Category Archives: SciComm

Glossary of geological terms: C

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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.

 

Calcite compensation depth (CCD): As ocean water depths increase, the partial pressure of CO2 increases and the temperature decreases – in both cases CaCO3 becomes increasingly soluble. An important consequence of this convergence is a decrease in CaCO3 saturation to the point where calcite and aragonite begin to dissolve. For calcite, the depths range from about 4.6 to 5.1 km. Aragonite is more soluble and the ACD depths are about 3 km. This means that the sea floor at or below these depth limits will tend to be devoid of calcareous sediment (particularly microfossils like foraminifera and coccoliths).

 

Calcite divide (geochemistry):  The stage during evaporation of brines where calcite precipitation determines the succession of minerals in waters subsequently depleted in Ca2+ and CO32-. It determines whether the brine subsequently evolves as HCO3 rich or  HCO3 poor.

 

Caldera: A large volcanic collapse basin resulting from withdrawal and eruption of large volumes of magma or explosive pyroclastics. Basin walls are initially steep but may become degraded over time. Many become lakes post-eruption. Calderas are the sites of some of the largest known eruptions (e.g. Yellowstone, Krakatoa, Taupo).

 

Caliche: Also called calcrete. Soil horizons in which carbonate precipitation results in a hardened crust. They develop in regions in which evaporation exceed precipitation, where periods of wetting alternate with drying. Thus, carbonate textures commonly show evidence of dissolution and reprecipitation. A common product is vadose pisoids that also show evidence of multiple episodes of dissolution and precipitation. They can develop in alluvial-lacustrine and intertidal-supratidal settings.

 

Capillary zone:  In hydrogeology, also called the capillary fringe.  It is a relatively narrow interval above the watertable where surface tension forces on aquifer materials cause water to rise and partly fill pore spaces. The capillary fringe is part of the unsaturated, or vadose zone.

 

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 predipitation 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 relatively deep basin. There are no significant margin builds such as reefs or mud mounds.

 

Carbonic acid: A weak acid that forms naturally from the reaction:

CO2 + H2O = H2CO3

It is the primary cause of slight acidity of rain (pH 5.5 to 5.8). It is an important component in the series of carbonate equilibria, particularly for pH buffering.

 

Carlsbad twins: Common twins in plagioclase and some potassium feldspars. It is an penetration twin with a plane that separates two crystal segments.

 

Cataclastite: Fine ground-up bedrock produced by grinding during faulting. The grain size range is >0.1 mm and <10 mm. Clasts are angular. There is no preferred orientation. The difference between a cataclastite and fault breccia is mainly in the degree of induration in the former.

 

Catastrophism: The principle that interpreted Earth landscapes and processes as the product of catastrophies. It had its origin in the Biblical Noachian deluge, and garnered support from events like volcanic eruptions and earthquakes. It was also compatible with Bishop Ussher’s estimate of age for the Earth computed from Biblical genealogy at about 6000 years. The principle was stood on its head in the late 18th century by James Hutton’s principle of Uniformity, later reinforced by Charles Lyell.

 

Celestial pole: An imaginary line drawn along Earth’s axis of rotation to the Pole Star, Polaris.  Because Polaris is very close to this axis, it appears to be stationary in the night sky, whereas all other stars appear to move from east to west. However, even this pole moves slowly with precession of the equinoxes, completing a complete cycle about every 25,000 years. Cf. geographic pole, magnetic pole.

 

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.

 

 

Chalcedony: A fibrous form of microcrystalline quartz, or chert. It commonly form radial clusters. Under crossed polars, extinction patterns are sweeping or radial.

 

Chemical equilibria: Chemical reactions normally written with the reactants on the left and products on the right. The two are separated by either:

  • An equal sign indicating equilibrium, where forward reactions (to the right) equal reverse reactions, or
  • By two opposing arrows that indicate forward and reverse reactions.

Equilibria should be charge and mass balanced. The quantities of reactants and products are written as concentrations or activities.

 

Chemical equilibrium: At equilibrium there is no net gain or loss of reactants (by convention, the left side of the equation) or products and no net change in energy. Note that this does not mean the system is static – even at equilibrium there are still collisions between ions (all reactions in solution involve collisions), but collisions on the left equal those on the right side of the equation.

 

Chemical facies: (hydrogeology) This is a useful concept to demonstrate the chemistry of groundwater in relation to aquifer rock-sediment composition, and the evolution of groundwater chemistry as it flows from one rock type to another. For example, flow from sandstone to limestone aquifers will be accompanied by a change in HCO3 and pH, plus the concentrations of cations like calcium and magnesium.

 

Chemical kinetics: Also called Reaction kinetics. This is the study of reaction rates and reaction pathways, and hence is distinct from thermodynamics that deals with energy transfer during reactions and is independent of rate. Kinetics is a measure of the rate of change (in concentration or activity) of both reactants and products, in reversible and irreversible reactions. It is particularly important in reactions that are slow relative to mass/solute transport. A good example if these conditions is the precipitation of dolomite under surface conditions – the reaction is thermodynamically favoured, but kinetically is very slow. Kinetics is related to thermodynamics in terms of equilibrium constants, the activation energy of reactions (i.e. Gibbs free energy), and temperature. As a general rule, the rate of 1st-order reactions doubles for every 10º increase in temperature.

 

Chemical stability (of sedimentary grains): The ability to resist dissolution or chemical change during sediment transport and burial.  Quartz tends to be chemically stable, compared with feldspar that may react, particularly during burial diagenesis to form new minerals such as clays. Minerals like zircon are extremely stable and can survive several sedimentary cycles.

 

Chemocline: A boundary within a water column at which there is a fairly abrupt change in chemical gradient. Examples include the boundary between fresh water and seawater, or changes in REDOX conditions, from oxidation to reducing.

 

Chemotroph: Organisms that obtain their metabolic energy and synthesize biomass (such as carbohydrates) from reduced elements like sulpur, sulphide, and ferrous iron, instead of sunlight.

 

Chert: SiO2. The general name given to micro- and cryptocrystalline quartz that precipitates in sediment, as biogenic products, in under volcanic and hydrothermal conditions. It may be bedded or laminated – a common occurrence in some carbonates associated with microbialites, or as siliceous oozes on the deep-sea floor and composed of diatoms or radiolarians. Sponges provide another source of fine particulate silica.  Nodular cherts tend to be diagenetic; classic examples are the nodules in Cretaceous chalks of southern England and Europe. Chalcedony is a fibrous form if microcrystalline quartz that under crossed polars shows sweeping or radial extinction. Recrystallization is common, in part because of the high surface area (and therefore reactivity) afforded by crystal size.

 

Chronostratigraphy: The part of stratigraphy that evaluates time relationships of rock units, whether as relative time like that determined from fossils or observing stratigraphic succession, or from numerical values of time measured by geochronology.

 

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.

 

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.

 

Clathrate: A general term for gas molecules that become trapped in an ice crystal cage. There are no chemical bonds between the gas and water ice and the gas can be released upon melting. Also called gas hydrates. Vast amounts of methane are trapped this way beneath the sea floor and in permafrost.

 

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.

 

Cleavage (crystallography): A plane of weakness within a crystal that will break with relative ease. It is a function of weak bonds between certain planes of atoms within a crystal lattice; the pattern of weakness repeats regularly through a crystal. Some minerals have poor or no cleavage (e.g. quartz, olivine); others have good cleavage along several lattice planes (e.g. calcite, feldspar). Cleavage can be a defining characteristic of a mineral, particularly in thin section.

 

Cleavage (structural): A regularly spaced planar to curviplanar foliation, that is a plane of weakness in metamorphic or strained rock, caused by the growth and alignment of platy minerals like muscovite. It is a penetrative fabric, pervasive at macro and micro-scales. In folded rock the intersection of cleavage with bedding is a lineation that parallels the fold axis.

 

Climbing ripples: Also called ripple drift. These structures are indicative of deposition from suspension load combined with bedload formation of ripples. In profile cross-section views, successive ripples climb over the stoss face of their nearest neighbours. They tend to form when there is an abrupt decrease in flow velocity, as at the mouth of a river, or in a waning turbidity current.

 

 

Clinoform: John Rich (1951) originally defined clinoforms as the depositional surface from wave base to the base of slope, including a shoreward undaform and a deep water fondoform. The latter two terms have been discarded. Clinoforms are now defined as the sinusoidal, chronostratigraphic surface extending from a shoreline across the adjacent shelf or platform to the slope and deep basin beyond. Clinoforms are important components of modern stratigraphic sequences.

 

Clints: Fracture networks in limestones formed by surface (meteoric) dissolution. They are common karst landscapes and occur sympathetically with grykes.

 

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.

 

Cognate epiclasts: Also called Accessory Pyroclasts. Pyroclasts derived from earlier-formed and co-magmatic volcanic rocks at the same volcano. Cf. Accidental pyroclasts, Juvenile pyroclasts.

 

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.

 

Colonnade jointing: Columnar ‘organ-pipe’ like cooling joints oriented at right angles to magma body margins. In lava flows, colonnades may be tiered, with a larger cross-section columns at the base and smaller columns that intersect the lava surface (and cooled more quickly). Cf. entablature.

 

Column collapse: Plinian and Vulcanian eruptions produce columns of hot, turbulent mixtures of juvenile fragmentals, gas and air. Gravity-induced collapse of the column produces hot pyroclastic flows and surges. This is the most common mechanism of PDC generation.

 

Columnar jointing: Regular arrays of joints formed during cooling and contraction of magma. They can occur in lava and hot ignimbrite flows, and intrusive dykes and sills. Cooling begins from the outer surfaces and progresses towards the centre of the magma body where joints are oriented normal to the outer surface. They form as straight to slightly curved columns with 4 to 8 sided polygonal cross-sections. Cf. colonnade, entablature, fracture porosity.

 

Compaction:  The process where sediment particles, once deposited, are pushed closer together to form a more tightly knit framework. Compaction begins almost immediately following deposition and continues during sediment burial. The normal compressive stress in this case is applied by the overlying sediment. Because porosity is also reduced, an additional requirement for compaction to take place is the release of interstitial water through aquifers. If fluid cannot escape (for example because of permeability barriers) then the rock body will not compact, and internal fluid pressures will rise – this is called overpressure. Mudrocks can compact to less than a tenth their depositional thickness. More rigid frameworks like sandstones compact far less. See also pressure solution, lithic fragments.

 

Compensation depth: Applies to models of isostasy where, at equilibrium there is a common depth at which lithostatic pressures are equal across all components of the lithosphere.

 

Composite terrane: See Superterrane.

 

Compressibility: The ability of a fluid or rock to change its volume in concert with changing stress, for example changing lithostatic pressures during sediment burial. It is usually expressed as the ratio of relative volume change (V) with pressure (P):

β = 1/V. (δV/δP)

Water has very low compressibility – at 6000 psi (41.4 MPa) (equivalent to 3.2 km water depth) the change in volume is 1.8%. Mudstone is highly compressible; halite is not. Compression results in a loss of porosity and permeability.

 

Compression: The application of stress that results in shortening of a rock body, or the reduction in volume of sediment, rock, or fluid. Shortening my occur through the entire body, or along faults.  It is the opposite of extension. Compression can be represented as an axis, or axes on a strain ellipse.

 

 

Concordia plot: (Geochronology). Devised by George Wetherill in 1956, For the Pb/U decay system (commonly used for zircon geochronology), the curve plots age against the three Pb/U ratios for the two U-Pb decay systems, assuming an ideal closed system (i.e. no loss of any isotope during a crystal’s lifetime). The measured isotope ratios for any crystal or batch of crystals are compared with this ideal curve; if the age from each decay system is the same and they lie on the curve, i.e. they are concordant, then that is the true age of the sample.

Discordance ages arise from Pb loss, inherited Pb from the time of crystallization, from the analysis of several crystals of different ages in a single sample, or from single zoned crystals where ages from the inner to outer most zones become progressively younger. A straight line discord will plot below the concordia and intersect it at two points – the oldest will be close to the age of crystallization.

 

Condensed section: (Stratigraphic condensation). Basically, very thin stratigraphic units that represent long periods of slow and non-deposition. They are characterised by: one or more biozone (depending on duration – 105 to 107 years); contain internal, non-depositional or erosional discordances, including omission surfaces; abundant authigenic minerals like carbonate, phosphate, chert, glauconite); commonly have hardgrounds or nodules of carbonate, phosphate, iron-manganese.

 

Conduction:  This is a diffusive process where heat is transferred via molecular vibrations. Conduction does not involve the transfer of mass, cf. convection, advection. It is a less efficient mechanism of heat transfer than convection.

 

Confined aquifer: see Aquifer-confined.

 

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.

 

Conjugate Riedel shears: See Riedel shears.

Conjugate faults: Fault pairs where the fault planes intersect at (commonly) 60o such that the direction of minimum extension bisects this angle, and the direction of maximum extension bisects the obtuse angle (~120o). Conjugate faults occur in dip-slip and strike-slip structural domains.

 

Contaminant: A chemical or substance that we would rather not be present in our environment, food, air, etc., but is present because of either natural occurrences and processes, or human-induced processes. For example, heavy metals like lead, mercury and arsenic can occur naturally concentrated in ore bodies, and released by natural weathering, or by mining, into local surface and groundwaters. Cf. pollutant.

 

Continental rifts:  Basins formed by lithospheric stretching and thinning, where the primary subsidence mechanism is faulting. Rifts commonly have high heat flow, manifested as volcanism, resulting from shallow mantle plumes (in part an isostatic response to crustal thinning). Sediment fill early in rift history tends to be terrestrial and coarse grained.

 

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.

 

Convection:  The flow of fluids en masse resulting from temperature and buoyancy gradients. Convection is the primary mechanism for transferring heat from Earth’s mantle to the lithosphere. Cf. conduction, advection.

 

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 branches, or nodular clusters around shells or pebbles (e.g. Lithothamnion). They are an important contributor to cool-water bioclastic limestones.

 

Correlative conformity: A surface marking the end of sea level fall (regression) that is correlative with the subaerial unconformity at the lowest shoreline. Its extension basinward takes it across the top of the lowstand deposits. The use of correlative conformities in sequence stratigraphy has been the subject of considerable debate.

 

Cosmogenic isotopes: Relatively rare isotopes formed on Earth surface materials (soils, rocks) and asteroid surfaces, by the interaction of cosmic rays and certain elements, such as beryllium (Be-7, Be-10), and chlorine (Cl-36). Half-lives are as short as 34 minutes (Ci-34) and as long as 15.7 million years (I-129). They can be used for dating of ice, groundwater, and exposure times at the surface.

 

Cosmopolitan taxa: Species that are distributed globally according to the appropriate environments in which they live. They are important for biostratigraphic correlations between sedimentary basins. Cf. endemic taxa.

 

Crater lakes: Water that accumulates in volcanic craters, extinct or active. On active volcanoes, eruption through a crater lake may have a strong phreatic or phreatomagmatic  imprint (depending on whether there is new magma) until all the water has been vapourised.

 

Craton: The stable interior of continents, usually made up of very old geological provinces including Precambrian shields and distinguished from more marginal mobile belts or collisional orogens. Note however that Cratonic rocks may be structurally involved during orogenic deformation.

 

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 taper theory: A mechanical theory used to explain the formation of wedge-shaped fold-thrust belts and accretionary prisms. During compression the slope, and therefore the angle of the wedge taper reaches a critical point depending on the strength of the materials, the frictional forces along the décollement, and the slope of that surface. As the critical slope or taper angle is approached, the materials within the wedge will deform – once the critical taper is reached, the entire mass slides along the décollement and there is little subsequent internal deformation of materials.

 

Crossbed: Refers to the dipping cross stratification, or foresets of bedforms like ripples and sandwaves. Foresets dip in the direction of flow (air, water).

 

Crossed polars (nicols: The upper polarizer, between the objective lenses and the Bertrand lens, filters out all remaining frequencies present in plain polarized light (PPL). The nicols can be moved in and out of the light path. If you look through the oculars when the polarizer is in the light path (i.e., crossed nicols) then no light will reach the oculars – all will be black. However, if a thin section is placed between the lower and upper polarizers, most minerals will reorient the PPL such that some of this light will pass through the upper polarizer; this light will contain slow (ordinary) and fast (extraordinary) vibration directions that will arrive at the eye pieces at slightly different times. The resulting interference produces the kaleidoscope of colour among all the minerals present. Minerals that permit the passage of light are called anisotropic; those that do not are isotropic.

 

Crust:  The outer layer of Earth. The boundary between crust and mantle is defined by the Moho. Oceanic crust is produced at oceanic spreading ridges. It averages 6 km in thickness, and consists primarily of basalt, gabbro, peridotite and ultramafic rock types. Continental crust averages 35 km thick but ranges to 70 km beneath some Cratonic blocks. Continental crust tends to be more felsic than oceanic crust. The crust is underlain by the mantle lithosphere.

 

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.

 

Cryptocrystalline: Materials like chert and chalcedony where visual resolution of individual quartz crystals require a very high powered microscope, or Scanning Electron Microscope.

 

Crystal symmetry: Symmetry describes the shape of an object and can be represented both mathematically and visually. In crystallography, the two most useful forms of symmetry are:

  1. Axes of rotation (crystallographic axes) where a particular crystal face will be repeated during rotation through 360o. The number of repetitions for a 360o rotation can be 2, 3, 4, or 6, that are referred to as two-fold, three-fold, four-fold, and six-fold (axial) symmetry respectively.
  2. Planes of symmetry where two parts of a crystal are mirror images. For an analogy, think of this concept in terms of the common bilateral symmetry in many living organisms, such as people, and many classes of mollusc. Note that planes of symmetry are NOT the same as twin planes.

Crystal systems: There are 6 crystal systems based on combination of the elements of symmetry; a seventh system – trigonal – is usually considered a subclass of the hexagonal system. The defining criteria are axial lengths, the angles between axes, and axial symmetry (the number of repetitions about an axis). The systems are: cubic (isometric – the most symmetrical), tetragonal, hexagonal, orthorhombic, monoclinic, triclinic (the least symmetrical).

 

Crystallographic axes: Three or four axes about which a crystal can be rotated through 360o.  The axes intersect at a single point (the centre of symmetry). They are labelled according to their lengths. If axes are the same length, then they are referred to as a1, a2, a3 etc. If they have different lengths, they are labelled a, b, and c. Thus, in the cubic (isometric) crystal system they are labelled a1, a2, a3, and in the tetragonal system a1, a2, c. The hexagonal system is the only one with four axes. Angles between axes are labelled α, β, γ.

 

Crystal zoning: Zoning commonly displays as concentrically arranged crystal growths, where the composition changes outwards from the crystal interior. The zones maintain the same crystallographic and optic axis. The changes in composition involve substitution of certain cations, for example in calcite Fe2+ and Mn2+ substitute for Ca2+, and in plagioclase sodium may substitute for calcium such that the inner core is a calcium anorthite and the outer zone is a sodium albite. Zoning indicates changing fluid or magma crystallization conditions. Zoned crystals may also be twinned.

 

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.

 

Cycles: The regular, periodic repetition of events. Measurement of cycle periodicities allows us to predict past and future events. In Earth sciences we recognise cycles at all scales of  time and space: daily ocean tides, revolutions around the sun, sea level rise and fall, Milankovitch orbitals, and perhaps the grandest cycle – Wilson cycles in the life and death of tectonic plates and sedimentary basins.

 

Cycle hierarchies: We recognise several orders of stratigraphic cyclicity that are usually inferred to have a causal relationship with cycles of relative sea level fluctuation. High order cycles are commonly nested, or superposed on lower order cycles:

  • 1st order cycles – about 50-100 Ma; Allogenic, depending on plate tectonic interactions.
  • 2nd  order cycles – about 5-50 Ma; Allogenic, depending on plate tectonic interactions.
  • 3rd order cycles – about 0.2-5 Ma;  Allogenic and autogenic processes.
  • 4th order cycles – about 100-200 thousand years (ka); Allogenic and autogenic processes.
  • 5th order cycles – 10 years -100 ka; Allogenic (e.g. Milankovitch orbitals) and autogenic processes.

 

Cyclothem: Cyclothems are the stratigraphic record of cycles. They consist of repetitive successions of marine sandstone, shale or limestone overlain by non-marine deposits such as coal, sandstone, and paleosols. Each cyclothem records a cycle of transgression and regression. The term was originally defined by European explorers for coal who recognised the repetitive nature of the sandstone-mudstone-coal successions. Harold Wanless (1932) extended it to include shale-limestone- paleosols cycles in the Pennsylvanian of central and eastern USA.

 

Cylindrical folds: Most folds can be described as cylindrical (imagine the cross-section of a soup can), or they contain segments that can be inscribed by (imaginary) cylindrical curves of different diameters. The concept is very useful for stereonet analysis because the dips and strikes on each limb (plotted as great circles) will intersect at a point corresponding to the fold axis bearing and plunge. All cylindrical folds have (straight) fold axes and (flat) axial planes.

 

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Glossary of geological terms: B

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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.

 

Back-arc basins: Basins behind magmatic arcs, floored by oceanic or continental crust. Oceanic back-arc basins are commonly formed by sea floor spreading; their continental cousins by rifted continental crust. In continental varieties, high heat flow is manifested as volcanic centres, and fault-controlled geothermal activity.

 

Back-bulge: A shallow basin between a flexural forebulge and the adjacent continent. It is regarded as a separate depozone in foreland basin systems.

 

Backflow: Flow on the lee side of bedforms that is opposite the overall direction of flow. Backflow may be strong enough to form small-scale beforms (commonly ripples) that migrate up the main bedform lee face or upstream.

 

Backstripping: A numerical geohistory analysis that calculates the contribution to total subsidence by tectonics – the method was developed by Watts and Ryan, 1976. The tectonic subsidence component is determined by subtracting the isostatic effect of sediment load. It is usually done in conjunction with decompaction and bathymetric corrections.

 

Back thrust: A thrust that has vergence opposite the dominant trend of a thrust system. In many cases back thrust vergence will be towards the hinterland, i.e. thrust plane dip is towards the foreland.

 

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.

 

BAF: The acronym for block and ash flows.

 

Ballistics (volcaniclastic): Blocks and bombs ejected by powerful explosive volcanic eruptions that follow a parabolic trajectory to be deposited as tephra.

 

Ball and pillow structure: Ball, or spheroidal shaped structures formed by soft sediment deformation of sandstone crossbeds during early differential compaction, that is promoted by sediment dewatering. The original cross laminations are folded, oversteepened, and even overturned. They are common in fluvial deposits.

 

Base: A base is a substance that gains a proton in aqueous solution. This can be written in a generalized way as  H+ + OH = H20.  Water can act as a base or an acid. Solutions with excess OH are basic with pH > 7.

 

Baseflow: (Hydrogeology) Baseflow is the subsurface discharge to streams from the watertable. The amount of discharge depends on the hydraulic gradient of the watertable with respect to the stream surface. During dry periods, baseflow may be the only source of water to maintain stream flow.

 

Baselevel: It is an imaginary or theoretical plane to which geological, geomorphic and geodetic measurements are referenced. The commonly accepted datum is sea level, although it is also recognized that this too changes with time. The choice is based on common sense and a recognition that shorelines are a natural boundary between marine and nonmarine realms. Other baselevels may be useful depending on the problem being investigated, for example the margin of endorheic lakes, or some arbitrary position on a deep-water submarine fan.

 

Base surge: Synonymous with pyroclastic surge. The term base surge was first used to describe turbulent, bottom hugging flows generated by nuclear test detonations in the late 1940s and early 1950s.

 

Basic (igneous petrology): Volcanic and intrusive rocks poor in free silica (quartz), but enriched in feldspars and alkali earth (calcium, magnesium) ferromagnesian minerals like amphiboles, pyroxenes and olivines. Typical examples are basalt, gabbro, and diorite. Cf. felsic.

 

Basin axis: Applies to elongate basins usually corresponding to the deepest part of a basin. The axis usually parallels orogenic belts (e.g. foreland basins) or volcanic arcs (forearc basins), rifted plate boundaries as in passive margins, or parallels fault displacement as in strike-slip basins. A basin axis may migrate laterally with changes in tectonic and/or sediment loads.

 

Basin-wide evaporites:  Accumulate on subsiding basin floor and as a consequence can reach several 100 m thick. Associated facies range from shallow marine shoreface to deeper slope and base-of-slope environments. Precipitation takes place either within the water column with crystals sinking to the basin floor (meromict), or at the sea floor (holomict). The particular mode of precipitation depends on the degree of brine stratification and evaporitic drawdown.

 

Beach: Obvious to most what this looks like. But from a sedimentological perspective it is the part of the coast, marine or lacustrine, 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.

 

Beachrock: Rapidly cemented carbonate and siliciclastic sand-gravel across a beach face; cementation occurs at or just beneath the surface. Cementation rates are measured in months. Once lithified, they can be eroded by storms into boulder deposits, that can then be re-cemented. Rapid lithification of beach sand-gravel changes the habitat for local benthic organisms. Cements are mostly aragonite and high magnesium calcite.

 

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.

 

Benioff zones: Identified in 1930s (before plate tectonics) as a zone of earthquakes associated with oceanic trenches, some as deep as 650 km.. Benioff (1949) suggested that they originated on large, continentward-dipping reverse faults where the direction of slip placed oceanic crust beneath continental crust. The Zone is now recognised as a subducting slab of oceanic crust.

 

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.

 

Bernoulli equation: Named after Daniel Bernoulli who in 1738 expressed the conservation of energy in a flowing fluid as:

Total energy E = ½ ρv2 + ρgz + P

Where ρ = fluid density, v = velocity, g = gravity constant, z = elevation with respect to a datum, P = fluid pressure.

The first term ½ ρv2 is kinetic energy; the term ρgh is potential energy; P is fluid pressure, or force per unit area. Because groundwater generally moves very slowly, the kinetic energy term is ignored. The equation allows us to express the potential energy, or hydraulic potential for groundwater flow, commonly referred to as total hydraulic head, in terms of two components – a pressure head, and an elevation head, relative to a datum. Thus hydraulic head can be expressed in terms of some height, or elevation (e.g. metres, feet etc.).

 

Biaxial minerals: Anisotropic minerals where plain polarized light entering at any angle, other than along two optic axes, is resolved into two planes of polarized light; these two planes each contain the fast and slow rays. The resulting colour depends on the different in the refractive indices of these two light paths – i.e., the birefringence. Minerals may be positively or negatively biaxial, depending on the orientation of fast and slow rays.

 

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).

 

Bioclast: Any clast derived from vertebrate or invertebrate organisms. Bioclasts may represent an entire organism, such as a bivalve shell or foraminifera, or fragments thereof. They are common framework constituents of siliciclastics and carbonates.

 

Biomould: Also biomold. The impression of an organism left in a rock following dissolution of the original skeletal mineral – commonly calcite and aragonite. A cast of this impression is formed if the mould is filled with a new precipitate or sediment.

 

Bioremediation: The use of living organisms to help clean up contaminated sites or aquifers, primarily using naturally available or introduced microbes. For example, certain bacteria will break oil down into manageable compounds like carbon dioxide or methane.

 

Biostratigraphy: The chronological ordering of strata based superposition of strata and the observed stratigraphic variations in fossils and fossil assemblages. The principle of faunal succession is based primarily on the appearance of specific organisms in certain strata that, in progressively younger rocks (deemed younger because they occur higher in the stratal succession), evolve into different, but related organisms.

 

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.

 

Birefringence: Plain polarized light that passes through a mineral is resolved into mutually perpendicular fast and slow rays that will each have different indices of refraction as (i.e., their refraction paths and velocities will be different). Birefringence is the maximum difference between these two index values. Under crossed nicols, the difference is manifested in the ‘intensity’ of interference colours.

 

Blind thrust: A thrust that does not breach the surface at the time of its formation. Blind thrust tip points (tip lines) typically contain fault propagation fold pairs.

 

Block and ash flow: Ground-hugging, concentrated PDCs characterised by a vast range of clast sizes, including blocks having dimensions measured in metres. They are usually derived from collapsing lava domes. They are commonly associated with pyroclastic surges. Deposits consist of a mix of ash and blocks; they are poorly sorted, usually matrix-supported, and poorly- or ungraded. Block angularity is highly variable.

 

Blocks/bombs (volcaniclastics): Both terms are used as textural descriptions for primary volcaniclastics (regardless of their origins). Clast sizes are 64 mm and coarser.

 

Boil over: PDCs can be generated by fire fountains that eject large volumes of fragmented lava over a crater rim. PDCs formed in this way are concentrated in lapilli, splatter, and flattened or aerodynamically shaped bombs.

 

Bombs – ballistics (volcaniclastic): Ejected lava fragments, or bombs (particularly in Hawaiian and Strombolian fire fountains), can be shaped aerodynamically into spindle-like ballistics while being flung through the air. If the lava is still molten when it lands it will spatter and cool in a variety of shapes (e.g. cow-pat, bombs, bread crust bombs).

 

Bomb sags (Volcaniclastic): Large, ballistic blocks and incandescent fragments of magma ejected during an eruption, may land on earlier deposited tephra causing the beds to sag. The bedding deformation may be accentuated during compaction.

 

Bottom simulating reflector (BSR): In some oceanic regions, methane hydrate layers (methane clathrate) just below the sea floor have much lower density than the overlying and underlying sediment. They present as a prominent reflector on seismic profiles.

 

Botryoidal cement: In limestones, this cement form is presented as radial clusters of fibrous or bladed calcite or aragonite that precipitate in more cavernous porosity. Common examples are found in reef frameworks, and fenestrae that form by mineral dissolution, gas bubbles, and crystal expansion (e.g. halite-gypsum crystal growth in sabkhas). Fenestrae are common in some cryptalgal laminates and mud mounds containing Stromatactis.

 

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:

  1. Massive muddy sandstone, with or without a scoured base.
  2. Graded and laminated muddy sandstone.
  3. Laminated with ripples and climbing ripples, commonly convoluted by soft sediment deformation.
  4. Graded, laminated siltstone-mudstone.
  5. A mix of turbidity current mud and hemipelagic mud, that are deposited from suspension.

 

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.

 

Bowen reaction series:  A predictable order of mineral crystallization in a cooling magma, after the early 20th Century geologist Norman Bowen. One of the first minerals to crystallize from magma is olivine (from about 1300o to 1200oC). Feldspar, the most common rock-forming mineral, begins to form below temperatures of about 1000oC, and one of the last to appear, quartz at about 800oC. Bowen’s discovery revolutionised the way we think about the evolution of igneous rocks.

 

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.

 

Branch point: Locations along a thrust where branching or fault splays are generated.

 

Breakup unconformity: A significant, commonly angular stratigraphic discordance between syn-rift deposits that are confined to fault-bound grabens and half grabens, and the base of more widespread post-rift strata that signal the change to a passive margin. In a plate tectonic context it signifies the transition from rifting to sea floor spreading (rift-drift).

 

Breccia: Consists predominantly of angular clasts larger than 2mm. Like conglomerates they are poorly sorted, clast-supported frameworks. The degree of clast angularity indicates little or no reworking.

 

Brines: Generally used for natural waters more saline than seawater. The main dissolved salt is sodium chloride (NaCl), but calcium and magnesium sulphates are also important constituents, and there are several important trace elements, such as lithium. The primary mechanism for brine concentration in ocean basins and saline lakes is evaporation. The saturation level for NaCl is about 357 ppt (normal seawater is 32 ppt).

 

Brittle behaviour (rheology): Most Earth materials behave elastically up to their elastic limit, beyond which deformation is irreversible. If the strain rate is high, this deformation will take place as sudden fracturing (e.g. broken glass). Brittle deformation is also enhanced by low confining pressures and low temperatures – these are the conditions that lead to faulting and fracturing during an earthquake. Cf. Ductile flow.

 

Bubble texture (volcaniclastic): A texture characteristic of volcanic ash presented as highly arcuate apophyses in shard walls, or as complete bubble outlines within shards. They commonly form during explosive eruptions, from the introduction of superheated steam when magma is in contact with water (as in phreatomagmatic eruptions), or from degassing of volatiles within the magma.

 

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.

 

Buoyant plume: A turbulent mix of gas, air and fine particles that is less dense than air. It develops above the main body of a pyroclastic flow or sediment gravity flow by elutriation of particles from the main flow. The plume dissipates as the particles settle gravitationally.

 

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Glossary of geological terms: A

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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.

Thanks.

 

Aa flows: Massive lava flows that grade upward to a chaotic jumble of blocky and clinker lava fragments. Aa flows tend to advance more slowly than pahoehoe flows, where broken lava blocks tumble down the flow front and are overridden by the oncoming mass. (cf. pahoehoe budding).

 

A/S ratio (Accommodation/supply): The ratio between the rate of change of accommodation and sediment supply rate. The concept recognises that the expected change in accommodation with relative sea level rise or fall can be offset by sediment supply. For example during rising sea level, high rates of sediment supply can produce progradation instead of retrogradation during transgression.

 

Abiotic: Physical and chemical conditions not directly associated with life forms, but interact with biotic conditions to form ecosystems. For example, salinity, pH, temperature, precipitation. The term includes organic compounds present in abiotic conditions such as comets. Cf. prebiotic.

 

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.

 

Absolute age: A term that should be abandoned. There are no absolute ages in geology, only relative ages or radiometric ages. Radiometric ages depend on isotope half life and blocking temperature; any measured age has errors.

 

Accidental pyroclasts: Fragmental debris derived from basement rocks during an explosive eruption. May occur with Juvenile and Cognate pyroclasts.

 

Accommodation: In depositional systems, it is broadly defined as the space available for sediment to potentially fill. It is usually referenced to baselevel that in marine systems is sea level.   This definition does not imply the mechanisms that create accommodation space. Accommodation space can increase during relative sea level rise or decrease during sea level fall. Such changes are caused by allogenic and autogenic processes. The concept of accommodation has evolved into one that also incorporates sediment supply.

 

Accretionary aggregates: The aggregation of fine ash into pellets, a few millimetres in diameter, within turbulent, wet ash columns and plumes derived by explosive phreatic and phreatomagmatic eruptions. Electrostatic charges in the turbulent plume play an important role. Experimental evidence also indicates that cementation by sulphates and other minerals can occur rapidly in the plume – this increases their preservation potential. Pellet cores may contain fine ash, or fine lapilli. Pellets may be completely unstructured, or consist of concentrically layered fine ash. Aggregates with multiple concentric layers constitute the well know accretionary lapilli. They may flatten in impact with the ground. They  range from about 5 – 25 mm diameter.  There is some evidence they have formed on Mars.

 

Accretionary lapilli Accretionary aggregates of fine ash surrounded by multiple, concentric layers (onion like) that form within turbulent, wet ash columns and plumes during explosive phreatic and phreatomagmatic eruptions.

 

Accretionary prism: Accretionary prisms are wedge-shaped stacks of oceanic sediment and some volcanic rock scraped from the top of the subducting lithosphere and plastered over the trench slope. Older autochthonous deposits on the upper plate may also be incorporated.  Each slice of sediment is separated by landward-dipping thrusts (i.e. verging towards the trench). Accretion begins at the frontal taper. Landward stacking of thrust panels occurs above a décollement, where the oldest panels are farthest from the trench.

 

Acid: A substance that releases or donates a proton when dissolved in water. The proton is a hydrogen ion that in solution associates with an H20  molecule to form H30+ , but is usually written as H+ . Acids react with bases (bases contain hydroxyl ions – OH ). Solutions with excess H+ are acidic, such that pH < 7.

 

Active rifting: Extension and stretching of the lithosphere, and development of rift basins promoted by a rising mantle plume. Cf. Passive rifting.

 

Activity (geochemical): Sometimes referred to as effective concentration. The activity of an ion is the ratio of its concentration versus some standard concentration and is therefore dimensionless (unlike concentration). The ratio is calculated using an activity coefficient. It is used in equilibria because it expresses the amount of an anion or cation that is available for reaction; compare concentration that measures the total amount of an ion. In a solution like sea water there are many different cations and anions, all reacting to collisions of various kinds. For example, the CO32- anion may collide with cations other than Ca2+ (Na+, Mg2+, K+ and so on), such that the amount of CO32- available to react with Ca2+ is less than the measured concentration. In other words, the amount of CO32- available in real solutions depends not just on its overall concentration, but also on its environment. For this reason, it is preferable to use activities in thermodynamic calculations, such as equilibrium constants. The activity of solids is usually taken as 1.

 

Activity coefficient:  The activity coefficient (γ) for a specific ion species is related to the degree of ionic interaction with other species in solution. For dilute solutions γ approaches 1 because there are few ion interactions (γ is dimensionless). Thus, the γ value for HCO3 in fresh river water averages about 0.95, but in sea water is much lower (0.57) because of ionic interactions. Activity (a) is calculated for specific ions from the relationship:

a = γ m where m is concentration.

 

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

 

Advective fluid flow:  The flow of fluids through a porous medium; in this case only the fluids move. Advective flow via aquifers is the most efficient mechanism for mass transfer of dissolved solids in the shallow crust. cf. convective flow, groundwater flow.

 

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.

 

Aerosol: Small droplets of liquid of solid particles suspended in air, mainly by air turbulence. Liquid aerosols commonly have dissolved compounds like sulphuric and hydrochloric acid derived from volcanic eruptions. They are important in Earth’s upper atmosphere because can they reflect incoming solar energy, resulting in cooling, or absorb heat that raises atmospheric temperatures.

 

Afar depression: Also called the Afar Triangle. The northernmost sector of the East African Rift System, characterised by alkaline volcanism and hypersaline-hyperacidic lakes. It is the most advance sector of continental rifting that appears to be transitional to incipient production of oceanic crust and sea floor spreading. It borders Ethiopia, Eritrea, and Djibouti. Its shores are washed by Red Sea and Gulf of Aden. In a plate tectonic context It is considered to be a triple junction  – the confluence of EARS, and the Red Sea – Gulf of Aden sea floor spreading.

 

Aggradation: The vertical accretion of strata when sediment supply greatly 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 has a significant vertical component.

 

Airfall ash/tephra: Volcanic ejecta (ash to block sizes) that falls to the surface from an eruption column. Deposits tend to mantle topography but may be reworking by precipitation runoff. Deposits may be size-sorted because of gravitational settling; individual beds become finer and thinner with distance from source.

 

Air sparging: A method of groundwater remediation that uses air forced down a borehole into an aquifer, to volatilize hydrocarbon contaminants. The produced vapour phase is extracted and scrubbed to remove the offending compounds.

 

Albite twins: Common twinning in plagioclases and potassium feldspars, presented as multiple, parallel lamellae that traverse the entire crystal section. The width of twin segments decreases and the number of lamellae increases in more calcic plagioclases.

 

Alizarin Red-S: This is a soluble organic acid that reacts with calcium. Distinguish between calcite (stains pink-red) and dolomite (no stain) can be easily done using this stain, on rock slabs or thin sections.

 

Alkalinity: Alkalinity is a measure of the amount of acid that can be added to an aqueous solution without causing significant changes to the pH; also referred to as the acid neutralizing capacity or buffering capacity. The total alkalinity of seawater is primarily determined by the major anions:

mHCO3 + 2mCO32- + minor constituents like borate, phosphate, and silicate anions.

 

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.

 

Allochthonous: In geology this means a sediment, rock, lithospheric block, or fluid body that has moved from the place where it formed to the place where it is now found. Cf. Autochthonous.

 

Allochthonous terrane: See Terrane.

 

Allodapic limestone: Slope and deeper basin limestones deposited by turbidity currents.

 

Allogenic processes: (allocyclic processes). Control of stratigraphic architectures and sea level by processes acting outside a depositional system. Typically, this includes regional subsidence, tectonics in sediment source areas (e.g. mountain building and erosion), climate, and glacio-eustatic sea level fluctuations. Cf. Autogenic https://www.geological-digressions.com/autogenic-or-allogenic-dynamics-in-stratigraphy/

 

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.

 

Amygdaloids: Vesicles that are filled with mineral precipitates (commonly calcite, zeolite, chlorite). Precipitation occurs after the magma has cooled. cf.  Spherulites.

 

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°.

 

Angular unconformity: A stratigraphic surface that separates two bodies of strata having different orientations, the underlying rocks being much older than those overlying. Iconic examples are Hutton’s unconformities at Lochranza and Siccar Point that record long hiatuses between periods of deposition and mountain building. Cf. disconformity.

 

Anhedral: Refers to crystal form where the original crystal faces have been removed (by abrasion or dissolution) or are not recognizable because of crystal intergrowth. Neomorphic crystals are commonly anhedral.

 

Anisotropic minerals: Minerals in thin section reorient plain polarized light, resolving it into two vibration directions that will pass through the upper polarizer when nicols are crossed. One direction contains a fast light ray (also called the extraordinary ray), the other a slow ray (ordinary ray); the fast and slow rays are perpendicular to each other. Anisotropic minerals are further divided into uniaxial and biaxial based on the presence of one or two optic axes.

 

Anisotropy: An aquifer or aquitard is considered anisotropic if its permeability or hydraulic conductivity is not the same in all directions; usually specified along three principal orthogonal axes. Most porous aquifer media are anisotropic because of sedimentary bedding, sedimentary structures like crossbedding, fracture and joint networks, or tectonically induced structures like cleavage, folds or faults. Cf. isotropy.

 

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: Ripple-like bedforms that develop in Upper Flow Regime conditions. The corresponding standing (surface) waves are in-phase with the bedforms. Unlike ripples, the accreting bedform face grows upstream – antidunes appear to migrate upstream, against the flow. When flow conditions wane, they tend to wash out – therefore, preservation potential is low.

 

Anticline: Convex upward or outward folds, where layers are stratigraphically younger  in the convex direction. Cf. antiform, syncline.

 

Antiform: Convex upward folds; the name is reserved for folds where stratigraphic younging, or facing directions are unknown. If younging direction is known, the name anticline is used. Cf. Synform.

 

Antithetic faults: Subsidiary or minor faults that have a sense of displacement opposite that of a master fault or principal deformation zone. They occur in extensional regimes (such as accommodation in the hanging wall of listric faults), compressional regimes as back-thrusts, and strike-slip faults. Opposing subsidiary structures are synthetic where the displacement has the same shear sense as the master fault. Antithetic and synthetic faults commonly occur in parallel arrays. Cf. Riedel shears.

 

Antoine Lavoisier: (1743-1794) published one of the first explanations of transgression and regression, and the relationship of grain size in marine environments that would later become important for the development of facies concepts.

 

Aphanitic: Used to describe fine-grained volcanic and intrusive rocks where individual crystals cannot be observed without a microscope. Cf. Phaneritic

 

Aquifer: A porous and permeable medium beneath the surface that permits groundwater flow.  In hydrogeology, the definition has a very pragmatic value, where the amount of groundwater flow is usable (as in extraction); everything else is an aquitard.

 

Aquifer – confined:  This term applies to aquifers that are bound above, below, and laterally by aquitards. Confined aquifers are always saturated. Their hydraulic potential is defined by a potentiometric surface.

 

Aquifer mining: Excess removal of groundwater from a confined aquifer will cause irreversible changes to the structure of the porous medium (commonly sand grains), causing the grains to pack more densely. Not only does this reduce porosity, permeability and therefore water production, it also causes a reduction in the solid volume of the aquifer. Excessive mining can eventually cause land subsidence.

 

Aquifer – unconfined:  The upper boundary of unconfined aquifers is at Earth’s surface. They contain a watertable, above which is an unsaturated zone where pore spaces are air-filled at atmospheric pressures, and a saturated zone below. Drainage of an unconfined aquifer is by gravity alone. Common examples include fluvial and alluvial gravels and sands.

 

Aquiclude:   An aquiclude prevents any kind of groundwater flow. Examples include granite-like lithologies, and thick sequences of halite (although even these lithologies have permeability, albeit extremely low. Other aquicludes involve artificial barriers designed to prevent or deflect contaminated groundwater flow.

 

Aquitard:  Any rock or sediment unit that retards groundwater flow. Common examples include mudstones and other mud-prone lithologies such as glacial diamictites. An important property of aquitards is their ability to release water by vertical seepage to confined aquifers.

 

Arctic circle: Currently at latitude 66°33′46.9″N, it is the southern limit of continuous 24 hour daylight (summer) or night (winter) – actually measured to the centre of the Sun. It is moving north at about 15m/year because the earth’s tilt moves about 3o over a 41,000 year cycle known as Obliquity. There is an corresponding polar circle in the southern hemisphere.

 

Arenite: Almost synonymous with sandstone, although its definition is a bit more precise: a rock composed mainly of sand-sized grains and having less than 15% matrix. Greater than 15% matrix and the rock is a wacke.

 

Argillite: The general name for highly indurated mudstone. They tend to have a greenish hue, in part because of chlorite cements, in addition to illite plus or minus calcite. In many argillites there is a subtle transition from burial diagenesis to incipient metamorphism with the alteration of illite clays to white micas, and the appearance of prehnite or pumpellyite.

 

 

Arkose: An arenite (sandstone) that has 25% and more feldspar grains.

 

Armillary sphere: A device with moveable, concentric spheres designed to map the heavens in three dimensions, mostly with Earth at the center; They were probably invented in China about the 3rd to 1st centuries BC, or ancient Greece, or both.  They became popular adornments in the 15th and 16th centuries. Their construction required remarkable craftsmanship.

 

Armoured mudball: A soft, cohesive ball of mud (mud rip-up) that picks up and embeds smaller clasts of rock as it is rolled along a channel floor.

 

Artesian spring: Flow from an aquifer is spontaneous if the watertable (unconfined aquifer) or potentiometric surface (confined aquifer) lies above the local ground surface. Spring flow is usually focused along faults, fractures, or bedding planes.

 

Artesian condition: The water level in a borehole drilled into a confined aquifer, will rise to a level that reflects the hydraulic potential of the aquifer at that location. If the water level lies above the top of the aquifer, the aquifer is said to be artesian. This is the most common condition for shallow, confined aquifers. See also potentiometric surface, hydraulic head.

 

Aspect ratio: In basin analysis this applies to the ratio of areal extent represented as along-strike length, to down-dip width. It is more commonly used in description of strike-slip basins that tend to be long and narrow.

 

Asthenosphere: The mechanically weak, upper mantle layer underlying the lithosphere, where deformation is primarily by viscous creep. The lithosphere-asthenosphere boundary is defined by the solidus – an isotherm at 1100o – 1300oC.

 

Asymmetric ripples: The most common ripple form in unidirectional flow. Bedforms are asymmetric  in profile views, with the lee face the steepest and facing downstream.

 

Atoll: Small oceanic islands and a lagoon protected by fringing coral reefs that sit atop an extinct submarine volcano, or sea mount. The volcanic edifice subsides during cooling of the sub-crustal rocks. Growth of the coral reef keeps pace with subsidence, but the atoll will eventually founder beneath the waves.

 

Aulacogen:  Failed rifts oriented at high angles to plate boundaries. They may represent rifting at triple junctions where one ‘arm’ of the rift system failed to progress to the drift, or sea floor spreading stage.

 

Autobreccia (volcanicliastic): The non-explosive brecciation of cooling lava as it flows can take place at the flow top, base, and margins. Breccia fragments can be incorporated into the bulk of the flow and, depending on their temperature, may bend or weld to other fragments. They are common in viscous, slow moving felsic magmas (rhyolites, dacites), but can occur in basaltic lavas.

 

Autochthonous: In geology this means a sediment, rock, or fluid body forming in the place where it is now found. Cf. Allochthonous.

 

Autogenic processes: (autocyclic). In stratigraphy, this refers to processes acting within a depositional system, that produce changes stratigraphic architecture and relative sea level. They can produce depositional patterns similar to those associated with allogenic processes like climate, tectonic, or sea level changes. Cf. Allogenic.

 

Autotroph: An organism that produces its own energy and food without the assistance of other organisms. Important groups include photoautotrophs that photosynthesize organic production, and chemoautotrophs that use inorganic chemical compounds.  The group includes microbes like algae, bacteria and fungi, plants, and phytoplankton. 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.

 

Axial plane: An imaginary plane that connects the fold axes for every layer in a fold. It is always flat. All cylindrical folds have an axial plane. Cf. Axial surface.

 

 

Axial surface: An imaginary plane formed by connecting the hinge lines of all layers in a fold. It may be flat or curved. If it is flat it is called an axial plane.

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Classification of sedimentary basins

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A schematic of sedimentary basins distributed across three continental blocks, an ocean basin (e.g. Pacific basin), and a remnant ocean basin (e.g. Juan de Fuca plate). Key tectonic elements are: subduction zones (black triangles) and associated basins, an orogenic thrust belt resulting from continent-continent or terrane-terrane collision (e.g. Alberta foreland basin) and associated unroofing of a metamorphic core complex (e.g. Omineca Belt, Canadian Rockies), an orogenic belt in the plate above a subduction zone associated with a volcanic arc, continental rifting with a nascent rift basin (e.g. Red Sea), and a much older passive margin sedimentary prism (east coast North America), ocean crust rifting at spreading ridges, basins associated with transform faults, and intracratonic (e.g. modern Hudson Bay) and intra-oceanic basins, the latter depicted as a moat around large sea mounts (e.g. Hawaii). Figure is modified from Ingersoll (1988) who modified it from Dickinson (1980).

A schematic of sedimentary basins distributed across three continental blocks, an ocean basin (e.g. Pacific basin), and a remnant ocean basin (e.g. Juan de Fuca plate). Key tectonic elements are: subduction zones (black triangles) and associated basins, an orogenic thrust belt resulting from continent-continent or terrane-terrane collision (e.g. Alberta foreland basin) and associated unroofing of a metamorphic core complex (e.g. Omineca Belt, Canadian Rockies), an orogenic belt in the plate above a subduction zone associated with a volcanic arc, continental rifting with a nascent rift basin (e.g. Red Sea), and a much older passive margin sedimentary prism (east coast North America), ocean crust rifting at spreading ridges, basins associated with transform faults, and intracratonic (e.g. modern Hudson Bay) and intra-oceanic basins, the latter depicted as a moat around large sea mounts (e.g. Hawaii). Figure is modified from Ingersoll (1988) who modified it from Dickinson (1980).

The rationale for classification of sedimentary basins

The formulation of plate tectonic theory in the late 1960s led to a complete rethink about how sedimentary basins form; how they subside and how the sedimentary fill is accommodated. The pioneering investigations by W.R. Dickinson and his colleagues laid the foundations for a geodynamic approach to basin analysis, governed principally by plate interactions based on:

  • the composition and rheology of the lithosphere (oceanic, continental, transitional)
  • proximity to plate boundaries, and
  • plate trajectories (extensional, convergent, transform).

Dickinson recognised that sedimentary basins evolve in concert with tectonic plates and plate boundaries.  For example, along convergent plate boundaries the relative trajectory may change from purely orthogonal to oblique, with subsequent transitions in the style and mechanisms of subsidence and sediment fill (he also pioneered concepts of sediment routing and provenance in relation to plate tectonic environments).

The plate tectonic environment also provides a rationale for the diversity of basin shapes, sizes, and longevities. For example, compare the rapid subsidence typical of strike-slip (wrench, pull-apart) basins (105 to 106 years), compared with 107 to 108 years for passive margins. The difference in size between these two basin types is also an order of magnitude or two.

[It is worth remembering at this point that all plate motions on Earth, as a sphere, are relative and dictated by poles of rotation, called Euler poles. Thus, orthogonal convergence at some location on a boundary, relative to a pole of rotation, will have corresponding strike-slip or oblique strike-slip motion (transpressional or transtensional), or extensional motion elsewhere. If the pole of rotation moves, then the sense of motion will also change. Motion relative to a pole of rotation can be plotted as a vector that has direction, and magnitude (velocity).]

The geodynamic context of sedimentary basins is embedded in several classification schemes, particularly those proposed by Dickinson (1974), Kingston et al. (1983) , and later iterations by Busby and Ingersoll (1995), Ingersoll (2012), and Allen and Allen (2013). The utility of such schemes lies not just in the convenient labeling of basins but placing them in a broader context of plate history.

Dickinson’s scheme and its later iterations are based on the four main types of plate boundary:

  • Divergent, extensional or rifted margins,
  • Convergent margins, mostly subduction and/or collision related
  • Transform margins bound by lithosphere-scale strike-slip faults, and
  • Intraplate settings, that are distant from plate boundaries but are subjected to long wavelength buckling stresses (a few 100 km) generated by plate interactions.

 

The Kingston et al. classification scheme

The basins classification proposed by Kingston et al. (1983) expanded Dickinson’s plate tectonic modus operandi by incorporating basin-scale depositional style. The rationale for this approach was dictated by the needs of petroleum exploration. Their Figure 1, modified slightly here, is a kind of flow chart that was intended to aid identification of ancient basins.

 

Sedimentary basin classification according to (and modified from) Kingston et al. (1983), presented as a flow chart. The three primary identification parameters are plotted against the two main crustal domains: continental and oceanic. I have added more recent basin terminology below their main basin types.

Sedimentary basin classification according to (and modified from) Kingston et al. (1983), presented as a flow chart. The three primary identification parameters are plotted against the two main crustal domains: continental and oceanic. I have added more recent basin terminology below their main basin types.

Classes of sedimentary basins in their scheme are based on:

  • Depositional cycles or stages that represent distinct tectonic episodes. These cycles are basin-scale in thickness and duration, are usually bound by unconformities, and have predictable stratigraphic transitions from marine to non-marine. They also refer to these as depositional sequences and depositional episodes and yet there is no explicit reference to the sequence stratigraphic methodologies of Sloss (1963), Frazier (1974), or the Exxon crew.
  • Basin-forming tectonics incorporates two main plate boundary interactions: divergence and convergence, and the proximity of basins to boundaries, i.e. plate margin or plate interior. They regard strike-slip systems as variations of either of these boundary types.
  • Basin-modifying tectonics recognises that changes in plate motions will result in significant changes in basin evolution wherein basins may become inverted (uplifted and eroded) or subside according to different mechanisms. This parameter explicitly recognises that many sedimentary basins represent multiple tectonic histories.

The Kingston et al. Scheme identifies 10 theoretical basin types, but for practical purposes excludes two of these (oceanic trench and oceanic fracture types) because they were deemed of little use to petroleum exploration.

 

The Dickinsonian classification schemes

In a series of papers, Ingersoll (1988, 2012), Busby and Ingersoll (1995) used the Dickinson’s framework to progress successively more complex classification schemes. The latest iteration lists 32 basin types (Ingersoll, 2012). These schemes are based on actual, modern basin analogues where we can be reasonably certain of the association between basins and their plate tectonic setting. Excluded from the classification criteria are the architecture and composition of the basin fill, setting it apart from the Kingston et al. scheme.

 

Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels

Panel 1. Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels – click on each for an enlarged view.

Panel 1. Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels

 

Panel 2. Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels

Panel 2. Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels

 

Panel 3. Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels

Panel 3. Ingersoll’s Table 1.2 (2012) of sedimentary basins has been modified slightly and split into three panels

From a purely geodynamic perspective, the Kingston et al. scheme is less useful because the 10 basin types refer to highly generalized plate tectonic settings, for example, Trench associated basins includes the actual deep trench, trench slope basins, forearc and interarc basins. There is also some ambiguity with their placement of foreland basins that overlie continental crust, but do not seem to fit any of the convergent margin designations for either continental or oceanic crust.

The successive Ingersoll schemes outline specific basin types, employing well-known terminology; the flow chart has been recreated from Ingersoll (2012, Table 1.2). It too is divided into the four main types of plate boundary, with 26 basins plus 6 ‘miscellaneous’ varieties. While 32 different basin types may seem unwieldy (Ingersoll offers a cryptic apology for this), each is much easier to apply because of the actualistic basis for the classification. In other words, the scheme is not encumbered by awkward details of stratigraphic architecture.

 

The Allen and Allen classification scheme

The third classification scheme considered here is from Allen and Allen (2013). It identifies basin types based on the primary mechanisms of subsidence and uplift, viz. isostasy, flexure, and the dynamic topography (dynamic topography is the subsidence or uplift of the surface caused directly by buoyancy-convection in the mantle. This is different to the topography generated during lithospheric adjustments towards isostatic equilibrium). Basins are further subdivided according to their location on continental and oceanic lithosphere.

Flexure here refers to loading by either or both tectonic and sediment loads. Foreland basins are classic examples where flexural subsidence is initiated by emplacement of thrust sheets but is augmented by the growing load of sediment. In contrast, subsidence in forearc basins, that sit atop a structurally telescoped wedge of cold oceanic crust, depends more on the tectonic load.

 

The basin classification according to Allen and Allen (2013), based on the principal subsidence mechanisms, including sediment load which is highest in continental basins, and of declining influence in oceanic basins.

The basin classification according to Allen and Allen (2013), based on the principal subsidence mechanisms, including sediment load which is highest in continental basins, and of declining influence in oceanic basins.

Although not explicitly stated, the inclusion of sediment loading as an important determinant of subsidence implies there must be some knowledge of sediment thickness and the timing of sediment fill; in other words, stratigraphic architecture. Timing is important here because periods of rapid sediment flux will have a greater impact on the isostatic and flexural response than periods of little sediment input. Extended periods of erosion will have the opposite effect, promoting uplift.

 

Afterword

Classification schemes will evolve as our knowledge of sedimentary basins improves. The schemes shown here all have a reasonably finite number of basin types, some more useful than others depending on one’s approach to basin analysis. Theoretically, there is potential for a larger number of basin types if one wished to split categories into a myriad hybrids. But the utility of such splitting is questionable – at some point classification schemes become unwieldy. At present, the Ingersoll and Allen x 2 schemes seem to strike different but reasonable balances, depending on which basin analysis outcome you chose to explore.

 

Topics in this series

Sedimentary basins: Regions of prolonged subsidence

Defining the lithosphere

The rheology of the lithosphere

Isostasy: A lithospheric balancing act

The thermal structure of the lithosphere

Stretching the lithosphere: Rift basins

Nascent conjugate, passive margins

Basins formed by lithospheric flexure

Accretionary prisms and forearc basins

Basins formed by strike-slip tectonics

Allochthonous terranes – suspect and exotic

Source to sink: Sediment routing systems

Geohistory 1: Accounting for basin subsidence

Geohistory 2: Backstripping tectonic subsidence

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Stratigraphic condensation – condensed sections

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An idealised cross-section of a condensed section at an omission surface. Condensation is a function of sediment removal by tidal currents, a sea floor starved of sediment, or sediment bypass to deeper waters. The original soft sediment biota (trace fossils, invertebrates) changes to a different set of fauna and flora as the hardground develops. Palimpsest shells become encrusted with barnacles and bryozoa. The hardground biota is characterised by boring and encrusting organisms.

An idealised cross-section of a condensed section at an omission surface. Condensation is a function of sediment removal by tidal currents, a sea floor starved of sediment, or sediment bypass to deeper waters. The original soft sediment biota (trace fossils, invertebrates) changes to a different set of fauna and flora as the hardground develops. Palimpsest shells become encrusted with barnacles and bryozoa. The hardground biota is characterised by boring and encrusting organisms.

Condensed sections record multifarious interactions of physical, biological, and geochemical processes played out over long time spans, all condensed into a few very thin beds.

 

The stratigraphic record is anything but continuous. We know that unconformities and disconformities represent missing geological time, significant gaps where the rock record either never developed or was removed by erosion. But what about the rest of the stratigraphic record? How complete is it?

Andrew Miall (2014) refers to the “emptiness of the stratigraphic record”, echoing Ager’s (1973) earlier comment that the “sedimentary record is more gap than record “. Take your average shallow marine, turbidite, or fluvial succession. At the most basic, bedding planes represent breaks in sedimentation, breaks that might be measured in minutes or centuries. Likewise, fluvial channel deposits that are replete with cross-cutting bedforms might represent 10s to 100s of years accumulation and yet we are only witnessing the final acts of preservation, wherein the record of all previous channel bedforms exists only in the facies models we derive. Miall’s assessment of the Late Cretaceous Mesaverde Group indicates that the actual record of deposition exposed in Book Cliffs may be as little as 10% of its 4.86 million-year duration.

Condensed sections are a special kind of ‘stratigraphic incompleteness’. Kidwell (1993), and a more recent review by Föllmi (2016 – who also provides a list of well-known examples), define a condensed section as:

  • Very thin units (decimetres) relative to
  • The span of time they represent (105 to 107 years). For example, the fossiliferous Late Oligocene Kokoamu Greensand in southern New Zealand, is commonly 1-5 m thick but represents a time span of about 4 million years.
  • Containing a biota representative of the span of time, meaning that over a period of 106 years, several faunal or floral biozones may be present. However, the paleontological record may be complicated by postmortem mixing, commonly by bioturbation.
  • Containing internal discordances and hiatuses caused by erosion or non-deposition. Bedload transport and erosion of sediment on the sea floor is typical of shoreface dynamics on siliciclastic shelves and carbonate platforms. Non-deposition may be due to sediment starvation (other than fine particles in suspension), for example during transgression when river supply is thwarted by rising baselevels; or situations where sediment bypasses a shelf or platform on its way to deeper waters.
  • Containing abundant authigenic minerals (i.e. precipitated at or close to the sea floor) such as carbonate, phosphate, silica (chert), glaucony (glauconite green sands), manganese and iron oxides.
  • Hardgrounds are common sites of condensation. Examples include carbonate hardgrounds on shallow carbonate platforms, that also provide domicile to boring and entrusting organisms; iron-manganese hardgrounds and nodules at much greater bathyal to abyssal depths, and phosphate nodules or hardgrounds in regions of nutrient upwelling.
Manganese nodules, 2 - 10 cm across, litter the deep sea floor off Cook Islands, South Pacific. They precipitate in-situ on the sea floor, under conditions of extremely low sedimentation

Manganese nodules, 2 – 10 cm across, litter the deep sea floor off Cook Islands, South Pacific. They precipitate in-situ on the sea floor, under conditions of extremely low sedimentation  Image credit: USGS, https://www.usgs.gov/media/images/cook-islands-manganese-nodules

The most likely place you will see a condensed section is in outcrop or core. The common array of wireline logging tools may also help identify these units, based on elevated gamma response (for example, responding to the elevated potassium in glauconite), or abrupt changes in density in sonic and density logs across hardgrounds.

 

Omission surfaces

Stratigraphic surfaces swept bare by erosion or starved of sediment are commonly referred to as omission surfaces. Omission surfaces are important components of condensed stratigraphic sections. Although not a necessary condition, the term is commonly reserved for surfaces that have been modified by benthic organisms; examples include encrusting molluscs such as oysters, bryozoa, corals, barnacles, and calcareous algae (e.g. Lithothamnion), plus critters that are capable of boring through firm or cemented sediment.

A condensed section in the Middle Jurassic Carmel Formation, Utah. The cross-section through a carbonate hardground shows encrusting oysters (upper half of image – black arrow) and cemented sands and muds bored by bivalves. The yellow arrow points to a possible geopetal structure within an oyster shell. Image credit: Wilson44691 2008, https://commons.wikimedia.org/wiki/File:CarmelHardgroundSection.jpg

Omission surfaces commonly develop on hardgrounds and firmgrounds, that also harbour specialized trace fossil assemblages, particularly those that can bore through cemented sediment. More specifically they are called ichnological omission surfaces. MacEachen et al (2012) listed several diagnostic criteria to help identify these surface (note that some of the criteria taken individually are common to many non-omission trace fossil suites, but taken collectively are convincing):

  • Trace fossils cut across sedimentary structures and other trace fossils; borings may also transect earlier-deposited or cemented shells.
  • If the boring organism is a shelly mollusc, such as a pholad bivalve, then preservation of the organism within its home is possible (unlike most other trace fossils). The bored hole will also fill be filled by sediment from the exposed surface.
  • Traces are unlined but may show wall markings such as scratches.
  • Hardgrounds are less likely to encourage critters that prefer soft substrates.
  • Burrows in hardgrounds are less likely to compact during sediment burial.
Modern Pholad bivalves have bored into indurated sandstone. The shells of some remain in the holes. Some abandoned bore holes have been preferentially cemented by calcite and are resistant to subsequent erosion.es

Modern Pholad bivalves have bored into indurated sandstone. The shells of some remain in the holes. Some abandoned bore holes have been preferentially cemented by calcite and are resistant to subsequent erosion (arrows).

 

A condensed section from the Middle Jurassic Bowser Basin

 

A middle Jurassic condensed section represents the transgressive component of a shelf parasequence. MRS = maximum regressive surface; MFS = maximum flooding surface. Arrow top left points to an ammonite. The section is 50 cm thick; the nderlying regressive component is about 10 m thick.

A middle Jurassic condensed section represents the transgressive component of a shelf parasequence. MRS = maximum regressive surface; MFS = maximum flooding surface. Arrow top left points to an ammonite. The section is 50 cm thick and records a passage of time similar to the underlying 10 m thick regressive component.

In this coarsening-upward parasequence, shoreface sandstones are terminated by a surface of maximum regression (MRS) above which are thin transgressive deposits. The preserved record of transgression is little more than 0.5 metre of thick, between the MRS and a flooding surface (MFS), but probably represents a passage of time the same order-of-magnitude as the underlying regressive portion of the parasequence. Thus, the record of transgression is condensed; it consists of two main components:

  • A fossiliferous, pebbly, sandy mudstone that overlies the MRS, and represents the reworking of coarse sediment, bivalves and ammonites left on the sea floor at the end of regression. Little new sediment was introduced to the sea floor at this time, other than some mud from suspension, i.e. the sea floor was essentially starved of new sediment. Some shells are encrusted with bryozoa that may have grown on the reworked debris.
  • Sediment starvation continued and calcite precipitated within the uppermost mud and silt. This component of the condensed section is overlain by the maximum flooding surface (MFS), that in turn is overlain by several metres of mud, silt and fine sand that herald the return to regression.

 

Condensed sections are fascinating in and of themselves because…

They are stratigraphically skinny, and yet they record the multifarious interactions of physical, biological, and geochemical processes, played out over long periods. They also contribute to stratigraphic analysis:

  • They indicate dramatic changes in sediment flux.
  • They frequently contain evidence for dramatic changes in biota, for example the shifting demographics of organisms as soft substrates transition to hardgrounds.
  • They help define key stratigraphic surfaces such as subaerial unconformities, the maximum regressive surface, and transgressive flooding surfaces and, therefore, changes in baselevel.
  • They help define clinoform lapout, for example along surfaces of (transgressive) onlap and (regressive) downlap.
  • They are useful stratigraphic markers within basins, and in some cases between sedimentary basins.

 

This post is part of the How To…series  on Stratigraphy and Sequence Stratigraphy

 

Other posts in this series on Stratigraphy and Sequence Stratigraphy

Stratigraphic surfaces in outcrop – baselevel fall

Stratigraphic surfaces in outcrop – baselevel rise

A timeline of stratigraphic principles; 15th to 18th C

A timeline of stratigraphic principles; 19th C to 1950

A timeline of stratigraphic principles; 1950-1977

All the stratigraphies

Baselevel, Base-level, and Base level

Sediment accommodation and supply

Facies and facies models

How to read a sea level curve

Autogenic or allogenic dynamics in stratigraphy?

Stratigraphic cycles: What are they?

Sequence stratigraphic surfaces

Parasequences

Shorelines and shoreline trajectories

Stratigraphic trends and stacking patterns

Clinoforms and clinothems

Stratigraphic lapout geometry

Depositional systems and systems tracts

Which sequence stratigraphic model is that?

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Stratigraphic lapout geometry

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This post about lapouts (onlap, toplap, downlap, offlap, truncation) is a companion to Clinoforms and clinothems

 

An important part of stratigraphic analysis involves the identification of stratigraphic boundaries or surfaces – namely, the boundaries between successions of strata that indicate substantial changes in the conditions of sediment accumulation. Such changes include fluctuations in baselevel and accommodation, sediment supply, and depositional environments. We may be lucky enough to find a stratigraphic discordance in outcrop, but the scale of such an observation generally prevents us from seeing the bigger geological picture – is the discordance of local significance, or does it extend across large tracts of the sedimentary basin? What is the angular relationship between the discordance and overlying strata – in outcrop they may appear parallel, but on a regional scale there may be a significant angular discordance? Regional mapping, surface and subsurface, will help answer these questions.

This is where seismic reflection comes into its own because the depth of stratigraphic resolution and its lateral extent greatly exceed what we see in outcrop. In a previous post, we saw that clinoforms, that in seismic profiles are recorded as inclined, arcuate or sinusoidal reflections, are commonly organised into progradational, aggradational, and retrogradational stacks. Identification of the boundaries that separate these clinoform packages is based on the geometry of the reflection at stratigraphic terminations; a relationship called lapout. The geometry and stratigraphic position of these terminations help differentiate the kinds of stratigraphic surfaces associated with stratigraphic sequences.

The common stratigraphic terminations are divided into two groups (Mitchum et al. 1977):

  • Onlap and downlap, that terminate above a surface,
  • Toplap and truncation, that terminate below a surface.
An idealised profile, parallel to depositional dip, showing the geometric relationships and lapouts among clinoform packages. Onlap surfaces also correspond to sequence boundaries in standard sequence stratigraphic schemas.

An idealised profile, parallel to depositional dip, showing the geometric relationships and lapouts among clinoform packages. Onlap surfaces also correspond to sequence boundaries in standard sequence stratigraphic schemas.

Onlap: Clinoforms and other stratal packages formed during transgression will terminate in a progressively landward position across the top of a surface. Each termination approximates a shoreline. Onlap units must have a dip shallower than the surface at which they terminate. Onlap commonly occurs across subaerial unconformities and surfaces of maximum regression.

 

Seismic profile of an onlap surface over an unconformity. Image: Geophysicus, 2017

Seismic profile of an onlap surface over an unconformity. Image: Wikicommons Geophysicus, 2017

Downlap: Downlapping clinoforms terminate with the basin floor (marine and lacustrine). Downlap units must have a dip greater than the surface at which they terminate. Clinoform profile is typically progradational. In this case, downlap commonly occurs across a depositional surface that represents the end of transgression – the surface of maximum flooding.

 

Downlap and toplap revealed by seismic reflection in the Plio-Pleistocene Giant Foresets Formation, offshore Taranaki, New Zealand. The succession is strongly progradational; the shelf-edge trajectory almost horizontal.

Downlap and toplap revealed by seismic reflection in the Plio-Pleistocene Giant Foresets Formation, offshore Taranaki, New Zealand. The succession is strongly progradational; the shelf-edge trajectory almost horizontal.

A smaller-scale, outcrop example of downlap in prograding outer shelf-slope mudstones. Paleocene Strand Bay Formation, Ellesmere Island. The outcrop is about 20 m high.

A smaller-scale, outcrop example of downlap in prograding outer shelf-slope mudstones. Paleocene Strand Bay Formation, Ellesmere Island. The outcrop is about 20 m high.

 

Toplap: From a geometric perspective, toplap is the mirror-image of downlap such that clinoforms or other stratal packages terminate beneath a surface. Toplap units must have a dip greater than the surface at which they terminate. Toplap completes the characteristic sinusoidal clinoform or stratal geometry typical of progradational successions. A toplap surface may be interrupted by a truncation surface.

 

An outcrop scale version of toplap developed between fan-delta foreset and topset beds. Middle Jurassic, Bowser Basin, British Columbia.

An outcrop scale version of toplap developed between fan-delta foreset and topset beds. Middle Jurassic, Bowser Basin, British Columbia.

Truncation: This applies to clinoforms and other stratal packages that terminate below an eroded surface. Commonly this applies to subaerial unconformities during regression, and shoreface ravinement surfaces during transgression.

Offlap: This category, added to the original Mitchum et al. list, describes a surface where stratigraphic terminations downstep basinward during forced regression. Depending on the degree of baselevel fall, the surface may become a subaerial unconformity or its marine equivalent – a surface of maximum regression. However, there may be additional complications if shoreface ravinement removes any part of these records during the subsequent transgression.

 

This post is part of the How To…series  on Stratigraphy and Sequence Stratigraphy

 

Other posts in this series on Stratigraphy and Sequence Stratigraphy

Stratigraphic surfaces in outcrop – baselevel fall

Stratigraphic surfaces in outcrop – baselevel rise

A timeline of stratigraphic principles; 15th to 18th C

A timeline of stratigraphic principles; 19th C to 1950

A timeline of stratigraphic principles; 1950-1977

All the stratigraphies

Baselevel, Base-level, and Base level

Sediment accommodation and supply

Facies and facies models

How to read a sea level curve

Autogenic or allogenic dynamics in stratigraphy?

Stratigraphic cycles: What are they?

Sequence stratigraphic surfaces

Parasequences

Shorelines and shoreline trajectories

Stratigraphic trends and stacking patterns

Clinoforms and clinothems

Stratigraphic condensation – condensed sections

Depositional systems and systems tracts

Which sequence stratigraphic model is that?

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Mineralogy of carbonates; Pressure solution

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pressure solution at ooid contacts

Pressure solution and stylolite formation in carbonates.

This is part of the How To…series  on carbonate rocks

Pressure solution takes place in lithified rock when stress at intergranular and intercrystalline boundaries exceeds normal hydrostatic pressure. Differential stress can develop at almost any stage of sediment burial and compaction, during tectonic deformation and metamorphism. This post will describe pressure solution in carbonates, but it is worthwhile remembering that it can occur in many sediment types, as well as deformed rock. For example, cleavage, a penetrative fabric commonly found in folded metamorphosed strata, develops via a combination of shortening, pressure solution of certain mineral components and precipitation of new minerals. It is generally understood that pressure solution will occur if three conditions are met:

  • Differential compressive stresses develop at intergranular contacts.
  • Dissolved components are transported from the grain contacts to regions of lower compressive stress; this requires efficient fluid movement, and
  • The solute reprecipitates some distance from its point of origin.

Diagram showing the prgression of pressure solution and burial compaction

In sedimentary rocks, pressure solution is commonly manifested as:

  • Discontinuous grain-to-grain contacts, where one grain crosscuts or penetrates another grain. Intergranular pressure solution was first described by Henry Clifton Sorby in 1863. Pressure solution in ooid grainstones is a good example because we know the original shape of the grains involved. Intergranular pressure solution can develop between quartz grains but is a slower process than in carbonates because of its much lower solubility.

 

Pressure solution in ooid grainstone; single and compound ooids. Note the interpenetration stylolites.

 

  • One of the earliest descriptions of stylolites was presented by K.F. Klöden, a German ‘naturalist’ who in 1928 provided us with the name stylolite, but interpreted them as fossil traces. We now know that stylolites represent pressure solution across an extensive surface, producing contacts with complex sawtooth and interdigitate geometries. Surfaces can extend laterally many metres; the relief on a stylolite surface ranges from millimetres to 10s of centimetres. Stylolite surface orientation is determined by the principal compressive stress which is close to vertical if the stress is borne of sediment compaction – therefore, surfaces commonly parallel bedding but may diverge from bedding if tectonic deformation is involved. The presence of stylolites indicates dissolution of carbonate and a reduction in stratigraphic thickness; losses of 10% thickness are common.

Stylolite seam in dolostone. There are four additional but more crypticseams

Stress, strain, and fluid flow

During compaction grains become closer packed, reducing porosity and expelling connate fluids. Sediment rigidity increases, aided by cementation. Fluid pressures at this stage are close to hydrostatic (the pressure due to the overlying column of water and air). At some point in this process differential stresses in excess of hydrostatic develop at grain contacts, raising the potential for calcite dissolution. Dissolution will be a maximum at grain contacts where pressure and strain energy are maximized. The ensuing solute concentration gradient between the contact and adjacent pore space will promote diffusion along a thin fluid film between the grains. Note, if solute cannot be transported from the contact then dissolution will cease. The ‘thin film’ hypothesis was first suggested by P.K. Weyl in 1959. It remains a popular mechanism for explaining solute transport by aqueous ion diffusion.

 

Diagram depicting pressure solution, stress, strain and diffusion of solute

Diffusion of solute will increase the activity (or concentration) product in the adjacent pore spaces, promoting reprecipitation of calcite (or dolomite); this may take place in the immediately adjacent pore spaces, or solute may be moved farther afield by advective flow (i.e. fluid transport en masse) and precipitate some distance away. Where precipitation occurs will depend on the efficiency of advective flow and the calcite activity product.

Reprecipitation reduces the effective porosity.

 

The influence of the mechanical properties of grains

One prerequisite for pressure solution to take place is that the grains in contact must be rigid. During compression (compaction) and at the relatively low burial temperatures where pressure solution occurs (less than 200oC) rigid grains will act elastically (unless strain rates are high in which case they behave as brittle materials). Elastic strain energy will contribute, along with compressive stress, to calcite dissolution.

If the materials are ductile then pressure solution is inhibited. Common examples in siliciclastics include lithic grains, and in carbonates non-lithified pellets and mud intraclasts.

As the pressure solution seam increases in length-area, there is a concomitant decrease in effective stress and strain rate. Thus, as seams grow, the tendency for dissolution to decrease acts in concert with a reduction in effective porosity resulting from reprecipitation (i.e. the porosity that permits fluid flow), as a kind of self-regulating process.

 

The influence of insoluble materials

Stylolites are usually outlined by dark seams of clay, organic matter, and iron oxides – it’s what makes them so recognisable. Clays are particularly important here because they enhance the transfer of dissolved mass away from grain contacts. Most of the insoluble residues were originally dispersed through the host rock and concentrated along the seam as carbonate minerals dissolved.

The role of clays has been debated at length (lots of topics in geology are debated at length). Do clays act as passive bystanders, left behind after all the action, or do they participate in the pressure solution process? A model presented by E. Aharonov and R. Katsman (2009), based on numerical simulation (and not experimental data) posits that clays act as a kind of catalyst to pressure solution. In this model, both clays and compressive stress act together to the extent that stylolites will not propagate if clays are not present.

 

The consequences of pressure solution

  • Pressure solution is the geochemical response to differential stress.
  • It is an important part of the diagenesis continuum that produces lithified rock.
  • It results in loss of stratigraphic thickness.
  • It promotes precipitation that reduces porosity.

Links to other posts in this series:

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates; Karst

Mineralogy of carbonates; Burial diagenesis

Mineralogy of carbonates; Neomorphism

 

References etc.

E. Aharonov and R. Katsman, 2009. Interaction between pressure solution and clays in stylolite development: Insights from modelling. American Journal of Science v. 309, p. 607-632.

Robin G.C. Bathurst, 1976. Carbonate Sediments and their Diagenesis. Elsevier, Developments in Sedimentology, 12. 658p. Particularly Chapter 11

R. Tada and R. Siever, 1989. Pressure solution during diagenesis. Annual Review of Earth and Planetary Sciences, v. 17, p. 89-118. 

H.C. Sorby, 1863. On the direct correlation of mechanical and chemical forces. Proceedings of the Royal society of London, v.12, p. 583-600. Henry Clifton Sorby was one of the architects of modern sedimentology

P.K. Weyl. 1959. Pressure solution and the force of crystallization – a phenomenological theory. Journal of Geophysical Research, v 64, p. 2001-2025.  Weyl promoted the thin fluid film idea in this seminal paper.

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Mineralogy of carbonates; Neomorphism

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Neomorphic calcite in bivalve

This is part of the How To…series  on carbonate rocks

Diagenesis of carbonate sediment begins soon after deposition and continues at all depths until metamorphic processes takes over (a statement that is about as diffuse as the boundary between diagenesis and metamorphism). Burial can also be expressed as a function of changing temperature, pressure, and fluid composition,  and is manifested as physical and chemical changes such as:

  • compaction and pressure solution.
  • replacement of metastable and unstable minerals like aragonite,  high-Mg calcite, and evaporites by calcite,
  • dolomitization,
  • creation of secondary porosity, and
  • neomorphism – recrystallization.

Continue reading

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Mineralogy of carbonates; Karst

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Karst depicted in late 14th century chinese painting

Karst landscapes – limestone dissolution, saturation and kinetics.

This is part of the How To…series  on carbonate rocks

Mountains in traditional Chinese painting are commonly depicted as pinnacles appearing out of some ethereal mist, bound by precipitous faces; symbols of some heightened awareness, an expression of deep time. And in the fertile valleys below an alternative, ephemeral human presence, almost an afterthought.

As surreal and metaphorical as these iconic images seem, they are rooted in real-world landscapes, the karst of southern China’s Guizhou, Guangxi, Yunnan and Chongqing provinces that in 2007 were designated a UNESCO World Heritage site. Its cones, pinnacles, sinkholes, caves, bridges and Shillin (Stone Forests) developed in thick Devonian to Triassic limestone. This is one of the classic regions for tropical and subtropical karst.

 

classic tropical karst in Guangxi Province, south China

Karst surface and subsurface structures are sculpted in limestone, dolostone and evaporites like gypsum and anhydrite. In all cases, the primary process is dissolution in the meteoric vadose and shallow phreatic zones. Therefore, karst is best developed in humid climates: prime examples include the tropical-subtropical landscapes of South China, temperate New Zealand (on Oligocene limestones), and the glacial – post-glacial of Ireland (the Burrens, that are underlain by Carboniferous limestone).

 

Pinnacle karst in Oligocene Te Kuiti Gruop limestone, Waitomo, NZ

 

 

Clints and grykes in Carboniferous limestone, the Burrens, west Ireland

Karst formation provides a good opportunity to examine two competing diagenetic processes –  dissolution and precipitation in terms of solution saturation, chemical kinetics, and groundwater flow.

 

Dissolution controlled by calcite saturation

Dissolution of limestone at the Earth’s surface is summarized in the following reaction (keeping in mind that all the carbonate equilibria are involved depending on pH, temperature, and concentrations or activities):

CaCO3 + H2O + CO2(aqueous) → Ca2+ + 2HCO3                   (1)

An important determinant for calcite dissolution is the degree of saturation. Dissolution will take place in solutions that are undersaturated, precipitation in solutions that are over- or supersaturated with respect to calcite. For calcite, the degree of saturation in a solution is calculated by comparing its ion-activity product with the solubility product (i.e. the ion product if that solution were at equilibrium under the same conditions of temperature and pressure). The ratio between these two activity products is called the saturation (Ω). Ω values less than one indicate undersaturation, values greater than one over-saturation; a value of one indicates the solution is in equilibrium with solid calcite.

Rainwater contains dissolved CO2 and carbonic acid; the average pH is 5.5 to 5.8. As it filters through soils it may pick up additional CO2 from plant decay. Dissolution of limestone in the vadose and shallow phreatic zones proceeds rapidly because the water is highly undersaturated. As residence time increases in the phreatic zone, so too do the concentrations of dissolved carbonate species; there is a concomitant decrease in undersaturation and as a consequence,  a decrease in the rate of limestone dissolution. At some point in this process, the saturation approaches one and dissolution ceases.

 

Dissolution controlled by kinetics

At this stage in our deliberations we should remind ourselves that the discussion of limestone solubility and groundwater saturation is based on thermodynamic parameters such as ion activity and energy transfer, that together determine whether a chemical reaction will proceed.  What thermodynamics doesn’t do is describe the paths which these reactions take. This is the role of Chemical Kinetics.  What does this mean?

Kinetics deals primarily with two parameters: the rate at which reactions take place, and the path that chemical species take to form a reaction. Reactions in aqueous solutions involve collisions between at least two ion species. They may combine directly such as:

A + B (reactant ions) → C (product)

or via a smallish number of intermediate steps until the final product is formed (these intermediate steps are called elementary reactions). Each reaction step requires that the ions have a certain amount of energy before it can proceed – this is the activation barrier.

A → A*  (fast)

B → B*  (slow)

A* + B* → C where A* and B* are short-lived intermediate species.

The overall rate of a reaction is determined by the slowest intermediate reaction – the one that finds it most difficult to reach its activation energy (in this case the reaction involving B).

Knowing something about the kinetics of calcite dissolution and precipitation can help us decipher which processes are important in diagenesis, whether it is cementation on the seafloor or the formation of karst.

We can now look at the picture of limestone dissolution in a different context, summarized in the diagram below. Dissolution is rapid at high degrees of undersaturation because:

  • There are very few ion species competing for space on active crystal faces, and
  • Dissolved mass is moved rapidly away. From a chemical kinetic perspective, the reaction is controlled by the rate at which the dissolved mass is removed from calcite crystal surfaces and transferred to some other site (transport-controlled reactions); in groundwater systems this depends on groundwater flow rates. Thus, the reaction is probably a relatively simple A + B → C type.

 

Gneral trends for water composition in karst, showing the range of calcite dissolution-precipitation, saturation and pCO2

As the concentration of dissolved species increases (i.e. greater degrees of saturation) the number of molecular collisions also increases at calcite crystal surfaces. Thus, at low levels of undersaturation the rate of dissolution is controlled more by what is happening at the crystal surface – it is a surface controlled reaction and sensitive to factors such as adsorption of ion species on the surface, and dehydration of adsorbed species. For example, Ca2+ and CO32- in solution are surrounded by water molecules and for them to combine at the crystal surface, they need to shed this water (dehydrate). Under these conditions, slow intermediate reactions will determine the overall rate of dissolution.

 

Precipitation

Drip cements produce stalactites if the balance of atmospheric CO2 in the cave allows for supersaturation with repsect to calcite.

 

At some point in their seepage journey karst fluids are capable of precipitating calcite, commonly as drip cements in caves as water filters through the vadose zone (providing us with the spectacle of stalactites and stalagmites), and in tufas where groundwaters emerge as springs. For this to happen the solutions must be over- or supersaturated with respect to calcite. However, we also know that as saturation levels approach one there is no further dissolution and therefore no mechanism to increase dissolved carbonate species to levels of oversaturation. Some other process must intervene here to produce supersaturated conditions.

It is generally understood that the partial pressure of CO2 in water passing through the vadose zone is greater than that in cave atmospheres. As water enters a cave, the various carbonate equilibria will accommodate this change in pCO2 by degassing CO2, reducing the concentration of H2CO3, and pushing reactions (1) and (2) to the left.

CO2(gas) ← CO2 (aqueous) (2)

CaCO3 + H2O + CO2(aqueous) ← Ca2+ + 2HCO3 (1)

From a kinetic perspective, calcite precipitation under these conditions is surface controlled, aided in part by the availability of nucleation sites on the crystal surface and the delivery or removal of ion species by fluid flow.

 

Links to other posts in this series:

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; meteoric hydrogeology

 

References and useful texts

D. Ford and P. Williams. 2007. Karst Hydrogeology and Geomorphology. John Wiley & Sons.

J.W. Morse and F.T. Mackenzie 1990. Geochemistry of sedimentary carbonates. Developments in Sedimentology 48. Elsevier, Amsterdam, 707 p.

P.A. Domenico and F.W. Schwartz, 1997. Physical and Chemical Hydrogeology, 2nd Ed. John Wiley & Sons. 506 p.  This book focuses on groundwater but has an excellent section on aqueous chemistry.

USGS Glossary of karst terminology. 1972.    PDF version

W.B. White, 2015. Chemistry and karst. Acta Carsologica, v.44/3, p. 349-362. Full text available

 

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Mineralogy of carbonates; meteoric hydrogeology

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Oligocene cool water limesontes exposed in karst at Punakaiki Pancake Rocks, NZ

 

Meteoric diagenesis of carbonates – a competition between dissolution and precipitation.

This is part of the How To…series  on carbonate rocks

 

Marine carbonates may, during their geological evolution, be exposed by a fall in relative sea level (eustatic, shoreline retreat forced by progradation, tectonic uplift). The subsequent displacement of seawater by freshwater is rapid, the resulting meteoric diagenesis is profound. The influx of freshwater changes the stability of those mineral phases that were stable or metastable in seawater – principally aragonite, high-Mg calcite in bioclasts, frameworks, ooids and cements. As such, meteoric diagenesis involves significant dissolution of CaCO3. However, experience shows that redistribution of dissolved carbonate results in precipitation in calcretes and caliches, eolianites (cemented dune sands), and deeper pore-filling calcite spar mosaics; most of these cements are low-Mg calcites.

 

Schematic of the components of meteoric hydrology and diagenesis

Meteoric diagenesis is entirely dependent on the flow of fresh water to remove dissolved mass (solute) and redistribute it to sites of calcite precipitation. The freshwater itself occurs in two broad settings:

  • the surface, where precipitation flows overland as sheet runoff or streams, and infiltrates soils and permeable rock, and
  • beneath the surface in aquifers.

Surface diagenesis begins with rain, that is weak carbonic acid because of dissolved CO2 – rain is usually in equilibrium with atmospheric CO2. The average pH of rain is 5.5 to 5.8 ( average seawater pH is 8.0 to 8.1). Corrosion of limestone building materials is an obvious manifestation of calcite dissolution.

 

Advanced acid rain dissolution of limestone in astatue from St Pauls Cathedral, London

 

Likewise, development of karst landscapes, sinkholes, caves and subterranean streams are all products of limestone dissolution. Some of the dissolved mass is precipitated as calcretes (caliche, paleosols), fracture-fill spar, stalactites (the ones that hang from the ceiling), stalagmites and cave pearls where waters reach saturation levels and partial pressure of CO2 is reduced.

 

Clints and grykes in karsted carboniferous limestone, Burrens, Ireland

 

Polished rock slab showing vadose pisoids in a Peleoproterozoic caliche

 

The unsaturated zone

The interval of infiltration is called the unsaturated zone by hydrogeologists, but limestone aficionados prefer to call it the vadose zone; it extends down to the watertable. It is the interval of porous and permeable sediment or rock in which pore spaces are mostly air-filled and where air pressure is close to atmospheric. Residual water may be present at grain contacts as menisci (because of surface tension); these may act as sites for meniscus cements.

Some of the water in the vadose zone will reach the watertable (groundwater recharge), some of it will be taken up by plants, and some released by evapotranspiration. Two processes act to modify rainwater chemistry:

  • Soil formation and break-down of organic matter to humus that releases humic acids and CO2; thus, the partial pressure of CO2 may increase and the pH decrease.
  • Dissolution of limestone and carbonate sediment produces Ca2+ and CO32- , but at pH between 5 and 6 the dominant species are HCO3 and H2CO3. A typical plot of species concentration versus pH is shown below.

All these dissolved products enter the groundwater system.

 

Bjerrum plot of activity versus pH for common aqueous carbonate species

 

The saturated zone

Pore spaces below the watertable are filled with water; this is the saturated or phreatic zone. Furthermore, groundwater is always on the move, driven for the most part by gravitational potential energy imparted by topography. Porosity is of several types: intergranular, intragranular (e.g. the chambers of gastropods), secondary porosity formed by dissolution of metastable components, fractures, and cavernous porosity formed by solution collapse (sink holes (dolines) and underground streams.

 

Aquifers

There are two fundamentally different kinds of aquifer:

  • Unconfined aquifers, sometimes called watertable aquifers. The watertable is the boundary between the vadose and saturated portions of the aquifer. Drainage in an unconfined aquifer is by gravity. If an unconfined aquifer is drained there is no change in the organization or packing of grains. The position of the watertable fluctuates seasonally, depending on the balance of evaporation, drawdown by vegetation (and people), and recharge.
  • Confined aquifers are bound by sediment or rock layers that retard flow (aquitards). They are always saturated (even when drawn down by pumping). In natural systems, there is usually a balance between recharge and discharge of water, but if this balance is upset (e.g. during an extended dry period), then the aquifer framework (i.e. clast support) will respond elastically.

The chemistry of groundwater evolves over time. Rainwater is usually devoid of dissolved salts, but once it enters a groundwater system, biochemical reactions in soils, and inorganic reactions involving carbonates, clays (particularly ion exchange) and other silicate minerals, will add or subtract ion species that can be involved in other reactions. Like their sedimentary cousins, groundwater chemistry can be represented by chemical facies, for example water my be dominated by Ca2+ or Fe3+, or CO32- or SO42-, depending on the composition of aquifer materials. Migration of groundwater through different rock-sediment types is commonly manifested in a change from one chemical facies to another. Tracking this chemical evolution is useful for two reasons: it provides a record of where the water has resided, and may provide clues to changes in the precipitation history of rocks. For example, partitioning of trace elements in zoned calcite cements will record changing pore water compositions.

 

Piper plot of groundwater chemical facies from different aquifers

Groundwater at the shoreline

The interaction between groundwater and seawater can have a profound effect on carbonate diagenesis. Groundwater doesn’t just cease to flow at the shoreline – with enough hydraulic drive it can extend many kilometres beneath the sea floor, exiting as freshwater seepage and springs. One celebrated example is located beneath the Florida carbonate platform where exploratory drilling in 1965 discovered fresh and brackish water in boreholes up to 120km offshore and 130m below the sea floor. In one borehole fresh water flowed 2m above the ship’s deck (Manheim, 1967). Seepage of fresh or brackish water will affect local biotas and the relative stability of metastable carbonates, both in the rocks through which the groundwater flows and at the seafloor. This is illustrated in a great example from Yucatan Peninsula (link kindly provided by Nigel Platt – © Nigel Platt, Edison E&P UK Ltd).  Here, fresh-water springs across the shallow platform at Casa Cenote, Tankah Beach upwell through the platform carbonates.  Freshwater is recharged from the highlands a few 100 km inland. Flow to the coast is focused through a complex network of  limestone caves.

The watertable in coastal unconfined aquifers will tend to merge with the shoreline. Coincidentally, a wedge of seawater will develop landward of the shore below the fresh groundwater. It’s position below the freshwater is dictated by density differences. The boundary between seawater and groundwater is a zone of mixing. A reasonable approximation of the depth to the groundwater-seawater boundary is expressed in the Ghyben-Herzberg equation, shown in the diagram above:

z = h.ρf / ρs – ρf

where z is the depth to the interface from sea level, h the watertable elevation, ρs (1.025 gm/cc) and ρf (1 gm/cc) the densities of seawater and freshwater respectively, such that

z = 40h

An important corollary is that for every unit decrease in watertable depth (h) there will be a corresponding 40 unit rise in the interface (and vice versa).

The interface is dynamic and will respond to both short term changes (e.g. seasonal fluctuations in recharge, pumping) and longer geological intervals, migrating with excursions of the shoreline during rising or falling sea levels. Again, from a geological perspective, this means that boundary between fluid environments that determine carbonate mineral stability, will also be dynamic.

 

Links to other posts in this series:

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; Karst

Mineralogy of carbonates; Burial diagenesis

Mineralogy of carbonates; Neomorphism

Mineralogy of carbonates; Pressure solution

Mineralogy of carbonates; Sabkhas

 

References and useful texts

Manheim, F.T. 1967, Evidence for submarine discharge of water on the Atlantic continental slope of the southern United States, and suggestions for further research New York Academy of Sciences Transactions, Series 2, v.29, p. 839-853.

Robin G.C. Bathurst, 1976. Carbonate Sediments and their Diagenesis. Elsevier, Developments in Sedimentology, 12. 658p. An example of the longevity and utility of one of the best on this topic. Now also as an ebook.

Noel James and Phillip Choquette.1984. Diagenesis 9. Limestones; The meteoric environment. The Canada Geoscience Series on Carbonate Diagenesis is available from the CGS archive.

Noel James and Brian Jones. 2015. The origin of carbonate sedimentary rocks. American Geophysical Union, Wiley works, 464p.An excellent recent update.

Peter Scholle and Dana Ulmer-Scholle, 2003. A colour guide to the petrography of carbonate rocks: grains, textures, porosity, diagenesis. AAPG Memoir 77. Loaded with images.

Erik Flugel. 2010. Microfacies of carbonate rocks: Analysis, interpretation and application. Springer. The ebook is cheaper

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