Please note – I no longer maintain Glossaries by alphabet; A, B, C… etc. All items on these pages have been moved to subject-specific glossaries such as Volcanology, Sedimentary facies and processes, and so on. The list of subject-based glossaries can be viewed in the drop-down menu on the navigation bar. These glossaries are continually updated.
QFL plots: Ternary plots of quartz, feldspar and lithics, and variations on this theme, to describe sediment composition (principally in sandstones and conglomerates), and to track stratigraphic and basinal changes in composition with reference to the provenance of epiclastic sediment.
Quartz: One of most common detrital constituents of sedimentary rocks, and second only to feldspar to all rocks in general. Its chemical formula is SiO2. Crystallographic form is hexagonal rhombohedral. It is uniaxial positive and non-pleochroic. Strained quartz will show undulatory extinction. It occurs in felsic igneous and metamorphic rocks, hydrothermal deposits, fracture fills, and as diagenetic products (e.g. cements). It forms chert and chalcedony in its micro- and cryptocrystalline form.
Quartz arenite: A sandstone with 95% and more quartz grains. Also called an orthoquartzite. The term quartzite tends to be reserved for metamorphic rocks.
Quartzite: An older term synonymous with quartz arenite and orthoquartzite. It is better applied to metamorphosed equivalents of these rock types.
Radiaxial fibrous cement: Radiaxial fibrous calcite and its companion fascicular optic calcite are common cavity-filling cements in ancient limestones. Both consist of radially fibrous calcite clusters but differ in their crystallographic orientation. Cleavage and twin planes in radiaxial crystals are concave toward the pore interior, in concert with optic axes that converge toward pore spaces (i.e. opposite the divergence in the actual crystals). In some cases, the fabric may represent calcite-dolomite replacement of initially fibrous aragonite. In others the fabric may precipitate directly.
Radiogenic heat: One of three heat sources in Earth’s interior, it is the heat generated by radioactive decay of natural isotopes, principally Uranium-238 and -235, Thorium-232, and Potassium-40. See also gravitational friction, primordial heat.
Radiolaria: Marine zooplankton that secrete a symmetrical, intricately structured silica (opal) skeleton; they have a very long geological range, from the latest Precambrian to Recent. They are very useful biostratigraphic indicators, particularly where calcareous microfossils are absent – this is commonly the case for deep ocean sediments (oozes) below the calcite (CCD) and aragonite compensation depths (ACD). The original opal converts to microcrystalline quartz during sediment burial.
Ravinement surface: A surface of erosion that accompanies the landward migration of a shoreline and associated shoreface. Ravinement may erode coastal dune, lagoon and estuarine deposits from the previous highstand. It may also remove some or all of the subaerial unconformity that formed during the previous sea level fall. They are one of the hallmarks of transgression.
Reactivation surface: The lee face of bedforms, such as ripples and larger subaqueous dunes, that is eroded during a reversal of current. When the current returns to its normal flow, the bedform will reactivate and continue to advance downflow. These structures are useful indicators of tidal current asymmetry (ebb-flood reversal). See also interference ripples, lenticular and flaser bedding.
Recrystallisation: In sedimentary rocks this involves the transformation or replacement of a mineral with itself, and usually entails changes in crystal size and shape (but not bulk composition): as in micritic calcite to sparry calcite, or aragonite to its polymorph calcite. The term was originally coined for the process of annealing in metals, which is a dry process. Recrystallisation in sedimentary rocks is always a wet process that involves dissolution of a mineral at grain boundaries, followed by precipitation. It tends to cross-cut original textures, destroying them in the process.
Recumbent fold: Any fold that has been rotated so that its axial surface is close to horizontal.
REDOX: Reactions in which oxidizing and reducing agents combine; thus one atom is oxidized and the other reduced simultaneously. For example, in the sour, toxic gas hydrogen sulphide (H2S), 2 H atoms lose an electron each to the sulphur atom; 2H+ S2-.
Reduction: When an atom gains electrons it becomes reduced. It has electrons to spare and can donate them to the atom of another element that has a deficit of electrons (i.e. it is oxidized). A reduced element that donates electrons is a reducing agent. Cf. Oxidation, REDOX.
Regolith: A surface covering or layer of loose, unconsolidated rubble. Clasts of all sizes are generally unsorted, commonly incorporating broken bedrock.
Regression: A general term for retreat of the sea and the accompanying seaward migration of shorelines and associated marine sedimentary facies and biotas. The process of regression results in subaerial exposure of former sea floor. Regression is the result of baselevel (sea level) fall. See Forced regression; Normal regression.
Regressive laminae: Bedform laminae that accrete down-flow (i.e. in the direction of current flow), most commonly from pyroclastic surges, but also in open flow antidunes. cf. progressive laminae.
Regressive surface of marine erosion: (RSME) The abrupt, erosional contact at the base of shoreface wedges that form during forced regression. It forms in concert with down-stepping shoreline trajectories. The RSME overlies highstand deposits that accumulated during an earlier stage of normal regression.
Regressive Systems Tract (RST): The regressive systems tract contains all strata deposited from the beginning to the end of regression; there is no subdivision into normal and forced regression. It is one of only two systems tracts in Transgressive-Regressive (T-R) sequence stratigraphic schemes. The base is the maximum flooding surface; the top is the subaerial unconformity and in the marine part of the succession, the maximum regressive surface. The RST in the T-R model contains the highstand, falling stage, and lowstand systems tracts.
Relief: (crystallographic) The refractive index (RI) of a mineral is the ratio of the velocity of light in air to that in the mineral. Indices differ amongst minerals such that:
- Isotropic minerals have a single RI
- Uniaxial minerals have two RI along different axes, and
- Biaxial minerals have 3 RI.
The different RIs for any mineral are manifested as relief in plain polarized light, where one mineral may appear to stand above or below its neighbours. The orientation of uniaxial and biaxial minerals will also determine their relief.
Releasing bend: Strike-slip motion at a bend in the PDZ that produces extensional structures. Pull-apart, or strike-slip basins are commonly developed at releasing bends. The left or right handedness of bends is determined by looking along the trend or strike of the fault – if the bend moves to the left, it is a left bend. Thus, a full description of the fault bend might be left-handed/releasing/sinistral or left-lateral strike-slip fault. Cf. Restraining bend, stepover.
Releasing bend basin: Strike-slip basins formed by extensional subsidence at releasing bends. The type is represented by the iconic Ridge Basin in southern California.
Restraining bend: Strike-slip motion at a bend in the PDZ that produces compressional structures such as thrusts and pop-up ridges. The left or right handedness of bends is determined by looking along the trend or strike of the fault – if the bend moves to the left, it is a left bend. Cf. Releasing bend, stepover.
Retrogradation: Back-stepping of sedimentary facies and parasequences during transgression. The shoreline trajectory also trends landward. The stacking pattern in a retrogradational succession will show progressive deepening from one parasequence to the next.
Reverse fault: Steep faults (>45o) where the hanging wall (the wall over your head) moves up relative to the footwall. It is the opposite of a normal fault. Thrust faults have reverse displacements but the fault plane is usually less than 45o.
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.
Rheology: Describes the mechanical response of materials to stress. It applies to solids and fluids in Earth systems and is usually expressed as a relationship between stress and strain, or in the case of viscosity the strain rate. The three end-member behaviours are elastic, plastic (including ductile flow), and viscous behaviour. The principles can be applied to materials at the scale of the lithosphere and asthenosphere, to the behaviour of fluids in a single turbidity current.
Rhodolith: Pebbles and shells encrusted with calcareous coralline red algae. They are important contributors to cool-water carbonate sediments and limestones A common example is the alga Lithothamnion.
Ridge transform: Transform faults that segment oceanic spreading ridges; they accommodate oblique spreading. Associated ridge fracture zones represent inactive ridge transforms.
Riedel shears: Parallel arrays of faults that form during the early stages of strike-slip formation. They are oriented at low angles to the principal displacement zone, commonly about 15o, and are synthetic to the PDZ. As deformation continues the Riedel shears become linked and part of the PDZ. A second set of faults at about 75o to the PDZ are conjugate to the Riedel shears and antithetic to the PDZ. The line bisecting the conjugate set parallels the direction of principal stress (on a strain ellipse this is the long axis); it also parallels the axes of en echelon folds. A third set of shears, P shears, at 10o and less to the PDZ, may form after the Riedel shears, and link with them to form the PDZ.
Rift basin: In a plate tectonic context, an elongate basin bound by normal, commonly listric faults and related grabens that forms during the stretching of continental lithosphere. Volcanism is common in active rifts where mantle plumes manifest at as effusive and explosive eruptions. Sediments include coarse grained alluvial and fluvial deposits, associated with the elevated fault topography and rift shoulders. Evaporites accumulate in arid settings, from saline lakes or marine incursions. See rift drift, rift shoulder.
Rift-drift: Refers to the transition from continental rifting during lithospheric extension where basin subsidence is a function of brittle failure of the crust, to the production of oceanic crust and initiation of sea floor spreading. The drift stage is accompanied by accumulation of a passive margin on the continent side (trailing edge) in concert with an ocean basin. The transition may be recorded stratigraphically by a breakup unconformity.
Rift shoulder: An area of elevated topography in the footwall of rift basin boundary faults. Elevations can be 100s of metres above the plateau or plain adjacent to the rift basin. The elevated topography is a primary source of clastic sediment shed into the rift basin.
Right-lateral displacement: Normally used for describing the sense of movement on strike-slip faults. For an observer, the far side of the fault (the fault block opposite) will appear to move to the right. Synonymous with dextral displacement or motion.
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.
Ripple: Bedforms that develop by movement of sand and coarser-grained sediment at the sediment-water interface under unidirectional and bidirectional flow. In unidirectional flow, ripples generate an asymmetric profile, with an upstream stoss face, and a downstream lee face. Grains move as bedload up the stoss face and tumble, or avalanche down the lee face. The lee face dips in the direction of flow. Symmetrical, bidirectional ripples form beneath waves in response to wave orbitals interacting (to and fro) with the 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 birdsfoot 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).
Roof-floor thrusts: Major thrusts providing the upper and lower boundary faults for duplexes. At the front of the duplex a roof thrust will step down and floor thrust step up to merge into a common zone of displacement.
Rotation (kinematics): Rigid body deformation where components of shear produce a change in orientation. It may take place in concert with rigid body translation, and or distortion by ductile flow.
Roundness: The textural quality that describes the degree of rounding, or angularity of clasts. It DOES NOT refer to the overall shape of the clast. Thus, a rod-shaped clast can be as well rounded as a spheroidal clast. Although roundness can be quantified, it is usually identified by comparing the clasts under investigation with grain silhouettes, as in a handy-dandy grain size comparator. The Powers scale used today identifies a range from very angular to very rounded.
Rudstone: A clast-supported framework where more than 10% allochems coarser than 2 mm (upper limit of coarse sand), supported by grains coarser than 2 mm. In other words, everything is coarser than 2 mm. Approximately equivalent to one of Folks poorly sorted, sorted, or rounded sparites, where the qualification might include bio, ooid, pellet and so on. Dunham’s (1962) limestone classification scheme reviewed and modified by Lockier and Junaibi (2016).
Rule of Vs: A technique, borrowed from geographers, that simplifies the field mapping of strata across valleys and ridges. If bedding dip is known, it is possible to predict where across a valley or ridge the strata should be located.
Run out: The distance traveled by a pyroclastic density current or sediment gravity flow, from start to finish.
