northern hemisphere, and left in the southern hemisphere). The resulting seaward hydraulic gradient results in offshore-directed currents. Cf. storm surge.
Coastline: The boundary between land and a body of water. The term is commonly used to mean a relatively broad, loosely defined zone that can include steep or subdued land forms (e.g. cliffs, coastal dunes) as well as beaches. Cf. shoreline.
Coccoliths: Marine phytoplankton that secrete calcium carbonate skeletons; they are one of the main constituents in natural chalk. Coccospheres are algal cells surrounded by coccoliths arranged into spheres tubes and cup-shaped bodies, up to 100 microns in diameter. They are one of the culprits responsible for marine algal blooms.
Codiacean algae: A group of green algae that precipitate aragonite needles 2-3µm long. Two common species are Halimeda and Penicillus that, across carbonate platforms and reefs, produce large volumes of aragonite mud. Cf. coralline algae.
Cohesionless grains: Grains (usually sand or silt) that do not stick together. This property is necessary for most sandy bedforms to form. Cohesion in finer grained particles prevents the formation of sediment bedload and saltation load movement.
Colluvium: Sedimentary particles of any size that accumulate near the base of, or on lower slopes, by continuous or discontinuous surface runoff, sheet flood, soil and rock creep, and solifluction. Cf. Alluvium.
Combined flow Flow induced by wave orbitals operating in tandem with unidirectional, bottom-hugging flows, such as turbidity currents. Combined flow is frequently invoked to explain hummocky and swaley cross stratification, based to some extent on flume experiments, and observations of coastal flow.
Conglomerate: Sedimentary rock where the framework consists of clasts coarser than 2 mm (granule). Clasts show variable degrees of rounding and shape. Sorting tends to be poor. The term gravel is used for modern sediments. They typically represent high energy conditions like those found in braided rivers, alluvial fans, and gravel beaches. Cf. breccia, pebbly mudstone.
Consolidation: Is broadly synonymous with compaction of sediment that results in a loss of porosity and bulk volume. It is the main physical process involved in sediment diagenesis.
Continental rise: The bathymetric transition from continental slope to abyssal plain. Gradients are less then those of continental slope, merging with the deep basin beyond. Water depths are commonly >3000 m. Much of the rise are is made up of submarine fans that are fed by submarine canyons and gullies on the adjacent slope. Mass transport deposits derived from the slope generally move across the rise.
Continental shelf: The submarine extension of a continent. Shelf inclinations are generally <1o averaging about 0.1o . Water depths range from about 60 m to 200 m. Shelves and their environments are sensitive to sea level fluctuations. During low sea levels (e.g. during glaciations) the shorelines migrate seawards and the shelf thus exposed is subjected to weathering and fluvial erosion . A significant change in slope at their seaward margin is called the slope break – it marks the bathymetric transition to continental slope. It also corresponds to the transition from continental to oceanic crust.
Continental slope: The bathymetric region beyond the shelf and shelf break, extending from about 100m to 3000 m, with gradients between 2o – 5o . Slopes are commonly transected by gullies and submarine canyons that focus sediment transport, some of which remains on the slope (finer-grained sediment), and some bypassing the slope on its way to the basin beyond; in this case sediment transport is commonly via turbidity currents and other types of sediment gravity flow. Gravitational failure also shapes the slope. Hemipelagic sediment is important to slope accumulations.
Convoluted laminae: Laminae that are initially parallel or crossbedded, will become folded and pulled apart during the early stages of compaction (soon after deposition) and dewatering. They are characteristic of turbidites where dewatering is hindered by muddy permeability barriers, such that local fluid pressures are elevated. They are also common in fluvial and other channelised sediments (here called ball and pillow structures).
Cool-water limestone: Predominantly bioclastic limestones typically made up of bryozoa, various molluscs, brachiopods, calcareous algae, barnacles, and echinoderms. Isopachous, micritic, and pore-filling cements are mostly calcite; aragonite cement is uncommon.
Coquina: A limestone made up of shells, shell fragments and other bioclasts, with a degree of sorting that indicates relatively high depositional energy. Where the fragments are mostly sand-sized, the Dunham limestone classification equivalent is grainstone.
Coralline algae: Calcite and high magnesium calcite precipitating red algae, that build upon substrates such as bioclasts and rock surfaces and other algae. All begin life as encrusters, but grow to different forms such as articulated, flexible, bush-like branches, or nodular clusters around shells or pebbles (e.g. Lithothamnion). They are an important contributor to cool-water bioclastic limestones. Both types contribute to temperate and tropical carbonate sediment. They are important components of coral and bryozoan reefs.
Coriolis effects: The result of (fictitious) Coriolis forces apply to rotating, non-inertial systems like Earth. The forces act orthogonal to the direction of movement such that deflections are to the right of the direction of forward motion in the northern hemisphere, and to the left in the southern hemisphere. Coriolis forces are directly proportional to linear velocity on the same rotating body. Coriolis effects increase towards the poles of rotation and are zero at the equator. The deflections apply to ocean water masses (gyres), contourites, and to weather systems.
Crevasse splay: A crudely fan-shaped body of sediment deposited on the flood plain when a river in flood breaks through its levee. The sediment is mostly fine sand and silt. Ripples and climbing ripples tend to form close to the levee breach where flow velocities are highest; erosional discordances are also common. Flow competence wanes rapidly as the flood waters splay across the floodplain, depositing progressively finer-grained sediment.
Critical flow: Also called Tranquil flow. The flow conditions for a Froude number of 1 , at some critical flow velocity and flow depth, where any surface wave will remain stationary (it will not move upstream or downstream). Surface waves will usually be in-phase with their bedforms, for example antidunes. See also subcritical and supercritical flows.
Critical shear stress: see Threshold shear stress for grain movement.
Cryptalgal laminates: A general term for laminated mats composed primarily of cyanobacteria, but like includes other microbes. The laminates may be flat and uniform, or tufted, pustulose, or polygonal, resulting from desiccation or, in arid environments, evaporite precipitation. In the rock record they are commonly found with stromatolites. The term microbialite is generally used in modern examples because there are several groups of microbes including bacteria, cycanobacteria, and red and green algae.
Cut bank: An outside river bank subjected to erosion. In meandering fluvial channels, cut banks are located opposite point bars (the inside channel margin on which deposition occurs). Channels tend to be deepest along the cut bank margin.
Cyanobacteria: Microscopic, single cell or colonial, prokaryotic organisms that today are aquatic and photosynthetic. They are likely the first known photosynthetic organisms on Earth, and were the primary builders of stromatolites and cryptalgal laminates (or microbialites) the oldest being about 3.4 Ga; as such they were responsible for producing free (molecular) oxygen in Earth’s ancient atmosphere. Precambrian fossil microbes, best preserved in cherts, are an assortment of filaments and coccoid colonies.
Cyclic steps Cyclic steps are basically trains of chutes and pools, where supercritical to subcritical transitions occur repeatedly downstream. At each transition there is a hydraulic jump – this is the step in each flow transition. As the hydraulic jumps move upstream they erode sediment that is then deposited on the stoss face immediately downstream. The wavelength of cyclic steps is potentially 100-500 times the water depth, and is significantly greater than that for stationary waves and their associated antidunes.
Debris flow: A type of sediment gravity flow containing highly variable proportions of mud, sand, and gravel, in which the two primary mechanisms for maintaining clast support are (mud) matrix strength (a function of viscosity) and dispersive pressures caused by clast collisions. Rheologically they behave as (non-Newtonian) plastics or hydroplastics. Unlike turbidites, there is no turbulence, hence normal grading is absent or poorly developed. Some debris flows develop significant internal shear that imparts a crude stratification and/or an alignment of clasts. Terrestrial flows include highly mobile mud flows, and lahars in volcanic terrains. The more mobile types may grade to hyperconcentrated flows
Deep water waves: Waves that do not interact with the sea floor. This applies to open ocean wind-driven waves, the speed of which depends only on the ratio of wavelength to wave period. Deep-water waves occur where water depth is greater than half the wavelength. Cf. shallow water waves.
Delta front: A general description of delta components, or subenvironments, at and beyond the mouth of distributary channels and the coastal margin, including distributary mouth bars and prodelta.
Delta plain: The portion of a delta that is transitional between fluvial and delta front environments. It is a low-gradient area that contains distributary channels, and overbank regions that include vegetated swamps, marshes, and ponded areas. It also includes interdistributary bays.
Depositional dip: Corresponds to the maximum slope of a depositional surface, normal to depositional strike.
Depositional environment: The physical, chemical, and biological conditions in which sediment is deposited or precipitated.
Depositional episode: Introduced by D. Frazier (1974) working on Gulf Coast stratigraphy. They are basically cyclic repetitions of strata packages that begin with sedimentary facies deposited as a prograding succession, and end with transgression. Cf. Genetic sequence.
Depositional system: A 3-dimensional assemblage of genetically related environments (in modern systems), and lithofacies in ancient systems. As an example, modern and ancient deltas contain distributary channels, delta plains, crevasse splays, beaches, bars, and prodelta slopes. All these environments are spatially and environmentally distinct and yet they are dependent, one on the other. Together they form a delta depositional system.
Deserts: Regions that receive less than 250 mm of precipitation a year and are generally in continuous moisture deficit. Whatever life forms live in these environments have adapted to the harsh conditions. Most modern hot deserts are located between the Tropics of Cancer and Capricorn that are bathed by the trade winds. The main cold deserts are at the two poles. There are also mid-latitude deserts (Sonoran in USA, Tengger in China) a,d coastal deserts such as Atacama that commonly receives <1mm rain a year.
Desert varnish: A coating of clays, iron-manganese oxides and amorphous silica that produce black to reddish hues on the surface of bedrock and sediment particles that are exposed for long periods in arid desert environments. Coatings are only a few microns thick.
Desiccation: The drying of sediment during subaerial exposure. In muddy sediment, the process commonly results in shrinkage and formation of mud cracks, or desiccation cracks. Cf. synaeresis.
Dewatering: This is the process where interstitial fluids are ‘squeezed’ from sediment during compaction, as sedimentary grains become more closely packed. The process of dewatering increases fluid pressures and promotes fluid flow in aquifer-like deposits. Fluid escape my be diffuse, or focused through narrow pipes and sheets. It is an important stage of mechanical diagenesis, but it also contributes to chemical diagenesis by transferring dissolved mass from one part of the sedimentary column to another. Cf. liquefaction, fluidization, fluid escape structures
Diamictite: Although the term is commonly used to describe glacial deposits, it more generally refers to extremely poorly sorted deposits in which there has been negligible reworking, containing angular clasts ranging in size from clay to boulders. In glacial depositional systems, they are they form from ablation of ice in lateral, terminal and medial moraines. Clast composition may be quite variable depending on changes in bedrock composition along the path of glacier flow.
Diapir: A buoyant, mobile body acting as a fluid that intrudes to shallower levels of the crust. Salt diapirs are common, but the process also occurs with mudstones and magmas. Positive buoyancy occurs when fluid forces acting on the body exceed the gravitational forces. Diapirism in salt produces many kinds of intrusive geometries, from dome-shaped, to laterally extensive walls, sheets, and salt-cored anticlines. During intrusion the stress on the surrounding strata is accommodated by faulting and folding. Salt diapirism results in salt withdrawal from stratiform evaporites at depth imposing a kind of supply and demand limit to the size and number of diapirs that might be generated from a particular evaporite unit. The increasing overburden load plays a critical role in initiating salt instability (buoyancy disparities) and diapir rise.
Dispersive pressure Pressures developed by clast collisions. Dispersive pressures are one of the main mechanisms that support non-turbulent sediment gravity flows such as dilute debris flows, and dilute pyroclastic density currents such as pyroclastic surges. They tend to be more important in flows where matrix viscosity and matrix strength are low or have been reduced by ingestion of fluid.
Distributary channel: Channel systems on a delta plain that represent the transition from fluvial to the delta front. Channels may be straight to sinuous, single or multiple. They commonly are contained by levees. Sediment within the channels tends to be sandy, and bedforms are typically those of other sandy fluvial channels. In marine delta systems, a tidal signal will extend some way upstream, depending on channel gradient and flow competence.
Distributary mouth bar: Sand-dominated subaqueous bars and platforms that form at the coastal outlets of distributary channels where there is an abrupt decrease in flow velocity. The coarsest sediment will be deposited close to channel mouths (outlets), with finer-grained material moved farther offshore.
Diurnal tides In areas where coastline shape and bathymetry interfere with the normal semidiurnal cycle, the tides become diurnal – one flood and one ebb tide in 24 hours.
Draa: The largest aeolian dune bedform that can be as high as 300 m and several kilometres long. They are usually compound structures consisting of smaller, amalgamated and superposed aeolian bedforms. Classic examples are found in the Sahara Desert. They are also found on Mars. Named after Draa Valley in Morocco.
Drowned valley: Drowning of coastal river valley systems during transgression results in a highly embayed coastline dotted with islands. Estuaries develop where the landward extent of transgression pushes tidal influences and the saline wedge up river channels. Here’s an example.
Ebb tide: The outgoing tide. Cf. flood tide
Ebb tidal delta: Delta-like platforms that accumulate at the seaward limit of tidal channels that drain harbours, bays and lagoons. Strong ebb tidal currents carry sand from the embayment; sand is also derived from the adjacent beach, shoreface and shelf. The delta platform is modified by transverse waves. Part of the platform may be exposed at low tide. Cf. Flood tidal deltas.
Eelgrass: The common name for thin bladed seagrasses like Zostera and Posidonia.
Ekman spirals – Ekman veering: The Coriolis deflection in the uppermost ocean waters is about 45o. Friction between this layer and waters of lower velocity immediately beneath it results in the second layer being dragged in the same direction, although the deflection is less because of energy losses. This process is repeated for deeper waters to depths of about 100-200 m. The result is a kind of deflection spiral, called an Ekman spiral (also referred to as Ekman veering) – named after Vagn Walfrid Ekman (Sweden, 1902). The actual depth of Ekman veering depends on wind strength. The net effect is a deflection of current flow about 90o to the wind direction – veering to the right in the northern hemisphere and to the left in the southern hemisphere.
Elutriation: Removal of fine particles by the upward flow of fluid or gas, through the body of a pyroclastic density current or sediment gravity flow. Elutriation is responsible for the development of a buoyant plume above such flows.
Endolithic algae: Eukaryotic algae that live in micropores of skeletons and shelly material, and in pore throats of granular sediment. They are capable of dissolving calcium carbonate and promoting early diagenesis or weakening organic structures that leads to their fragmentation. They also play a role in micritisation of bioclasts. The term also applies to endolithic fungi and bacteria.
Endorheic lake: A water body that has no surface outflow drainage, and is surrounded by drainage divides. In most cases inflow from surface runoff and groundwater discharge is balanced or exceeded by evaporation.
Ephemeral: An event that is short-lived, transitory, here one minute and gone the next. Such events may appear ephemeral because they have low preservation potential in the rock record. Sedimentological examples are flash floods, hurricanes (from a geological perspective), bedforms like antidunes or rain-drop impressions, student examinations.
Epifauna: Marine and non-marine benthic organisms that live on a substrate – the sediment-water interface, shells, aquatic plants, other organisms. They may be permanently attached (e.g., barnacles, forams, calcareous algae), or mobile (e.g., gastropods, many bivalves, forams, ostracods).
Epiflora: Marine and non-marine benthic plants that live on a substrate – the sediment-water interface, shells, aquatic plants, other organisms. Common examples include macroalgae, calcareous red and green algae.
Estuary: An inland arm of the sea that is linked to terrestrial drainage and is influenced by tides. In map view they are commonly funnel-shaped, broadest at their seaward margins. Estuaries are common in regions where a rise in relative sea level has drowned coastal valleys. Hence, they are part of, and merge into bays, harbours, and lagoons. They are commonly protected by barrier islands, spits, and bars. The influence of tides can extend 80-100 km inland although this does not mean the salt wedge extends that far. Saline and brackish conditions have a strong influence on biological activity. Tidal ranges vary from place to place; In the Bay of Fundy tides and tidal bores are as high as 14 m. They are common habitats for mangroves and salt marshes.
Euhaline Aquatic systems with salinity of 30.0-40 parts per thousand derived primarily from marine salts.
Euxinic conditions: Ocean waters that are depleted in dissolved oxygen (anoxic) and are sulphidic. The sulphide is primarily dissolved H2S. Euxinia can occur in highly stratified water bodies, such as lakes and enclosed seas where there may be an the anoxic layer occurs beneath shallower waters with varying amounts of dissolved oxygen. However, euxinia may also have occurred in larger oceanic water masses in the geological past.
Excess weight forces: A term introduced by Myrow and Southard (1996) for the density-enhanced mass of suspended sediment in the water column. Thus, these forces tend to act downslope (seaward) and contribute to the distribution of sediment across a shelf or delta during storms.
Exfoliation: A surface process where weathered layers peel from the rock surface. Layers usually conform to the exposed rock surface. Spheroidal weathering is one type of exfoliation that appears onion-like. The process involves a combination of chemical dissolution and mechanical removal of the weakened layer.
Facies: Sedimentary facies are descriptions that encapsulate the essential physical, biological, and chemical attributes of rocks and sediments, at whatever scale an observer chooses (e.g. single beds, or groups of beds); facies reflect the conditions in which they formed. Amanz Gressly (1836) originally defined facies to reflect objective descriptions; this purpose is still regarded as important. However, modern usage commonly adds a genetic reference, such as tidal flat facies. Experience shows that many facies repeat through geological time. This is an important attribute because it provides us with a sound basis for interpreting sedimentary rocks and ancient environments. See Facies associations; Facies models.
Facies association: Sedimentary facies that occur together, forming associations that are repeated in time and place (e.g. different sedimentary basins). For example, facies that describe fluvial overbank deposits will be associated with facies that define fluvial channels, swamps, peat bogs, paleosols, and oxbow lakes. It is these associations that provide the real clues to interpreting paleoenvironments.
Facies models: Facies models are simplified descriptions of a complex sedimentary universe, a scaled-down version of a depositional systems like submarine fans, or high sinuosity fluvial channels. They contain facies and facies associations visualised in the context of a theoretical framework of processes. Models allow us to visualize and interpret our observations within an established framework – that framework may be mathematical, conceptual, or empirical. Models allow us to predict outcomes where direct observations or measurements are not possible.
Fairweather wave base: The maximum depth at which wave orbitals impinge the sea floor and sustain sediment movement, during normal fair weather. The actual depth is about half the wavelength. Cf. Storm wavebase.
Fall-line: The line where coastal plain deposits onlap rocky hinterlands, plateaus, and piedmonts. They are characterised by a change in relief and slope between the bedrock terrain and adjacent gently sloping coastal plains. The changes in relief are commonly presented as narrow bands of waterfalls and rapids along rivers that transect both geomorphic regions. An iconic example is located along the eastern United States seaboard, where a fall line exists between the Appalachian piedmont (west) and the Atlantic coastal plain, and extends about 1400 km along strike from New York to Georgia.
Fan delta: Fan deltas are like alluvial fans except they dip their toes in lakes and shallow seas. So, in addition to the alluvial component, there is subaqueous deposition down a relatively steep, angle-of-repose slope. Large, basinward-dipping foresets are a defining characteristic. They are generally coarse-grained. Fluvial distributary systems tend to be braided.
Fetch: The distance the wind travels over open water. A large fetch usually means larger, longer period waves. Fetch is an important consideration for studies of coastal wave dynamics.
Firn: Snow that is at least one season old and has undergone some compaction. It is much less dense than glacier ice, but more dense than névé. Firn transforms to glacier ice during subsequent burial.
Floodplain: The flat area extending outward from the river channel margins that is frequently inundated during flood events. Floodplains are commonly vegetated and can have well developed soils. They tend to be fertile regions that historically have been the focus of agriculture. They are also popular places to live which for obvious reasons is frequently problematic.
Flood tidal delta: A delta-like platform that accumulates on the inward part of tidal channels at the entrance to harbours, bays and lagoons. Strong flood tidal currents carry sand from the beach, shoreface, and shelf and channels that drain the embayment. Cf. Ebb tidal delta.
Flow regime: A useful model for deciphering the hydraulic conditions of deposition and bedforms for unidirectional flow, introduced by Harms and Fahnstock, 1965. The model partitions bedforms according to flow velocity and the configuration of surface waves. There are two fundamental types of flow:
- Lower Flow Regime – at the lowest flows laminated sand, and with increasing velocity, a transition from ripples to larger dune bedforms. For the latter, the surface waves are out of phase with the bedforms.
- Upper Flow regime – includes parallel laminated sand (the type that produces parting lineations), and at higher velocities, antidunes (where the surface waves, or standing waves are in-phase with the bedforms), and chute and pool. A hydraulic jump forms when Upper Flow weakens to Lower Flow regime.
Flow separation (granular): At high Re values the boundary layer detaches from the particle/grain surface at the point where the solid surface curves away from the direction of flow; the boundary layer is no longer attached on the downflow side of the particle. Incipient flow separation probably begins at Re values <500 and is fully developed where turbulence dominates.
Fluid drag force: Objects that move through a fluid experience opposing frictional forces that are caused primarily by fluid viscosity, and properties of the object such as size, shape, and surface roughness. Estimation of drag force magnitude is an important part of sediment transport modelling, for example estimating sedimentation rates in oceans, lakes, and volcanic eruption columns. An important solution to the drag force problem was developed by George Stokes (1851), now known as Stokes Law. Drag is also an important component of the forces that initiates and maintains movement of a grain as bedload or suspension load.
Fluid lift force: The force acting on a grain in a flowing fluid that provides a vertical component of lift. Lift forces develop because flow velocity across the grain boundary is lower than that higher above the bed – the velocity difference creates an upward directed pressure gradient. Lift forces can also develop in turbulent flow.
Fluidization: The process where sedimentary particles are suspended, or float in the interstitial fluid by the upward flow of fluid. In contrast, the fluid in a liquefied sediment is largely static. Fluidization in sediment may be caused by escaping, overpressured fluids (dewatering).
Framestone: A limestone composed of in situ frameworks build by organisms (i.e. not transported). Common examples include corals, stromatoporoids, and oysters. The matrix between framework components should be described separately.
This term was introduced by Embry and Klovan (1971) as a modification of Dunham’s (1962) limestone classification scheme; see review and modification by Lockier and Junaibi (2016).
Free stream flow: In sedimentology, the flow through a water column, between the granular or bed flow boundary and the water surface. Flow is characterised as laminar or turbulent. Flow velocity is commonly quoted as an average over a certain depth.
Froude number: A dimensionless number that expresses the characteristics of flow, including surface waves and bedforms, as the ratio between gravitational forces and inertial forces:
Fr = V/√g.D
Where V is bulk flow velocity that reflects the dominant effect of gravity on surface flows, and the inertial component is √g.D where g is the gravitational constant, and D is water depth. The denominator represents the speed of a surface wave relative to the bulk flow velocity. Whether the surface wave is faster, slower or the same speed as the bulk flow will depend on its resistance to move, or its inertia.
Geostrophic flow: Generally considered for air or water flow that parallels lines or contours of hydraulic pressure or air pressure (isobars), where there is a balance between Coriolis forces and pressure forces. In the oceans it is a product of Coriolis deflections and Ekman current veering.
Gilbert delta: Originally described by G. Gilbert for coarse-grained deltas that display a 3-fold architecture: horizontal to shallow dipping topset beds (analogous to a delta plain), foresets beds, and bottom set beds. They form where coarse bedload rivers empty into lakes and marine basins. They are included in the general category of fan deltas.
Glacial outwash: Deposits, usually coarse-grained, deposited downstream of glacier ice fronts by fluvial processes. Streams are commonly braided. Outwash streams may be linked to subglacial channels. Small outwash fans may also form where subglacial streams exit the ice. Outwash streams and fans may drain into or from lakes.
Glaciofluvial: A broad term that includes a variety of fluvial environments and processes associated with glaciers, ice caps and ice sheets. This includes subglacial and other ice contact deposits (such as eskers), as well as outwash streams originating at ice fronts. Most are coarse-grained.
Glaciolacustrine: Lakes that form from glacier or ice cap meltwaters, and receive glacial outwash sediment. Lakes may be located in antecedent drainage lows, or damming of outwash streams by ice or landslides. Coarse-grained deposits will form as beaches or small deltas (e.g. Gilbert-type deltas). Mud carried by outwash streams will settle in the lower energy parts of lakes. Lake varves are a characteristic product of seasonal freeze-thaw.
Grain flow: Sediment gravity flows consisting mostly of sand, in which the primary mechanism of grain support are dispersive pressures generated by grain-to-grain collisions. Maintenance of grain flows requires relatively steep slopes compared with debris flows and turbidity currents.
Grainstone: The cousin to siliciclastic sandstones, where sand-sized carbonate particles (<2 mm) form a grain-supported framework, relatively free of or carbonate mud (micrite).Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).
Gravel The unconsolidated equivalent of conglomerate, composed of varying proportions of pebbles, cobbles, and boulders.
Hemipelagic sediment: Very fine-grained siliciclastic sediment (clays, silt) that is deposited from suspension in the ocean water column; it may be mixed with pelagic sediment. Hemipelagite tends to accumulate in relatively deep-water slope, rise, and ocean basins remote from strong bottom currents.
Heterotroph: An organism that requires the assistance of other organisms to generate energy and food. In other words, they eat other heterotrophs and autotrophs. The group includes omnivores, herbivores, carnivores, and critters or plants that use decomposition processes. cf. autotroph.
Hjulström diagram: Filip Hjulström’s iconic, empirically derived graph of fluid flow velocity against grain size, that shows the domain where grain movement is initiated, and the domain where there velocities are not sufficient to move grains. The graph encompasses grain sizes from clay to cobble. Both variables in this graph are dimensional – cf. Shields the diagram where shear stress and Reynolds Number are non-dimensional.
Homopycnal flow: Homopycnal flows form when the density of riverine water masses that flow into a water body, is about the same as that of the receiving water body (i.e., the density contrast approaches zero). The momentum of the plume diminishes abruptly and most of the sediment accumulates in delta-like mouth bars, the adjacent delta slope, or Gilbert delta foresets. Bedload movement beneath the plume can form various dune bedforms.
Hummocky cross stratification (HCS) On bedding they present as low amplitude mounds adjacent to dish-shaped depressions, or swales. Mounds are approximately equidimensional to slightly asymmetric in map view. Mound spacing ranges up to 5-6 m. In cross section they are found in sharp-based, fine- to medium-grained sandstone beds. In cross-section, hummocks are composed of sandstone laminae a few millimetres thick, shaped to conform to the mound (or swale) surfaces – i.e., the laminae are continuous from trough to apex, and again to trough. Cross laminae dips are generally less than 15°. They represent deposition between fairweather and storm wavebase. The popular hypothesis is that they form during combined oscillatory flow (generated by storm waves) and unidirectional, possibly turbidity current flow.
Hurricane: A tropical cyclone that has sustained wind speeds of 119 km/hr (74 miles/hr) and more. The term is reserved for northern hemisphere storms east of the International Dateline (Greenwich Meridian). Hurricane strength is categorized in the Saffir-Simpson Hurricane Wind Scale: 1 119-153 km/h, 2 154-177 km/h, 3 178-208 km/h, 4 209-251 km/h, 5 252 km/h or higher. cf. Typhoon.
Hydraulic jump: A region of turbulence and an increase in water depth that develops in channels when Froude supercritical (Upper Flow Regime) conditions slow to subcritical conditions (tranquil, Lower Flow Regime).
Hydraulics: The study of fluids at rest or in motion, and for the latter the conditions promoting flow in water, air, and sediment-water mixtures, and the processes of sediment movement and deposition. Involves consideration of flow velocity, turbulence, laminar flow, frictional drag, and shear stress. cf. Hydrodynamics.
Hydrodynamics: The study of fluids in motion and their interactions with solid particles – a more specific branch of hydraulics.
Hydroperiod: The duration of tidal flooding and inundation over a salt marsh – flooding only occurs during spring tides and storm surges.
Hydroplaning A term applied to sediment gravity flows and dilute pyroclastic density currents – where the head of these flows lifts above the substrate. Flume experiments show that a layer of water/fluid beneath the flow can reduce drag, such that the flow head rises and in doing so increases its velocity. If the velocity increases is sufficient, the head can detach (at least temporarily) from the main body of the flow. This mechanism offers one explanation for surging at the head of many flows.
Hyperconcentrated flow: Sediment laden flows that behave mechanically between two end-member flow types: normal stream flow with little or no suspended sediment load, and debris flows having high matrix content. Hyperconcentrated flows have no yield strength, like water, but do have a viscosity that depends on strain rate. Rheologically, they behave somewhere between a Newtonian fluid and a plastic (or hydroplastic). A typical example is a mud-laden river flood.
Hyperpycnal flow: A hyperpycnal flow develops when the density of a flood-derived fluvial plume is greater than that of the receiving lacustrine or marine water body. Sediment on the dense plume (freshwater plus sediment) will plunge towards the sea or lake bed forming a bottom-hugging, turbidity current.
Hyperpycnite: The deposit resulting from a hyperpycnal flow. A range of lithofacies and sedimentary structures have been proposed, ranging from normal graded turbidites, reverse then normal grading, or non-graded fine-sandstone beds, to full-blown Bouma or partial Bouma sequences and debris flows.
Hypersaline: Having salinity greater than seawater (>35 parts/1000). Modern hypersaline environments are most common between the tropics but are found in such diverse places as the Antarctic dry valleys. Plant and animal life require specialized adaptations to survive these conditions. Prolonged hypersalinity may result in evaporite deposits in lakes and seas.
Hypopycnal flow: A river generated flow that forms when plume density is less than the lake or sea; the plume is buoyant and will tend to disperse across the top of the water body. Coarsest sediment will fall rapidly out of suspension close to the river mouth forming mouth bars, and finer-grained sediment progressively farther from shore – the latter will form laminated hemipelagites or prodelta deposits. Hypopycnal plumes can extend several 10s of kilometres from their river mouths. They can also be deflected by wind and tide currents.
Ichnology: The study of trace fossils, the behaviour of the critters that made them, the environment they lived, fed, escaped, and traveled in, and their relationship with other sedimentary facies and stratigraphic surfaces. See Lebenspurren.
Ignitive turbidity current: Refers to sediment gravity flows, principally turbidity currents, that form from pre-existing deposits, and are triggered by processes such as slope failure, seismicity, and canyon-margin collapse, or transform from debris flows. Ignition in this sense means flow acceleration and entrainment of sediment that produces what G.Parker (1982) referred to as a “self-sustaining turbidity current.
Incised channel: A geomorphic term normally applied to channels that have been deepened by relatively rapid changes in base-level (rapid sea level fall, tectonic uplift), such that the original channel shape is preserved (e.g., incised meandering fluvial channel). The term applies to fully fluvial and to estuarine channels.
Inertia: Inertia is generally defined as a force that resists the change in motion of a body; here motion refers to a vector that describes velocity and direction, and ‘body’ refers to anything composed of matter, including a body of fluid. Inertia was codified by Newton in his Laws of Motion – in the 1st Law as the Law of Inertia, and in the 3rd, as the Action-Reaction law. Inertial forces are central to the quantification of fluid mechanics expressed in Froude and Reynolds numbers.
Infauna: Marine and non-marine benthic organisms that live or feed within sediment, usually the upper few centimetres below the sediment-water interface. Common examples include molluscs and crustaceans. Infaunal activity produces bioturbation.
Intertidal: Literally means between tides. It is the region above mean low tide, and below mean high tide. Its morphology is that of a beach, tidal flat, and tidal or estuarine channel. Seaward is the subtidal zone (rarely exposed) that includes the shoreface; also called the littoral zone. Landward is the supratidal zone.
Isobar: Contour lines connecting points of equal air pressure. If wind flow is geostrophic then the air mass flows parallel to the isobars.
Isopach: A contour that delineates a sedimentary, volcanic or volcaniclastic unit thickness, either as a single bed or succession of beds. Unit thickness is measured directly in the field, from core or borehole logs (gamma and SP logs are commonly used to do this), or from seismic reflection traces. Isopachs are used to map thickness trends.
Isotropic HCS: Applies to hummocky cross-stratification where the geometry and dip of laminae are the same for profiles viewed at different orientations of the same hummock. Cf. non-isotropic HCS.
Jacob’s Staff: A measuring stick with an inclinometer at one end, that is used to measure directly true stratigraphic thickness in dipping beds. Use as a surveying instrument dates to Medieval times.
Jet flow (river mouth): Turbulent riverine flow that extends as a relatively narrow band beyond the river mouth into a receiving water body. The initial dimensions of the jet will be those of the channel. The distance the jet will flow depends on the density contrast between the river water and receiving basin water (including the suspended sediment load), and wind, wave and current conditions offshore. River jets are the primary mechanism for generation of hypopycnal, homopycnal, and hyperpycnal flows.
Jökulhaup: (also Jökulhlaup) An outburst from a glacial lake, commonly caused by failure of natural ice dams, but can also occur during rapid ice and snow melting during volcanic eruption, including subglacial eruptions. They can have catastrophic consequences. Water flows during an outburst are capable of carrying large blocks of rock and ice. From Icelandic jökull meaning glacier, and hlaup meaning flood.
Lagoon: A shallow bay protected from ocean swells, and to some extent storms, by barrier islands, spits, and bars. Extensive tidal flats commonly border the landward margins of lagoons, crossed by estuaries and small tidal channels. In tropical and temperate climates, mangrove swamps provide breeding grounds for all manner of critters. There is regular tidal exchange of ocean seawater through large channels; delta platforms at the inner or seaward channel exits are called flood- and ebb-tidal deltas respectively. There is a continuous exchange of sediment (mostly sand) between the lagoon, barrier and coastal dunes, and the open sea shelf.
Laminar flow: Defined and quantified by Osbourne Reynolds, laminar flow is described conceptually as flow lines that are parallel, or approximately so, and relatively straight. The flow velocity will be the same across each flow line. Expressed in terms of Reynolds numbers (Re), it is the flow condition when Re < 2000. The transition to turbulent flow is usually abrupt.
Lateral moraine: The accumulation of rocky debris at the surface of glacier margins, derived by scraping and erosion of adjacent bedrock valley walls. They form as parallel ridges in the ablation zone of glacier margins. As deposits they are characteristically a very poorly sorted mix of rock flour to boulder size, angular clasts.
Levee: Natural levees are linear, mound-like deposits that accumulate along the banks of many fluvial, delta distributary, tidal, and submarine channels; they act as a partition between an active channel and adjacent floodplain. Deposition occurs during channel flooding. If levee accretion is significant it may prevent regular flood plain inundation. Rippled and laminated fine-grained sand tend to be deposited during rising flood stages, and silt-mud veneers during waning flow stages. In some cases, vegetation will stabilize the levee, and dampen overbank flow to the floodplain. Levees that rim submarine channels accumulate during the passage of turbidity currents. See crevasse splay.
Liquefaction: If water-saturated sediment is disturbed, for example by earthquake ground shaking, the grains begin to separate until they are ‘floating’ in the interstitial water. At this point, the fluid now consists not only of water but also the floating grains and a consequence of this is that fluid pressures increase. The sand is now liquefied. It no longer has shear strength and cannot support surface loads. Eventually the grains will settle and at this point the excess water will escape to the surface. Cf. dewatering, fluidization, sand volcanoes.
Littoral zone: The nearshore region of marine and lacustrine environments. In the marine setting it extends from high tide to shallow offshore depths. The term is used primarily to designate ecological environments for diverse marine organisms.
Littoral drift: Synonymous with longshore drift.
Longshore drift: Drift of water masses, sediment, and swimmers occurs when waves approach a beach at an angle. Here, water moving up the beach (wave swash) returns farther along the beach. Longshore drift (or along shore drift) is an important coastal process that contributes to coastline straightening by sand bars, and to the formation of sand spits and barrier islands. See undertow, rip currents.
Mangrove: Flowering shrubs and small trees that are salt tolerant, living on sandy and muddy tidal flats and salt marshes. Most common between the subtropics but do extend beyond these latitudinal limits. They deal with salt uptake by excreting it from their leaves. They have complex root systems that help stability under conditions of shifting sediment and tides. Mangroves provide important habitats and breeding grounds, and also help protect coasts from storm wave surges and erosion. See also paralic, marsh.
Marsh: A wetland dominated by herbaceous plants, that is transitional between a lake or sea and terrestrial environments. In paralic settings they form salt marshes that are inundated during spring tides and contain plant species that have adapted to saline conditions, such as the succulent Salicornia. Fresh and salt water marshes are important habitats and breeding grounds for many vertebrate and invertebrate species. Cf. Swamp
Meandering channel: One of the fundamental fluvial channel types, also known as high sinuosity channels. They are generally single channels organised as sinuous loops. Channel thalweg is constantly on the move such that meanders migrate laterally and downstream. Abandoned meanders may be preserved as ox-bow lakes. Deposition takes place in three main settings: the main channel, point bars, and flood plain (that includes swamps, lakes, and vegetated areas).
Meromict: A stratified lake or enclosed sea where the layers do not mix. Bottom water layers may become anoxic as dissolved oxygen is used up by organisms. In saline waters it applies to salt crystals that precipitate within saturated layers and then sink to the bottom.
Mesosaline: Waters with salinity of 5 to 18 ppt derived from land-derived salts.
Mixohaline Water with salinity of 0.5 to 30 ppt. derived from ocean salts.
Molar Tooth structure: Crumpled to sinuous, occasionally cross-cutting, vein-like structures in calcareous to dolomitic mud rocks; in places they superficially resemble deformed burrows. Typically, a few millimetres wide, and extending 20-30 cm from bedding; they are filled with micritic calcite or dolomite. Their name is derived from the bedding plane expression where they appear like elephant molar teeth. Most common in shallow water Precambrian carbonate and siliciclastic rocks. They have been ascribed to desiccation, syneresis, and fossil algae, but the most convincing explanation is that they were seismically induced fractures during shallow burial (B. Pratt, 1998 – PDF, link above).
MTD (Mass Transport Deposits): MTD is the acronym given to soft sediment slumps, slides and debris flows, mostly generated on relatively high angle slopes between the shelf or platform margin, and deep-water settings at the base-of-slope and beyond. The term is generally reserved for sediment packages at or close to the sea floor, that move and deform en masse under the influence of gravity, commonly in multiple events.
Mud: A mix of silt- and clay-sized particles. On the Wentworth scale it includes all sizes smaller than 0.0625 mm, or 4 phi. Grain size analysis of unconsolidated mud samples is usually by pipette, or Laser Size Analyser.
Mud cracks: See Desiccation cracks.
Mud volcano: Small cone-shaped buildups associated with erupting mud, ranging from about a metre to 10s of metres high. Eruptions may be quiet where mud flows, slithers and slides down slope, or more violent, reminiscent of lava fire fountains, shooting mud 10s of metres into the air (or water). If methane is present in the mud, the eruptions can ignite. They form on land and on the sea floor.
Neap tide: The lowest tides during a full tidal cycle, occurring when the Sun and Moon are at right angles to each other. They occur 7 days after a spring tide.
Negative buoyancy: The condition where upward-directed buoyancy forces on an object suspended in a fluid, are less than gravity forces such that the object falls.
Newtonian fluid: A rheological class wherein a fluid has no yield strength (cf. plastics), and deforms continuously (strain) with increasing stress, independent of viscosity. Water is the best known example. cf. Plastic, hydroplastic rheology
Ocean gyres: Large scale (100s to 1000s of kilometres wide) ocean circulation cells driven primarily by wind, but strongly influenced by Coriolis deflections and geostrophic flow. There are 5 main gyres in our modern oceans: North and South Pacific and Atlantic gyres, and the Indian Ocean Gyre. There are also several smaller-scale circulation cells.
Oscillatory flow: Flow created by gravity wave orbitals; flow is successively offshore-onshore. This kind of flow influences sediment distribution and bedforms on the shoreface (above fairweather wave-base), and is also involved in formation of tempestites during storms, where it can combine with either offshore directed unidirectional currents or shore-parallel geostrophic currents, particularly in the formation of hummocky cross-stratification.
Overbank deposits: This applies to channels that, during flood stage, spill water and sediment over the adjacent bank or floodplain in the case of fluvial and delta distributary channels, or the submarine fan lobe adjacent to submarine channels. They tend to be fine-grained. In terrestrial environments the overbank deposits may bury floodplain vegetation and soils.
Paleocurrent: The direction of flow and sediment transport in ancient environments can be estimated from directional sedimentary structures such as crossbeds and sole marks, and from mapped facies changes such as grain size trends. The strength of paleocurrents can also be approximated by the size of bedform, and the size or density of clasts. Paleocurrent analysis is basically an exercise in statistics where flow directions are expressed as means.
Paleoslope: An ancient depositional surface that has a dip referenced to established datums such as ancient shorelines or shelf-platform margins, and in terrestrial settings the regional drainage patterns.
Paleosol: The general name for all manner of paleo-soils. Their identification in the rock record adds considerable value to assessment of subaerial exposure, unconformities, and paleoclimates.
Palimpsest deposits Deposits formed under one set of environmental conditions and processes, and overprinted to varying degrees by a new set of processes under different environmental conditions. Modern continental shelves are known to contain such deposits, where, for example, fluvial sands deposited during low sea level are stranded during the subsequent rise in sea level and partly or completely reworked into shelf sand bars. Recognition of palimpsest deposits requires that some of the original lithofacies are preserved.
Palustrine: The most common type of non-tidal wetland, where water is sourced from rain, surface runoff, or groundwater, and not directly associated with lakes, rivers, or marine and tidal influences. Palustrine environments may have low marine-derived salt content (<0.5 ppt) but remain non-tidal.
Panne: Shallow ponds on salt marsh platforms. They are usually recharged by saline water during spring tides, but the pond salinity can vary because of precipitation.
Paralic: Coastal environments (and their deposits) that are characterised by interfingering shallow marine and non-marine conditions. It includes deltas (delta plains, interdistributary bays, and channels), lagoons, and estuaries. Paralic systems are susceptible to even minor changes in sea level and sediment supply, recorded for example as shoreline trajectories. They are stratigraphically important because they record the transition from fully marine to terrestrial.
Patterned ground: Characteristic surface structures in seriously cold, periglacial regions, particularly tundra and other regions underlain by permafrost. The patterns include symmetrical polygons, stripes and circles that have diameters generally <10-15 m, although some patterns are >100 m. Collectively, the patterns may covers many square kilometres. Their formation is related to cryogenic processes such as freeze-thaw expansion and contraction, frost heave, and ice wedging. They have also been observed on Mars.
Periglacial environments: Cold environments associated with glaciers and ice sheets, that are subject to seasonal freeze and thaw. Some periglacial regions are underlain by permafrost. Most contain some kind of vegetation and organic soil cover, that is modulated by perennial snow cover. Common landforms include patterned ground, ice wedges, melt-water ponds, fluvial channels of varying sinuosity, small fan deltas or Gilbert deltas, and thermokarst.
Permafrost: Ground that remains frozen for at least 2 years. It consists of soil, sediment, and fractured bedrock bound by ice. It may also include methane clathrates, and significant volumes of dispersed organic carbon. Shallow melting of permafrost produces thermokarst.
Photosynthesis: A process that converts sunlight energy to chemical energy in plants, cyanobacteria, and algae. One of the chemical products is molecular oxygen(O2), that in plants is formed from carbon dioxide reacting with water in plant cells to produce sugars and oxygen. It is generally understood that most of Earth’s free oxygen was produced during the Precambrian by cyanobacterial stromatolites.
Photic zone: The uppermost layer of the oceans and lakes where light penetrates; the base of the zone is at about 1% of incident sunlight. On average it is about 200 m deep. It is the layer where more than 95% of photosynthesis by marine organisms takes place.
Plane bed: Refers to hydraulic conditions where parallel laminations form; it is an important component of the Flow Regime hydraulic model. There are two plane bed conditions: (1) Where velocity flow in the Lower Flow Regime (LFR) is sufficient to move sand grains, but not sufficient to form ripples. (2) Under Upper Flow Regime (UFR) conditions, where flow washes out LFR dune bedforms to form parallel laminated sand; under these conditions plane bed indicates the transition from LFR to UFR.
Planktic: Used as an adjective to describe a diverse group of single and multi-celled organisms (plankton) that live within a water mass. Thus, planktic foraminifera are one of two major groups – the other being benthic foraminifera. It has been argued that this is the correct derivation from an original Greek word, rather than the commonly used alternative Planktonic.
Plastic (rheology): A material or fluid behaves plastically if it has the strength to resist deformation up to its yield strength, beyond which it deforms continuously as stress is applied, independent of viscosity. The mode of deformation is also called ductile flow.
Platform evaporites : Marine evaporites dominated by gypsum and halite, generally a few 10s of m thick, that accumulate on shallow platforms isolated from fresh seawater and groundwater influx, and where evaporation exceeds new water input. Commonly interfinger with shallow water siliciclastic and carbonate facies and their associated faunas and floras, including shoreface and sabkha facies. Cf. basin-wide evaporites
Playa lake: From the Spanish word for ‘beach’, its meaning has morphed to a dry lake, usually floored by evaporitic minerals, that intermittently becomes flooded. Cf. Salina.
Plume (river): A water mass that enters a receiving basin (lake or sea) at a river mouth, and is distinguished by its suspended sediment or chemical load. Sediment plumes commonly develop during river flood events associated with storms, spring thaw, natural and artificial dam collapses. The margins of sediment plumes are initially well defined, but gradually become diffuse as mixing and dilution occur.
Plunge line (sediment plumes): For river-derived sediments that are more dense than the receiving water body, this corresponds to the region where sediment begins to settle, or plunge from the plume toward the sea or lake bed. This is one mechanism for the generation of hyperpycnal flows.
Pocket beaches Pocket beaches are common along rocky coasts, between closely spaced headlands and rocky promontories. Beach sediments are commonly gravel or a mix of gravel and sand. They tend to be high energy beaches.
Point bar: An accumulation of sand and mud on the inside, or accretionary margin of a channel bend. They are a characteristic bedform in high sinuosity rivers and in many estuaries. Internally they are organised into continuous or discontinuous, channel-dipping foresets of sand and mud; sand is more dominant near the channel, mud, silt and carbonaceous material on the upper surface where there is also a transition to the adjacent flood plain. Each foreset contains laminated and crossbedded sandstone. Foresets may also contain discordances from local erosion. A stratigraphic column drawn from the channel, through the point bar to flood plain presents a classic fining upward facies succession.
Preservation potential: A nebulous expression that is generally used to express the relative potential for preservation of sedimentary structures and fossils. Thus, the soft part of an animal has very low potential because it degrades rapidly or is consumed by other critters; the shelly exoskeletons, shells, test, and internal skeletons have significantly higher potential. Likewise, plant leaves, flowers, fruits and seeds have relatively low potential – although pollen, because of its composition, are commonly well preserved.
Prodelta: Develops basinward of the steeper gradient delta front, as gently dipping stratal units that eventually merge with the basin floor. The prodelta is below wave base. It derives its mainly muddy-silty sediment from the distal limits of turbidity currents, from suspension, and from hypopycnal flows of mud.
Progradation: The basinward accretion of sediment when sediment supply keeps pace with or exceeds the generation of accommodation, either at the beginning or end of sea level rise. In a sequence stratigraphic context, it occurs during normal regression. The shoreline trajectory is approximately horizontal.
Radian: An angular measure commonly used in mathematical expressions involving rotation and moving bodies, for example angular velocity. 2π radians is equivalent to 360o.
Reworking: The condition where sediment is frequently moved by air or water currents and waves (e.g. channel beds, beaches, the shoreface, sand dunes). Reworking commonly improves the degree of grain size sorting by winnowing that separates lighter from heavier sediment fractions. Under some conditions of deposition, such as sediment gravity flows (e.g. turbidity currents) there is little opportunity for reworking of entrained sediment.
Reynolds number: Derived by Osbourne Reynolds in the mid 19th century, to describe the transition from laminar to turbulent flow. Reynold’s number Re expresses the ratio of inertial (resistance) forces to viscous (resistance) forces:
Re = ρVD/μ
with fluid density = ρ, fluid viscosity μ, mean velocity of flow V, that reflects shear rate and inertia forces, and Tube diameter D that influences the degree of turbulence. Re is dimensionless.
Rhizome: Fibrous or woody plant structures that grow within a soil, from which stems, leaves, and roots extend. In plants such as seagrasses they can develop dense mats just below the sediment-water interface. They are one of the main mechanisms for expansion of plant growth.
Rip currents: Rip currents are flows a few 10s of metres wide that move rapidly offshore; current speeds of 4m/second have been recorded. They form when seawater that has moved up a beach reverses its flow, focused into narrow channels by sand bars and holes. The currents are powerful because so much water is being focused through a relatively narrow gap. Rips can appear suddenly on any beach where there is appreciable wave activity.
Riparian zone: The area of land in immediate contact with a river, lake or tidal zone. It is commonly considered to be a buffer zone that is reflected in the type of vegetation, such as marsh or wetland, meadows or forests, as well as a zone of protection and management. or example a well-developed riparian vegetation and soil will help trap and sequester land-derived nutrients and sediment.
River-dominated delta: Deltas where fluvial processes tend to overcome opposing coastal processes such as waves, tides, or long-shore currents. They tend to be strongly lobate – the classic modern example is the Mississippi birds-foot delta, with relatively small number of major distributary channels. Sediment is dominated by silt and mud. the entrance of sediment laden river flow into a lake of sea is dominated by the relative differences in buoyancy of the river plume – more dense coarser-grained flows across the substrate (hyperpycnal flow), less dense as a muddy plume in the upper part of the water column (hypopycnal), and more general mixing with waters of equal density (homopycnal).
Sabkha: Broad, flat areas of evaporitic sand-mud flats that form in arid to semi-arid climates. Modern coastal sabkhas are part of the intertidal realm, occupying the supratidal zone that is infrequently flooded by seawater by very high tides and storm surges. Sabkhas can also occur in interdune areas where the local watertable is close or at the surface. Common mineralogy includes gypsum, anhydrite and halite. Precipitation of evaporites takes place at the surface and within the shallow sediment column. Sabkhas also have specialised invertebrate faunas, and microbial communities that form extensive, desiccated mats.
Salina: A salt-water pond, spring or lake, either natural or artificial. From the Spanish for salt pit, and earlier Latin salinus meaning saline. Cf. Playa Lake.
Saline intrusion: See seawater intrusion.
Saline lake: A terrestrial water body where evaporation exceeds surface freshwater influx and fresh groundwater seepage. Recharge may be seasonal and intermittent. Intense evaporation results in precipitation of salts, commonly halite and gypsum. Lakes may be connected to inflowing and outflowing drainage, or they may be endorheic. See also Playa Lake
Saline lake brines: Unlike seawater, terrestrial brines have widely variable compositions, depending on local soil and bedrock compositions, groundwater chemistry, and the degree of evaporitic drawdown. Typical brines contain Na+, Ca2+, Mg2+, Cl–, SO42-, HCO3–, CO32-, and SiO2, but concentrations are highly variable. pH ranges from highly alkaline to highly acidic. Evaporation pathways produce a succession of different minerals. See also calcite-gypsum divides.
Saline wedge: Saltwater wedge. The relatively dense seawater layer that extends upstream and beneath the freshwater layer in tidal channels, particularly estuarine channels. There can be varying degrees of salt-freshwater mixing at the interface depending on the severity of turbulence created by tidal stress and channel morphology.
Salt marsh: A marsh dominated by salt-tolerant herbaceous plants and microbial mats in upper intertidal to supratidal areas, usually flooded during spring tides and storm surges. They are important habitats for invertebrates and vertebrates. Drainage is principally by shallow tidal creeks. Sediment is commonly a mix of fine sand and mud. A degree of sediment desiccation may occur during prolonged dry periods. See also sabkha, tidal flats.
Saltation load: Grains that temporarily leave the sediment-water-air interface, for example by bouncing along the surface under high flow velocities, but where fluid forces are not sufficient to maintain suspension. The saltation load is part of the bedload. See also Traction carpet.
Sandspit: An emergent sand bar at the entrance to a bay or estuary. At one end the spit is attached to headlands; at the other an open tidal channel that allows seawater exchange between the bay and open sea. Larger spits may also have a veneer of sand dunes. Cf. Barrier island; Tombolo.
Seagrass: Seagrasses are monocotyledons, the group of angiosperms that evolved a tolerance to saline conditions from their Late Cretaceous terrestrial ancestors. They inhabit low to moderate energy, intertidal and shallow subtidal environments, and develop extensive root systems, produce flowers, and are pollinated while submerged. They are one of the most productive marine ecosystems, act as nurseries and habitats to many infaunal-epifaunal invertebrate and vertebrate species and dampen waves and tidal currents. Seagrass communities frequently coexist with mangrove forests, salt marshes, and coral reefs.
Sea stack Coastal landforms where stacks of bedrock, commonly shaped as columns or blocks that extend above sea level, have been isolated from an adjacent bedrock landmass by wave erosion and weathering processes such as salt expansion, precipitation, and wind. They are common on rocky, cliffed coasts.
Seawater intrusion: (saline intrusion) A term used in hydrogeology to indicate the replacement of fresh groundwater by an intruding wedge or lens of seawater. This commonly occurs in coastal aquifers where excessive fresh groundwater withdrawal results in a fall in the local watertable, and a corresponding rise in the fresh water/seawater interface by 40 times the amount the watertable has fallen. Sea water intrusion is, for practical purposes, irreversible. See Ghyben-Herzberg principle. Not to be confused with saline wedge.
Second cycle sediment Sediment of any grain size derived from older sedimentary rocks. ‘Cycle’ in this context refers to the inferred history of the older rocks, that began life as loose sediment, were buried, lithified, uplifted and eroded, providing sediment for a new geological cycle. These determinations usually require detailed analysis of grain provenance, composition, texture, and degree of alteration. Zircon age and crystal-zone systematics play an important part in modern provenance analyses. Second cycle sediment usually contains high proportion of stable minerals, such as quartz.
Sediment gravity flow: Sediment-water mixtures that flow downslope under the influence of gravity. Each flow is a single event. In marine and lacustrine environments such flows include grain flows, turbidity currents and debris flows. They are the main depositional components of submarine fans. Each flow type has a distinctive rheology. Each leaves a characteristic sedimentologic signature depending on the degree of turbulence within the body of the flow, the amount of mud in the sediment mix, and whether the flow is supported by matrix strength, turbulence, or shear. Flows may be initiated by seismic events, gravitational instability of sediment, or storm surges. The terrestrial equivalents include mud flows and lahars.
Seismite: Deformation of soft or firm sediment during seismic events (commonly earthquakes). Soft sediment deformation occurs during liquefaction, fluidization, and mobilization of single beds or thick sediment packages, producing folding, normal and reverse-thrust faults, dewatering and flow structures. Spectacular examples crop out along the margins of Dead Sea.
Semidiurnal tides: Two tides every 24 hours. Diurnal tides (one every 24 hours) occur in areas where coastline shape and bathymetry interfere with the normal semidiurnal cycle.
Settling velocity (Terminal velocity): Under the influence of gravity the settling velocity of an object (commonly written as Ws) is the point where the submerged weight of the object equals the fluid drag force on that object. At this point, the fall velocity is constant. From Stokes Law, Ws can be calculated from Ws = 1/18. (γ D2/ μ) where γ is the submerged weight per unit volume calculated from the expression γ = (ρs – ρw)g where ρ is the density of the solid grains and water respectively. D is grain diameter.
Shallow water waves: Waves whose orbitals interact with the sea or lake floor at the point where water depth is about half the wavelength. Open ocean deep water waves eventually become shallow water waves as they approach the shoreline. Here, some of their energy is transferred to the sea floor, and to conserve momentum the waves slow down but increase in amplitude. Tsunamis are considered to be shallow water waves because their wavelengths are measured in 10s to 100s of kilometres.
Sheetfloods: Intermittent sheet-like flow during flood events, that is not confined to a channel by spreads laterally. They develop mostly on alluvial fans. Depending on their competence, they carry mud, sand, and gravel. Deposits may show crude grain size grading and ripples. Flow in some sheetfloods is hyperconcentrated.
Shields diagram: A plot of the Shields parameter against the grain Reynolds Number that defines two fundamental domains – that where grain movement is initiated (creating bedload or suspension load conditions), and that were there is no grain movement. Both variables are dimensionless.
Shields parameter: The Shields Parameter Θ is an empirical function that is used to calculate the shear stress required to initiate grain movement along a sediment bed. It is written as: Θ = τc.D2/(ρs – ρw)gD3 where τc = critical stress at the grain boundary; D = mean grain diameter, and ρ the density of the solid grains and water respectively. The value (ρs – ρw)g is the submerged specific weight of a grain. The numerator τc.D2 is proportional to the fluid force acting on a grain; the denominator (ρs – ρw)gD3 is proportional to the weight of the grains. Θ is dimensionless. Θ is the independent variable in the Shields diagram, plotted against the grain Reynolds Number.
Shoreface: The shallow marine environment extending from the low tide zone to fairweather wave base. The sea floor in this region is constantly impinged by wave orbitals. Bedforms of various sizes will form, depending on wave energy and tidal currents. Benthic flora and fauna have adapted to conditions of constant water motion and movement of sediment.
Shoreline: The boundary between land and a body of water. It is a more specific term than coastline – it is usually taken as the line at the top of the wave-washed shore (beach).
Sieve diameter: The minimum diameter of a grain that will pass through a particular sieve mesh size. The measurement is used for grain size analyses of unconsolidated or disaggregated sands and gravels.
Sinkhole: Also called Dolines, are collapse structures formed by removal of subsurface rock, either by erosion of dissolution within the vadose and saturated (phreatic) zones, are typical of limestone terrains; they can also occur in landscapes underlain by evaporites. They tend to be circular in cross-section. Collapse usually occurs rapidly into large, subsurface caverns. They are common in karst landscapes.
Sinuosity (fluvial geomorphology): The ratio of river length (along its axis, or thalweg) between two locations, divided by the straight-line distance between the same locations. Meandering rivers have high sinuosity – >1.5 (all those loops); braided rivers (with multiple channels) have low sinuosity (<1.1). Straight channels have a sinuosity of one.
Slack-water: The brief period between high and low tide reversals when tidal height or depth neither increases or decreases and when tidal currents flow ceases (there may still be water movement from waves). On tidal curves (height/time) this corresponds to the curve peaks and troughs.
Spring tides: The highest tides during a full tidal cycle, occurring when the Sun and Moon are aligned (the Moon can be in full or new phase).
Stationary waves: Also called standing waves. Surface waves formed during the transition from subcritical to supercritical flow. They are the surface manifestation of, and are in-phase with antidune bedforms on the channel floor; the waves migrate upstream in concert with the deposition of backset laminae on the stoss slopes of antidunes. Stationary waves that break (upstream) have become unstable. Unstable wave eventually decay and surge downstream.
Stokes Law: George Stokes determined the mathematical solution to the problem of fluid drag forces acting on a particle that is settling through a viscous fluid (published 1851). Thus Fluid drag Fd
Fd = 6πμVR where
μ is viscosity, V is mean velocity, and R is particle radius (the equation is often written as Fd = 3πμVD where D is particle diameter).
Stokes Law applies under conditions of laminar flow and Reynolds Numbers <1. Stokes Law enables the derivation of an equation expressing settling velocities – this has important implications for sedimentology, aerodynamics, volcanology, and other problems involving fluid flow.
Storm berm/ridge Low amplitude mounds, a few centimetres to decimetres high that have gently rounded surfaces on the seaward margin but may be steeper landward. They form when storm waves move gravel from the shallow shoreface to the beach and beyond the high or spring tide limit.
Storm-flood-dominated delta: a category of delta proposed by Lin and Bhattacharya (2021) where prodelta, delta front lobes, and to a lesser extent distributary channels are profoundly influenced by storms and hyperpycnal floods, and contain a significant proportion of tempestites in their stratigraphic record.
Storm surge: The landward surge of water caused by increased sea levels during storm coastal setup. The magnitude of the surge depends on storm duration, wind direction and strength, wave fetch, and the amplifying effects of coastal geomorphology. Storm surges can be very destructive.
Storm tide: A storm tide is a landward surge that coincides with high tides, particularly spring tides, such that the sea level elevation is greater than from the coastal setup alone.
Storm wave base: The maximum depth at which storm-generated waves impinge the sea floor and are capable of moving sediment. Storm wave base is deeper than fairweather wave base.
Strandline: A more-or-less linear platform containing remnants of ancient beaches above an active high tide level (e.g., berms, storm ridges).
Strandplain: Also called chenier plains. A belt of sand along and above the active shoreline that contains roughly parallel sand or gravel ridges that represent former shorelines. Strandplains are attached landward; there are no lagoons or embayments (cf. barrier islands and sandspits). They commonly develop at river mouths where there is sufficient sediment supply to promote progradation of the shoreline. Each sand ridge can be interpreted as a step in the relatively flat shoreline trajectory.
Subcritical flow: Defined by Froude as the conditions in surface flows where inertial forces dominate and Fr<1. It corresponds to lower flow regime bedforms such as ripples and larger dune structures, that usually are out of phase with surface waves. Also called tranquil flow. cf. antidunes, supercritical flow.
Submarine canyon: Like their terrestrial counterparts, they are narrow, deep, steep sided valleys that extend from a continental shelf or platform to the slope, terminating near the base of slope or rise, where they merge with submarine channels. Their location may be structurally controlled, initiated by paleodrainage, or focusing of sediment gravity flow during low sea levels. They are important conduits for sediment delivery to submarine fans. Canyon wall collapse may produce significant tsunamis. Canyon heads may approach within a few 100 m of shorelines (e.g. Monterey Canyon, California, Hikurangi Canyon, New Zealand).
Submarine fan: Fan-shaped depositional systems that accumulate at the base of slope, continental rise and adjacent basin floor. Sediment is usually fed via a large submarine channel or canyon that may bifurcate into multiple channels down gradient. The channels feed sediment to lobes that prograde basinward; lobes may be inactive for a period. Deposition is dominated by sediment gravity flows – turbidity currents, debris flows. Mass transport deposits (slumps, slides) are common in some fan systems.
Submarine gullies: Like their terrestrial counterparts, gullies are steep sided depressions that form where there is an abrupt change in slope, typically at the marine shelf-slope break. They can form by erosion via some pre-existing depression, or by slope failure. Gullies become the focus for transfer of sediment from the shelf-platform to the deeper basin via submarine channels and channel complexes.
Submerged specific weight: The weight of a solid in a fluid is less than its weight in a vacuum because of buoyancy forces. In water this is calculated as (ρs – ρw)g where ρ is the density of the solid and water respectively, and g is the gravity constant. See also Stokes Law.
Subtidal zone: A nebulous term for the sea floor below mean low tide. It includes the shoreface and the littoral zone.
Supercritical flow: Defined by Froude as the conditions in surface flows when gravitational forces dominate (over inertial forces) and the Froude number Fr > 1. The corresponding stream flow surface conditions manifest as an acceleration of flow such that stationary waves (critical flow) break upstream forming chutes. This corresponds to upper flow regime conditions. cf. subcritical flow.
Supratidal zone: The region above spring tides that is inundated only sporadically by storm surges. On low relief coasts it can be an extensive flat, including salt marsh, or sabkhas in arid climates. On high relief, rocky coasts it refers to the splash zone that is rarely inundated by tides.
Suspension load: The part of the sediment load held in suspension in water or air by turbulence and buoyancy. See bedload also Stokes Law
Swaley cross bedding Formed in conjunction with hummocky cross stratification. They occur as low relief depressions, where infilling laminae are continuous from crest to crest, and dip less than 15°. They also occur in beds lacking HCS and may represent preservation above fairweather wavebase (HCS are usually found below fairweather wavebase). A possible reason for this is that the swales are negative features on the sea floor that can avoid truncation and reworking by fairweather waves. HCS on the other hand are more likely to be reworked by fairweather wave orbitals.
Swamp: A wetland in freshwater or coastal (paralic) seawater environments that has a vegetation cover dominated by trees (cf. marsh).
Swash zone: The portion of a beach subject to wave run-up. Run-up velocity depends on the momentum produced by breaking waves and the beach gradient. It is usually sufficient to move sand and shells, and remove fine-grained sediment. Cf. Backwash.
Synaeresis cracks: Cracks in sediment formed by compaction, changes in salinity, and in some cases by dewatering of sediment during seismic events. They are not formed by subaerial exposure and desiccation. Their shape and geometry is superficially like that of mud cracks; V-shaped in cross-section, straight to slightly curved strands in plan view, and occasionally polygonal.
Syndepositional processes: Strictly speaking, processes that take place during sedimentation, although the term is often extended to include processes ‘soon after’ deposition. Common examples are deformation that influences sedimentation (syndepositional faulting, slumping), geochemical processes such as sea floor cementation, and biogenic activity. A common synonym is synsedimentary.
Tabular crossbedded lithofacies: A lithofacies characterised by crossbeds having a planar bottom set (boundary) across which foresets are in tangential or abrupt angular contact. Also called 2D subaqueous dunes. This definition generally follows that of McKee and Weir, 1953.
Talus: Angular, poorly sorted rubble that accumulates at the base of steep rock faces or slopes, typically associated with exposed fault planes. If the source of eroded material is focused, a talus fan may form.
Tempestite: The deposit and/or erosional surface developed during a storm. Onshore and offshore erosional surfaces usually form as the storm waxes; tempestites usually accumulate during the waning stage of a storm. Typical sedimentary structures include HCS, SWS, modified wave ripples, combined flow climbing ripples, upper plane-bed laminae, and graded beds including turbidites.
Terminal moraine: An accumulation of rocky debris at the snout of a glacier (also called an end moraine). The debris is derived from bedrock plucked from the valley walls (lateral moraines) and glacier base and dumped during ice ablation. The moraines mark the maximum advance at any particular time of a glacier’s history.
Terminal velocity: See Settling velocity
Thalweg: In river systems, an imaginary line connecting the deepest parts of a channel along its length is the thalweg, or talweg.
Thermal equator: The latitude where Earth’s surface temperatures are a maximum. It is currently about 10o north of the geographic equator, because the northern hemisphere averages 1.2o to 1.5o higher than the southern hemisphere. There is some seasonal variation in its location. There have been major latitudinal shifts in its position over the past 100,000 years, that have led to significant, frequently abrupt climate change.
Threshold shear stress: The shear stress imparted by a flowing fluid on a sedimentary grain that can initiate grain movement. Movement will occur when fluid drag and lift forces exceed the combined gravity, viscous shear, and grain contact forces.
Threshold velocity: In sedimentary hydrodynamics, this is the velocity at which fluid forces overcome gravity and friction forces acting on grains. This boundary condition depends on grain size, density and shape, and on the roughness at the sediment-water interface – that is roughness caused by grains of different sizes.
Tidal current asymmetry The ebb and flood of semi-diurnal or diurnal oceanic tides results in either the reversal of current flow directions or, the weakening of flow during one or other of the tides. Common sedimentary structures that reflect these conditions include herringbone crossbeds, lenticular and flaser crossbeds, interference ripples, tidal bundles, and reactivation surfaces.
Tidal deltas: Sandy, delta platforms that accumulate at entrance to tidal channels that drain harbours, bays, and lagoons. They are classified as ebb or flood deltas; ebb tidal deltas form on the seaward margin of the channel entrance and can be modified by marine processes. Deposits typically are sand-dominated, and comprise trough crossbedded channel facies, and on the adjacent (submerged) platform ripples and sandwaves.
Tidal flat: Broad, low relief and low gradient expanses, extending from high tide to low tide limits. They are exposed during ebb tides. They are commonly home to a diverse benthic fauna and flora, and are important breeding and feeding grounds for many marine organisms. Sediment is commonly a mix of sand and mud. Mud-prone versions are sometimes called mud-flats. They may be drained by tidal channels.
Tidal range: This is the range between mean high water and mean low water. It varies from place to place because of coastal geomorphology and bathymetry. In some places it can be amplified (Bay of Fundy has a range to 14 m) or weakened – ranges in the Mediterranean are very low. A commonly used scale for tidal ranges is:
- Micro-tidal < 2 metres.
- Meso-tidal 2 – 4 metres.
- Macro-tidal > 4 metres.
Tidal wave: The cycle of tidal highs and lows that move along a coastline. If the waves have a period of 12 hours (i.e. two tides per day) then they are semidiurnal. Movement of tidal waves around ocean margins is caused by Earth’s rotation relative to the tidal bulge produced by gravitational forces from the Moon and Sun. Movement is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere. Tidal waves are NOT synonymous with Tsunami.
Tide-dominated deltas: Characterised by seaward-trending sand bars and ridges where river sediment supply is contained on the delta plain during high tides, and accreted to bars via distributary channels during ebb tides. The sediment ridges tend to develop over the mid- and outer delta plain. The delta plain may extend seawards to extensive tidal flats. An excellent example is found in the modern Mahakam River delta, eastern Borneo.
Tombolo: An emergent sand bar that connects headlands and islands, and is not cut by tidal channels. Aupouri Peninsula, northernmost NZ, is a good example, constructed during several stages of glacio-eustatic sea level rise and fall during the Pleistocene.
Traction carpet: Above the flow threshold velocity, non-cohesive grains at the sediment-water interface move by rolling, jostling, and sliding. Grain movement is contained within the bedload. See also saltation load, suspension load.
Tranquil flow: See subcritical flow.
Tropical cyclone: the general name given to strong tropical and subtropical storms that have a well defined eye around which winds rotate. Much of the heat energy that drives TCs comes from the ocean. In the northern hemisphere, TCs are called Hurricanes if they occur east of the International Dateline, and Typhoons if they are west of the Dateline.
Trough crossbedded lithofacies: A lithofacies defined by crossbeds having concave, spoon-shaped basal contacts that truncate previously formed crossbeds. Foresets tend to mimic the basal contact geometry and generally are tangential with the base. Also called 3D subaqueous dunes. They are common under conditions of confined, channelised flow. Found in gravel and sand facies. This definition generally follows that of McKee and Weir, 1953.
Tsunami: (plural Tsunamis). A wave generated by a sudden pulse of energy – an earthquake, subaerial and submarine landslide, volcanic eruption or sector collapse, or asteroid impact.The waves can travel at speeds of several 100 km/hour. In mid ocean they may pass unnoticed, but increase in amplitude across a shallow shelf as they interact with the sea floor. Tsunamis act as shallow water waves. Waves on open coasts may be many metres high; in confined embayments like fiords, they can reach 10s to several 100 m high. Wave run-up extends to even greater heights.
Tundra: A region that is treeless because of extreme cold and where growing seasons are brief. Although treeless, they are home to many grasses, low shrubs, and flowering plants that support a variety of wildlife. In mountainous regions, tundra is located at elevations above the tree-line. Vast expanses of tundra occur in the Arctic and subarctic. Tundra is commonly underlain by permafrost. It is the coldest of all biomes.
Turbidity current: A sediment-water mixture that flows downslope under the influence of gravity. The sediment mix is most commonly sand, silt, and mud. During flow, sedimentary grains are kept in suspension by turbulence. Scouring of the underlying bed may occur at the head of the flow. Deposition from turbulent flow produces graded bedding plus a characteristic suite of sedimentary structures exemplified by the Bouma Sequence. They form in lacustrine and marine settings that have modest depositional slopes. In marine environments, they are generated on continental slopes and in submarine canyons; they are one of the main components of submarine fans.
Turbulent flow: Turbulence is described by flow lines that constantly change direction and velocity. In a flowing stream this is manifested as eddies, boils, and breaking waves. In sedimentary systems, turbulence is an erosive process, and an important mechanism for maintenance of sediment suspension through water columns and in sediment gravity flows. It was first quantified by Osbourne Reynolds for conditions where Reynolds numbers Re > 2000.
Typhoon: A tropical cyclone that has sustained wind speeds of 119 km/hr (74 miles/hr) and more. The term is reserved for northern hemisphere storms west of the International Dateline (Greenwich Meridian). cf. Hurricane.
Undertow: On all beaches, the return flow of water produces an undertow that flows beneath the incoming waves. Undertow occurs everywhere along a beach. Its influence is generally confined to the surf zone, and for the most part is not dangerous. Cf. rip current.
Viscosity: Viscosity is used to describe a material in which its strength depends on the rate of deformation, or strain rate. From a practical point of view, it is a measure of its resistance to deformation, or flow. It is normally applied to fluids, including rocks that may behave as fluids under high confining pressures and low strain rates. In the Earth sciences, viscosity is applied to phenomena like surface water flows (as in Reynolds numbers), sediment gravity and pyroclastic flows, lava flows and ice sheets, and to rocks-magmas in the lithosphere and asthenosphere.
Wave base: The maximum water depth where wave orbitals impinge and interact with the sea/lake floor. The distinction is made between fairweather wave base, and storm wave base. Wave base depth is about half the wavelength.
Wave-dominated delta: More common along high wave-energy coastlines where sand-prone sediment delivered to the coast by distributary channels, is reworked and redistributed by marine processes. Distributary mouth bars form at channel exits; long-shore movement of sand provides sediment nourishment for beaches, sandspits, and barrier islands. The delta edge tends to be lobate and smoothed or locally straightened by these processes. A classic modern example is Nile delta. See river-dominated deltas, tide-dominated deltas.
Wave orbitals: The circular motion of water beneath transverse waves. Orbital diameter is greatest beneath the wave crest and diminishes with depth. The maximum depth that orbitals interact with the sea/lake floor is called the wave base.
Wavelength (Oceanography): The distance between crests or troughs of successive water waves; the same definition applies to bedform – the distance between crests of successive ripples.
Wetland: The region between terrestrial and fully aquatic systems, where the watertable is very shallow or at the surface for a significant period such that hydrophytic plants thrive. Wetlands may be tidal or non-tidal. Wetland waters may be fresh, brackish (riverine, lacustrine), or partly saline from marine derived salts (e.g. estuarine, coastal plain, delta plain).
Wind shear: Usually applied to the frictional drag of wind over water that produces waves and contributes to the build up of storm surges. About 2% of the wind energy is transferred to the uppermost water mass.
Winnowing: Removal of lighter grains by wind or flowing water, leaving denser material behind. The degree of winnowing depends on the strength, or carrying capacity of air/water flow. Borrowed from an old English agricultural term for removing wheat from the chaff. Derived from Old English windwian, meaning ‘from the wind’.
Yield strength Viscous fluids have finite strength, called the yield strength where the fluid will not deform or flow below a critical stress. Fluids (or solids) that behave in this manner are referred to as hydroplastic or plastic.
Zero shear stress boundary: See Flow boundary.
