Glossary of sequence stratigraphy, stratigraphic terminology, stratigraphic concepts and philosophy, and depositional systems.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
Baselevel model: One of the principle models in sequence stratigraphy that relates, in a theoretical way, regression and transgression (retrogradation) in relation to changing baselevels (usually sea level). The model is based on identification of sedimentary, chemical, and biofacies, stratigraphic trends, theoretical considerations, and numerical modelling.
Bed: The pre-eminent unit in sedimentary geology and stratigraphy, that represents a period of relatively uninterrupted deposition – the duration can range from seconds to multiple years. Sediment composition and texture are relatively uniform within a single bed but can vary between beds. Beds are bound top and bottom by bedding planes. Their areal extent can exceed many 10s of square kilometres and be as little as a few decimetres. Thicknesses also vary greatly, from a few millimetres to 10s of metres.
Bedding plane: The bounding surfaces of beds. The upper plane represents cessation of the depositional event that was responsible for creating the bed. If erosion of the upper bedding plane occurs, it is usually associated with the next depositional event. Differential compaction by overlying beds can also change the form of the bedding plane. The lower bedding plane coincides with the top of the underlying bed. The geometry of the bedding planes defines styles of bedding such as parallel bedding, lenticular bedding, wedge-shaped bedding.
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.
Cambrian: The first period in the Paleozoic Eon, dating from 541 to 485.4 Ma, preceded by the latest Precambrian Ediacaran. The Cambrian witnessed an explosion of invertebrate animals including abundant shelly faunas, and primitive vertebrates. The Burgess Shale is an iconic example of soft-body preservation on some of these species. Prominent among invertebrate species were the Trilobites, molluscs, and brachiopods. It is hypothesized that the dramatic change occurred during a rise in atmospheric oxygen, abruptly, or as more recent investigations suggest, more gradually (Stockey et al., 2024; OA) The Cambrian ended with major extinctions caused by an oxygen crisis. The period was named by Adam Sedgwick in 1835.
Carboniferous: A subdivision (Period) in the Paleozoic, named from the ubiquitous coal deposits in fluvial-related strata. It spans 359.3 Ma to 289.9Ma. It is underlain by the Devonian and overlain by the Permian. The two main subperiods the older Mississippian and Pennsylvanian represent two different plate configurations where the assembly of Pangea was complete only in the later part of the Pennsylvanian. Glaciation was more prominent in the younger subperiod.
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.
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. cf. geochronologic units.
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.
Concordia plot: For the U-Pb system, the curve plots the expected (theoretical) 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). Measured isotope ratios for any crystal or batch of crystals are then 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. Discordant ages usually plot below the concordia. In this case, if the line segment (a discordia) connecting two or more discordant ages intersects the concordia, then the upper age intersection is taken as the maximum age of the samples.
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.
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.
Cretaceous: The geological period that witnessed the end of some major fossil groups (e.g. dinosaurs, ammonites) and ushered in modern animal and plant groups – the first flowering plants, many insect groups, and mammals. It extends from 145 – 66 Ma, the last period of the Mesozoic Era. The climactic finale was an asteroid impact – the Chicxulub impact. Pangea was in its final breakup throes. There were no polar icecaps. Arctic regions were home to many boreal plant species and animals including those more attuned to warmer climes like snakes and crocodilians.
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.
Diastem: A brief hiatus, or short break in deposition or erosion that is considered a normal part of the conditions in a particular environment. For example, the break between turbidites on deep water submarine fans, or the erosional contact beneath a storm deposit on a tidal flat.
Disconformity: An unconformity where the overlying and underlying strata have the same orientation.
Downlap: Downlapping clinoforms terminate on top of 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. See also onlap, toplap, offlap.
Eccentricity: One of the Milankovitch orbital cycles. Earth’s orbit around the sun is an ellipse where the degree of ellipticity (the length of the ellipse axes) changes about 5% over 100,000 years. Both Precession and Obliquity are superimposed on this longer-term cycle.
Ecliptic (astronomy): The plane in which Earth orbits the Sun. The name derives from the fact that any eclipse can only occur when the Moon also enters the same plane (the Moon’s orbit of Earth is slightly oblique to the ecliptic). See Milankovitch, precession, obliquity, eccentricity.
Ediacaran fauna: Seemingly, the beginning of life forms 575-542 million years ago, that were more complex than procaryotic algae and cyanobacteria of the preceding 3 billion years. They appeared immediately prior to the Cambrian explosion. Iconic fossil forms include fossils that are petal-, feather-, or sea-pen-like. All were soft-bodied.
Endemic taxa: Species restricted to certain geographic or oceanic regions, or specific sedimentary basins. Cf. Cosmopolitan taxa.
Facies association: Sedimentary facies that occur together, forming associations that are repeated in time and place (e.g. different sedimentary basins). For example, facies that describe fluvial overbank deposits will be associated with facies that define fluvial channels, swamps, peat bogs, paleosols, and oxbow lakes. It is these associations that provide the real clues to interpreting paleoenvironments.
Facies models: Facies models are simplified descriptions of a complex sedimentary universe, a scaled-down version of a depositional systems like submarine fans, or high sinuosity fluvial channels. They contain facies and facies associations visualised in the context of a theoretical framework of processes. Models allow us to visualize and interpret our observations within an established framework – that framework may be mathematical, conceptual, or empirical. Models allow us to predict outcomes where direct observations or measurements are not possible.
Falling Stage Systems Tract (FSST): Forms during relative sea level fall when sedimentation rates are low. The FSST is bound at the base the basal surface of forced regression; at the top by a subaerial unconformity and its correlative conformity. Depositional systems include the shoreface and deeper shelf, and base of slope.
Fission tracks: Fission track technology allows us to unravel the thermal history of sedimentary basins. Certain minerals like apatite contain small amounts of Uranium-238. Uranium decay produces a visible track, a few microns long, where the apatite crystal structure has been damaged by radiation. If we can measure the amount of uranium present, and knowing the half-life of uranium-238, we can determine the age of the crystal by counting the number of tracks. Fission tracks are annealed at temperatures >110oC, for example during sediment burial. If the rock is then cooled below 110oC fission tracks will again begin to develop. Thus, the age of cooling below 110oC can also be determined.
Flysch: The German word for flow, was applied in the early 19th C as a stratigraphic descriptor for thick successions of interbedded shale and sandstone (plus a few subordinate lithologies). It was primarily a European term used to describe rocks associated with the Tertiary Alpine Orogeny. Flysch sandstone are invariably graded – this rock type was where some of the earliest turbidites were described.
Fondoform: A term introduced by John Rich (1951) to encompass the region between the base of a slope (continental, delta) and the deep basin beyond. Fondoform was part of his system that included clinoform and undaform. The terms undaform and fondoform have all but faded into obscurity.
Foraminifera: A large group of single-celled marine protists that secrete chambered tests (shells), each chamber being added as the organism grows. They range from about 100 microns to several centimetres. Depending on the species, secreted tests consist of a chitinous material (organic), agglutinated sand grains, calcite or aragonite. Of about 4000 living species, 40 are planktic, the remainder are benthic. Their biostratigraphic range is Cambrian to Recent. They are one of the most important groups of microfossils in biostratigraphy and paleoecology.
Forced regression: Forced regression occurs during fall in baselevel (sea level) if the rate of fall (the rate of change of negative accommodation) exceeds sedimentation. In this case, shorelines and associated shallow marine deposits are forced seawards; the shoreline trajectory is also down depositional dip. Cf. Normal regression.
Formation: A formal lithostratigraphic unit based on rock composition and mapability; it must have well defined and easily identifiable surface or subsurface contacts. Formations boundaries have no chronostratigraphic significance. Formations are inherently diachronous.
Ga: The abbreviation for giga-annum, or 109 years.
Genetic sequences: An alternative to the standard Exxon sequence stratigraphic model, introduced by W. Galloway (1989). Genetic sequence boundaries are bound by maximum flooding surfaces, rather than subaerial unconformities. They are based in part on D. Frazier’s (1974) depositional episodes, that begin with a period of progradation and end with transgression (maximum flooding).
Geochronologic units: Division of the geological column into units that refer only to geological time and not process. cf chronostratigraphic units. e.g., Jurassic, Devonian.
Geochronology: The study of rocks and minerals to determine their age. Modern techniques use the decay profiles of radioactive isotopes, particularly uranium, lead, thorium, and potassium, to give us numerical ages (in comparison fossils provide relative ages).
Geographic poles: The points where lines of longitude converge, north and south. These poles are close to Earth’s pole of rotation; the coincidence is not exact or permanent because Earth wobbles about the rotation axis. Cf. Magnetic poles.
Geoid: A hypothetical surface of equal gravitational potential, that coincides with sea level in the absence of tides, waves, currents, and changes in air pressure. Sea level in this context is an ideal surface. Because it depends on gravitational potential, the geoid, and therefore sea level will not be a smooth surface, but will have long wavelength hill and valley like relief. Satellite altimetric measurements if sea level are referenced to the geoid.
Half life: The time taken to reduce a quantity of a radioactive isotope by half. This means that after one half life, it is probable that 50% of the isotope will remain; after the second half life 25%, and so on. Isotope decay is exponential.
Hiatus: (plural hiatuses). A stratigraphic surface that records a break in sedimentation. A hiatus can be any duration. A depositional hiatus may be a few minutes or 1000s of years. The hiatus recorded by angular unconformities can be many millions of years. The term was introduced by A.W. Grabau to describe the absence of a rock record between pulsations, or cycles. See also Lacuna.
Highstand systems tract (HST): Formed during the late stage of relative sea level rise and the beginning of sea level fall. Depositional systems that make up the HST can include fluvial, delta, barrier island – lagoon, coastal, and shelf deposits. The lower boundary is a maximum flooding surface. The upper boundary is a sequence boundary that includes the subaerial unconformity, and offshore the basal surface of forced regression (FSST). The HST underlies the FSST and overlies the TST.
Ichnology: The study of trace fossils, the behaviour of the critters that made them, the environment they lived, fed, escaped, and traveled in, and their relationship with other sedimentary facies and stratigraphic surfaces. See Lebenspurren.
Interglacial: The period between glaciations. Periods of warming controlled by Milankovitch orbitals and Solar insolation, changing atmospheric carbon dioxide, albedo, and ocean currents.
Iridium anomaly: Anomalously high concentrations of Ir derived from meteorite impacts incorporated into sediments, particularly mudrocks. It was first recognized at the Cretaceous-Tertiary (K-T) boundary, corresponding to the widespread distribution in aerosols generated by the Chicxulub impact.
Jacob’s Staff: A measuring stick with an inclinometer at one end, that is used to measure directly true stratigraphic thickness in dipping beds. Use as a surveying instrument dates to Medieval times.
James Hutton: (1726-1797). The founder of modern geology, he proposed the principle of uniformity (later called uniformitarianism), recognised the unfathomable depths of geological time and the value of unconformities, and sorted the problem for the origin of granites – viz. from melts, rather than ocean precipitates.
Johannes Walther: (1860-1937). We remember Walther primarily for his ‘Law’, that is an essential part of any modern analysis of sedimentary facies and depositional systems: ‘‘. . . only those facies and facies-areas can be superimposed primarily which can be observed beside each other at the present time’’ (Walther 1894). It provides a rational means for interpretation of ancient environments by inviting us to examine modern analogues.
Ka: The abbreviation for kilo-annum, or 103 years.
Lacuna: Introduced by Harry Wheeler (1964), it encompasses the total time missing at the unconformity, and is divided into an hiatus which is a non-depositional or erosional episode above the unconformity, and an erosional or degradational vacuity below it (i.e. the time represented by rocks removed by erosion). The contact separating these two domains is the base level transit, which defines stratigraphic onlap and offlap geometries. These concepts pre-empted by about 20 years the fundamentals of sequence stratigraphy.
Lagoon: A shallow bay protected from ocean swells, and to some extent storms, by barrier islands, spits, and bars. Extensive tidal flats commonly border the landward margins of lagoons, crossed by estuaries and small tidal channels. In tropical and temperate climates, mangrove swamps provide breeding grounds for all manner of critters. There is regular tidal exchange of ocean seawater through large channels; delta platforms at the inner or seaward channel exits are called flood- and ebb-tidal deltas respectively. There is a continuous exchange of sediment (mostly sand) between the lagoon, barrier and coastal dunes, and the open sea shelf.
Lapout: The geometry of stratigraphic termination of clinoforms relative to some through-going surface. Although these can be observed in some outcrops, seismic reflection profiles provide the best opportunity to identify lapout classes. See Onlap, Downlap, Offlap, Toplap.
Lithostratigraphy: Formal lithostratigraphy is concerned with the description and mapability of rocks, using physical, fossil, and mineralogical attributes. The basic lithostratigraphic unit is the Formation. There is no reference to time.
Lowstand Systems Tract (LST): Forms at the end of sea level fall and the beginning of sea level rise. Depositional systems include submarine fan, base of slope and mass transport deposits. The top of the LST is the maximum regressive surface; the base is the correlative conformity equivalent to the subaerial unconformity (also a sequence boundary).
Ma: The abbreviation for mega-annum, or 106 years.
Magnetic reversals: Reversal of Earth’s magnetic field has occurred many times, and over the last few million years this has happened about every 200,000 to 300,000 years. The last reversal took place 780,000 years ago; this is called the Brunhes-Matuyama Reversal. Reversals are recorded by iron-bearing minerals in volcanic and sedimentary rocks where the minerals act as tiny magnets – the direction of polarity (i.e. magnetic N and S) is locked in mineral at the time of lava solidification or sedimentation, and this remnant magnetism can be measured.
Magnetostratigraphy: The formal stratigraphic measure is the magnetostratigraphic polarity unit that is used to subdivide and correlate rock units according to whether polarity is Normal (north pointing) or Reversed for a body of rock.
Maximum flooding surface: The MFS represents the sea floor at the end of transgression, marking the change to regression, and where the shoreline trajectory reverses from landward to seaward. It commonly forms a resistant topographic bench where it overlies a (transgressive) condensed section and in turn is overlain by mudrocks and fine-grained sandstones of the succeeding normal regression.
Maximum regressive surface: (MRS) Also called the Transgressive surface. This surface represents the sea floor at the time when regression ends and transgression begins, and the shoreline trajectory reverses from seaward to landward. Depending on its location on the lowstand shelf it may be an erosional discordance or a conformable surface. Overlying strata will tend to onlap the MRS.
Milankovitch (Milutin): 1879 – 1958. A Serbian mathematician and engineer, best remembered for the eponymous Milankovitch orbital cycles. Earth’s rotation and orbit around the sun is an ellipse that is perturbed by ever-changing wobbles and tilts. Milankovitch’s mathematical theory outlines three kinds of orbital cycle: Precession, Obliquity and Eccentricity, that are influenced by gravitational interactions between the earth, sun, moon, and to a lesser extent the planets. These astronomical cycles have a direct impact on solar insolation and therefore global climates, on a scale of 100s to 10,000s of years.
M.y. – m.y.: The abbreviation for the duration of time, rather than a specific age expressed as Ma. In this case millions of years.
Neptunism: The theory that all rocks had precipitated at different times from a universal ocean. One of its chief proponents in the 18th century was Abraham Werner.
Nicolaus Steno: (1638-1686), is credited with establishing three axioms, or principles that are fundamental to geology and stratigraphy (published 1669):
- The law of superposition is an axiom that is fundamental to geology, archaeology, and other fields dealing with geological stratigraphy. In plain language, it states that in undeformed stratigraphic sequences, the oldest beds will be at the bottom of the sequence.
- The Principle of Original Horizontality states that layers of sediment are originally deposited horizontally under the action of gravity.
The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. This concept is central to geological mapping and correlation of beds or successions of beds.
Nonconformity: A stratigraphic surface separating underlying, eroded igneous or metamorphic rocks, from younger sedimentary or volcanic strata. Cf. angular unconformity, disconformity.
Normal regression: In the baselevel sequence stratigraphic model, normal regression occurs during the final stage of sea level rise and the beginning of sea level fall (repeated at the other end of the sea level curve), if sedimentation rates equal or exceed the rate of change of accommodation. The resulting stratigraphic trends are progradation commonly with some aggradation. Cf. Forced regression.
Obliquity: One of the Milankovitch astronomical orbitals. The earth’s axis is presently tilted at 23.5o to the ecliptic (the plane of Earth’s orbit around the Sun). However, the plane of Earth’s orbit around the sun is also precessing (wobbling) with a period of 71,000 years. The combined effects of axial precession and ecliptic precession cause earth’s tilt to move between 21.5o and 24.5o, a shift that takes 41,000 years. Changes in obliquity impact the severity of seasons. Cf. Precession, Eccentricity.
Offlap: Stratigraphic terminations that downstep basinward during forced regression. The surface may become a subaerial unconformity or its marine equivalent – a surface of maximum regression. See also onlap, downlap, toplap.
Omission surface: Depositional surfaces swept bare by erosion or starved of sediment. Omission surfaces are important components of condensed stratigraphic sections. They are commonly modified by encrusting and boring organisms.
Onlap: Clinoforms and other stratal packages formed during transgression that 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. See also downlap, toplap, offlap.
Outcrop: Visible rock, soil, or sediment exposed at the surface (of a planetary body), although not necessarily accessible. cf. subcrop.
Paleosol: The general name for all manner of paleo-soils. Their identification in the rock record adds considerable value to assessment of subaerial exposure, unconformities, and paleoclimates.
Palynomorphs: A group of microscopic animal and plant structures composed of resistant organic compounds. The animal representatives comprise a significant part of the plankton biomass, and include dinoflagellates, chitinozoans, and acritarchs. The plant side includes well known pollen and spores. They have biostratigraphic significance. Pollen and spores provide paleoenvironmental and biostratigraphic information on terrestrial environments. Colour alteration of pollen and spores can also be used to determine the thermal history of sedimentary basins.
Parasequence: A relatively conformable succession of genetically related beds bounded by marine flooding surfaces. Parasequences represent relatively short-lived, cyclical periods of progradation that are superimposed on or punctuate 3rd order regressive or transgressive cycle trends. They are the stratigraphic building blocks of shallow marine systems tracts, forming under conditions of normal and forced regression, and transgression depending on the rates of sediment supply versus the rates of change of accommodation.
Parasequence set: A stack of parasequences that are genetically related; in other words, parasequences that show consistent trends through a stratigraphic succession, and consistent shoreline trajectories that are progradational, aggradational, retrogradational, or degradational. Stated another way, a parasequence set is a cycle made up of higher-order cycles.
Pleistocene: From the Greek pleistos meaning much or more, and the word element -cene meaning recent – the most recent geological period. The name was introduced by Charles Lyell in 1839 to describe the shelly fauna in Sicily that includes modern species; he initially referred to the interval as Newer Pliocene. It extends from 2.58 – 0.0017 Ma, and includes several major glaciations. It is preceded by the Pliocene. It is also the period when Homo sapiens evolved.
Pliocene: Initially defined by Lyell in 1833, with two divisions – the Older and Newer Pliocene – the Newer is now the Pleistocene. Hence its etymology is similar to that of Pleistocene – from the Greek pleistos meaning much or more, and the word element -cene meaning recent. The Latin form of -cene is recens from which we also get recent. The Pliocene spans 5.333 – 2.58 Ma. It is the ultimate Series in the Neogene System.
Radiolaria: Marine zooplankton that secrete a symmetrical, intricately structured silica (opal) skeleton; they have a very long geological range, from the latest Precambrian to Recent. They are very useful biostratigraphic indicators, particularly where calcareous microfossils are absent – this is commonly the case for deep ocean sediments (oozes) below the calcite (CCD) and aragonite compensation depths (ACD). The original opal converts to microcrystalline quartz during sediment burial.
Ravinement surface: A surface of erosion that accompanies the landward migration of a shoreline and associated shoreface. Ravinement may erode coastal dune, lagoon and estuarine deposits from the previous highstand. It may also remove some or all of the subaerial unconformity that formed during the previous sea level fall. They are one of the hallmarks of transgression. The two main processes forming these surfaces involve erosion by waves and tidal currents – the distinction between the two in the stratigraphic record is based on facies analysis. Ravinement surfaces are also divided into unconformable (SR-U) that have regional extent and commonly erode underlying subaerial unconformities, and diastemic SR surfaces of more local extent and minimal erosion.
Regolith: A surface covering or layer of loose, unconsolidated rubble. Clasts of all sizes are generally unsorted, commonly incorporating broken bedrock.
Regression: A general term for retreat of the sea and the accompanying seaward migration of shorelines and associated marine sedimentary facies and biotas. The process of regression results in subaerial exposure of former sea floor. Regression is the result of baselevel (sea level) fall. See Forced regression; Normal regression.
Regressive surface of marine erosion: (RSME) The abrupt, erosional contact at the base of shoreface wedges that form during forced regression. They form in concert with down-stepping shoreline trajectories. The RSME overlies highstand deposits that accumulated during an earlier stage of normal regression.
Regressive Systems Tract (RST): The regressive systems tract contains all strata deposited from the beginning to the end of regression; there is no subdivision into normal and forced regression. It is one of only two systems tracts in Transgressive-Regressive (T-R) sequence stratigraphic schemes. The base is the maximum flooding surface; the top is the subaerial unconformity and in the marine part of the succession, the maximum regressive surface. The RST in the T-R model contains the highstand, falling stage, and lowstand systems tracts.
Retrogradation: Back-stepping of sedimentary facies and parasequences during transgression. The shoreline trajectory also trends landward. The stacking pattern in a retrogradational succession will show progressive deepening from one parasequence to the next.
Rule of Vs: A technique, borrowed from geographers, that simplifies the field mapping of strata across valleys and ridges. If bedding dip is known, it is possible to predict where across a valley or ridge the strata should be located.
Sequence stratigraphy: A method of stratigraphic analysis that recognises that the sedimentary record is organized into discrete, but genetically related stratal packages bound by key stratigraphic surfaces, surfaces that repeat through time and are dynamically controlled by changes in baselevel, accommodation, and sediment supply.
Shelf break: (or shelf edge) A relatively narrow submarine zone marking the transition from a continental shelf to steeper inclined continental slope – slopes commonly 2o – 5o . The break may interrupted by gullies eroded by rivers during sea level lowstands, or formed by submarine slope failures.
Stacking patterns (stratigraphy): The stratigraphic trend of repeated depositional cycles (at any scale). In sequence stratigraphy the stacking of parasequences is the basis for identifying systems tracts. For example, successive parasequences that indicate progressive deepening in a succession would be included in a retrogradational systems tract that develops during transgression. The stacking pattern is also reflected in the shoreline trajectory.
Stratigraphic cycles: The periodic repetition of sedimentary facies, fauna and flora associations, sediment chemistry, and hiatuses or discordant surfaces, that represent changing depositional environments, fluctuations in relative sea level and sediment accommodation, migrating shorelines, and the changing conditions of sediment storage and release. Cycles range in thickness from mm to 100s of metres, and duration from minutes to millions of years. They develop from both allogenic and autogenic processes.
Stratigraphic trends: A stratigraphic trend is the relatively ordered, vertical and lateral changes in bed geometry, sediment composition, sedimentary structures, and fossil – trace fossil content. Stratigraphic trends are found at all geological scales, from a few centimetres to 1000s of metres. Typically, we observe them as fining- and coarsening-upward trends. They are a fundamental element of stratigraphy, particularly sequence stratigraphy because identification of parasequences relies on recognition of such trends. Repeated trends comprise stratigraphic stacking patterns. See also shoreline trajectory.
Stratigraphic units: The International Commission on Stratigraphy defines three principle units:
- Time/geochronologic units (Era, Period) that refer only to geological time and not process.
- Time-rock/chronostratigraphic/time-stratigraphic units; rocks formed during a specific interval of time (System, Epoch).
- Rock/lithostratigraphic Units that refer only to the composition and mapability of strata (Formations, Groups).
Subaerial unconformity: In sequence stratigraphy, subaerial unconformities develop during regression and sea level lowstand, and for at least some of the subsequent transgression until the shelf is completely flooded. The shelf or platform is exposed to erosion and meteoric diagenesis. The hiatus is least at the final position of the shoreline, and greatest landward. They are generally considered to be chronostratigraphic surfaces. They are sequence boundaries.
Subcrop: Subsurface rock bodies and stratigraphic or structural surfaces, the extent of which can be mapped by well intersections, seismic profiles, and potential field data. The term is broader in scope than outcrop because it includes surfaces such as unconformities and paleotopography, and boundaries such as depositional or erosional limits of strata, and deformation boundaries.
Systems – history: A stratigraphic system is a chronostratigraphic unit that is represented by rocks formed during a specific interval of time; standard examples include Devonian, Jurassic etc. The geochronological equivalent is a Period (that represents time only). All major system names had been proposed by the late 19th century:
- Sedgwick – Cambrian, 1835
- Lapworth – Ordovician, 1879
- Murchison – Silurian, 1835
- Murchison & Sedgwick – Devonian, 1839
- Conybeare & Phillips – Carboniferous, 1822
- Murchison – Permian, 1841
- Von Alberti – Triassic, 1834
- Von Humboldt – Jurassic, 1799
- D’Halloy – Cretaceous, 1841
- Aduino – Tertiary, 1760
- Denoyers – Quaternary, 1829
Systems tracts: Systems tracts consist of depositional systems that are contemporaneous and genetically linked. They contain relatively comfortable stratigraphic successions (i.e. no major unconformities). When first used in sequence stratigraphy by Vail and others (1977) they were conveyed as representing positions on a eustatic sea level curve. The terms Highstand, Lowstand, Transgressive, and Falling Stage, and Regressive systems tracts are now associated only with relative sea level. Systems tracts can be used at any scale.
Toplap: Clinoforms and 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 geometry typical of progradational successions. A toplap surface may be eroded by a truncation surface. See also onlap, downlap, offlap.
T-R sequences: An alternative sequence stratigraphic model. They are bound by subaerial unconformities and their marine equivalents, maximum regressive surfaces (MRS). The regressive component assembles the highstand, falling stage, and lowstand systems tracts of depositional and Genetic sequences into a single stratigraphic entity – a Regressive Systems Tract.
Trace fossils: Trace fossils are what is left of the activity of some ancient critter, from dinosaurs to worms, that moved, burrowed, bored, rested, walked within or along a sediment surface. They represent animal behaviour rather than the animal itself and thus reflect feeding, resting, creating a home, escaping, or just wandering around.
Transgression: An advance of the sea over land as sea level rises, and the accompanying landward migration, or retrogradation of shorelines and associated marine sedimentary facies and biotas. The process of transgression may result in landward-progressing erosion, or ravinement. On a standard sea level curve transgression begins soon after baselevel begins to rise if the rate of sedimentation exceeds accommodation.
Transgressive surface: See Maximum regressive surface.
Transgressive systems tracts (TST): Form during rising sea levels where accommodation exceeds sedimentation rates. The base is a maximum regressive surface (MRS); the upper bounding surface is the maximum flooding (MFS). The TST overlies the HST and, where it is developed, the FSST. Depositional systems commonly include fluvial, delta and shelf strata.
Unconformity: A stratigraphic surface that indicates a break in deposition, and across which there is a significant hiatus. The full meaning of unconformities was discovered in the late 18th century by James Hutton. Their importance lies in the unraveling of major geological events such as mountain building or rifting continents and breaks in the sedimentary record resulting from changes in climate, tectonic events, or fluctuations in sea level. Subaerial unconformities define the boundaries of stratigraphic sequences. See angular unconformity, disconformity, non-conformity.
Undaform: A term introduced by John Rich (1951) to encompass the region between the shoreline and wave base of a shelf or platform – what we now call the shoreface. Undaform was part of his system that included clinoform and fondoform. The terms undaform and fondoform have all but faded into obscurity.
Uniformitarianism: One of the most important statements made by James Hutton in 1785, https://www.geological-digressions.com/a-chance-encounter-with-james-ussher-circa-1650/ was that the natural processes that today produce landscapes, mountain belts and oceans, progressed with the same intensity and as uniformly in the distant past (a past that he understood to have “…no vestige of a beginning…”). This is the principle of uniformity. In 1832, William Whewell coined the cumbersome extension uniformitarianism. Later still, Archibald Geikie coined the phrase The Present is the Key to the Past. Stephen J Gould has argued that the principle goes much further than using the present as analogy for past events and processes – it provides us with the philosophical warrant, or justification for induction to rationalise the past. See also Actualism.
Ussher, Bishop James: Bishop of Amargh, Primate of all Ireland and noted scholar, he is best known to the geological world for his scholarly “Annals of the Old Testament, deduced from the first origins of the world” (1650) where he concluded that the universe, and everything in it, began at noon, October 23, 4004 BC. His deduction was based on a careful teasing apart of Biblical genealogy.
Zircon geochronology: Zircon is a ubiquitous trace mineral in sedimentary and volcanic rocks. It utilises 238U→206Pb (the half-life of 4.47 Ga is almost the same as the age of the Earth), 235U→207Pb (half-life of 0.70 Ga), and 232Th→208Pb (half-life 14.01 Ga – getting close to the age of the universe) (1 Ga = 1 billion years). Methods are now available to measure ages not just in single zircon crystals, but in crystal zones, where each zone from the crystal centre outwards has a progressively younger age.
