Glossary

Stratigraphy terms

Plain-language definitions of the terms used across this site. New to the subject? The field guide walks through the core principles in order.

Accretionary wedge: A pile of ocean-floor sediment scraped off a descending plate and stacked up at a subduction zone — the Makran coast is a living example.
Angular unconformity: An unconformity where older, tilted or folded strata are overlain by younger, more horizontal layers.; See also Unconformity
Bed: The smallest formal lithostratigraphic unit — a single distinct layer within a member or formation.; See also Formation Member
Chronostratigraphy: Subdivision of rocks by their age and time of formation, independent of rock type.; See also Lithostratigraphy Geochronology (era, period, epoch)
Conformable: Describes strata deposited in an unbroken sequence, with no significant gap in time between them.; See also Unconformity
Conglomerate: A coarse sedimentary rock made of rounded, gravel-sized clasts cemented together.
Correlation: Establishing that strata in different places are equivalent in age or stratigraphic position.; See also Faunal succession
Diachronous: Describes a rock unit whose age differs from place to place, because the environment that formed it migrated over time.
Disconformity: An unconformity between parallel strata, where the missing time sits along an irregular or buried erosion surface.; See also Unconformity
Evaporite: A rock such as rock salt or gypsum formed when mineral-rich water evaporates; the Precambrian Salt Range Formation is a classic example.
Facies: A body of rock whose characteristics reflect the conditions under which it formed, distinguishing it from neighbouring rock of the same age.; See also Lithology
Faunal succession: The principle that fossil organisms succeed one another through time in a predictable order, so strata can be dated and correlated by the fossils they contain.; See also Law of superposition Correlation
Foreland basin: A basin that forms in front of a growing mountain belt, pushed down by the weight of the rising range and filled with debris eroded from it.; See also Sedimentary basin Orogeny
Formation: The fundamental mappable unit of lithostratigraphy — a distinctive body of rock extensive enough to show on a geological map, formally defined at a type locality.; See also Member Group Bed Type locality / type section
Geochronology (era, period, epoch): The dated divisions of geological time. Eras contain periods, which contain epochs — the framework laid out in the geological time scale.; See also Chronostratigraphy
Gondwana: The southern supercontinent — including the Indian plate — from which the land now forming Pakistan rifted and drifted northward.; See also Tethys
Group: Two or more associated formations that share characteristics, ranked one level above the formation.; See also Formation
Law of superposition: In an undisturbed sequence, each layer is younger than the one below it and older than the one above.; See also Original horizontality Faunal succession
Lignite: A soft, brown, low-grade coal; the Thar coalfield of Sindh is one of the largest lignite deposits in the world.
Lithology: The physical character of a rock — its composition, grain size, texture and colour.; See also Facies
Lithostratigraphy: Subdivision of rocks into units — formations, members and so on — based on their physical, lithological character.; See also Chronostratigraphy Formation
Marl: A lime-rich mudstone, intermediate in composition between limestone and clay.
Member: A named subdivision of a formation with its own recognisable character, but not mapped separately at the same scale.; See also Formation Bed
Nonconformity: An unconformity where sedimentary rock rests directly on older igneous or metamorphic rock.; See also Unconformity
Original horizontality: The principle that sediments are originally deposited in roughly horizontal layers; strata now tilted were moved after deposition.; See also Law of superposition
Orogeny: A mountain-building episode, typically driven by the collision of tectonic plates.; See also Foreland basin Suture zone
Outcrop: A place where bedrock is exposed at the surface rather than buried under soil or vegetation.
Regression: A relative fall of sea level that moves the shoreline seaward, exposing what was formerly seabed.; See also Transgression
Sedimentary basin: A region of the crust that subsides over long periods so sediment accumulates in it, building up a thick stratigraphic succession.; See also Foreland basin
Stratigraphy: The branch of geology concerned with the order, arrangement and age relationships of rock layers, and what they record about the history of the Earth.; See also Lithostratigraphy Chronostratigraphy
Stratum (strata): A layer of sedimentary rock with consistent characteristics that set it apart from the layers above and below. “Strata” is the plural.
Suture zone: The boundary along which two continental blocks were welded together after the ocean between them closed.; See also Orogeny Tethys
Tethys: The ancient ocean that once lay between Gondwana and the northern continents, squeezed out of existence by the collision that built the Himalaya.; See also Gondwana Suture zone
Transgression: A relative rise of sea level that moves the shoreline landward, depositing marine sediments over what was formerly land.; See also Regression
Type locality / type section: The specific place where a stratigraphic unit is formally defined, and against which other occurrences of it are compared.; See also Formation
Unconformity: A surface in the rock record representing a gap in time, where deposition paused or erosion removed rock before younger layers were laid down.; See also Disconformity Angular unconformity Nonconformity

Definitions are independent plain-language summaries for learners. For formal usage see the Geological Survey of Pakistan and the International Commission on Stratigraphy.